# Rocks/Glaciers/Glaciology

< Rocks‎ | Glaciers(Redirected from Glaciology)
The image shows a tidewater glacier. Credit: U.S. Fish and Wildlife Service.

Glaciology studies the internal dynamics and effects of glaciers. More than one glacier with a common source is an ice field. Several ice fields can become an ice cap. When the ice cap becomes large enough it is an ice sheet.

## Astronomy

The Tibetan plateau, often called the third pole, will be monitored by balloons, drones and ground sensors. Credit: Wolfgang Kaehler/LightRocket via Getty.

"Sitting at an average height of around 4,000 metres above sea level, the plateau protrudes into the middle of the troposphere, where most weather events originate. As the biggest and highest plateau in the world, it disturbs this part of the atmosphere like no other structure on Earth."[1]

"The plateau’s remoteness, altitude and harsh conditions — it is often called the third pole because it hosts the world’s third-largest stock of ice — mean that even basic weather stations are few. Satellite data are also plagued by large errors owing to lack of calibration from ground observations."[1]

In 1941, Hole-in-the-Wall Glacier surged. Credit: W.O. Field, World Data Center for Glaciology, Boulder, CO.

The image shows a glacial surge from the Sermersauq Ice Cap. Credit: Robert Gilbert, Niels Nielsen, Henrik Möller, Joseph R. Desloges, and Morten Rasch.

Def. "a glacier that experiences a dramatic increase in flow rate, 10 to 100 times faster than its normal rate; usually surge events last less than one year and occur periodically, between 15 and 100 years"[2] is called a surging glacier.

"In 1941, Hole-in-the-Wall Glacier [imaged at the right] surged, also knocking over trees during its advance."[2]

An "outlet glacier of the Sermersauq Ice Cap [on Disko Island, West Greenland, shown at the left with progressive surges marked] has surged 10.5 km downvalley to within 10 km of the fjord. [...] surging of the glacier, begun in 1995, was undetected until July 1999, when it was discovered during a geomorphic survey of the valley. Mapping from TM, Landsat and SPOT satellite imagery, and subsequent field work have documented the history of the event. On 17 June 1995 the terminus of the glacier was about where it appears in the 1985 air photography [...]. By 24 September 1995 the glacier had advanced 1.25 km and by 12 October another 1.25 km (mean advance during the second period : 70 m day-1). The advance slowed from 18 m day-1 in 1996 to 5 m day-1 in 1997 and <1 m day-1 between 1997 and 1999. By summer 1999 the advance ceased; the maximum extension of the terminus, about 10.5 km down-valley to about 10 km from the head of the fjord, was mapped from imagery on 9 July 1999 [...]."[3]

## Planetary sciences

Geologic time is annotated with glacial or ice age periods. Credit: William M. Connolley.

Earth at the last glacial maximum of the current ice age. Credit: Ittiz, based on: "Ice age terrestrial carbon changes revisited" by Thomas J. Crowley (Global Biogeochemical Cycles, Vol. 9, 1995, pp. 377-389.

Recent (black) and maximum (grey) glaciation of the northern hemisphere are during the Quaternary climatic cycles. Credit: Hannes Grobe/AWI.

Recent (black) and maximum (grey) glaciation of the southern hemisphere are during the Quaternary climatic cycles. Credit: Hannes Grobe/AWI.

The ice ages or glaciations on Earth occurred from the early Proterozoic (Huronian), late proterozoic (Cryogenian), early Paleozoic (Andean-Saharan) during the Ordovician and Silurian periods, late Paleozoic (Karoo Ice Age) during the Carboniferous and early Permian periods, and lately the Quaternary glaciation.

Although these ice ages are widely separated in geological time, "in most parts of the Earth major climatic and palaeoenvironmental units typically have a duration of the order of half a precession cycle (around 10 ka) rather than half an eccentricity cycle (around 50 ka) so that the level of stratigraphic resolution provided by the Middle Pleistocene [Marine Isotope Stage] MIS (typical duration 50 ka) is not sufficiently fine to constitute a universal stratigraphic template."[4]

## Colors

This image shows the blue water ice, or blue ice, of a glacier. Credit: .

Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of a glacier ... blue ice was observed in Tasman Glacier, New Zealand in January 2011.[5] ... ice is blue for the same reason water is blue: it is a result of an overtone of an oxygen-hydrogen (O-H) bond stretch in water which absorbs light at the red end of the visible spectrum.[6]

## Firns

At the Dye 3 location in south east Greenland, "it takes roughly 100 years before the surface snow is compressed into solid ice. During this slow process (firnification) a given snow layer sinks to a depth of 80 m below the new surface formed under constant deposition of 1m of snow per year in South Greenland."[7]

In places, "surface melting often occurs in the summer time. The melt water seeps through the porous snow and refreezes somewhere in the cold firn, which disturbs the layer sequence, of course."[7]

"Firn air is the air that is trapped in the porous medium of firn, which is typically the first one hundred meters of an ice core."[8]

At the South Pole, the firn-ice transition depth is at 122 m, with a CO2 age of about 100 years.

## Theoretical glaciology

The diagram is a cross-section of a glacier showing facies at the end of the balance year from glaciological field observations. Credit: Jan-Gunnar Winther.

The diagram illustrates the interrelationship of glaciology terms. Credit: .

Def. "a mass of ice that originates on land, usually having an area larger than one tenth of a square kilometer"[2] is called a glacier.

"[M]any believe that a glacier must show some type of movement; others believe that a glacier can show evidence of past or present movement."[2]

Def. the study of the internal dynamics and effects of glaciers is called glaciology.

At the center above is an idealized diagram of an alpine or mountain glacier. "Glaciers are composed of an ablation and an accumulation area. Within these two areas several facies might be present [as indicated in the center diagram]. The facies represent distinctive areas with characteristics that reflect the environment under which the snow or ice was formed."[9]

"The accumulation area is typically composed of wet-snow facies, percolation facies and dry-snow facies [...] due to long periods of mild weather which influence the glacier surface at all altitude levels, the accumulation area [may consist] predominantly of wet- snow facies at the end of the ablation period."[9]

"Superimposed ice is formed by (1) the refreezing of meltwater during the autumn and/or during the ablation period and (2) the refreezing of meltwater on the glacier surface below the snow line at the end of the ablation period [...] net loss by melting occurs in the ice facies."[9]

Def. "a current of ice in an ice sheet or ice cap that flows faster than the surrounding ice"[2] is called an ice stream.

Def. a network of interconnected glaciers or ice streams having a common source or a large expanse of floating ice (several miles long) is called an ice field.

Def. "a dome-shaped mass of glacier ice that spreads out in all directions"[2] is called an ice cap.

Def. "a dome-shaped mass of glacier ice that covers surrounding terrain and is greater than 50,000 square kilometers (12 million acres)"[2] is called an ice sheet.

## Aufeis

A group of hikers travels over a large sheet of aufeis in the Anaktuvuk River Valley. Credit: Paxson Woelber.

Laminations of ice occur in a sheet of aufeis. Credit: Nswanson.

A sheet of aufeis occurs in a glacial valley in Mongolia. Credit: Nswanson.

Ice layers in the trees are formed by an earlier winter flood. Credit: Doronenko.

Def. a sheet-like layered mass of ice is called aufeis.

Def. a sheet-like layered mass of ice formed in freezing temperatures from the freezing of successive flows of ground water over previously formed layers of ice is called naled.

## Little Ice Age

Changes in the 14C record, which are primarily (but not exclusively) caused by changes in solar activity, are graphed over time. Credit: Leland McInnes.

The Little Ice Age (LIA) appears to have lasted from about 1218 (782 b2k) to about 1878 (122 b2k).

A "climate interpretation was supported by very low δ’s in the 1690’es, a period described as extremely cold in the Icelandic annals. In 1695 Iceland was completely surrounded by sea ice, and according to other sources the sea ice reached half way to the Faeroe Islands."[7]

In the image at the top, "before present" is used in the context of radiocarbon dating, where the "present" has been fixed at 1950. The apparent decreases in solar activity are called the "Maunder Minimum", "Spörer Minimum", "Wolf Minimum", and "Oort Minimum".

"Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes."[10]

Precisely "dated records of ice-cap growth from Arctic Canada and Iceland [show] that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430-1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium. [Explosive] volcanism produces abrupt summer cooling at these times, and that cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed. [The] onset of the LIA can be linked to an unusual 50-year-long episode with four large sulfur-rich explosive eruptions, each with global sulfate loading >60 Tg. The persistence of cold summers is best explained by consequent sea-ice/ocean feedbacks during a hemispheric summer insolation minimum; large changes in solar irradiance are not required."[10]

## Early history

The early history period dates from around 3,000 to 2,000 b2k.

## Subatlantic history

The "calibration of radiocarbon dates at approximately 2500-2450 BP [2500-2450 b2k] is problematic due to a "plateau" (known as the "Hallstatt-plateau") in the calibration curve [...] A decrease in solar activity caused an increase in production of 14C, and thus a sharp rise in Δ 14C, beginning at approximately 850 cal (calendar years) BC [...] Between approximately 760 and 420 cal BC (corresponding to 2500-2425 BP [2500-2425 b2k]), the concentration of 14C returned to "normal" values."[11]

## Subboreal history

The "period around 850-760 BC [2850-2760 b2k], characterised by a decrease in solar activity and a sharp increase of Δ 14C [...] the local vegetation succession, in relation to the changes in atmospheric radiocarbon content, shows additional evidence for solar forcing of climate change at the Subboreal - Subatlantic transition."[11]

The "Subboreal period is characterized by the highest accumulations of diatom frustules and chrysophyte cysts in Lake Baikal sediments. The siliceous microfossil record suggests that the Holocene climatic optimum in this interior part of Asia corresponds to the Subboreal period 2.5–4.5 ka and not to the Atlantic period 4.6–6 ka. Although quite different from Holocene reconstructions for the European part of Eurasia, the Holocene sedimentary record from Lake Baikal shows good correlation with palynological and soil climatic records from southeast Siberia and Mongolia where similar responses of the terrestrial biosphere are also documented. A distinctive monospecific lamina of Synedra acus diatom species, coincident with the maximum of chrysophyte cyst accumulation during the Subboreal period, argues for the possible short-term changes of the trophic state of Lake Baikal from oligotrophic, with a cold-water diatom assemblage, to eutrophic with a thermophilic monospecific diatom flora."[12]

## Atlantic history

The "Atlantic period [is] 4.6–6 ka [4,600-6,000 b2k]."[12]

"During this [Atlantic] period the pollen indicates that the vegetation was quite clearly dominated by Alnus, with Betula and the trees of the mixed oak forest present in some quantity. The pollen of Corylus and Pinus, which were the dominants in the previous period, has decreased in amount. The low values for herbaceous types are maintained."[13]

"The end of the Atlantic period was marked by the decline of the elm, and was followed by a series of forest clearances which destroyed the mixed oak forest and produced the present vegetational landscape."[13]

"Some mountain glaciers in both the Northern and Southern Hemispheres advanced in late Atlantic and early sub-Boreal time, between about 5,200 and 4,600 radiocarbon years ago, and several in the Southern Hemisphere reached their greatest post-glacial extents."[14]

## Boreal transition

"In some cores a narrow band of clay interrupts the organic muds, at the horizon of the Boreal Atlantic transition."[13]

"In the woodland the dominant trees are Betula, Pinus and Corylus. Ignoring the problem of over-representation of Pinus in the deep-water cores (cf. Pennington, 1947) the woodland remains a birch-pine woodland with an increasing amount of Corylus. Pinus decreases in the early part of zone V and Betula is also at lower values than previously. Towards the end of the period Pinus again becomes important."[13]

## Ancient history

The ancient history period dates from around 8,000 to 3,000 b2k.

## Pre-Boreal transition

The last glaciation appears to have a gradual decline ending about 12,000 b2k. This may have been the end of the Pre-Boreal transition.

"About 9000 years ago the temperature in Greenland culminated at 4°C warmer than today. Since then it has become slowly cooler with only one dramatic change of climate. This happened 8250 years ago as shown in detail to the left of the main record in [the figure at the right]. In an otherwise warm period the temperature fell 7°C within a decade, and it took 300 years to re-establish the warm climate. This event has also been demonstrated in European wooden ring series and in European bogs."[7]

"The last remains of the American ice sheet disappeared about 6000 years ago, the Scandinavian one 2000 years earlier."[7]

"The Pre-boreal period marks the transition from the cold climate of the Late-glacial to the warmer climate of Post-glacial time. This change is immediately obvious in the field from the nature of the sediments, changing as they do from clays to organic lake muds, showing that at this time a more or less continuous vegetation cover was developing."[13]

"At the beginning of the Pre-boreal the pollen curves of the herbaceous species have high values, and most of the genera associated with the Late-glacial fiora are still present e.g. Artemisia, Polemomium and Thalictrum. These plants become less abundant throughout the Pre-boreal, and before the beginning of the Boreal their curves have reached low values."[13]

## Younger Dryas

Percentages of Neogloboquadrina pachyderma are shown with depth and 14C dates from cores. Credit: Scott J. Lehman & Lloyd D. Keigwin.

"The Younger Dryas interval during the Last Glacial Termination was an abrupt return to glacial-like conditions punctuating the transition to a warmer, interglacial climate."[15]

"From former cirque glaciers in western Norway, it is calculated that the summer (1.May to 30.September) temperature dropped 5-6°C during less than two centuries, probably within decades, at the Alleröd/Younger Dryas transition, some 11,000 years ago."[16]

"From the same data the Younger Dryas summer temperature is estimated to have been 8-10°C lower than at present, and from fossil ice wedges the mean annual temperatures 13°C lower than at present in the same area."[16]

"At the time of the Alleröd/Younger Dryas transition, the Scandinavian ice-sheet was still a major element in the climate system. The record from the Younger Dryas is distinct, consisting of ice-marginal deposits that are mapped nearly continuously around Scandinavia [...], showing that the climate turned to a more glacial regime in both the continental climate area of USSR/Finland and the oceanic climate area of Western Norway. This suggests that lower summer temperatures, and not increased winter precipitation, was the climatic factor that determined the major pattern of glacial response."[16]

An "amplification of the re-advance in Western Norway compared to the easterly areas, due to higher winter precipitation along the western flank of the ice-sheet, and topographical and glaciological factors [...] The re-advance also caused a relative rise in sea level in Western Norway through the combination of increased gravitational attraction and a halt in glacio-isostatic uplift (Anundsen, 1985)."[16]

The diagrams on the right show percentages of the planktonic foraminifera Neogloboquadrina pachyderma from two cores: "a" Troll 3.1 (60° 46.7' N, 3° 42.8' E, 332 m water depth) in the Norwegian Trench and "b" V23-81 located off Ireland.[17]

"Annual layer counting through the most recent of these [sudden changes in the temperature of precipitation] indicates that a warming of ~7 °C occurred within a 50-yr period during the transition from the Younger Dryas cold phase (~11-10 kyr BP) to the present interglacial2."[17]

## Allerød Oscillation

"During the Allerød Chronozone, 11,800 to 11,000 years ago, western Europe approached the present day environmental and climatic situation, after having suffered the last glacial maximum some 20,000 to 18,000 years ago. However, the climatic deterioration 11,000 years ago led to nearly fully glacial conditions on this continent for some few hundreds of years during the Younger Dryas. This change is completely out of phase with the Milankovitch (orbital) forcing as this is understood today, and therefore its cause is of major interest."[16]

"During the Allerød a branch of the North Atlantic Current entered the Norwegian Sea (Ruddiman and Mclntyre, 1973, 1981)."[16]

"Recent stratigraphical achievements and long time established chronologies exist for the Late Weichselian, i.e. 10-25 ka BP. During this period Denmark experienced the complex Main-Weichselian glaciation from 25 to about 14 ka BP (Jylland stade, Houmark-Nielsen 1989) followed by the Late Glacial climatic amelioration including the interstadial Bølling-Allerød oscillation (13-11 ka BP), finally leading to the interglacial conditions that characterize the Holocene (Hansen 1965)."[18]

The "large, but so far largely ignored eruption of the Laacher See-volcano, located in present-day western Germany and dated to 12,920 BP, had a dramatic impact on forager demography all along the northern periphery of Late Glacial settlement and precipitated archaeologically visible cultural change. In Southern Scandinavia, these changes took the form of technological simplification, the loss of bow-and-arrow technology, and coincident with these changes, the emergence of the regionally distinct Bromme culture. Groups in north-eastern Europe appear to have responded to the eruption in similar ways, but on the British Isles and in the Thuringian Basin populations contracted or relocated, leaving these areas largely depopulated already before the onset of the Younger Dryas/GS-1 cooling."[19]

Evidence "from the Hudson Valley and the northeastern U.S. continental margin [...] establishes the timing of the catastrophic draining of Glacial Lake Iroquois, which breached the moraine dam at the Narrows in New York City, eroded glacial lake sediments in the Hudson Valley, and deposited large sediment lobes on the New York and New Jersey continental shelf ca. 13,350 yr B.P. Excess 14C in Cariaco Basin sediments indicates a slowing in thermohaline circulation and heat transport to the North Atlantic at that time, and both marine and terrestrial paleoclimate proxy records around the North Atlantic show a short-lived (<400 yr) cold event (Intra-Allerød cold period) that began ca. 13,350 yr B.P."[20]

## Neolithic

The base of the Neolithic is approximated to 12,200 b2k.

