ES 331/767 Lecture 2

James S. Aber

Table of Contents
Glacier ice Glacier budget
Glacier movement Columbia Ice Field
Greenland Ice Sheet Antarctic Ice Sheets
Ice-sheet meteorites Polar science online

Glacier ice

Modern glaciers and ice sheets cover approximately 10% of the world's land areas. Of this total, the Antarctic and Greenland Ice Sheets account for 84% and 11% each, with all other glaciers being only 5% (Flint 1971). Glaciers are inherently continental features that form on land or shallow continental shelves. They may extend as floating ice shelves from land into deeper water, but cannot exist in deep ocean settings otherwise. The study of modern glaciers and their behavior is glaciology.

National Snow and Ice Data Center.
Satellite Image Atlas of Glaciers.

Glaciers are composed of four components: ice, water, air, and rock debris. Glacier ice is a unique form of natural ice that is dynamic; it moves internally and over the substratum and is thus a powerful agent for modifying the landscape. Glacier ice is fundamentally different from other kinds of ice, such as sea ice, lake ice, permafrost, etc. Ice is the main component of all glaciers, and ice has several special properties that are important for understanding how glaciers operate.

Ice on Lake Kahola, east-central Kansas. The swirling, frothy appearance was created by freezing of foam (bubbles) into the ice. © by J.S. Aber.
Broken shards of ice piled on shore of Melvern Reservoir, east-central Kansas. Lake ice froze during an extended cold period--temperature <10°F, and then was broken up by wind and wave action. © by J.S. Aber.
Closeup view of ice shards piled on shore of Melvern Reservoir. Fragments are 3-4 inches (10 cm) thick. Individual ice crystals are oriented in a vertical position within shards, which indicates the ice formed by freezing from the lake surface downward. © by J.S. Aber.

First, ice behaves as a plastic material under relatively low pressure. This accounts for internal deformation and flowage that takes place in all glaciers. The hardness of ice varies with temperature. At 0°C, ice has a hardness of 1.5 on the Mohs scale; at -70°C the hardness is 6 (Nesje and Dahl 2000). However, temperature below -30°C is rarely achieved in glaciers. Next ice is subject to pressure melting In other words, ice can be melted at temperatures below 0°C by application of pressure--see Fig. 2-1. Because of this and because water has a higher density than ice, water is able to exist under high pressure beneath many glaciers. Finally ice and water have relatively high heat capacities, and phase changes between ice and water involve large heat transfers. Thus the nature of a glacier depends in large part on its thermal regime.

Glacier ice is formed by a process of snow metamorphism, which may be accompanied by seasonal melting and refreezing in some glaciers. The transition from new-fallen snow to glacier ice takes place over a period of years as the snow becomes buried and gradually recrystallized. The process is evident in three main stages:

  1. New snow: Density varies from 0.05 to 0.4 g/cm³, and porosity is generally high. Distinctly layered for individual storms. Highly variable in texture and crystal form--see Fig. 2-3.

  2. Firn: "Old snow" that has survived at least one summer melt season. Density between 0.4 and 0.8 g/cm³ with porosity around 50%. Granular texture with annual layering clearly visible.

    View of firn exposed on the margin of accumulation zone in Vatnajökull, Iceland. Notice banding and bright white color. This firn section represents several years of snow accumumlation. © by J.S. Aber.

  3. Glacier ice: Solid, impermeable ice with density >0.8 g/cm³ and little porosity. Isolated air bubbles under high pressure; large crystals up to 30 cm long aligned in preferred fabric parallel to flow. Glacier ice often displays a distinctive aquamarine (cyan) color--see ice spectrum and Fig. 2-2.

    View inside glacier cave on Mt. Rainier, Washington. Typical blue-green (cyan) color of dense glacier ice is visible. Photo © PANA-VUE (S1979).

The metamorphism of snow to ice normally takes about 20-30 years in small alpine or maritime glaciers; however, the process may require a century or more in the interior of large ice sheets. The rate of conversion depends mainly on temperature and amount of annual snow accumulation.

Glacier budget

The glacier budget is a balance of mass gains and losses during the course of a year. Accumulation of snow is the primary means by which most glaciers gain mass. Snow may accumulate from direct air fall, or may be blown or avalanched from adjacent areas. Freezing rain, hail, hoarfrost, or other forms of ice accumulation are negligible.

Loss of mass, or ablation, takes place by melting at the surface, within, or beneath a glacier. Melt water may exit under pressure from the snout of a glacier, thus forming springs and fountains. Glaciers that enter lakes or seas may also lose mass by calving off icebergs. Some glaciers in extremely cold, dry climates lose mass by wind erosion and direct sublimation.

