Shrinking Glaciers

At the opposite end of climate extremes from deserts, Earth has experienced at least three major periods of long-term frigid climate and ice ages, interspersed with periods of warm climate. Most glaciers around the world are currently shrinking at rapid rates as a result of global warming. The earliest well-documented ice age is the period of the "Snowball Earth" in the late Proterozoic, although there is evidence of several even earlier glaciations. The late Paleozoic saw another ice age lasting about 100 million years, from 350-250 million years ago. The planet entered the present ice age about 55 million years ago. The underlying causes of these different glaciations is varied and includes anomalies in the distribution of continents and oceans and associated currents, variations in the amount of incoming solar radiation, and changes in the atmospheric balance between the amount of incoming and outgoing solar radiation.

Glaciers are any permanent body of ice (recrystallized snow) that shows evidence of gravitational movement. Glaciers are an integral part of the cryosphere, which is that portion of the planet where temperatures are so low that water exists primarily in the frozen state. Most glaciers are presently found in the polar regions and at high altitudes. However, at several times in Earth history glaciers have advanced deeply into mid-latitudes and the climate of the entire planet was different. Some models suggest that at one time the entire surface of Earth may have been covered in ice, a state referred to as the Snowball Earth.

Photo of ice wedge clinging to the top of Mount Kilimanjaro, Tanzania (Corbis)

Glaciers are dynamic systems, always moving under the influence of gravity and changing drastically in response to changing global climate systems. Thus, changes in glaciers may reflect coming changes in the environment. There are several types of glaciers. Mountain glaciers form in high elevations and are confined by surrounding topography, like valleys. These include cirque glaciers, valley glaciers, and fiord glaciers.

Piedmont glaciers are fed by mountain glaciers, but terminate on open slopes beyond the mountains. Some piedmont and valley glaciers flow into open water, bays, or fiords, and are known as tidewater glaciers. Ice caps form dome-shaped bodies of ice and snow over mountains and flow radially outward. Ice sheets are huge, continent-sized masses of ice that presently cover Greenland and Antarctica. These are the largest glaciers on Earth. Ice sheets contain about 95 percent of all the glacier ice on the planet. If global warming continues to melt the ice sheets, sea level could rise by up to 230 feet (66 m). A polar ice sheet covers Antarctica, consisting of two parts that meet along the Transan-tarctic Mountains. It shows ice shelves, which are thick glacial ice that floats on the sea. These form many icebergs by calving, which move northward into shipping lanes of the Southern Hemisphere.

Polar glaciers form where the mean average temperature lies below freezing, and these glaciers have little or no seasonal melting because they are always below freezing. Other glaciers, called temperate glaciers, have seasonal melting periods, where the temperature throughout the glacier may be at the pressure melting point, when the ice can melt at that pressure and both ice and water coexist. All glaciers form above the snow line, which is the lower limit at which snow remains year-round. It is at sea level in polar regions and at 5,000-6,000 feet (1,525-1,830 m) at the equator (Mount Kilimanjaro in Tanzania has glaciers, although these are melting rapidly).

This chapter examines the formation of glaciers and specific hazards that are caused by glaciation. Glaciers present two main categories of hazards. The first affects those who are working or living on or near glaciers, or transporting goods by sea, river, or land in glacially influenced areas. The second set of hazards is more global in nature and reflects climate change that brings on widespread glaciations. Glaciers also represent sensitive indicators of climate change and global warming, shrinking in times of warming and expanding in times of cooling. Glaciers may be thought of as the "canaries in the coal mine" for climate change. In the Swiss Alps, some ski resorts are covering their slopes with reflective foil in times of non-use in an effort to reduce melting from global warming. The last part of the chapter examines the implications of melting glaciers in terms of climate change.

Formation of Glaciers

Glaciers form mainly by the accumulation and compaction of snow and are deformed by flow under the influence of gravity. When snow falls it is very porous, and with time the pore spaces close by precipitation and compaction. When snow first falls, it has a density of about one-tenth that of ice; after a year or more, the density is transitional between snow and ice, and it is called firn. After several years, the ice has a density of 0.9 gm/cm3, and it flows under the force of gravity. At this point, glaciers are considered to be metamorphic rocks, composed of the mineral ice.

The mass and volume of glaciers are constantly changing in response to the seasons and to global climate changes. The mass balance of a glacier is determined by the relative amounts of accumulation and ablation (mass loss through melting and evaporation or calving). Some years see a mass gain leading to glacial advance, whereas some periods have a mass loss and a glacial retreat. The glacial front or terminus shows these effects.