## Mesolithic

The mesolithic period dates from around 13,000 to 8,500 b2k.

## Older Dryas

Comparison of the GRIP ice core with cores from the Cariaco Basin shows the Older Dryas. Credit: Konrad A Hughes, Jonathan T. Overpeck, Larry C. Peterson & Susan Trumbore.

The Older Dryas is a "century-scale cold [event]".[21]

"The most negative δ 18O excursions seen in the GRIP record lasted approximately 131 and 21 years for the [inter-Allerød cold period] IACP and [Older Dryas] OD, respectively. The comparable events in the Cariaco basin were of similar duration, 127 and 21 years. In addition to the chronological agreement, there is also considerable similarity in the decade-scale patterns of variability in both records. Given the geographical distance separating central Greenland from the southern Caribbean Sea, the close match of the pattern and duration of decadal events between the two records is striking."[22]

In the figures on the right, especially b, is a detailed "comparison of δ 18O from the GRIP ice core24 with changes in a continuous sequence of light-lamina thickness measurements from core PL07-57PC. Both records are constrained by annual chronologies, although neither record is sampled at annual resolution. The interval of comparison includes the inter-Allerød cold period (12.9-13 cal. kyr BP) and Older Dryas (13.4 cal. kyr BP) events (IABP and OD from a). The durations of the two events, measured independently in both records, are very similar, as is the detailed pattern of variability at the decadal timescale."[22]

## Bølling Oscillation

The Greenland ice-core oxygen isotope (δ 18O) stratigraphy. Credit: Barbara Wohlfarth.

The "intra-Bølling cold period [IBCP is a century-scale cold event and the] Bølling warming [occurs] at 14600 cal [calendar years] BP (12700 14C BP)".[21]

The "Bølling was originally defined as starting from 13000 14C BP (calibrated to ~15650 cal BP; Stuiver et al., 1998). [...] independent annual chronology indicate a much later onset of the Bølling (e.g., 14600 cal BP".[21]

"During the IBCP and perhaps also IACP, δ 18O values inversely correlate with δ 13C, but during the OD δ 18O shows positive correlation with δ 13C, suggesting dry conditions with high evaporation, as well as cold."[21]

The "δ 18O record shows late-glacial climatic deterioration beginning in latest Bølling time and culminating in a Younger Dryas reversal. The vegetation record shows only a small increase in non-arboreal pollen in Younger Dryas time, reflecting some openings in the forest cover. The alpine moraine record shows widespread Egesen moraines of Younger Dryas age (Ivy-Ochs et al. 1999). In sharp contrast to the situation in Great Britain, Younger Dryas cooling is not reflected in the insect record at Lobsigensee."[23]

"During the early Bølling warming, the boreal insect assemblage is replaced by a plant-independent temperate assemblage that reflects a mean July temperature close to interglacial values, just as in Great Britain. A shift in water plants supports such marked warming. The early Bølling climate was warm enough to support broad-leafed deciduous trees such as oak or hazel, which do not appear until 3000 years later because of migrational lags."[23]

The Bølling interstadial corresponds to GIS 1 as shown in the diagram on the right.[24]

## Oldest Dryas

"During the Late Weichselian glacial maximum (20-15 ka BP) the overriding of ice streams eventually lead to strong glaciotectonic displacement of Late Pleistocene and pre-Quaternary deposits and to deposition of till."[18]

"The synchronous and nearly uniform lowering of snowlines in Southern Hemisphere middle-latitude mountains compared with Northern Hemisphere values suggests global cooling of about the same magnitude in both hemispheres at the [Last Glacial Maximum] LGM. When compared with paleoclimate records from the North Atlantic region, the middle-latitude Southern Hemisphere terrestrial data imply interhemispheric symmetry of the structure and timing of the last glacial/interglacial transition. In both regions atmospheric warming pulses are implicated near the beginning of Oldest Dryas time (~14,600 14C yr BP) and near the Oldest Dryas/Bølling transition (~12,700-13,000 14C yr BP). The second of these warming pulses was coincident with resumption of North Atlantic thermohaline circulation similar to that of the modern mode, with strong formation of Lower North Atlantic Deep Water in the Nordic Seas. In both regions, the maximum Bølling-age warmth was achieved at 12,200-12,500 14C yr BP, and was followed by a reversal in climate trend. In the North Atlantic region, and possibly in middle latitudes of the Southern Hemisphere, this reversal culminated in a Younger-Dryas-age cold pulse."[23]

"The Oldest Dryas part of the Lobsigensee records [on the Swiss Plateau] features a boreal and boreal-montane insect assemblage, along with Betula nana."[23]

## Heinrich event H1

This stadial starts about 17.5 ka, extends to about 15.5 ka and is followed after a brief warming by H1.

"After c. 22 ka BP during the Jylland stade (Houmark-Nielsen 1989), Late Weichselian glaciers of the Main Weichselain advance overrode Southeast Denmark from the northeast and later the Young Baltic ice invaded from southeasterly directions. Traces of the Northeast-ice are apparently absent in the Klintholm sections, although large scale glaciotectonic structures and till deposits from this advance are found in Hjelm Bugt and Møns Klint (Aber 1979; Berthelsen 1981, 1986). At Klintholm, the younger phase of glaciotectonic deformation from the southeast and south and deposition of the discordant till (unit 9) were most probably associated with recessional phases of the Young Baltic glaciation. In several cliff sections, well preserved Late Glacial (c. 14-10 ka BP) lacustrine sequences are present (Kolstrup 1982, Heiberg 1991)."[18]

The "Allarp Till (Berglund & Lagerlund 1981), was deposited in connection with the first Late Weichselian ice advance in southern Sweden. Petrographic studies (Bose 1990) indicate that the first Late Weichselian ice advance which overrode northern Germany and reached the Brandenburg stage has a composition comparable to the Allarp till and the bedded diamictons from Klintholm."[18]

## Letzteiszeitliches Maximum

Diagram showings the position of the Lascaux interstadial (marked in red and orange) within the time range 10 to 30 ky BP. Credit: Rudolf Pohl.{{free media}}

"One wave was called Murrayians. This is an Ainu or Vedda-like group from the Thailand area. Skulls from Thailand 25,000 YBP resemble Aborigines."[26]

"Australoid types were present long before in India and Southeast Asia as skulls from India and Thailand 25,000 YBP are said to resemble Aborigines."[27]

Heinrich Event 2 (H2) extends "22-25.62 ka BP".[25]

The δ18O values from GISP-2 follow the diagram of Wolfgang Weißmüller. The positions of the Dansgaard-Oeschger events DO1 to DO4 and the Heinrich events H1 to H3 are also indicated. DV 3-4 and DV 6-7 are cold events marked by ice wedges in the upper loess of Dolní Veštonice.

## Heinrich Event 3

Heinrich Event 3 (H3) "occurs at 26.74 ka BP, coincident with the start of the transition into GIS 4."[25]

MIS Boundary 2/3 is at 29 ka.[28]

This advance occurred after the Møn and ended with GIS 6.[24]

## Heinrich Event 4

Heinrich Event 4 "33-39.93 ka BP".[25]

The "Hasselo stadial [is] at approximately 40-38,500 14C years B.P. (Van Huissteden, 1990)."[29]

The most pronounced cold interval in The Netherlands is the Hasselo Stadial (van Huissteden, 1990) at ca. 41-38.5 ka, followed by the warm Hengelo Interstadial (Zagwijn, 1974)."[30]

"One of two strongly rounded fragments of the mammoth maxilla from the Shapka Quarry in the southern Leningrad region was recently dated at 38450 + 400/–300 years (GrA-39 116) and rhinoceros remains (spoke bone), back to 38360 + 300/–270 years ago (GrA-38 819) [7]. The maxilla fragments occurred in sediments of the Leningrad Interstadial, which correspond to the transition between the Hasselo Stadial (44–39 ka ago) and the Hengelo Interstadial (38–36 ka ago)."[31]

The Hasselo stadial corresponds to the Skjonghelleren stadial in Norway but to the Sejrø interstadial in Denmark.[24]

"Paleomagnetic samples were obtained from cores taken during the drilling of a research well along Coyote Creek in San Jose, California, in order to use the geomagnetic field behavior recorded in those samples to provide age constraints for the sediment encountered. The well reached a depth of 308 meters and material apparently was deposited largely (entirely?) during the Brunhes Normal Polarity Chron, which lasted from 780 ka to the present time."[32]

"Three episodes of anomalous magnetic inclinations were recorded in parts of the sedimentary sequence; the uppermost two we correlate to the Mono Lake (~30 ka) geomagnetic excursion and 6 cm lower, tentatively to the Laschamp (~45 ka) excursion."[32]

"Some 41,000 years ago, a complete and rapid reversal of the geomagnetic field occured. Magnetic studies on sediment cores from the Black Sea show that during this period, during the last ice age, a compass at the Black Sea would have pointed to the south instead of north."[33]

"[A]dditional data from other studies in the North Atlantic, the South Pacific and Hawaii, prove that this polarity reversal was a global event."[33]

"The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today's configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today's field."[33]

"The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast."[33]

"During this period, the field was even weaker, with only 5% of today's field strength. As a consequence, Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure."[33]

"This is documented by peaks of radioactive beryllium (10Be) in ice cores from this time, recovered from the Greenland ice sheet. 10Be as well as radioactive carbon (14C) is caused by the collision of high-energy protons from space with atoms of the atmosphere."[33]

"The polarity reversal [...] has already been known for 45 years. It was first discovered after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central, which differed significantly from today's direction of the geomagnetic field. Since then, this geomagnetic feature is known as the 'Laschamp event'."[33]

The "new data from the Black Sea give a complete image of geomagnetic field variability at a high temporal resolution."[33]

## Marine Isotope Stage 3

Stone hand axes found at Lynford Quarry
 Discoid hand axe
 Cordiform hand axe
 Amygdaloid hand axe

Inca Huasi was a paleolake in the Andes named by a research team in 2006.[34]

It existed about 46,000 years ago in the Salar de Uyuni basin.[34] Water levels during this episode rose by about 10 metres (33 ft). Overall, this lake cycle was short and not deep,[34] with water levels reaching a height of 3,670 metres (12,040 ft). The lake would have had a surface of 21,000 square kilometres (8,100 sq mi).[35] Most water was contributed to it by the Uyuni-Coipasa drainage basin, with only minimal contributions from Lake Titicaca.[36] Changes in the South American monsoon may have triggered its formation.[37]

Radiocarbon dates on tufa which formed in Lake Inca Huasi were dated at 45,760 ± 440 years ago.[34] Uranium-thorium dating has yielded ages between 45,760 and 47,160 years.[34] Overall the lake existed between 46,000 and 47,000 years ago.[35] The Inca Huasi cycle coincides with the marine isotope stage 3,[38] the formation of a deep lake in the Laguna Pozuelos basin and the expansion of glaciers in several parts of South America.[39]

This lake cycle took part during a glacial epoch, along with the Sajsi lake cycles.[34] A more humid climate in northeastern Argentina and elsewhere in subtropical South America has been linked to the Inca Huasi phase.[37] However, rainfall might not have increased by much on the Altiplano during the Inca Huasi cycle.[35]

Other paleolakes are Coipasa, Ouki, Minchin, Sajsi, Salinas and Tauca.[34] Research made in 2006 attributed the "Lake Minchin" to this lake phase.[37]

In archaeology, a bout-coupé is a type of handaxe that constituted part of the Neanderthal Mousterian industry of the Middle Palaeolithic. The handaxes are bifacially-worked and in the shape of a rounded triangle. They are only found in Britain in the Marine Isotope Stage 3 (MIS 3) interglacial between 59,000 and 41,000 years BP, and are therefore considered a unique diagnostic variant.[40][41]

Lynford Quarry is the location of a well-preserved in-situ Middle Palaeolithic open-air site near Mundford, Norfolk.[42]

The site, which dates to approximately 60,000 years ago, is believed to show evidence of hunting by Neanderthals (Homo neanderthalensis). The finds include the in-situ remains of at least nine woolly mammoths (Mammuthus primigenius), associated with Mousterian stone tools and debitage. The artefactual, faunal and environmental evidence were sealed within a Middle Devensian palaeochannel with a dark organic fill. Well preserved in-situ sites of the time are exceedingly rare in Europe and very unusual within a British context.[43]

The site also produced rhinoceros teeth, antlers, as well as other faunal evidence. The stone tools on the site numbered 600, made up of individual artefacts or waste flakes. Particularly interesting were the 44 hand axes of sub-triangular or ovate form.[44]

The site was dated to Marine Isotope Stage 3 using Optically Stimulated Luminescence dating of the sand from the two layers of deposits within the channel.[44]

Eruptions occurred at Monte Burney (a volcano in southern Chile, part of its Austral Volcanic Zone which consists of six volcanoes with activity during the Quaternary) during the Pleistocene. Two eruptions around 49,000 ± 500 and 48,000 ± 500 years before present deposited tephra in Laguna Potrok Aike,[45] a lake approximately 300 kilometres (190 mi) east of Monte Burney;[46] there they reach thicknesses of 48 centimetres (19 in) and 8 centimetres (3.1 in) respectively.[45] Other Pleistocene eruptions are recorded there at 26,200 and 31,000 years ago,[47] with additional eruptions having occurred during marine isotope stage 3.[47]

Three Neanderthal individuals were recovered from the cave. The first, Mezmaiskaya 1, was recovered in 1993 and is an almost complete skeleton in a well preserved state due to calcite cementation that covers and holds the arrangement in place. It was assessed to be an infant about two weeks old, making it the youngest Neanderthal ever recovered. Although no burial pit was found, circumstances suggest that the body was buried intentionally, explaining the good state of preservation and the lack of scavenger marks. Mesmaikaya 1 was recovered from Layer 3, the oldest Late Middle Paleolithic layer at the site. Mezmaiskaya 1 is indirectly dated to around 70-60,000 years old.[48]

Additionally, 24 skull fragments of a 1-2 year-old Neanderthal child - Mezmaiskaya 2 - were found in 1994.[48] A recovered tooth was assigned to Mezmaiskaya 3.[49] Mezmaiskaya 2 was recovered from Layer 2, the youngest Late Middle Paleolithic layer, and directly dated to around 44,600-42,960 BP. DNA analysis reveals that Mesmaiskaya 2 was male.[48]

The middle of the Glacial (mid Marine Isotope Stage-3)is likely the age of Bisitun Cave .[50]

This map of Australia combines genetic and archaeological data to show the movements of Aboriginal populations going back tens of thousands of years. Credit: Alan Cooper and Ray Tobler, University of Adelaide.{{fairuse}}

"Genetics suggests Neanderthal numbers dropped sharply around 50,000 years ago. This coincides with a sudden cold snap, hinting climate struck the first blow."[51]

This corresponds to the Skjonghelleren Glaciation of Scandinavia where ice crosses the North Sea between 50-40 ka BP.