The higher, central surface of most glaciers is the zone of accumulation, also called the névé, in which firn is present. The ablation zone is located in the lower, marginal portions of glaciers. The accumulation zone is considerably larger in area than the ablation zone for most glaciers--see Fig. 2-4. Any healthy glacier will, over an extended time, have a budget in which accumulation equals or exceeds ablation. As the volume of accumulation and ablation in a single year is only a tiny fraction of the total mass of most glaciers, yearly variations in net accumulation are not significant. Small glaciers respond to climatic changes of decade-long duration, whereas large ice sheets take centuries to react to climatic changes.

View over portion of névé, accumulation zone on Vatnajökull, Iceland.
Snow tractor/sledge in center for scale. © by J.S. Aber.

The position on the glacier surface of balance between net accumulation and net ablation is called the equilibrium line altitude (ELA). This line separates the zone of accumulation (névé) above from the zone of melting and calving below. Changes in position of the ELA over time represent changes in the glacier's budget. Increasing height of the ELA indicates a negative budget with ice loss; decrease in ELA represents a positive budget with gain of ice. Nesje and Dahl (2000) have demonstrated that the ELA depends on two climatic factors--winter accumulation and mean summer temperature (at the ELA position). A change in either of these factors will lead to a shift in the ELA.

Glacier movement

Glaciers move. Glacier movement was illustrated dramatically in 1991, when the mummified body of a Bronze Age man emerged from a glacier in Austria. Similar accounts of bodies and artifacts carried by glaciers have been documented for many other sites. For a glacier in balance, it is necessary to transfer material from the accumulation area to the ablation zone in order to maintain a constant surface. Ice-flow velocities vary from imperceptible to several m/day, although most glaciers move <1 m/day. The following general ice movements are developed in most glaciers--see Fig. 2-5.

Glaciers move in three ways: internal deformation, basal slip, and deformation of the substratum. Below the shallow surface zone of crevasses, ice behaves in a plastic manner, flowing in response to the pressure gradient. The rate of flow depends on several factors, of which temperature and pressure are most important. Warmer ice generally flows more readily than colder ice, and ice under greater pressure is softer than ice at lower pressure. Because of this, most movement by plastic flow takes place near the base (lowest 100 m) of glaciers. This explains how "cold" glaciers that are frozen to the substratum are able to move, and why many glaciers move faster in the summer than in the winter.

Glaciers that have a "warm" base at the pressure-melting temperature also move by basal slip. In effect the glacier rides on local films or small pools of water that support and lubricate ice movement. Most alpine and maritime glaciers move by a combination of internal deformation and basal slip. However, the contribution of each movement mechanism may vary substantially between nearby glaciers and even within different parts of a single glacier. Seasonal variations also take place in glacier flow, as illustrated by Forbes bands.

In some cases, subglacial melt water may build up so much that nearly all the glacier is floating, or a deformable bed of high-pressure, fluidized sediment may form. When this happens, rapid ice advance, called a surge, may take place. Surging glaciers have been known to advance at rates >100 m/day. During a surge, much mass is quickly transferred from the accumulation area to the ablation zone during a period of weeks or months. The glacier usually remains stagnant for several years after a surge. Certain glaciers surge periodically, whereas other nearby glaciers never surge. Surges have not been observed in ice sheets, but surging is considered an important possibility for ice-sheet movement.

The Bering Glacier, Alaska began to surge between March and May 1993. During the next several months, the surge affected more than half of the glacier's 5,175 km² area. Surge-related features included: deeply crevassed bulges and pressure ridges, extensional fractures, intricate patterns of crevasses, tear faults, ephemeral lakes, etc. The ice margin advanced in places more than 1500 m into a proglacial lake, and the rate of advance approached 100 m/day. Bering Glacier is one of more than 200 temperate glaciers known to surge in the western United States and Canada. Surging glaciers also are quite common in eastern Canada, Iceland, and on the archipelago of Svalbard.

Columbia Ice Field

The Columbia Ice Field is the largest glacier complex in the Rocky Mountains south of Alaska today. It is located on the continental divide in Alberta and British Columbia. The accumulation zone is situated on mountain plateaus and generally exceeds 2500 m elevation. Snow Dome, the high point of the ice field, reaches 3460 m (>11,000 feet) and represents the triple point in the drainage divide system of North America: Arctic, Atlantic, and Pacific.

Scale model of Columbia Ice Field, B.C./Alberta, Canada.
View straight down from above ice field; south toward top.