Glaciers have two main zones, best observed at the end of the summer ablation period. The zone of accumulation is found in the upper parts of the glacier, and is still covered by the remnants of the previous winter's snow. The zone of ablation is below this, and is characterized

Headwall

Bergschrund Arete Firn field

Rock step

Headwall

Bergschrund Arete Firn field

Rock step

Cross section of glacier showing different zones and landforms

by older, dirtier ice, from which the previous winter's snow has melted. An equilibrium line, marked by where the amount of new snow exactly equals the amount that melts that year, separates these two zones.

Movement of Glaciers

When glacial ice gets thick enough, it begins to flow and deform under the influence of gravity. The thickness of the ice must be great enough to overcome the internal forces that resist movement, which depend on the temperature of the glacier. The thickness at which a glacier starts flowing also depends on how steep the slope it is on happens to be— thin glaciers can move on steep slopes, whereas glaciers must become very thick to move across flat surfaces. The flow is by the process of creep, or the deformation of individual mineral grains. This creep leads to the preferential orientation of mineral (ice) grains, forming foliations and lineations, much the same way as in other metamorphic rocks.

Some glaciers develop a layer of meltwater at their base, allowing basal sliding and surging to occur. Where glaciers flow over ridges, cliffs, or steep slopes, their upper surface fails by cracking, forming large, deep crevasses, which can be several hundred feet (100 m) deep. A thin blanket of snow sometimes covers and hides these crevasses, making for very dangerous conditions for people or animals crossing the glacier.

Ice in the central parts of valley glaciers moves faster than ice at the sides, because of frictional drag against the valley walls on the side of the glacier. Similarly, a profile with depth into the glacier would show that they move the slowest along their bases, and faster internally and along their upper surfaces. When a glacier surges, it may temporarily move as fast along its base as it does in the center and top. This is because during surges, the glacier is essentially riding on a cushion of meltwater along the glacial base, and frictional resistance is reduced during surge events. During meltwater enhanced surges, glaciers may advance by as much as several miles in a year. Events like this may happen in response to climate changes. As the climate warms, glacial melting has increased, and many glaciers are surging forward internally even as their snouts are retreating.

Calving refers to a process in which icebergs break off from the fronts of tidewater glaciers or ice shelves. Typically, the glacier will crack with a loud noise that sounds like an explosion, and then a large chunk of ice will splash into the water, detaching from the glacier. Tidewater glaciers retreat rapidly by calving.

glaciation and glacial Landforms

Glaciation is the modification of the land's surface by the action of glacial ice. When glaciers move over the land's surface, they plow up the soils, abrade and file down the bedrock, carry and transport the sedimentary load, steepen valleys, then leave thick deposits of glacial debris during retreat.

In glaciated mountains, a distinctive suite of landforms forms from glacial action. Glacial striations are scratches on the surface of bedrock, formed when the glacier dragged boulders across the bedrock surface. Roches moutonnées and other asymmetrical landforms form when the glacier plucks pieces of bedrock away from a surface and carries them away. The step faces in the direction of transport. Cirques are bowl-shaped hollows that open downstream and are bounded upstream by a steep wall. Frost wedging, glacial plucking, and abrasion all work to excavate cirques from previously rounded mountain tops. Many cirques contain small lakes called tarns, which are blocked by small ridges at the base of the cirque. Cirques continue to grow during glaciation, and where two cirques form on opposite sides of a mountain, a ridge known as an arête forms. Where three cirques meet, a steep sided mountain

forms, known as a horn. The Matterhorn of the Swiss Alps is an example of a glacial carved horn.

Valleys that have been glaciated have a characteristic U-shaped profile, with tributary streams entering above the base of the valley, often as waterfalls. In contrast, streams generate V-shaped valleys. Fiords are deeply indented glaciated valleys that are partly filled by the sea. In many places that were formerly overlain by glaciers, elongate streamlined forms known as drumlins occur. These are both depositional features (composed of debris) and erosional (composed of bedrock).

Glacial Transport

Glaciers transport enormous amounts of rock debris, including some large boulders, gravel, sand, and fine silt. The glacier may carry this at its base, on its surface, or internally. Glacial deposits are characteristically poorly sorted or non-sorted, with large boulders next to fine silt. Most of a glacier's load is concentrated along its base and sides, because in these places plucking and abrasion are most effective.

Active ice deposits till as a variety of moraines, which are ridge-like accumulations of drift deposited on the margin of a glacier. A terminal moraine represents the farthest point of travel of the glacier's terminus.