"The first humans probably reached Australia around 50,000 years ago, which is the age of the oldest human skeletons and tools found."[52]

All "the Aborigines likely descend from a single population, which reached the Australian continent 50,000 years ago. Populations then spread rapidly – within 1,500 to 2,000 years – around the east and west coasts of Australia, meeting somewhere in South Australia. Over the following millennia, the population groups remained practically isolated."[52]

"Australia 50,000 years ago was part of the same landmass as New Guinea. So that the first Aborigines could have reached New Guinea by way of South East Asia and then have gone farther to Australia. There, they settled in groups over the whole continent."[52]

Many "groups of Aborigines used similar tools and shared a similar language. If humans did not move, how could tools and languages?"[53]

The Karmøy stadial begins in the high mountains of Norway about 58 kyr B.P. and expands to the outer coast by 60 kyr B.P.[24]

The Schalkholz Stadial in North Germany is equivalent.

The glacial episode of Marine Isotope Stage 4, about 57-71,000 years ago, resulted in cooler and drier climatic conditions and the expansion of grassland vegetation. Credit: ROCEEH.{{free media}}

Neanderthal skull, Museum d'Anthropologie, campus universitaire d'Irchel, Université de Zurich (Suisse), is imaged. Credit: Guerin Nicolas.{{fairuse}}

Wisconsinian glacial began at 80,000 yr BP.[54]

Marine Isotope Stage 4.

"During the Middle Stone Age of Southern Africa, technological and behavioral innovations led to significant changes in the lifeways of modern humans. The glacial episode of Marine Isotope Stage 4, about 57-71,000 years ago, resulted in cooler and drier climatic conditions and the expansion of grassland vegetation. Sea level dropped by as much as 80 meters below its current level. During this period the cultural phase known as the Howieson’s Poort appeared across much of Southern Africa, peaking at about 60-65,000 years ago, and then disappeared. The lithic industry of the Howieson’s Poort is exemplified by changes in technology, such as the use of the punch technique, an increase in the selection of fine-grained silcrete, and the predominance of retouched pieces including backed tools, segments, scrapers and points. Segments are the type fossil of the Howieson’s Poort and represent multi-purpose armatures that were hafted onto wooden spear shafts. The standardized design and refined style of segments convey information about the behavior of their makers and provide insight about group identity. Increasing use of ochre, the presence of engraved ostrich eggshells, and a bone tool industry are associated with these stone artifacts. Also evident is an intensified use of space. Taken together, these behaviors suggest that the Howieson’s Poort represents a clear marker of modern human culture."[55]

"Using stone tool residue analysis with supporting information from zooarchaeology, we provide evidence that at the Abri du Maras, Ardèche, France, Neanderthals [a skull is imaged on the left from Abri du Maras] were behaviorally flexible at the beginning of MIS 4. Here, Neanderthals exploited a wide range of resources including large mammals, fish, ducks, raptors, rabbits, mushrooms, plants, and wood. Twisted fibers on stone tools provide evidence of making string or cordage."[56]

MIS Boundary 5.5 (peak) is at 123 ka.[28]

Marine Isotope Stage 5 or MIS 5 is a Marine Isotope Stage in the geologic temperature record, between 130,000 and 80,000 years ago.[57]

MIS Boundary 5/6 is at 130 ka.[28]

## Illinois Episode glaciation

An almost complete adult Homo sapiens mandible is discovered at the Jebel Irhoud site in Morocco. Credit: Jean-Jacques Hublin/Max Planck Institute for Evolutionary Anthropology.{{fairuse}}

A composite reconstruction was made of the earliest known Homo sapiens skull from Jebel Irhoud in Morocco. Credit: Philipp Gunz/Max Planck Institute for Evolutionary Anthropology.{{fairuse}}

Stone tools have been found at the Jebel Irhoud site in the same level as Homo sapiens fossils. Credit: Mohammed Kamal/Max Planck Institute for Evolutionary Anthropology.{{fairuse}}

The Jebel Irhoud site in Morocco is shown. Credit: Shannon McPherron/Max Planck Institute for Evolutionary Anthropology.{{fairuse}}

"Ages of sediments immediately beneath the oldest till (Kellerville Mbr.) in the bedrock valley average 160 ka and provide direct confirmation that Illinois Episode (IE) glaciation began in its type area during marine isotope stage (MIS) 6. The oldest deposits found are 190 ka fluvial sands on bedrock in the deepest part of the valley. These correlate to earliest MIS 6. We now correlate the lowest deposits to the IE (Pearl Fm.)."[58]

"Illinoian [is] (ca. 220,000-430,000 yr BP)".[54]

"The [Jebel Irhoud site] Moroccan fossils [...] are roughly 300,000 years old. Remarkably, they indicate that early Homo sapiens had faces much like our own, although their brains differed in fundamental ways."[59]

"We did not evolve from a single 'cradle of mankind' somewhere in East Africa. We evolved on the African continent."[60]

"It now looks like Denisovans can be placed at the site from close to 300,000 years ago to about 50,000 years ago, with Neandertals there for periods in between."[61]

MIS Boundary 7/8 is at 243 ka.[28]

## Kansan glacial

Kansan glacial spans 500,000-600,000 yr BP.[54]

MIS Boundary 14/15 is at 563 ka.[28]

MIS Boundary 13/14 is at 533 ka.[28]

An analysis of hominid tooth evolution, including specimens from Spanish Neandertals (top row), pushes back the age of a common Neandertal-human ancestor to more than 800,000 years ago. Credit: A. Gómez-Robles, Ana Muela and Jose Maria Bermudez de Castro.{{fairuse}}

Nebraskan glacial spans ca. 650,000-1,000,000 yr BP.[54]

On the right is an image showing an "analysis of hominid tooth evolution, including specimens from Spanish Neandertals (top row), pushes back the age of a common Neandertal-human ancestor to more than 800,000 years ago. The bottom row shows Homo sapiens teeth."[62]

"During hominid evolution, tooth crowns changed in size and shape at a steady rate."[62]

"The Neandertal teeth, which date to around 430,000 years ago, could have evolved their distinctive shapes at a pace typical of other hominids only if Neandertals originated between 800,000 and 1.2 million years ago."[62]

The magnetic field reversal to the present geomagnetic poles (Brunhes chron) occurred at 780,000 yr BP.

"The R1-till group includes two till units that overlie the 1.3 Ma Mesa Falls ash, thus indicating at least two glaciations between 1.3 Ma and 0.8 Ma."[63]

The magnetic field reversal to the opposite geomagnetic poles (subchron) occurred at 900,000 yr BP.

MIS Boundary 27/28 is at 982 ka.[28]

MIS Boundary 26/27 is at 970 ka.[28]

MIS Boundary 25/26 is at 959 ka.[28]

MIS Boundary 24/25 is at 936 ka.[28]

MIS Boundary 23/24 is at 917 ka.[28]

MIS Boundary 22/23 is at 900 ka.[28]

MIS Boundary 21/22 is at 866 ka.[28]

MIS Boundary 20/21 is at 814 ka.[28]

MIS Boundary 19/20 is at 790 ka.[28]

MIS Boundary 18/19 is at 761 ka.[28]

MIS Boundary 17/18 is at 712 ka.[28]

MIS Boundary 16/17 is at 676 ka.[28]

## Biber ice age

The base of the marly layer overlying sapropel MPRS 250, located at 62 m in the Monte San Nicola section, is the defined base of the Gelasian Stage. Credit: D. Rio, R. Sprovieri, D. Castradori, and E. Di Stefano.

Some number of N tills occurred during the Olduvai subchron.[63]

The magnetic field reversal to the present geomagnetic poles (Olduvai subchron) occurred at 2,000,000 yr BP.

The oldest till group, R2 tills, consists of till units with a reversed polarity and >77% of sedimentary clasts. Low amounts of expandable clays, substantial amounts of kaolinite, and the absence of chlorite characterize the clay mineralogy of R2 tills. The mineralogy of the silt fraction of R2 tills is rich in quartz and depleted in calcite, dolomite, and feldspar. This till group includes a till unit that underlies the 2.0-Ma Huckleberry Ridge ash, thus indicating deposition sometime between ~2.5 Ma (onset of Northern Hemisphere glaciations) (Mix et al., 1995) and 2.0 Ma.[63]

During the Gelasian the ice sheets in the Northern Hemisphere began to grow, which is seen as the beginning of the Quaternary ice age. Deep sea core samples have identified approximately 40 marine isotope stages (MIS 103 – MIS 64) during the age. Thus, there have probably been about 20 glacial cycles of varying intensity during the Gelasian.

In the regional glacial history of the Alps this age is now called Biber. It corresponds to Pre-Tegelen and Tegelen in Northern Europe.[64]

During the Gelasian, the Red Crag Formation of Butley, Suffolk, the Newbourn Crag, the Norwich Crag Formation and the Weybourne Crag Formation (all from East Anglia, England) were deposited. The Gelasian is an equivalent of the Praetiglian and Tiglian stages as defined in the Netherlands, which are commonly used in northwestern Europe.

Biber or the Biber Complex is a timespan approximately 2.6–1.8 million years ago in the glacial history of the Alps. Biber corresponds to the Gelasian age in the international geochronology, which since 2009 is regarded as the first age of the Quaternary period. Deep sea core samples have identified approximately 20 glacial cycles of varying intensity during Biber.[65]

In 1953, Schaefer defined the Biber glaciation, Biber Glacial, or Biber Ice Age from gravel landforms of the Stauden Plateau in the area of the Iller-Lech Plateau and in the Aindling river terrace sequence, by grouping together the so-called Middle and Upper Cover Gravels or Deckenschotter. This corresponded to the Staufenberg Gravel Terrace on the Iller-Lech Plateau, identified in 1974 by Scheunenpflug, and the so-called High Gravels of the Aindling region.[66] The rich crystalline sedimentary facies, that Löscher distinguished in 1976 in the area of the Rhine Glacier of the western Riß-Iller Plateau may also be paralleled with these glacial landforms.[67] The gravels in the Iller-Lech region ascribed to the Biber glaciation are generally heavily weathered and originate from the Northern Limestone Alps. Löscher's Kristallinreiche Liegendfazies, by contrast, originates from the bedrock of the molasse zone.

The term Biber glaciation was not part of the traditional four-stage glaciation schema of the Alps by Albrecht Penck and Eduard Brückner, but was named after the Biberbach river north of Augsburg in 1953 by Ingo Schaefer, based on the naming system of the traditional Penck schema.[68][69] Its type locality or type region is the Stauden Plateau in the Iller-Lech Plateaux and the Staufenberg Gravel Terrace in the area of Aindling. The Biber glaciation was thought to be followed by the Biber-Danube interglacial and the Danube glacial.

The absolute timing and the connexion with the glacial classification of North Germany and the Netherlands has been problematic. The Biber glacial was fought to correlate either to the Eburonian complex or the Pre-Tiglian complex in the Netherlands. In the former case it would correspond to Marine isotope stage (MIS) 56 to 62, which would place it in the period between 1.6 and 1.8 million years ago,[70][71] in the latter case it would roughly correspond to MIS 96 to 100, and would therefore have taken place about 2.4 to 2.588 million years ago.[71][72][73] The correlation was fraught with problems however due to the fact that the corresponding depositions in the Netherlands were probably not governed by climatic changes. Similar doubts on climatic grounds for the depositions assessed as Biber-related also exist in the Alpine region. It is possible that there were tectonic influences perhaps in the wake of the uplift phases of the Alps. The succession and appearance of the gravel bodies makes it possible that during their formation there were several periods of alternating fluvial erosion and accumulation. The Biber cold period at least corresponds partly with the Swiss cover gravel glaciations.[74]

The 2016 version of the detailed stratigraphic table by the German Stratigraphic Commission firmly places Biber in the Gelasian and gives a correspondence to Pre-Tegelen and Tegelen in the glacial geology of northern Europe. There is continuity between Biber and the glacial cycles of the following Danube stage[65]

Deep sea core samples have identified approximately 40 marine isotope stages (MIS 103 – MIS 64) during Biber.[65] Thus, there have probably been about 20 glacial cycles of varying intensity during Biber. The dominant trigger is believed to be the 41 000 year Milankovitch cycles of axial tilt.[75][76]

Gravels ascribed to Biber (also called the Highland Gravel or Oldest Gravel occur northwest of Augsburg as the Stauffenberg Gravel, as well as northeast as the Hohenried Gravel and southwest of Augsburg as the Stauden Plateau Gravel. Also included are isolated gravels of the Hochfirst near Mindelheim and the Stoffersberg near Landsberg am Lech.[77] There may also be gravels in the Sundgau from the Biber ice age.

## Pliocene

The Pliocene ranges from 5.332 x 106 to 2.588 x 106 b2k.

The northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma.[78]

The formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios and ice-rafted cobbles in the North Atlantic and North Pacific ocean beds.[79] Mid-latitude glaciation was probably underway before the end of the epoch. The global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas.[80]

## Holarctic-Antarctic Ice Age

"This late Cenozoic ice age began at least 30 million years ago in Antarctica; it expanded to Arctic regions of southern Alaska, Greenland, Iceland, and Svalbard between 10 and 3 million years ago. Glaciers and ice sheets in these areas have been relatively stable, more-or-less permanent features during the past few million years."[81]

"During the last one million years, large ice sheets developed in North America, Eurasia, the Andes, and elsewhere. These ice masses were unstable, growing and self-destructing in cycles averaging about 100,000 years, which correspond to eccentricity in the Earth's orbit around the Sun (Mangerud et al. 1996). The most recent great ice sheets disappeared only 10,000 years ago, but the Holarctic-Antarctic Ice Age still continues in regions of stable glaciation."[81]

## Karoo Ice Age

Ice flow in the Karoo basins over southern Africa during maximum glaciation is indicated. Credit: J. N. J. Visser.

North-south section across the Kalahari and Karoo basins illustrating the relief of the basin floor and the lithostratigraphic units. Credit: Visser.