Several large outlet glaciers descend from the accumulation zone into surrounding valleys. Among these, the Athabasca Glacier and Saskatchewan Glacier have been studied in considerable detail. Both are fed by ice falls, and both exhibit typical (non-surge) flow conditions for alpine valley glaciers--see Fig. 2-6. A remarkable story dramatically illustrates ice movement from the Athabasca Glacier. In April 1965, Julia Oko (age 44) was skiing on the glacier. She apparently broke through an ice bridge and fell 120 feet into a crevasse. She suffered a fractured spine, but was rescued and survived. Eleven years later, her belongings melted out at the glacier snout; these included her skis, rucksack and frozen lunch. At her home, she had the skis and poles mounted in the family room, which she enjoyed until her death in 2005.

Telephoto view of Saskatchewan Glacier descending into valley from the Columbia Ice Field (background). Glacier is fed by two streams, whose boundary is marked by a medial moraine (dark stripe) on the ice surface. The ice front is a few 100 m (several 100 feet) high. Photo © by J.S. Aber.

The Columbia Ice Field was considerably smaller during the early to mid-Holocene. Wood fragments found at the snout of the Athabasca Glacier are radiocarbon dated at around 8000 years old. The pine (Pinus sp.) and fir (Abies sp.) fragments are thought to represent forest trees that grew on the valley floor now occupied by the glacier (Luckman 1988). Outlet glaciers of the Columbia Ice Field expanded to maximum late Holocene positions circa A.D. 1840-1850, and glaciers remainded extended until the end of the 19th century. During this century, the outlet glaciers of the ice field have retreated significantly.

Greenland Ice Sheet

The Greenland Ice Sheet covers an estimated area of 1.7 million km² and is the second largest ice sheet existing today--see title image. Its maximum dimensions are 2460 km N-S, 1100 km E-W, and >3000 m elevation, with an average thickness of about 2 km Weidick (1975). The ice sheet represents about 11% of the total volume of glacier ice in the world today, an amount equivalent to 6 m of sea-level rise.

SeaWiFS satellite image over the whole of Greenland. Date 07/15/2000; obtained from NASA's Visible Earth.
Space-shuttle photograph of southern Greenland. Low-oblique view toward north, color-visible, 70-mm format, 12/88. All land areas are snow covered in this mid-winter scene. The interior ice sheet is smooth, and coastal mountains appear rugged. Deep glacier-carved fjord valleys cut through the coastal mountain ranges. NASA Johnson Space Center, Imagery Services, STS027-32-17.
Southeastern margin of Greenland
Taken from commercial flights at 36,000 feet (~11,000 m).
Images S.W. & J.S. Aber.

Southwestern margin of Greenland

The Greenland Ice Sheet is scientifically important because it is the only remaining ice sheet in the northern hemisphere, and it is located at the approximate center of ice-sheet development during the Pleistocene. Greenland is relatively accessible, because of the permanent human population located mainly in the southwestern coastal region. The Greenland Ice Sheet represents a bridge to understanding other northern hemisphere ice sheets of the Pleistocene.

Research camp of the Free University of Amsterdam (Netherlands) on the Greenland ice sheet. The surface is hard frozen snow and ice, but percolation melt water collects about ½ m below the surface. Foot pressure is sometimes enough to break through the hard surface, and people suddenly find themselves knee-deep in slush. Photo © by J.J. Zeeberg; used here by permission.

A good way to visualize the present state of the ice sheet is to examine the glaciation limit. This represents the lowest elevation at which firn can exist on the ice sheet. This limit is generally lowest in the northwest and northeast (200-400 m) and highest at the southern tip (1600-1800 m). All parts of the ice sheet above the glaciation limit are in the accumulation area (névé). Within the névé, snow exists in various facies depending on its water/ice content--see Fig. 2-7.

Dry-snow facies covers the high, central and northern portions of the ice sheet. Percolation facies covers most of the southern portion and lower northern parts; wet-snow facies is found only in a narrow marginal zone. A narrow marginal zone of the ice sheet is subject to loss, mainly by melting on land and by calving of icebergs where ice reaches the sea in large fjords.

Overview of margin of Greenland ice sheet. Note braided streams of melt water flowing over the broad valley. Thick outwash sediment of sand and gravel accumulates on the valley plain or sandur. Photo © by J.J. Zeeberg; used here by permission.
Outwash valley plain almost completely covered by sediment-laden melt water. Photo © by J.J. Zeeberg; used here by permission.
Outlet glacier on Greenland's west coast. This glacier and many others break off as icebergs, a process called glacier calving. Photograph by Preben Jensen, Denmark; reproduced here by permission.
View of massive iceberg floating in coastal waters of western Greenland. Only about 10% of the iceberg's volume is visible, the rest is below sea level. Photograph by Preben Jensen, Denmark; reproduced here by permission.