Glacial debris left on the sides of glaciers forms lateral moraines, whereas where two glaciers meet, their moraines merge and are known as a medial moraine.

Rock flour is a general name for the deposits at the base of glaciers, where they are produced by crushing and grinding by the glacier to make fine silt and sand. Glacial drift is a general term for all sediment deposited directly by glaciers, or by glacial meltwater in streams, lakes, and the sea. Till is glacial drift that was deposited directly by the ice. It is a non-sorted random mixture of rock fragments. Glacial marine drift is sediment deposited on the seafloor from floating ice shelves or bergs. Glacial marine drift may include many isolated pebbles or boulders that were initially trapped in glaciers on land, then floated in icebergs that calved off from tidewater glaciers. These rocks melted out while over open water and fell into the sediment on the bottom of the sea. These isolated stones are called dropstones and are often one of the hallmark signs of ancient glaciations in rock layers that geologists find in the rock record. Stratified drift is deposited by meltwater, and may include a range of sizes, deposited in different fluvial or lacustrine environments.

Glacial erratics are glacially deposited rock fragments that are different from underlying rocks. In many cases the erratics are composed of rock types that do not occur in the area they are resting in, but are found only hundreds or even thousands of miles away. Many glacial

Glacier from Chugach Mountains, Alaska, showing large medial moraine. The toe of the glacier is floating on the water in the fiord, so this is a tidewater glacier. (T. Kusky)

erratics in the northern part of the United States can be shown to have come from parts of Canada. Some clever geologists have used glacial erratics to help them find mines or rare minerals that they have found in an isolated erratic—they have used their knowledge of glacial geology to trace the boulders back to their sources following the orientation of glacial striation in underlying rocks. Recently, diamond mines were discovered in northern Canada (Nunavut) by tracing diamonds found in glacial till back to their source region.

Sediment deposited by streams washing out of glacial moraines is known as outwash and is typically deposited by braided streams. Many of these form on broad plains known as outwash plains. When glaciers retreat, the load is diminished, and a series of outwash terraces may form.

Types of Glacial Hazards

Glaciers present several hazards to those who live near or travel on the ice. Most people do not experience these hazards unless they are on the glacier. Other glacial hazards are more global, including hazards to shipping and rising sea levels caused by melting glaciers.

CREVASSES

Crevasses are extremely hazardous and may be hidden under fresh layers of snow or wind blown drifts. They form most typically over bedrock ridges where the upper surface of the glacier must bend in extension, opening crevasses. Crevasses may be long and narrow but up to several hundred feet (~100 m) deep. There have been a number of accidents on glaciers where hikers, explorers, or natives of glaciated lands have fallen into crevasses. The result is unpleasant and most typically deadly. As a person falls into a crevasse, he or she gets wedged tightly into the bottom of the crevasse, often injured from the fall but still alive. The person's body heat slowly melts an envelope around the person, who sinks slightly deeper into the crevasse. Gradually, the person gets squeezed into a smaller and smaller place and either suffocates from constriction or freezes from hypothermia. Glaciers are typically in remote locations, so death usually results before help can arrive.

CALVING

Tidewater glaciers and ice shelves are prone to spectacular calving events, where huge pieces of the front of the glacier break off and plunge into the water. While calving is one of the more spectacular

Ice block calving off (60 foot [18 m] high) toe of tidewater glacier, Alaska (T. Kusky)

events offered by nature, it may also be hazardous. Many a small boat, kayak, and sightseer have been plunged into icy water when they have gotten too close to the front of a tidewater glacier. These glacial fronts are extremely unstable, and may calve at any instant, sending hundreds of thousands of tons of ice plunging into the water in one thunderous crash. This generates large waves, often many tens of feet high, capable of capsizing even moderate-sized boats.

AVALANCHES

Glaciers and recently glaciated valleys have abundant very steep surfaces that are prone to avalanches. Active glaciers are continuously plucking material away from the bases of mountain valleys, causing higher material to avalanche onto the glacier where it gets carried away. Many glaciers are so covered with avalanche debris that they look like dirt-filled valleys. Snow also avalanches onto the main glacial surfaces, adding to the material that may get converted to ice. In areas where glaciers are retreating, recently deglaciated valleys have many avalanches, particularly during wet periods and during shaking by earthquakes.

ICEBERGS AND SEA ICE

Ice that has broken off an ice cap or polar sea or calved off a glacier and is floating in open water is known as sea ice. Sea ice presents a serious

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