A "glacial marine facies [occurs] on the Falkland Islands [Frakes and Crowell, 1967]."[82]

In "a complex situation, like the Karoo Basin and adjoining highlands [...] a marine ice sheet bounded the highlands during the last phase of glaciation".[82]

The "influence of Gondwana topography on glaciation about 275 to 300 m.y. ago, [...] is preserved as thick (up to 700 m) glacial and proglacial sequences in the Karoo, Kalahari, and Warmbad basins as well as other smaller basins toward the north. These deposits, known as the Dwyka Formation, cover an area close to a million square kilometers in southern Africa".[82]

"Valleys that had been incised into the Windhoek Highlands attained lengths up to 250 km, had striated floors and walls, and contained roches moutonnées [Martin, 1961]."[82]

"The Whitehill Formation (White Band) was taken as a datum [in the stratigraphic diagram on the lower right]. The Permo-Carboniferous boundary on the platform is based on microflora assemblages [Anderson, 1977]. Ms is massive; St, stratified; dmt, diamictite; Drg, dropstone argillite; Cb, carbonaceous; mds, mudstone; Sst, sandstone; Lm, laminated; cgl, conglomerate; sh, shale; Fs, fossiliferous; Cc, carbonate; and conc, concretions."[82]

"Glaciation is known from all continents that were once part of Gondwana, including: Africa, South America, Antarctica, India, Arabia and Australia. Glaciation began in the early Carboniferous (360 Ma), reached a peak in late Carboniferous, continued into early Permian, and mostly came to an end by late Permian (260 Ma) time, thus spanning 100 million years. Multiple glacial centers were active; each experienced repeated glacier advances and retreats. Particularly well-known glacial strata include the Dwyka Tillite (Karoo basin) in South Africa, Talchir Boulder Beds in India, and Wynyard Formation of Tasmania. Overall, two major glacial cycles took place. Both expanded gradually over periods of about 20 million years each to reach their maximum extents in late Pennsylvanian and early Permian times. Each major cycle then ended abruptly during only 1-10 thousand years (Gastaldo et al. 1996)."[81]

"Although precise dating is not possible for many of the Gondwana glacial deposits, a general migration of glaciation through time is apparent. Carboniferous glaciation took place mainly in South America, southern Africa, India, and western Antarctica; whereas Permian glaciation was located mostly in Australia and eastern Antarctica. This migration corresponded to the drifting of Gondwana over the South Pole [...] The Karoo ice age is marked by cyclothems, cyclic sedimentary sequences in continental areas that were located in low latitudes. Pennsylvanian and Permian cyclothems are well known throughout the mid-continent of the United States, particularly eastern Kansas. The cyclothems were created by repeated marine transgressions and regressions over a stable continental platform. These cycles are interpreted as results of frequent changes in global sea level associated with glaciation in Gondwana. Glacial cycles and variations in sea level are documented in oxygen-isotope variations within fossils of Pennsylvanian cyclothems [...]. Late Pennsylvanian sea-level fluctuations were at least 80 m and likely greater than 100 m in amplitude (Soreghan and Giles 1999)."[81]

## Andean-Saharan ice age

The "glacial episodes that occurred on Earth during the Palaeozoic (the Andean-Saharan between 450 and 420 Ma, and the Karoo between 360 and 260 Ma) did not achieve a global extent."[83]

"Glaciation is known from Arabia, central Sahara, western Africa, the lower Amazon of Brazil, and the Andes of western South America. Spectacular erosion of underlying rocks took place over large areas of the Sahara; whereas a good sedimentary record is preserved in Arabia. Continental ice sheets were developed in Africa and eastern Brazil, while alpine glaciers formed in the Andes region. The center of glaciation appears to have migrated through time: Ordovician (450-440 Ma) in Sahara, and Silurian in South America (Brazil 440-430 Ma, and Andes 430-420 Ma). The two continents were joined as parts of Gondwana, which was located over the South Pole".[81]

The Gaskiers glaciation is a period of widespread glacial deposits (e.g. diamictites) that lasted under 340 thousand years, between 579.63 ± 0.15 and 579.88 ± 0.44 million years ago – i.e. late in the Ediacaran Period – making it the last major glacial event of the Precambrian.[84]

Deposits attributed to the Gaskiers - assuming that they were all deposited at the same time - have been found on eight separate palaeocontinents, in some cases occurring close to the equator (at a latitude of 10-30°), where the 300 m-thick name-bearing section at Gaskiers-Point La Haye (Newfoundland) is packed full of striated dropstones.[85] Its δ13
C
values are really low (pushing 8 ‰), consistent with a period of environmental abnormality.[85] The bed lies just below some of the oldest fossils of the Ediacaran biota, where there is in fact a 9 million year gap between the diamictites and the 570 Ma macrofossils.[85]

## Varanger glaciation

The Varangian apparently spans 610 to 575 Ma.

## Elatina glaciation

Elatina Formation diamictite is below the Ediacaran Global Boundary Stratotype Section and Point (GSSP) site in the Flinders Ranges National Park, South Australia. An Australian \$1 coin is for scale. Credit: Bahudhara.{{free media}}

"The Elatina glaciation has not been dated directly, and only maximum and minimum age limits of c. 640 and 580 Ma, respectively, are indicated."[86]

"The Elatina glaciation is of global importance for several reasons:

1. its diverse and excellently preserved glacial and periglacial facies represent a de facto type region for late Cryogenian glaciation in general;
2. the Elatina Fm. has yielded the most robust palaeomagnetic data for any Cryogenian glaciogenic succession; and
3. the recently established Ediacaran System and Period (Knoll et al. 2004, 2006; Preiss 2005) has its Global Stratotype Section and Point (GSSP) placed near the base of the Nuccaleena Fm. overlying the Elatina Fm. in the central Flinders Ranges [...]."[86]

"Feeder dykes for volcanic rocks near the base of the [Adelaide Geosyncline] sedimentary succession have been dated at 867 ± 47 and 802 ± 35 Ma (Zhao & McCulloch 1993; Zhao et al. 1994) and 827 ± 6 Ma (Wingate et al. 1998)."[86]

"No volcanism is known in the region during the Elatina glaciation."[86]

"The Neoproterozoic–early Palaeozoic succession in the Adelaide Geosyncline was deformed by the Delamerian Orogeny at 514 – 490 Ma (Drexel & Preiss 1995; Foden et al. 2006)."[86]

"The Yerelina Subgroup at the top of the Cryogenian Umberatana Group embraces all the glaciogenic formations of the Elatina glaciation (Preiss et al. 1998)."[86]

"The Yerelina Subgroup is unconformably to disconformably overlain by the Ediacaran Wilpena Group."[86]

"Deposition in the North Flinders Zone commenced, possibly following an erosional break, with the 1070-m-thick Fortress Hill Fm., which comprises laminated siltstone with gritty lenses and scattered dropstones, some faceted, marking the onset of glacial deposition (Coats & Preiss 1987; Preiss et al. 1998). Clast lithologies include granite, quartzite, limestone, oolitic limestone and dolostone. The Fortress Hill Fm. is typical of the dominantly fine-grained units of the Yerelina Subgroup that are interpreted by Preiss (1992) as outer marine-shelf deposits."[86]

"The Fortress Hill Fm. is sharply overlain by sandstone and conglomerate at the base of the Mount Curtis Tillite (90 m) that may record a lowering of relative sea level and mark a sequence boundary (Preiss et al. 1998)."[86]

"The Mount Curtis Tillite is a sparse diamictite with erratics of pebble to boulder size, some faceted and striated, in massive and laminated, grey-green dolomitic siltstone. Clast lithologies are mostly quartzite, limestone and dolostone, but also include granite and porphyry (Coats & Preiss 1987). Granite boulders attain 3 x 8 m."[86]

"The Mount Curtis Tillite is overlain by the medium-grained, feldspathic Balparana Sandstone (130 m), which contains interbeds and lenses of calcareous siltstone and pebble conglomerate."[86]

"The Balparana Sandstone is disconformably overlain by the Wilpena Group. The main source for the glaciogenic deposits may have been the Curnamona Province to the present east [...] and possibly the now-buried Muloorina Ridge immediately north of the North Flinders Zone (Preiss 1987)."[86]

"The lower-most, laminated siltstone facies of the Fortress Hill Fm. shows progressively greater amounts of scattered, ice-rafted granules and pebbles. The shallow-water Gumbowie Arkose (45 – 90 m) disconformably overlies these early deposits at a possible sequence boundary and is conformably succeeded by the Pepuarta Tillite (120 – 197 m), which is a sparse diamictite with scattered clasts up to boulder size in massive and laminated, grey calcareous siltstone. Faceted and striated boulders reach 2.5 m in diameter. Clast lithologies include pink granite, granite gneiss, grey porphyry, quartz-granule conglomerate, various quartzites, and vein quartz. The siltstone facies with scattered large clasts of extrabasinal provenance implies deposition from floating ice."[86]

"The widespread Grampus Quartzite (60 m) disconformably overlies the Pepuarta Tillite, possibly at a sequence boundary defining a third genetic sequence of the Yerelina Subgroup (Preiss et al. 1998)."[86]

"It is conformably overlain by the laminated to cross-laminated, calcareous, pale grey Ketchowla Siltstone (271 m) (Preiss 1992). The Ketchowla Siltstone contains scattered ice-rafted granules, pebbles and boulders up to 1 m across, and is ascribed by Preiss (1992) to outer marine-shelf deposition under generally waning glacial conditions. It is overlain disconformably by the Nuccaleena Fm., with any Ketchowla Siltstone deposited in the North Flinders Zone having been completely removed by erosion at this sequence boundary (Preiss 2000)."[86]

"The outer marine-shelf successions of the Fortress Hill Fm. and Ketchowla Siltstone record the waxing and waning of glacial conditions, respectively. The Pepuarta Tillite and the correlative Mount Curtis Tillite mark the glacial maximum of the Elatina glaciation (Preiss et al. 1998)."[86]

"A U–Pb age of 657 ± 17 Ma was obtained for a zircon grain of uncertain provenance from the Marino Arkose Member of the underlying Upalinna Subgroup (Preiss 2000). Re – Os dating gave an age of 643.0 ± 2.4 Ma for black shale from the Tindelpina Shale Member at the base of the Tapley Hill Fm., which overlies glacial deposits of Sturtian age in the Adelaide Geosyncline (Kendall et al. 2006). Zoned igneous zircon from a tuffaceous layer near the top of the Sturtian-age glaciogenic succession gave a SHRIMP U – Pb age of c. 658 Ma (Fanning & Link 2006). Mahan et al. (2007) reported a Th–U–total Pb age of 680 ± 23 Ma for euhedral laths of monazite, interpreted as authigenic, from the Enorama Shale of the Upalinna Subgroup."[86]

## Nantuo glaciation

The Nantuo glaciation apparently occurred 654 ± 3.8 Ma.

## Ice Brook glaciation

The Ice Brook glaciation apparently spans 651 to 659 Ma.

## Ghaub glaciation

"Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units—the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland)—these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ13C excursions, lasting at most ∼50–80 m.y."[87]

"Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting."[88]

## Marinoan glaciations

This diamictite is from the Neoproterozoic Pocatello Formation, a 'Snowball Earth'—type deposit. Credit: Qfl247.{{free media}}

The term Marinoan glaciation has been applied globally to any glaciogenic formations assumed (directly or indirectly) to correlate with the Elatina glaciation in South Australia.[89] Recently, there has been a move to return to the term Elatina glaciation in South Australia because of uncertainties regarding global correlation and because an Ediacaran glacial episode (Gaskiers) also occurs within the wide-ranging Marinoan Epoch.[90]

The Marinoan glaciation was a period of worldwide glaciation that lasted from approximately 650 to 635 Ma and may have covered the entire planet, in an event called the Snowball Earth, where the end of the glaciation may have been sped by the release of methane from equatorial permafrost.[91][92] Great uncertainty surrounds the dating of pre-Gaskiers glaciations: the status of the Kaigas is not clear; its dating is very tentative and many researchers do not recognize it as a glaciation.[93]

During the Marinoan glaciation, characteristic glacial deposits indicate that Earth suffered one of the most severe ice ages in its history: glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[94][95]

Apparently the major glacial period the Marinoan occurred during the Cryogenian.[96]

A similar period of rifting, to the break up along the margins of Laurentia, at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia.[97]

The Marinoan glaciation ended approximately 635 Ma, at the end of the Cryogenian.[98]

The Marinoan glaciation was a period of worldwide glaciation that lasted from approximately 650 to 635 Ma, where the end of the glaciation may have been sped by the release of methane from equatorial permafrost.[98][99]

The name is derived from the stratigraphic terminology of the Adelaide Geosyncline (Adelaide Rift Complex) in South Australia and taken from the Adelaide suburb of Marino to subdivide the Neoproterozoic rocks of the Adelaide area and encompass all strata from the top of the Brighton Limestone to the base of the Cambrian.[100] The corresponding time period, referred to as the Marinoan Epoch, spanned from the middle Cryogenian to the top of the Ediacaran and included a glacial episode within the Marinoan Epoch, the Elatina glaciation, after the 'Elatina Tillite' (now Elatina Formation).[101] The term Marinoan glaciation came into common usage because it was the glaciation that occurred during the Marinoan Epoch.[100]

The term Marinoan glaciation was applied globally to any glaciogenic formations assumed to correlate with the Elatina glaciation in South Australia.[102] The Elatina glaciation in South Australia and the Gaskiers also occurs within the wide ranging Marinoan Epoch.[90]

The Earth may have underwent a number of glaciations during the Neoproterozoic era.[103]

There were three (or possibly four) significant ice ages during the late Neoproterozoic, periods of nearly complete glaciation of Earth are often referred to as "Snowball Earth", where it is hypothesized that at times the planet was covered by ice 1–2 km (0.62–1.24 mi) thick.[104]

During the Marinoan glaciation, characteristic glacial deposits indicate that Earth suffered one of the most severe ice ages in its history, where glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[105][106]

The melting of the Snowball Earth is associated with greenhouse warming due to the accumulation of high levels of carbon dioxide in the atmosphere.[107]

Glacial deposits in South Australia are approximately the same age (about 630 Ma), confirmed by similar stable carbon isotopes, mineral deposits (including sedimentary barite), and other unusual sedimentary structures.[104]

Two diamictite-rich layers in the top 1 km (0.62 mi) of the 7 km (4.3 mi) Neoproterozoic strata of the northeastern Svalbard archipelago represent the first and final phases of the Marinoan glaciation.[108]

The Marinoan "is separated from the Sturtian by a thick succession of sedimentary rocks containing no evidence of glaciation. This glacial phase could correspond to the recently described Ice Brooke formation in the northern Canadian Cordillera."[97]

## Gucheng

The Gucheng is apparently comparable to the Marinoan.

## Jiangkou

The Jiangkou spans the Chang'an through the Gucheng.

## Chang'an

The Chang'an occurred about 715.9 ± 2.8 Ma.

The Port Askaig glaciation is above the Elbobreen-Wilsonbreen glaciation.

## Elbobreen-Wilsonbreen glaciation

The Elbobreen-Wilsonbreen glaciation in Svalbard occurred c. 720 Ma.

## Cryogenian ice age

The Cryogenian Ice Age, or the Stuartian-Varangian Ice Age, a "Late Proterozoic ice age was apparently the greatest of all. Glacial strata are known from all modern continents (except Antarctica) with an overall time range of about 950 to 600 million years old. Glacial strata from several intervals during this time are well preserved in Africa, China, Australia, Europe, Arabia, North America, and elsewhere. Multiple glaciations are the rule. In Scotland and Ireland, for example, three glacial episodes took place between 700 and 580 million years ago (McCay et al. 2006)."[81]

It apparently consists of

1. glaciation of the Lower Congo region, Africa occurring 950-750 and 620-600 Ma,
2. Stuartian glaciation, Australia, 800-780 Ma,
3. Sinian glaciation, China, 800-760, 740-700, and 600 Ma,
4. glaciation in Western Canada/U.S.A., 850-800 Ma,
5. glaciation of the Saharan region, Africa, 730-650 Ma,
6. Marinoan glaciation, Australia, 690-680 Ma, and
7. Varangian glaciation, Norway, about 650 Ma.[81]

"Late Proterozoic glaciogenic deposits are known from all the continents. They provide evidence of the most widespread and long-ranging glaciation on Earth."[97]

Def. "a geologic period within the Neoproterozoic era from about [720] to 600 million years ago"[109] is called the Cryogenian.