The ice sheet occupies a shallow basin that is depressed below sea level and surrounded by mountains--see Fig. 2-8. In effect the ice sheet creates its own topography; it is the highest large plateau in the northern hemisphere. The coldest portion of the ice sheet is the high central zone (below -30°C), with warmer temperatures deeper and toward the margins, where pressure-melting conditions exist at the base of the ice sheet.

Several attempts have been made to estimate the ice-sheet budget. These attempts were difficult in the past because of the great size of the ice sheet and limited information concerning accumulation, melting, and calving of icebergs. Remote sensing observations have improved our ability to gauge changes in the ice sheet. Altimeter measurements from the ERS-1 and ERS-2 satellites documented an overall increase in elevation of the ice surface during the period 1992-2003 (Johannessen et al. 2005). Throughout the vast interior of the ice sheet, surface elevations increased at an average rate of ~6 cm/year. The narrow marginal zone suffered a decrease at ~2 cm/year. Over the whole of the ice sheet this represents an average increase of ~5 cm/year, or 60 cm total for the 11-year period of observation. When corrected for crustal depression, the total increase is estimated to be 54 cm. In other words, the Greenland Ice Sheet has gained more than half a meter of surface accumulation overall since the early 1990s, which suggests a strongly positive budget.

Ice movement is generally downward and outward from two major domes in the east-central and southern portions. Movement within most of the accumulation zone is slow, only a few m/year, with movement increasing to about 80 m/year near the firn line. The two domes are separated by a lower saddle from which several fleuves de glâce flow westward. These concentrated, high-velocity ice streams follow subglacial valleys leading westward to Disko and Umanak Bays, where a large volume of icebergs is discharged. Near the calving margin, ice-flow velocities are 2-10 km/year, and much melt water emerges from beneath the ice.

Aerial view of a modern nunatak surrounded by the Greenland ice sheet. Note crevasses and pressure ridges in ice around the nunatak. Ice flow from upper right toward lower left of view is indicated by the medial moraine extending away from the nunatak. Photo © by J.J. Zeeberg; used here by permission.
Overview of margin of Greenland ice sheet; outlet glaciers extend into valleys toward the left. The longest of these is the Russell Glacier. Photo © by J.J. Zeeberg; used here by permission.
Russell Glacier. Note the lateral moraine (far side of valley) and push moraine (glacier snout). The push moraine is composed of sand and gravel shoved up from the sandur plain. Research camp of the University of Amsterdam and Utrecht University (Netherlands) is visible in the lower right part of view. Photo © by J.J. Zeeberg; used here by permission.

The Greenland Ice Sheet was considerably larger during times of maximum Pleistocene glaciation. It averaged about 500 m thicker and extended to near the edge of the continental shelf. In spite of its greater thickness, the ice sheet did not cover all of the marginal mountains; many nunataks remained in which some plant species may have locally survived glaciation.

On the continental shelf, sea level was some 100-150 m lower, so some banks were exposed as dry land at the edge of the ice sheet. The ice sheet may have been confluent with the Ellesmere Ice Cap to the northwest, although this is a debatable interpretation. During maximum glaciation, the coastal areas were depressed as a result of ice loading on the crust; these areas have now rebounded by 50-200 m.

The ancestral Jakobshavn ice stream expanded several 100 km to the shelf edge on the western side of Greenland at the last glacial maximum ~20,000 years ago (Cofaigh et al. 2013). It began to retreat by 14,800 calibrated-years ago; however, it readvanced significantly during the short-lived Younger Dryas phase about 12,000 calibrated-years ago.

It is generally thought that the Greenland Ice Sheet formed in the late Pliocene or early Pleistocene by coalescence of ice caps and glaciers. New evidence from offshore marine deposits suggests that Greenland had a partial ice cover as far back as seven million years ago (late Miocene). The marine deposits contain large stones transported by drifting icebergs derived from Greenland glaciers.

Antarctic Ice Sheets

The Antarctic continent is 95% ice covered, and large ice shelves extend into adjacent Ross and Weddell Seas. Two ice sheets are actually present: the East Antarctic Ice Sheet and the smaller West Antarctic Ice Sheet--see Fig. 2-9. They are separated by the Transantarctic Mountains. The East Antarctic Ice Sheet represents about 60 m of sea-level equivalent, and the West Antarctic Ice Sheet contains water equivalent to another 5 m of sea level rise (Alley 1989).