The end of the period also saw the origin of heterotrophic plankton, which would feed on unicellular algae and prokaryotes, ending the bacterial dominance of the oceans.[110]

Apparently two major glacial periods occurred during the Cryogenian: the Marinoan and the Sturtian,[96][85] formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.

The Cryogenian is a geologic period that lasted from 720-635 Mya.[111]

The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy.[112]

Several glacial periods are evident, interspersed with periods of relatively warm climate, with glaciers reaching sea level in low paleolatitudes.[97]

Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[113]

The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".[114][115]

"Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors."[97]

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.[116]

During the Cryogenian period, the oldest known fossils of sponges, Otavia the first sponge-like animal[117] (and therefore animals) make an appearance.[118][119][120]

New groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.[121]

The base of the period is defined by a fixed rock age, that was originally set at 850 million years,[122] but changed in 2015 to 720 million years.[111]

## Sturtian

The Sturtian glaciation was a glaciation, or perhaps multiple glaciations,[123] during the Cryogenian Period.[96][85]

The break up along the margins of Laurentia at about 750 Ma occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia.[97]

The Sturtian glaciation persisted from 720 to 660 million years ago.[98]

A Sturtian age was assigned to the Numees diamictites.[124]

The duration of the Sturtian glaciation has been variously defined, with dates ranging from 717 to 643 Ma.[125][126][123] Or, the period spans 715 to 680 Ma.[127]

"Glaciogenic rocks figure prominently in the Neoproterozoic stratigraphy of southeastern Australia and the northern Canadian Cordillera]. The Sturtian glaciogenic succession (c. 740 Ma) unconformably overlies rocks of the Burra Group."[97]

The Sturtian succession includes two major diamictite-mudstone sequences, which represent glacial advance and retreat cycles, stratigraphically correlated with the Rapitan Group of North America.[97]

The Sturtian is named after the Sturt River Gorge, near Bellevue Heights, South Australia.

Reusch's Moraine in northern Norway may have been deposited during this period.[128]

## Numees

The Numees has a Sturtian age.

## Tereeken

The Tereeken occurred < 727 ± 8 Ma.

## Rapitan glaciation

"The Rapitan Group (Cryogenian) of western Canada is similar to the Chuos Formation in both lithofacies and basin context, representing deposition in a paraglacial rift basin (Young, 1976; Eisbacher, 1985). An iron-rich, dropstone-bearing unit (the Sayunei Formation) is capped by a diamictite unit (the Shezal Formation) (Hoffman and Halverson, 2011). Measured sections (Fig. 3 of Eisbacher, 1985) illustrate that the most complete successions have a basal ferruginous shale sequence bearing occasional dropstones. These deposits pass gradationally upward, via 5–40 m jaspillite-hematite ironstone at the top of the Sayunei Formation, into diamictites. The ironstone is laterally persistent in depocentres (Eisbacher, 1985). Sea-ice removal may have triggered local grounding line advance, resulting in deposition of the Shezal Formation (Eisbacher, 1985): Hoffman and Halverson (2011) recognised this as a possible catalyst for ironstone precipitation. In addition to an abiotic “rusting of the seas” model, a biologically-mediated mechanism was also considered. Once “the ice cover thinned and finally disappeared, anoxic and oxygenic photosynthesis could have precipitated Fe2O3-precursor from anoxic Fe(II)-rich basin waters” (Hoffman et al., 2011). [...] Such a biogenic mechanism for ironstone precipitation, via for example photosynthetic stromatolites, would be in agreement with our observations in Namibia."[129]

## Port Nolloth

The Port Nolloth extends from the Kaigas formation upwards to the Murmees.

## Kaigas formation

The Kaigas glaciation was a hypothesized snowball earth event in the Neoproterozoic Era, preceding the Sturtian glaciation inferred based on the interpretation of Kaigas Formation conglomerates in the stratigraphy overlying the Kalahari Craton as correlative with pre-Sturtian Numees formation glacial diamictites;[130] however, the Kaigas formation was later determined to be non-glacial, and a Sturtian age was assigned to the Numees diamictites.[131]

## Vendian

The Vendian occurred about 740 Ma.

## Chuos glaciation

"The "grainstone prism" was a major submarine drainage system localized in a paleovalley carved during the Chuos glaciation, which was occupied by a transverse ice-stream that cut the Duurwater trough during the Ghaub glaciation."[88]

"Despite early indications of two distinct glaciations (Kröner and Rankama, 1972; Guj, 1974), the prevailing view of a single glaciogenic horizon that could serve as a basis for correlation throughout the Otavi Group (Hedberg, 1979; SACS, 1980; Miller, 1997) led to the former "Otavi Tillite" (le Roex, 1941) being assigned to the Chuos Formation of Gevers (1931), a glaciogenic diamictite with an intimately associated banded iron formation that is widely distributed within the orogens bounding the Otavi platform (Martin, 1965a, 1965b). More recently, two glaciations have been firmly established in the Otavi Group (Hoffmann and Prave, 1996; Hoffman et al., 1998a; Hoffman and Halverson, 2008), the older Chuos Formation and a younger glaciation represented by the "Otavi Tillite" (le Roex, 1941), and its correlative carbonate-clast breccia unit of the Fransfontein homocline (Frets, 1969; Guj, 1974). Hoffmann and Prave (1996) renamed this younger glaciogenic unit the Ghaub Formation, after a farm near the section originally described by le Roex (1941)."[88]

The "Chuos glaciation occurred during a period of active faulting, which is reflected by the diversity of its debris and a low-angle (1.5°) structural unconformity [...] that cuts out ~2 km of strata (Hoffman et al., 1998a)."[88]

The Rasthof Formation [is] the postglacial cap carbonate overlying the Chuos diamictite".[88]

Below the Chuos glaciation is the Naauwpoort dated at 746 ± 2 Ma giving an upper age limit to the base of the Chuos.[88]

"U–Pb ages from the Askevold Formation (Hoffman et al., 1996) [Nabis Formation 747 ± 2 Ma (Hoffman et al., 1996)] are from further west: this formation is not preserved beneath the Chuos Formation in [the Ghaub and Varianto farm areas of the Otavi Mountain Land]."[129]

"Earlier analyses of the Chuos Formation concentrated on meta-sediments in the vicinity of its type section south of Windhoek and in the Damara Belt (Gevers, 1931; de Kock and Gevers, 1933; Martin et al., 1985; Henry et al., 1986; Badenhorst, 1988). More modern stratigraphic analyses several hundred kilometres to the west of the Otavi Mountain Land demonstrate that the Chuos Formation is cradled in a rift-related, fault bounded palaeotopography (Hoffman and Halverson, 2008), and hence its substrate also changes along strike, across the southern flank of the Owambo Basin. In the area of Ghaub and Varianto farms, the study interval comprises the Nabis Sandstone Formation of the Nosib Group, overlain by the Chuos Formation and succeeded by the Berg Aukas Formation [...]. This particular area has been mapped at the 1:250,000 scale (Geological Survey of Namibia, 2008). Age constraints include 747 ± 2 Ma from the Naauwport volcanics, locally beneath the Chuos Formation (Hoffman et al., 1996) and 635 ± 1 Ma from ash beds in the younger Ghaub Formation (Hoffmann et al., 2004)."[129]

## Beiyixi glaciaton

The Beiyixi is later than 755 Ma.

## Makganyene glaciation

"In its eastern domain, the Transvaal Supergroup of South Africa contains two glacial diamictites, in the Duitschland and Boshoek Fms. The base of the Timeball Hill Fm., which underlies the Boshoek Fm., has a Re-Os date of 2,316 ± 7 My ago (13). The Boshoek Fm. correlates with the Makganyene diamictite in the western domain of the Transvaal Basin, the Griqualand West region. The Makganyene diamictite interfingers with the overlying Ongeluk flood basalts, which are correlative to the Hekpoort volcanics in the eastern domain and have a paleolatitude of 11° ± 5° (14). In its upper few meters, the Makganyene diamictite also contains basaltic andesite clasts, interpreted as being clasts of the Ongeluk volcanics. The low paleolatitude of the Ongeluk volcanics implies that the glaciation recorded in the Makganyene and Boshoek Fms. was planetary in extent: a snowball Earth event (15). Consistent with earlier whole-rock Pb–Pb measurements of the Ongeluk Fm. (16), the Hekpoort Fm. contains detrital zircons as young as 2,225 ± 3 My ago (17), an age nearly identical to that of the Nipissing diabase in the Huronian Supergroup."[132]

The "Makganyene glaciation begins some time after 2.32 Ga and ends at 2.22 Ga, the three Huronian glaciations predate the Makganyene snowball."[132]

## Huronian ice age

Proposed correlation is of the Huronian Supergroup and the upper Transvaal Supergroup. Credit: Robert E. Kopp, Joseph L. Kirschvink, Isaac A. Hilburn, and Cody Z. Nash.

The Huronian Ice Age is known "mainly from Canada and the United States in North America, where dated rocks range from 2500 to 2100 million years old. The Gowgonda Formation of Ontario is especially noteworthy for its excellent preservation of glaciogenic strata dated about 2300 million years old. Other glacial deposits are found in Wyoming, Michigan, Quebec, and the Northwest Territories. These rocks record extensive Early Proterozoic continental glaciation through a time span of about 400 million years, during which three or more glacial expansions took place. The configuration of the continents during this time is highly speculative."[81]

"The period from 2.45 Ga until some point before 2.22 Ga saw a series of three glaciations recorded in the Huronian Supergroup of Canada (11) [in the above centered image]. The final glaciation in the Huronian, the Gowganda, is overlain by several kilometers of sediments in the Lorrain, Gordon Lake, and Bar River formations (Fms.). The entire sequence is penetrated by the 2.22 Ga Nipissing diabase (12); the Gowganda Fm. is therefore significantly older than 2.22 Ga."[132]

"The three Huronian glacial units, penetrated and capped by the Nipissing diabase, predate the Makganyene diamictite in the Transvaal. The uppermost Huronian glacial unit, the Gowganda Fm., is overlain by hematitic units, perhaps reflecting a rise in O2. The basal Timeball Hill Fm. contains pyrite with minimal [mass-independent fractionation] MIF (26), whereas the upper Timeball Hill Fm., which we suggest is correlative to the Lorrain or Bar River Fms., contains red beds. The Makganyene diamictite records a low-latitude, snowball glaciation (29), perhaps triggered by the destruction of a CH4 greenhouse. It is overlain by the Kalahari Mn Field in the Hotazel Fm., the deposition of which requires free O2."[132]

"In its eastern domain, the Transvaal Supergroup of South Africa contains two glacial diamictites, in the Duitschland and Boshoek Fms. The base of the Timeball Hill Fm., which underlies the Boshoek Fm., has a Re-Os date of 2,316 ± 7 My ago (13). The Boshoek Fm. correlates with the Makganyene diamictite in the western domain of the Transvaal Basin, the Griqualand West region. The Makganyene diamictite interfingers with the overlying Ongeluk flood basalts, which are correlative to the Hekpoort volcanics in the eastern domain and have a paleolatitude of 11° ± 5° (14). In its upper few meters, the Makganyene diamictite also contains basaltic andesite clasts, interpreted as being clasts of the Ongeluk volcanics. The low paleolatitude of the Ongeluk volcanics implies that the glaciation recorded in the Makganyene and Boshoek Fms. was planetary in extent: a snowball Earth event (15). Consistent with earlier whole-rock Pb–Pb measurements of the Ongeluk Fm. (16), the Hekpoort Fm. contains detrital zircons as young as 2,225 ± 3 My ago (17), an age nearly identical to that of the Nipissing diabase in the Huronian Supergroup. As the Makganyene glaciation begins some time after 2.32 Ga and ends at 2.22 Ga, the three Huronian glaciations predate the Makganyene snowball."[132]

"In contrast to the Makganyene Fm., the three Huronian diamictites are unconstrained in latitude. Poles from the Matachewan dyke swarm, at the base of the Huronian sequence, do indicate a latitude of ≈5.5° (18), but ≈2 km of sedimentary deposits separate the base of the Huronian from the first glacial unit (19), which makes it difficult to draw conclusions about the latitude of the glacial units based on these poles. Low latitude poles in the Lorrain Fm. (20, 21), which conformably overlies the Gowganda diamictite, are postdepositional overprints (22)."[132]

"Some of the earliest continental red beds were deposited in the Firstbrook member of the Gowganda Fm. and in the Lorrain and Bar River Fms. in Canada, as well as in the upper Timeball Hill Fm. in South Africa. The basal Timeball Hill Fm. has recently been dated at 2,316 ± 7 My ago (13). In our proposed correlation, all of the red bed-bearing units were deposited after the last Huronian glaciation and before the Makganyene glaciation. The formation of the red beds could involve local O2, although it does not demand it (34). Syngenetic pyrite from the basal Timeball Hall Fm. shows only slight MIF of S (26), consistent with the initiation of planetary oxygenation or enhanced glacial activity."[132]

## Pongola glaciation

The Pongola glaciation is dated "at 2.9 Ga".[132] It extends to 2780 Ma.

"The oldest known midlatitude glaciation, recorded in the Pongola Supergroup diamictite, occurred at 2.9 Ga (10)."[132]

## Glacial ice petrogony

The Ablation Zone is the section of the glacier where melting dominates. Credit: Mauri S. Pelto.

The Accumulation Zone is the zone of the glacier where snowfall exceeds snowmelt. Credit: Mauri S. Pelto.

Usually glacial ice starts with the meteoritic accumulation of snow. Depending on temperature, latitude, altitude, and occasional other factors, snow melts to water, sublimates to water vapor, or accumulates progressively.

"Snow develops into ice much more rapidly on glaciers in temperate regions, where periods of melting alternate with periods when wet snow refreezes, than in [regions] where the temperature remains well below the freezing point throughout the year."[133]

Different "mechanisms [occur] in different areas. We have to subdivide glaciers, and even different parts of the same glacier, into different categories according to the amount of melting that takes place."[133]

"The zones differ one from another in the temperature and physical characteristics of the material near the surface."[133]

"New snow (immediately after falling in calm conditions) [has a typical density of] 50-70 [kg m-3.] Damp new snow [ranges from] 100-200 [s]ettled snow 200-300 [d]epth hoar 100-300 [w]ind-packed snow 350-400 [f]irn 400-830 [v]ery wet snow and firn 700-800 [and] [g]lacier ice 830-923 [...] The difference between firn and ice is clear; firn becomes glacier ice when the interconnecting air- or water-filled passageways between the grains are sealed off, a process known as pore close-off."[133]

"Crystal size [grain size] increases with time and depth as snow transforms to ice."[133]

"The characteristics of the [glacier] zones [start] from the highest elevations (the head of a glacier or center of an ice sheet)."[133]

1. Dry-snow zone: "No melting occurs here, even in summer. The dry-snow line marks the boundary between this zone and the next one."[133]
2. Percolation zone: "Some surface melting occurs in this zone. Water can percolate a certain distance into snow at temperatures below 0 °C before refreezing. If the water encounters an impermeable layer it may spread out laterally. When it refreezes an ice layer or an ice lens forms. The vertical water channels also refreeze, when their water supply is cut off, to form pipe-like structures called ice glands."[133]
3. Wet-snow zone: In this zone, by the end of summer, all the snow deposited since the end of the previous summer has warmed to 0 °C."[133]
4. Superimposed-ice zone: In the percolation and wet-snow zones, the material consists of ice layers, lenses, and glands, separated by layers and patches of snow and firn. At lower elevations, [...] so much meltwater is produced that the ice layers merge to a continuous mass, called superimposed ice. [...] The snow line refers to the boundary between the wet-snow and the superimposed-ice zones."[133]
5. Ablation zone: the area below the equilibrium line [the lower boundary of the superimposed-ice zone.] Here the glacier surface loses mass by the end of the year."[133]

Def. "the section of the glacier where melting dominates"[134] is called the ablation zone.

"By the end of the summer this area [ablation zone] is exposed glacier ice."[134]

Def. "the zone of the glacier where snowfall exceeds snowmelt"[134] is called the accumulation zone.

## Greenland ice sheet petrogony

On the way from Mestersvig to Renland is Stauning’s Alps. Credit: Willi Dansgaard.

This is Antarctic Havn, Greenland, specifically the southern shore of King Oscar Fjord. Credit: Jerzy Strzelecki.

Folding on an island is visible from the Caledonian orogeny where Segelselskapets Fjord mouths into King Oscar Fjord, at 72°28'0.90"N, 24°41'55.44"W. The rock is of precambrian-cambrian age. Credit: Håvard Berland.

δ-changes (isotopic fractionation) in vapour and precipitation by evaporation from the sea (to the left) and by formation of precipitation from a cooling air mass during its move towards higher latitudes and/or higher altitudes, e.g. along a warm front or over the inland ice (to the right). Credit: Willi Dansgaard.

This is a plot of mean annual δ of snow versus temperature at stations on Greenland. Credit: Willi Dansgaard.

Mean annual δ versus temperature at Atlantic island, coast and continental stations suggest (1) a separate tropical-subtropical, a temperate and perhaps a Polar distillation column, and (2) the influence of re-evaporated freshwater on δ at coastal and particularly continental stations. Credit: Willi Dansgaard.

Def. the study of the structure of rocky objects, or rocks, is called petrognosy.

Def. the study of the origin of rocky objects, or rocks, is called petrogony.

"The ice cap covers 85% of the total 2.2 mill. km2 area of Greenland [...]. It measures 2500 km from north to south and about 750 km from west to east in Mid Greenland. The surface reaches 3250 m [above sea level] a.s.l. at Summit and 2850 m on a minor dome in South Greenland."[7]

"Unlike the American and Scandinavian ice sheets built up during each glaciation in the past, most of the Greenland ice cap survived the intervening warm periods for two reasons: Firstly, the high surface elevation (at present 2/3 of its area lies more than 2000m a.s.l.) is associated with low surface temperatures (present annual mean values down to -32°C at Summit), and melting with run-off only in the marginal areas. Secondly, ample supply of precipitation from the Atlantic Ocean is given off by low-pressures moving northward along the coasts, sometimes even crossing the ice sheet."[7]

"The annual accumulation ranges from 13 cm of water equivalent in the central part of northeast Greenland to more than 100 cm along the southeastern coast, a total supply of about 500 km3 of water equivalent per year. The total annual loss of material is approximately of the same magnitude, of which 50% is given off as melt water by mainly coastal surface and bottom melting and an equal amount as icebergs."[7]

The diagram at the right shows "δ-changes (isotopic fractionation [...]) in vapour and precipitation by evaporation from the sea (to the left) and by formation of precipitation [snow] from a cooling air mass during its move towards higher latitudes and/or higher altitudes, e.g. along a warm front or over the inland ice (to the right)."[7]

"The main reason why δ varies in the water cycle is that the heavy H2O18 molecules have a 10 ‰ lower tendency to evaporate from a water surface than the light H2O16 molecules, and a 10 ‰ higher tendency to condensate from water vapour – or in physical terms: The vapour pressure of H2O18 is 10 ‰ lower than that of H2O16. Hence, vapour in equilibrium with sea water (SMOW) has a δ-value of -10 ‰. [indicated in the diagram to the right] and, conversely, when the vapour cools off droplets are formed with a δ-value 10 ‰ higher than that of the vapour, at first therefore 0 ‰."[7]

"When a cloud gives off rain, it loses more H2O18 than corresponding to the concentration in the vapour. The remaining vapour thereby gets a lower δ-value. This decrease in δ will continue by further cooling, and so will the δ of the rain as the precipitation process proceeds. If the air mass does not take up new vapour on the way, the δ-value of the precipitation decreases approximately by 0.7 ‰ per °C cooling."[7]

"In [the diagram at the right] the primary evaporation takes place from a sub-tropical ocean surface to the left, and the horizontal arrows follow the humid air mass toward north while cooling to the dew point, when the first rain is formed (with δ = 0 ‰). During the proceeding cooling, when the air mass crosses a continent, or flows over the inland ice (to the right in the figure), or ascent along a warm front, it gives off precipitation and acquires steadily decreasing δ’s for both the vapour and the precipitation. This explains [...] the generally very low δ values in rain from the top part of the warm front [...]."[7]

The second figure at the right "shows how the mean δ of snow is correlated with the local mean annual air temperature measured 10 or 20m below surface. Disregarding some stations on the eastern slope of the ice cap indicated by crosses, 1 centigrade lower temperature corresponds to 0.7 ‰ lower δ-value. The deviation of the stations on the eastern slope is probably due to these stations receiving some snow from western directions, i.e. from air-masses that have passed the high ice-ridge thereby losing considerable amounts of the isotopically heavy components of the water vapour, whereas the temperature is determined by adiabatic ascent from the east."[7]

"In [the third lower figure at the right] the Atlantic [United Nations’ International Atomic Energy Agency] IAEA stations are divided into groups of ocean, coast and continental stations. Further grouping in a tropical-subtropical (red), a temperate (green) and a polar (violet) category shows linear relationships between annual δ versus temperature within each category, which suggests (1) the influence of re-evaporated fresh water from the continents, and (2) the existence of at least two more or less separate distillation columns in the Atlantic Ocean, a tropical-subtropical and a temperate one, perhaps even a polar one. This is an effect of the air taking up new vapour during its travel toward higher latitudes, i.e. a precipitation pattern more complicated than that considered in [the first diagram at the right]."[7]

## Largest objects

The Aletsch glacier is the largest in the Alps of Switzerland. Credit: Dirk Beyer.

The Quelccaya ice cap is the largest glaciated area in the tropics. Credit: Edubucher.

The Aletsch glacier in the image on the right is the largest in the Swiss Alps.

The Quelccaya ice cap in the image on the left is located in southern Peru in the Cordillera Vilcanota and is the largest glaciated area in the tropics.

## Longest objects

Examples of glacier length records from different parts of the world are graphed. Credit: J. Oerlemans.

The Baltoro Glacier in the Karakoram, Baltistan, Northern Pakistan, at 62 kilometres (39 mi) in length, is one of the longest alpine glaciers on Earth. Credit: Guilhem Vellut from Paris.

There is a "link between climate processes and glacier mass balance (3, 4)."[135]

"The response of a glacier to climate change depends on its geometry and on the climatic setting. To unravel the climate signal con- tained in the glacier length records, it is necessary to discriminate with respect to the climate sensitivity c and to the response time τ (the time a glacier needs to approach a new equilibrium state). Similar to other climate proxies, glacier length fluctuations are the product of variations in more than one meteorological parameter. Glacier mass balance depends mainly on air temperature, solar radi- ation, and precipitation."[135]

The "primary source for melt energy is solar radiation but that fluctuations in the mass balance through the years are mainly due to temperature and precipitation (25, 26). Mass-balance modeling for a large number of glaciers has shown that a 25% increase in annual precipitation is typically needed to compensate for the mass loss due to a uniform 1 K warming (3, 27). These results, combined with evidence that precipitation anomalies normally have smaller spatial and temporal scales than those of temperature anomalies (2), indicate that glacier fluctuations over decades to centuries on a continental scale are primarily driven by temperature. Here, the climate sensitivity c is therefore defined as the decrease in equilibrium glacier length per degree temperature increase."[135]

"Climate sensitivity depends in particular on the surface slope (a geometric effect) and the annual precipitation (a mass-balance effect)."[135]

In "the sample of 169 glaciers, c varies by a factor of 10, from ~1 to ~10 km K–1 [...]. The response time is to a large extent determined by the slope and the balance gradient (the rate at which the mass gain or loss changes with elevation). Values of τ vary from about 10 years for the steepest glaciers to a few hundreds of years for the largest glaciers in the sample with a small slope (the glaciers in Svalbard). Most of the values are in the range of 40 to 100 years [...]."[135]

The Baltoro Glacier in the Karakoram, Baltistan, Northern Pakistan, in the image on the lower right, at 62 kilometres (39 mi) in length, is one of the longest alpine glaciers on Earth.

## Emissions

Antarctica's major ice shelf areas are indicated. Credit: National Snow & Ice Data Center.

This is a schematic of glaciological and oceanographic processes along the Antarctic coast. Credit: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.

A 430-square-kilometer section of the 13,680-square-kilometer Wilkins Ice Shelf on the Antarctic Peninsula rapidly disintegrated. Credit: National Snow & Ice Data Center.

This satellite image shows floating chunks of ice from the 2008 Wilkins Ice Shelf collapse. Credit: Cheng-Chien Liu and An-Ming Wu, National Space Organization, Taiwan.

Def. a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface is called an ice shelf.

"Ice shelves are permanent floating sheets of ice that connect to a landmass."[136]

"Most of the world's ice shelves hug the coast of Antarctica [as shown in the image on the right]. However, ice shelves can also form wherever ice flows from land into cold ocean waters, including some glaciers in the Northern Hemisphere. The northern coast of Canada's Ellesmere Island is home to several well-known ice shelves, among them the Markham and the Ward Hunt ice shelves."[136]

"Ice from enormous ice sheets slowly oozes into the sea through glaciers and ice streams. If the ocean is cold enough, [...] newly arrived ice doesn't melt right away. Instead it may float on the surface and grow larger as glacial ice behind it continues to flow into the sea. Along protected coastlines, the resulting ice shelves can survive for thousands of years, bolstered by the rock of peninsulas and islands. Ice shelves grow when they gain ice from land, and occasionally shrink when icebergs calve off their edges."[136]

The schematic on the right presents glaciological and oceanographic processes along the Antarctic coast. Snow falling in the accumulation zone creates an upstream stress. The ice shelf has built up to a thickness of about 4 km. The ice flows along the glacier and in ice streams within. On the coast the ice loses contact with its bedrock at the grounding line and becomes significantly thinner by some 100 m. It forms an ice shelf over the continental shelf. At the edge of the continental shelf, tabular icebergs calve.

"Satellite imagery [third image on the right] revealed that the western front of the 13,680 square kilometer (5,282 square mile) Wilkins Ice Shelf began to collapse because of rapid climate change in a fast-warming region of Antarctica."[136]

"This satellite image [fourth on the right] shows floating chunks of ice from the 2008 Wilkins Ice Shelf collapse."[136]

"Most ice shelves are fed by inland glaciers. Together, an ice shelf and the glaciers feeding it can form a stable system, with the forces of outflow and back pressure balanced. Warmer temperatures can destabilize this system by increasing glacier flow speed and—more dramatically—by disintegrating the ice shelf. Without a shelf to slow its speed, the glacier accelerates. After the 2002 Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula accelerated up to eight times their original speed over the next 18 months. Similar losses of ice tongues in Greenland have caused speed-ups of two to three times the flow rate in just one year."[136]

"Ice shelves fall into three categories: (1) ice shelves fed by glaciers, (2) ice shelves created by sea ice, and (3) composite ice shelves (Jeffries 2002). Most of the world's ice shelves, including the largest, are fed by glaciers and are located in Greenland and Antarctica."[136]

## Meteors

This shows calving by the Perito Moreno Glacier, in Los Glaciares National Park, southern Argentina. Credit: Luca Galuzzi.

These Multi-angle Imaging SpectroRadiometer (MISR) images show the progression of a "loose tooth"—an iceberg calving from the Amery Ice Shelf. Credit: NASA Earth Observatory, Clare Averill and David J. Diner, Jet Propulsion Laboratory; and Helen A. Fricker, Scripps Institution of Oceanography.

Retreating calving front of the Jacobshavn Isbrae glacier in Greenland from 1851 - 2006. Credit: NASA Earth Observatory, Cindy Starr, based on data from Ole Bennike and Anker Weidick (Geological Survey of Denmark and Greenland) and Landsat data.

Photos show the A54 iceberg calving from the Scar Inlet Shelf (the remainder of the Larsen Ice Shelf). Credit: Ted Scambos, National Snow and Ice Data Center, University of Colorado, Boulder, and NASA Moderate Resolution Imaging Spectroradiometer images courtesy NASA Earth Observatory.

The image on the right shows calving by the Perito Moreno Glacier, in Los Glaciares National Park, southern Argentina.

"Calving of huge, tabular icebergs is unique to Antarctica, and the process can take a decade or longer. Calving results from rifts that reach across the shelf. In the case of Antarctica's Amery Ice Shelf, the calving area resembles a loose tooth [images on the second right]." per Clare Averill and David J. Diner, and Helen A. Fricker, State of the Cryosphere: Ice calves at .

On a stable ice shelf, calving is a near-cyclical, repetitive process producing large icebergs every few decades. The icebergs drift generally westward around the continent, and as long as they remain in the cold, near-coastline water, they can survive decades or more. However, they eventually are caught up in north-drifting currents where they melt and break apart.

In Greenland, floating ice tongues downstream from large outlet glaciers are more broken up by crevasses. Calving of the ice tongues releases armadas of smaller, steep-sided icebergs that drift south sometimes reaching North Atlantic shipping lanes. Calving of the large glacier, Jacobshavn, on the east coast of Greenland is responsible for the majority of icebergs reaching Atlantic shipping and fishing areas off of Newfoundland and most likely shed the iceberg responsible for the sinking of the Titanic in 1912. The Petermann Glacier in northwestern Greenland also shed a large ice island in August 2010. These denizens of the ocean are now tracked by the National Ice Center in the United States, along with other organizations.

By 2006, the Jacobshavn Glacier, third image on the right, had retreated back to where its two main tributaries join, leading to two fast-flowing glaciers where there had previously been just one.

The rapidly retreating Jakobshavn Glacier in western Greenland drains the central ice sheet. This image, third one on the right, shows the glacier in 2001, flowing from upper right to lower left. Terminus locations before 2001 were determined by surveys and more recent contours were derived from Landsat data. The recent stages of retreat have widened the ice front, placing more of the glacier in contact with the ocean.

In recent years, calving of the largest ice tongues in Greenland (in particular, Jacobshavn, Helheim, and Kangerdlugssuaq) has accelerated probably due to warmer air and/or ocean temperatures. As the ice tongues have retreated, the reduced backpressure against the glacier has allowed these glaciers to accelerate significantly.

The images, fourth set of images on the right, show a tabular iceberg calving from an ice shelf. This iceberg happens to be calving from the remnant piece of the Larsen B ice shelf at the southwestern corner of the embayment. While the Larsen B Ice Shelf underwent disintegration [...], this was a normal calving event.

Large tabular iceberg calvings are natural events that occur under stable climatic conditions, so they are not a good indicator of warming or changing climate. Over the past several decades, however, meteorological records have revealed atmospheric warming on the Antarctic Peninsula, and the northernmost ice shelves on the peninsula have retreated dramatically (Vaughan and Doake 1996).

The most pronounced ice shelf retreat has occurred on the Larsen Ice Shelf, located on the eastern side of the Antarctic Peninsula's northern tip. The shelf is divided into four regions from north to south: A, B, C, and D.

## Gamma rays

"Airborne gamma ray spectrometry data provide information about the distribution of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh) in the upper 1 m of surficial materials [...] Knowledge of the distribution of surface materials is useful for interpreting gamma ray data because gamma radiation is attenuated by water and unconsolidated sediments [...] Ratio patterns of K, eU, and eTh in surficial materials can enhance subtle variations in the elemental concentrations caused by lithological variation of the alteration processes associated with mineralization."[137]

## Visuals

At the mouth of Schlatenkees glacier, water comes rushing out. Credit: SehLax.