Antarctica--a mosaic of 40 Galileo images taken through red, green, and violet filters. Antarctica is about 4000 km across from top to bottom of view. The South Pole is just right of center, and Ross Ice Shelf is just above the center. Dark zones are open-water seas around the margin of the ice sheet. Derived from NASA Goddard Space Flight Center, Earth-Galileo imagery.

Landsat image mosaic of Antarctica--LIMA.

Most of the East Antarctic Ice Sheet rests on crust that is above sea level. Recent geodetic surveys have shown that ice at the South Pole moves nearly 10 meters per year toward the northwest (Mullins 1999). Much of the base of the West Antarctic Ice Sheet lies far below present sea level. In other words, the East Antarctic Ice Sheet is land based, whereas the West Antarctic Ice Sheet is marine based. Floating ice shelves and ice tongues have distinctive ice-flow dynamics that involve interaction between the inland ice and the sea.

Marine-based ice sheets are inherently unstable. Such an ice sheet is held in check by back stress from the adjacent ice shelf by shear stress at basal pinning points and along lateral margins--see Fig. 2-10. A slight rise in relative sea level (crustal subsidence), decrease in ice flow, or thinning of the ice shelf would reduce the back stress and could lead to rapid collapse of the ice sheet (Alley 1989). Antarctic ice shelves are sources of enormous tabular icebergs.

The West Antarctic Ice Sheet is drained by a series of ice streams that feed into the Ross Ice Shelf--see Fig. 2-11. These ice streams flow through a region of relatively thin ice (about 1000 m), but they maintain flow rates of around 500 m per year, and they accelerate downstream. They are characterized by highly crevassed surfaces in contrast to the adjacent smooth surface of slow-moving ice. The individual ice streams appear to have complex and different histories of movement. For example, ice stream C seems to have stopped moving around 100 years ago. Most of the rapid motion of ice streams is accomplished by basal sliding over water-lubricated beds or by deformation within a thin, water-saturated sediment layer below the ice (Alley 1989).

The Ross Ice Shelf has fluctuated considerably during the past 45,000 years--see Fig. 2-12. Between 45,000 and 27,000 years ago, the ice edge was quite similar to now, then expanded reaching a maximum position ~13,000 years ago. During the Holocene, the ice margin retreated considerably reaching a minimum position in the middle Holocene between 4000 and 2000 years ago.

Antarctica was glaciated much earlier than was Greenland. A semipermanent ice sheet was established by the end of the Eocene or earliest Oligocene, about 34 million years ago (Ivany et al. 2006). Glaciation in other high-latitude southern lands also started early. Tasmania supported local glaciers in the early Oligocene, around 36 million years ago, when it was a mountainous peninsula of Australia at about 55-56S latitude (Macphail et al. 1993). The Antarctic ice sheet experienced a major expansion during the early Pliocene, 5-4 million years ago (Bart 2001). This expansion took place in spite of relatively high sea level and warm temperature globally, and the ice volume may have been significantly larger than present ice volume.

Ice-sheet meteorites and cosmic dust

Many meteorites have been found in Greenland, especially in the Thule area. The same is true for Antarctica, where literally thousands of meteorites have been collected from the surface in "blue ice" areas. In fact, the number of meteorites found during the past few years in Antarctica is about 25% of all meteorites ever found worldwide. It is estimated that more than 750,000 meteorites lie buried within the ice sheet (Lucchitta et al. 1987). Ice flow locally concentrates meteorites at the surface in areas of strong wind ablation, where ice is locally trapped behind mountains--see Fig. 2-13.

Large nickel-iron meteorite, named Agpalilik, recovered from the edge of the Greenland ice sheet, near Thule. The meteorite is about 4 feet (1.2 m) long, and was one of several larger and smaller fragments found in the vicinity. This specimen on display at the Geological Museum, Copenhagen, Denmark. Photo © J.S. Aber.

The Greenland Ice Sheet has yielded a rich collection of cosmic dust (particles <1 mm in size). About 85% of the cosmic dust particles are carbonaceous chondrites, thought to be samples of the primitive solar system. Cosmic dust falls constantly on the ice sheet and is carried by ice flow to the margin. In some places, ice melts in local basins and the particles settle to the bottom of "blue ice lakes," where they accumulate year after year (Episodes 9, 1986).

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ES 331/767 J.S. Aber (2013).