At the mouth of Schlatenkees glacier, in the image on the right, water comes rushing out along with sediment that alters the color of the ice and the water.

## Blues

The Perito Moreno Glacier, in Los Glaciares National Park, southern Argentina, exhibits a typical blue color of the ice. Credit: Martin St-Amant.

Even when a grotto forms underneath a glacier, the ice still exhibits its typical blue color as with the Perito Moreno Glacier, in Los Glaciares National Park, southern Argentina, in the image on the right.

"Majestic glaciers and thick snow banks act like filters that absorb red light, making a crevasse or deep hole appear blue."[138]

## Cyans

Glacier Flour causes the blue green color in this Lake on Mt. Daniels. Credit: Mauri S. Pelto.

"Blue to blue-green hues are scattered back when light deeply penetrates frozen waterfalls and glaciers."[138]

"Glacier Flour causes the blue green color in this Lake on Mt. Daniels. The flour is clay sized particle resulting from the glacier eroding the rock at its base."[134]

## Oranges

Holgate Glacier, Kenai Fjords National Park, Alaska, has an orange color. Credit: Beeblebrox.

The color of the sediment deposited on or scraped away from the confining rocks often colors a glacier and its meltwater. Here it does so to the Holgate Glacier imaged on the right.

Canadian RADARSAT image shows the shelf in August 2002, when a crack made its way down the length of the shelf. Credit: Alaska Satellite Facility, Geophysical Institute, University of Alaska Fairbanks.

This is a Radarsat image of ice streams flowing into the Filchner-Ronne Ice Shelf. Credit: .

"One example of an ice shelf composed of compacted, thickened sea ice is the Ward Hunt Ice Shelf off the coast of Ellesmere Island in northern Canada. Canadian RADARSAT image shows the shelf in August 2002, when a crack made its way down the length of the shelf."[136]

The image on the left is a Radarsat portrayal of ice streams flowing into the Filchner-Ronne Ice Shelf. This image uses data from the Radarsat RAMP 125m Mosaic. The dataset is freely available from the National Snow and Ice Data Center.

## Gaseous objects

At some altitudes when warm, moist air comes in contact with a glacier, water vapor condenses and forms fog as here on the Gorner Glacier in Switzerland. Credit: Roy Lindman.

Ozone-depleting gases in Greenland firn.

36Cl from 1960s nuclear bombs in US glacier ice.

At some altitudes when warm, moist air comes in contact with a glacier, water vapor condenses and forms fog as imaged on the right on the Gorner Glacier in Switzerland.

The "firn air analyses of Site M in Dronning Maud Land, Antarctica (15°E, 75°S, 3453 m.a.s.l) are described. These firn air analyses were measured with gas chromatography, yielding concentration profiles with depth for a wide variety of trace gases."[8]

The "firn air analyses are focussed on the non-methane hydrocarbons (NMHCs): ethane, propane and acetylene, and methyl chloride. The NMHCs were studied because very little is known about their long-term and seasonal trend in the atmosphere around Antarctica and Southern Hemisphere in general whereas these NMHCs play an important role in the atmospheric oxidation chemistry. Studying the long-term and seasonal trend for methyl chloride is very interesting because this gas shows a large spatial variability although this is not expected because of its large lifetime."[8]

The "location and age of the oldest firn air, being 156±22 years for the mean age of CO2 at pore close-off depth at 82°E, 83°S on the Antarctic plateau."[8]

The "chemical signature based on 16 trace elements, a chemical fingerprint, of an unknown 1500-year old volcanic horizon is measured with inductively coupled plasma mass spectrometry (ICP-MS). [The] trace metal chemistry is compared to the DEP record directly measured in the field. With the chemical fingerprint for the 1500-year old volcanic horizon, which has been scaled and normalised to allow comparison among various geochemical data, Mount Erebus is identified as a good candidate for the source."[8]

Below the firn is a zone in which seasonal layers alternately have open and closed porosity. These layers are sealed with respect to diffusion. Gas ages increase rapidly with depth in these layers. Various gases are fractionated while bubbles are trapped where firn is converted to ice.[139]

The surface layer is snow in various forms, with air gaps between snowflakes. As snow continues to accumulate, the buried snow is compressed and forms firn, a grainy material with a texture similar to granulated sugar. Air gaps remain, and some circulation of air continues. As snow accumulates above, the firn continues to densify, and at some point the pores close off and the air is trapped. Because the air continues to circulate until then, the ice age and the age of the gas enclosed are not the same, and may differ by hundreds of years. The gas age–ice age difference is as great as 7 kyr in glacial ice from Vostok.[140]

Gases involved in ozone depletion, CFCs, chlorocarbons, and bromocarbons, were measured in firn and levels were almost zero at around 1880 except for CH3Br, which is known to have natural sources.[141]

Similar study of Greenland firn found that CFCs vanished at a depth of 69 m (CO2 age of 1929).[142]

Analysis of the Upper Fremont Glacier ice core showed large levels of chlorine-36 that definitely correspond to the production of that isotope during atmospheric testing of nuclear weapons. This result is interesting because the signal exists despite being on a glacier and undergoing the effects of thawing, refreezing, and associated meltwater percolation.[143]

36Cl has also been detected in the Dye-3 ice core (Greenland)[144], and in firn at Vostok.[145]

Studies of gases in firn often involve estimates of changes in gases due to physical processes such as diffusion. However, it has been noted that there also are populations of bacteria in surface snow and firn at the South Pole, although this study has been challenged.[146][147]

It had previously been pointed out that anomalies in some trace gases may be explained as due to accumulation of in-situ metabolic trace gas byproducts.[148]

## Liquid objects

The image shows an opening of a moulin on the Greenland ice sheet. Credit: NASA/JPL.

An aerial view atop the Greenland ice sheet shows water converging on the moulin. Credit: NASA/JPL.

Def. "(1) a large outburst flood that usually occurs when a glacially dammed lake drains catastrophically (2) any catastrophic release of water from a glacier"[2] is called a jokulhlaup.

"Proglacial stream water is a mixture of supraglacial and subglacial meltwater with groundwater inputs (Raiswell 1984). The term supraglacial is used for both superglacial (surface melt) and englacial (throughput) meltwaters that do not come into contact with subglacial water until just before or after the portal of the glacial stream. Subglacial meltwaters are characterized by high suspended and dissolved solids and residence times on the order of several days, while supraglacial meltwaters are typically characterized by very low dissolved and suspended solids, atmospheric pCO2 levels (10-3.5 atm), and residence times of < 24 h (Raiswell 1984). Collins (1979) found that supraglacial meltwater accounted for a large percentage (50-80%) of the total meltwater discharge of two alpine glaciers during warm summer months. Mixing of very dilute supraglacial meltwaters with subglacial water rich in finely ground glacial flour creates a reactive environment for ion exchange (Lemmens and Roger 1978) and mineral dissolution (Raiswell1984). An enhanced reactivity of minerals in glacial flour due to small particle size has been proposed frequently (e.g. Reynolds and Johnson 1972; Raiswell 1984) and has been qualitatively simulated in the laboratory (Keller and Reesman 1963)."[149]

A moulin is a narrow, tubular shaft in a glacier that provides a pathway for water to travel from the glacier's surface to its bottom. [Alberto] Behar and co-investigator Dr. Konrad Steffen of the University of Colorado, Boulder, led an expedition to send a JPL-built probe down into these glacial chutes in the remote and isolated Pakisoq region of the West Greenland Ice Sheet. That dynamic area of Earth's northern polar region is not well understood and is responding rapidly to climate change. Previous NASA measurements there using global positioning system data show the ice there moves an average of about 20 centimeters (8 inches) a day, accelerating to about 35 centimeters (14 inches) a day during the summer melt. The scientists set out to see if these moulins, or pathways, within and beneath these mountains of ice can shed new light on how glacial water flows from the ice sheet to the sea.

How water is distributed within a glacier and the rate at which it permeates through to a glacier's bottom affects the glacier's ability to store water, its pressure, and the speed at which a glacier moves. Scientists have long known that ponds of melted ice on the surfaces of glaciers and the moulins they create allow glaciers to flow faster. Our study -- the first of its kind in this region -- we hope will provide a better understanding of the factors at work here.

In glaciological terms, moulins (French for "mill") are essentially vertical "rivers" that serve as a glacier's internal plumbing system, carrying water out of the glacier from melt water lakes on the surface. They can be hundreds of meters deep and up to 10 meters (33 feet) wide. The melt water lakes are typically found in the undulations of the ice sheet all around Greenland between 500 and 1,500 meters (1,640 to 4,920 feet) above sea level. Occasionally, the lower end of a moulin may be exposed in the face of a glacier.

In Greenland, the surface of the ice sheet moves at varying speeds, on both seasonal and shorter-term time scales. Seasonally, glacial water penetrates to the glacier bed through significant thicknesses of cold ice. Occasionally, however, there are periods when water flows rapidly into glacial drainage systems. For example, early in the melt season, new drainage connections are established between glacial surfaces and their beds, resulting in sudden new flows of water out of the glaciers. In the middle of the melt season, surface melting resumes after periods of cold weather, which can partially close sub-glacial channels.

Once the probe descended to 110 meters (361 feet), it encountered horizontally flowing water and debris about one to two meters (3.3 to 6.6 feet) deep. In this particular moulin, the water flows out in well-developed channels to the edge of the ice sheet. At the time of the experiment, the scientists measured the water flow rate of the surface melt rivers feeding the moulin at approximately 15 cubic meters a second (about 238,000 gallons a minute).

Video data from the probe revealed enormous ice caverns formed by the moulin deep beneath the glacial surface. This particular moulin appears to fill up with snow, which hardens during the winter. When the surface melts in the summer, this ice forms snow bridges.

## Hydrology

Glacier runoff such as this waterfall from the Colonial Glacier is the melt from the entire glacier. Credit: Mauri S. Pelto.

A proglacial lake is a lake beyond the terminus of a glacier. Credit: Mauri S. Pelto.

The image shows a channel cut by a supraglacial stream on the Rainbow Glacier. Credit: Mauri S. Pelto.

Def. "the melt from the entire glacier"[134] is called glacier runoff.

"It is highest on warm dry days when runoff from other sources is at its lowest."[134]

Def. "a lake beyond the terminus of a glacier"[134] is called a proglacial lake.

Def. a stream running on top of a glacier or cutting a channel into a glacier from the top is called a supraglacial stream.[134]

"Almost every general textbook on hydrology includes a chapter on snow: measurement, melting, calculation of snowmelt runoff, and forecasting of snowmelt floods."[150]

Measurement "of glacier growth and wastage, predicting glacier runoff, the buffering effect of glaciers on streamflow variations, glacier outburst floods, or the internal structure and hydraulic properties of glaciers [are how] glacier hydrology differs, both qualitatively and quantitatively, from the hydrology of conventional streams and even the hydrology of snow. Yet about three-fourths of all the freshwater on the earth is temporarily detained as glacier ice (equivalent to the world's precipitation for about 60 years), and in many parts of the world hydroelectric, irrigation, and domestic water resources are directly dependent on glacier runoff."[150]

"Hydrologically, perhaps the most significant aspect is that when the glacier is in the process of adjusting to climate change (this occurs continually because climate changes quickly and glaciers slowly), precipitation is being stored or released from storage [3]. Not everyone is aware that, during the 1920-45 period when many runoff "base" or "normal" periods are defined, glaciers in most populated parts of the world were receding and releasing streamflow greatly in excess of precipitation [4]."[150]

"Mountain snow cover is a critical resource as these high elevation mountain regions provide the majority of fresh water supply in arid and semi-arid environments to more than a billion of the Earth’s population [Bales et al., 2006]. The duration of snow pack in mountain regions critically controls the timing and magnitude of water supplies, power generation, agriculture timing, and forest fire regimes [Westerling et al., 2006], as well as the duration over which glacial ice is exposed to absorption and enhanced ablation. Some studies already suggest that climate change has induced earlier snowmelt-fed runoff [Mote, 2003; Stewart et al., 2005]."[151]

Earlier "snowmelt and its effects on mountain water resources and glacial extent is a likely scenario in many of the world’s mountain ranges under enhanced dust deposition."[151]

## Rocky objects

Frying Pan Glacier is almost entirely covered by rocks and debris. Credit: George L. Snyder.

Def. "looks like a mountain glacier and has active flow; usually includes a poorly sorted mess of rocks and fine material; may include: (1) interstitial ice a meter or so below the surface (ice-cemented), (2) a buried core of ice (ice-cored), and/or (3) rock debris from avalanching snow and rock"[2] is called a rock glacier.

At the right, "Frying Pan Glacier, Colorado, is almost entirely covered by rocks and debris in this photograph from 1966."[2]

## Ice-rafted debris

This debris covered iceberg was calved from the terminus of Alaska's Sheridan Glacier. Credit: Bruce Molnia US Geological Survey.

"Correlations between North Atlantic marine sequences and Greenland ice-core δ 18O records showed that high amounts of ice-rafted debris (IRD) and extremely cold [sea surface temperatures] SSTs occurred prior to GIS 12, 8, 4, 2 and 1 (Bond et al. 1993). These so-called Heinrich events [in the diagram of the Bølling section], which originated from surges of Northern Hemisphere ice sheets (Hemming 2004), caused a rise in global sea level (Chappell 2002; Lambeck et al. 2002; Rohling et al. 2004; Rohling & Pälike 2005), which in turn could have caused further destabilization of the large ice sheets. Provenance analyses of Heinrich-layer detritus in North Atlantic marine sediments indicate that the IRD is derived from the Hudson Strait region/Laurentide ice sheet (Hemming 2004), although precursor events could possibly have originated from European and Icelandic ice sheets (Grousset et al. 2000, 2001)."[24]

## Moraines

A lateral moraine on a glacier joining the Gorner Glacier is left of center. Credit: Adrian Pingstone.

Lateral and terminal moraines of a valley glacier are shown. Credit: Natural Resources Canada, Terrain Sciences Division, Geological Survey of Canada.

"Lateral and terminal moraines of a valley glacier, Bylot Island, Canada [are shown in the image at the left]. The glacier formed a massive sharp-crested lateral moraine at the maximum of its expansion during the Little Ice Age. The more rounded terminal moraine at the front consists of medial moraines that were created by the junction of tributary glaciers upstream."[2]

## Glacial landscapes

These are Landsat surficial predictions. Credit: I.M. Kettles, A.N. Rencz, and S.D. Bauke.

In the two images on the right, (a) is a predictive "surficial geology map based on Landsat TM. [The outline of an area] covered by the traditional surficial geology map is shown [outlined by] a black line. (b) [is the traditional] surficial geology map, based on 1:50,000-scale field mapping and aerial photograph interpretations".[137]

## Trimlines

The arrow shows where the glacier was just a few years ago. Credit: .

Def. a clear boundary line on the wall of a glacier valley that delineates the maximum recent thickness of a glacier is called a trimline.

## Isostasy

The figure shows isostatic rebound in Canada. Credit: Steve Dutch.

This map shows isostatic uplift in Scandinavia. Credit: Steve Dutch.

Def. the state of balance or pressure equilibrium thought to exist within the Earth's crust, whereby the upper lithosphere floats on denser magma beneath is called isostasy.

"It's probably no accident that two of the largest epicontinental seas (seas extending deep into the continent) on Earth are Hudson Bay and the Baltic, both dead center in areas of active isostatic uplift. In all likelihood, the crust in these regions is still depressed and has not finished rising, and when uplift is complete both seas will mostly or entirely disappear. Gravity measurements suggest that the crust in the Hudson Bay region has another 100 meters still to rise."[152]

## Oxygens

Many rocky objects are composed of oxide minerals. Oxygen has three known stable isotopes: 16O, 17O, and 18O.

The stable isotopic compositions of low-mass (light) elements such as oxygen, hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, fluorine, neon, sodium, magnesium, aluminum, silicon, phosphorus, and sulfur are normally reported as "delta" ([δ]) values in parts per thousand (denoted as ‰ per mille) enrichments or depletions relative to a standard of known composition.[153]

For 18O to 16O:

${\displaystyle \delta ^{18}O={\Biggl (}{\frac {{\bigl (}{\frac {^{18}O}{^{16}O}}{\bigr )}_{sample}}{{\bigl (}{\frac {^{18}O}{^{16}O}}{\bigr )}_{standard}}}-1{\Biggr )}*1000\ ^{o}\!/\!_{oo}}$

The ratio by convention is of the heavy to light isotope in the sample or standard.

Various isotope standards are used for reporting isotopic compositions; the compositions of each of the standards have been defined as 0‰. Stable oxygen and hydrogen isotopic ratios are normally reported relative to the SMOW standard ("Standard Mean Ocean Water" (Craig, 1961b)) or the virtually equivalent VSMOW (Vienna-SMOW) standard. Carbon stable isotope ratios are reported relative to the PDB (for Pee Dee Belemnite) or the equivalent VPDB (Vienna PDB) standard. The oxygen stable isotope ratios of carbonates are commonly reported relative to PDB or VPDB, also. Sulfur and nitrogen isotopes are reported relative to CDT (for Cañon Diablo Troilite) and AIR (for atmospheric air), respectively.

## Spectrometers

Snow has the highest albedo of any naturally occurring surface on Earth. Credit: Thomas H. Painter, Andrew P. Barrett, Christopher C. Landry, Jason C. Neff, Maureen P. Cassidy, Corey R. Lawrence, Kathleen E. McBride, and G. Lang Farmer.

"Snow has the highest albedo of any naturally occurring surface on Earth. However, when impurities such as dust or soot are present, snow albedo decreases (particularly in visible wavelengths) [Conway et al., 1996; Warren and Wiscombe, 1980] [Figure at right]. With enhanced absorption by dust, grain growth rates increase and further depress snow albedo. Dust has high potential to sustain shortwave radiative forcing after deposition because particles tend to accumulate near the snow surface as ablation advances [Conway et al., 1996]. Deposition in mountain ranges comes primarily in the spring when frontal systems entrain dust particles from disturbed and loose soils [Wake and Mayewski, 1994], coinciding with solar irradiance approaching its annual maximum."[151]

## Earth

Earth has ice sheets, ice caps, ice fields, and glaciers.

## Sea ices

This is an aerial view of the pack ice off the eastcoast of Greenland. Credit: .

This is pack ice off the coast of Vaxholm, Sweden. Credit: Cyberjunkie.

Pack-ice-covered Auke Bay Harbor, Alaska, in winter. Credit: David Csepp, NOAA/NMFS/AKFSC/ABL.

Def. "frozen ocean water"[136] is called sea ice.

"It forms, grows, and melts in the ocean."[136]

Def. a large consolidated mass of floating sea ice is called pack ice.

Pack ice in the image on the right is drifting southward in the East Greenland current during July 1996.

## Drift ices

A photograph of the breakwater of Lake Michigan at Milwaukee, Wisconsin, in the winter, shows pancake ice. Credit: Benn Newman.

Drift ice is captured in the river Spree with "Bode-Museum" and "Fernsehturm" ashore. Credit: Michael Fiegle.

When waves buffet the freezing ocean surface, characteristic "pancake" sea ice forms. Credit: Ted Scambos, NSIDC.

Def. one or more floating slabs of ice which have become detached from larger sheets or shoreline glaciers and which are moved by wind or sea currents is called drift ice.

Def. a form of ice that forms on water covered to some degree in slush is called pancake ice.

In the first image on the left, when "waves buffet the freezing ocean surface, characteristic "pancake" sea ice forms."[154]

"Sheets of sea ice form when frazil crystals float to the surface, accummulate and bond together. Depending upon the climatic conditions, sheets can develop from grease and congelation ice, or from pancake ice."[155]

"If the ocean is rough, the frazil crystals accummulate into slushy circular disks, called pancakes or pancake ice, because of their shape. A signature feature of pancake ice is raised edges or ridges on the perimeter, caused by the pancakes bumping into each other from the ocean waves. If the motion is strong enough, rafting occurs. If the ice is thick enough, ridging occurs, where the sea ice bends or fractures and piles on top of itself, forming lines of ridges on the surface. Each ridge has a corresponding structure, called a keel, that forms on the underside of the ice. Particularly in the Arctic, ridges up to 20 meters (60 feet) thick can form when thick ice deforms. Eventually, the pancakes cement together and consolidate into a coherent ice sheet. Unlike the congelation process, sheet ice formed from consolidated pancakes has a rough bottom surface."[155]

## Locations on Earth

"The coordinates should describe the location of a glacier as accurately as possible. In the former [World Glacier Inventory] WGI it was recommended to place this point in the upper part of the ablation area near the centre of the main stream. This recommendation is still valid for manual assignment of the coordinates. Considering the available GIS-based methods, it is also acceptable to create a label point inside a glacier polygon automatically, and add the x-y coordinates of this point to the attribute table of the respective glacier".[156]

"In regions where glacier changes are small and the coordinates from a former glacier inventory are available, it should be determined whether the former coordinates are still within the current outlines. If they have been stored with a sufficient number of digits, it is possible that only a few label points have to be shifted slightly. This way of assigning label points is preferable in order to establish a link to the former inventory."[156]

"Highest, mean and lowest glacier elevation are also basic entries in the former WGI. It is recommended that they be derived from glacier-specific statistical analysis using the elevation information from the [digital terrain model] DTM and a local glacier ID as an identifier for the respective glacier or zone."[156]

"The aspect or orientation of a glacier is a useful parameter for all kinds of modelling (e.g. Evans, 2006). In the former UNESCO guidelines this variable was restricted to eight directions: ‘Orientation of the down-glacier direction ac- cording to the eight cardinal points ... should be given.’ The mean aspect as derived from a DTM allows one to consider the value of all individual cells covered by the glacier and to derive a mean value in the full 0–3608 range. It must be taken into account that aspect is a circular parameter, which means that mean values must be derived by a decomposition in the respective sine and cosine values (Paul, 2007; Manley, 2008)."[156]

Def. the data present in contour lines; measurement of altitude or depth versus area is called hypsography.

Def. the measurement of elevation relative to sea level is called hypsometry.

## Stratigraphy

A sample is taken from an outcropping of melting permafrost. Credit: Sergei Zimov.

Sampling the surface of Taku Glacier in Alaska demonstrates that there is increasingly denser firn between surface snow and blue glacier ice. Credit: .

Glaciology like many fields within geology often requires field work.

The image at right shows a glaciologist taking a sample of a melting permafrost outcropping.

The second image at the right shows sampling of the Taku Glacier surface in Alaska. Such sampling demonstrates that there is increasingly denser firn between surface snow and blue glacier ice.

## Glacial physics

Annual flow velocities on the Austre Torellbreen tongue are derived from displacement of crevasses on a pair of ASTER images. Credit: Małgorzata Błaszczyk, Jacek A. Jania and Jon Ove Hagen.

Examples of ASTER geocoded images (321 bands) of glaciers classified into different groups of flow dynamics. Credit: Małgorzata Błaszczyk, Jacek A. Jania and Jon Ove Hagen.

A Stagnant Glacier is a glacier that is not moving significantly, notice lack of crevassing. Credit: Mauri S. Pelto.

An Active Glacier is moving at reasonable pace, has crevassing, and is eroding its bed. Credit: Mauri S. Pelto.

"Glacier length is the most demanding parameter regarding additional manual work and uncertainty."[156]

"The definitions used in the former guide are:"[156]

Def. the "average of the lengths of each tributary along its longest flowlines to the glacier snout"[156] is called the mean length.

Def. the "longest flowline of the whole glacier"[156] is called the maximum length.

"When a DTM of sufficient quality is available, automated techniques can be used to identify the highest glacier point and then follow the steepest downward gradient until the curvature of the glacier surface changes from concave to convex. In this region – in general, the ablation area – manual digitization close to the central flowline of the main trunk might be more efficient. For manual digitization of the length, the flowline should cross elevation contours perpendicularly. Uncertainty of the result is thereby reduced if flowline digitization starts at the lower end of the glacier."[156]

Large apparently solid objects of ice flow. Gravity is a likely force acting on large solid ice objects. Many glaciers slide over their beds. Ice "itself can flow like a very viscous fluid, [...] increases in velocity after heavy rain [...] showed that water helps a glacier to slide. [...] the velocity is greatest in the central part and decreases progressively toward each side. [...] a glacier moves more slowly near its head and terminus than elsewhere. [...] velocity vectors do not parallel the glacier surface; relative to the surface, they are inclined slightly downward in the higher parts of the glacier, where snow accumulates, and slightly upward in the lower reaches to compensate for ice lost by melting."[133]

Ice "moves more rapidly at the surface than at depth. [...] in general, although slow across-glacier extrusion flow sometimes occurs in narrow valleys."[133]

There is a "time lag between the advance of the terminus and the increase in snowfall that produced it."[133]

"Mean slope is a value that could be derived from elevation range and glacier length and was thus not listed in the guidelines by UNESCO/IASH (1970). Mean slope as derived for each glacier from the DTM with zone statistics is independent of the glacier length and refers to all individual cells of the DTM (Manley, 2008). [Mean] slope is a good proxy for other parameters like mean thickness (Haeberli and Hoelzle, 1995) [...] A large number of further parameters characterizing individual glaciers (e.g. driving stress, slope-dependent thick- ness, volume, thermal conditions, response and reaction times) can be derived or estimated from the basic parameters".[156]

"Annual flow velocities on the Austre Torellbreen tongue [are] derived from displacement of crevasses on a pair of ASTER images (2005 and 2006) [in the image at right]: black lines – location of crevasses in 2005, white lines – location of crevasses in 2006, blue line – front position in 2005. The background image is a portion of the FCC of ASTER scene (acquired on 23.07.2006)."[157]

The mean flow velocity of the Svalbard tidewater glacier Austre Torellbreen is 260 m yr-1.[157]

"Examples of ASTER geocoded images (321 bands) of glaciers [in the second image on the right are] classified into different groups of flow dynamics: a) Negribreen – very slow or stagnant glacier (5.08.2003) [date surveyed 05 August 2003 at a mean flow velocity of <30 m yr-1]; b) Storbreen – slow−flowing glacier (7.08.2004) [80 m yr-1]; c) Austre Torellbreen – fast−flowing glacier (23.07.2005) [260 m yr-1]; [and] d) Perseibreen – active surge glacier (7.08.2004) [730-910 m yr-1]."[157]

Def. "a glacier that is not moving significantly, notice lack of crevassing"[134] is called a stagnant glacier.

Def. a glacier that is

1. "moving at reasonable [a] pace",[134]
2. "has crevassing",[134] and
3. "is eroding its bed"[134] is called an active glacier.

When the stress of the layer above exceeds the inter-layer binding strength, it moves faster than the layer below.[15]

The geothermal heat flux becomes more important the thicker a glacier becomes.[16]

## Ice streams

These animations show the motion of ice in Antarctica. Credit: .

This is a velocity map of Antarctic ice streams. Credit: Jonathan Bamber, University of Bristol.

The image on the right shows animated motions of ice flowing across Antarctica.

The second image on the right shows the ice stream velocities of Antarctic ice from zero (black) up to 250m/yr (cream white).

"Although they account for only 10% of the volume of the ice sheet, ice streams are sizeable features, up to 50 km wide, 2000 m thick and hundreds of km long. Some flow at speeds of over 1000 m per year and most of the ice leaving the ice sheet passes through them."[158]

"Ice streams generally form where water is present, but other factors also control their velocity, in particular whether the ice stream rests on hard rock or soft, deformable sediments. At the edges of ice streams deformation causes ice to recrystallise making it softer and concentrating the deformation into narrow bands or shear margins. Crevasses, cracks in the ice, result from rapid deformation and are common in shear margins."[158]

## Fracture zones

Shown are shear or herring-bone crevasses on the Emmons Glacier. Credit: Walter Siegmund.

Ice cracks are imaged in the Titlis Glacier. Credit: Audrius Meskauskas.

A graduate student crossing a crevasse in the Easton Glacier, North Cascades. Credit: Mauri S. Pelto.

Glacier mass balance is assessed in crevasses such as this on Easton Glacier. Credit: Mauri S. Pelto.

An Icefall is an area of rapid movement on a steep slope with extensive open crevassing. Credit: Mauri S. Pelto.

This is an illustration of water and ice pressure on the ice shelf close to the calving front. Credit: Arne Keller, Kolumban Hutter.

Shear or herring-bone crevasses on the Emmons Glacier shown on the right often form near the edge of a glacier where interactions with underlying or marginal rock impede flow. In this case, the impediment appears to be some distance from the near margin of the glacier.

Ice cracks are also imaged in the Titlis Glacier shown on the second right.

In the third image on the right, a graduate student crosses a crevasse on the Easton Glacier, Mount Baker, in the North Cascades of the United States.

The fourth image on the right shows a crevasse on the Easton Glacier.

"Glacier mass balance is assessed in crevasses such as this on Easton Glacier. Note the horizontal horizon marking the 2007 summer surface with 3.2 m of 2008 winter snowpack on top."[134]

"Crevasses [o]pen because of an acceleration of the glacier."[134]

Def. "an area of rapid movement on a steep slope with extensive open crevassing"[134] is called an icefall.

The "ice in the immediate vicinity of the shelf front is being weakened due to accumulating fractures (which may be transported from further inland), which finally leads to calving events. This calls for the incorporation of some kind of damage or fracture parameterization into ice-shelf models, in order to predict/assess their stability and possible onsetting instabilities, which have been shown to fall apart on a surprisingly short timescale."[159]

"The great unsolved problem in ice-shelf dynamics (perhaps in the whole of glaciology) is the treatment of the shelf front."[159]

"No response of an ice-sheet/ice-shelf system to climate variations can be computationally forecasted, unless this boundary condition is properly parameterized."[160]

"Dynamically, an equivalent description of the calving rate – the second condition – is the relevant climatological statement, describing the mass loss of the shelf, for which only first estimates exist ... The difficulty with parameterizations of the calving rate is its inter- mittent non-smooth occurrence in nature. Such discon- tinuous behavior of the mass loss by ice shelves is most likely not adequately parameterizable, but smeared over long time scales (of decades) a smooth parameterization may well be possible".[160]

"[A]long-flow ice shelf spreading [may be] the dominant control on calving".[161]

The "influence of water penetrating into any crack which opens at the base of the ice shelf [is] at a pressure close to the ice overburden pressure[. It] tends to counterbalance the action of the latter as an effect opposing crack opening."

${\displaystyle p_{eff}=p-p_{w},}$

where ${\displaystyle p_{w}}$  is the water pressure at a given depth [in the figure on the left]. This change accounts for the fact that the water pressure reduces the contribution to failure of the local mean pressure in ice near the base of the ice shelf that is exposed to crevasses. It assures that damage evolution in an ice shelf always starts at the top or base, not somewhere in between."[159]

## Technology

This is a drill rig for producing boreholes to monitor permafrost. Credit: Charles Harris, University of Cardiff.

"Satellite imagery and data from ground surveys are used to reconstruct the integrated pattern of the principal longitudinal and transverse features produced on a continent-wide scale by the last ice sheets in Europe and North America."[162]

Boreholes made by rigs like that imaged at the right in Svalbard, Norway, for example, indicate that ground temperatures rose 0.4C over the past decade, four times faster than they did in the previous century.[163]

## Hypotheses

1. Glaciers have occurred throughout Earth history.

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