Glaciers and Glaciation

Jordan Leslie

KAMSC Geology Term Paper
Mr. Sinclair
January 10, 2013

A glacier is basically a thick ice mass that originates on land from the accumulation, compaction, and recrystallization of snow. Since glaciers are agents of erosion, they must also flow. Similar to running water, groundwater, waves, and wind, glaciers are dynamic forces that are capable of accumulating, transporting, and depositing sediment. Glaciers are found in many parts of the world today. However, they are mostly found in remote areas. Thousands of relatively small glaciers exist in lofty mountain regions, where they usually follow valleys originally occupied by water. Unlike the rivers that previously flowed in these valleys, glaciers move very slowly, approximately a few centimeters per day. Based on their location, glaciers are narrowed down to two categories: valley glaciers and alpine glaciers. Each is a stream of ice, bounded by precipitous rock walls, that flows down valley from an accumulation center near its head. Like rivers, valley glaciers can be long or short, wide or narrow, single or with branching tributaries. Generally, the widths of alpine glaciers are small compared to the length. Some glaciers extend for just a fraction of a kilometer, whereas others go on for tens of kilometers.

The picture above shows the Lateral moraine on a glacier joining the Gorner Glacier, in Switzerland. The Gorner Glacier runs along the bottom of the picture. The moraine bank runs up the left hand side of the picture and results from rocks and earth falling onto the glacier and from rocks being pulled out by the moving ice. If the glacier then melts a little, the moraine bank is left clearly visible which is why it is visible in the picture.

In contrast to valley glaciers, ice sheets exist on a much larger scale. These enormous masses flow out in all directions from one or more centers and completely obscure all but the highest areas of underlying land. Even sharp variations beneath the glacier usually appear as a relatively subdued undulation on the ice surface. These differences, however, affect the behavior of the ice sheets. This is most common near the margins, and happens by guiding the flow in certain directions and creating regions of slower and faster movement. Although many ice sheets have existed in the past, just two achieve this status today. Greenland is covered by an ice sheet that occupies about 1.7 million square kilometers, or about 80 percent of the island. In specific areas, the ice extends 3000 meters above the island’s bedrock floor. In the south polar realm, the huge Antarctic Ice Sheet attains a maximum thickness of nearly 4300 meters and can cover an area about 14 million square kilometers. Because of these huge proportions, the term ice sheet is usually preceded by the world continental. The combined areas of present day continental ice sheets represent about 10 percent of the Earth’s land area. Along portions of the Antarctic coast, glacial ice flows into bays, creating features called ice shelves. These are large, relatively flat masses of floating ice that extend towards the sea from the coast but remain attached to the land along one or more sides. The shelves are thickest in their landward sides and thin seaward. They are also sustained by ice from the adjacent ice sheet as well as being nourished by snowfall and the freezing of seawater to their bases. Antarctica’s ice shelves, for example, extend over nearly 1.4 million square kilometers. The Ross and Filchner shelves are the largest. The Ross Ice Shelf alone covers an area nearly the size of present day Texas.

This photo comes from our geology textbook. It shows the aerial view of the South Cascade Glacier in Washington, a valley glacier about 3 kilometers long. The snowline and cracks are called crevasses, which you can see are labeled in the display and clearly visible

In addition to valley glaciers and ice sheets, other types of glaciers are also identified. Covering some uplands and plateaus are masses of glacial ice called ice caps. They resemble ice sheets but are much smaller in size than the continental-scale features that have been discussed. Ice caps occur in many places, including Iceland and several large islands throughout the Arctic Ocean. Often ice caps and sheets feed outlet glaciers. These tongues of ice flow down valleys extending outward from the margins of larger ice masses. The tongues are essentially valley glaciers that are avenues for ice movement from an ice cap or ice sheet through the mountainous terrain to the sea. Where they encounter the ocean, some outlet glaciers spread out as floating ice shelves. Often large numbers of icebergs are produced as a result. Piedmont glaciers occupy broad lowlands at the bases of steep mountains and form when one or more alpine glaciers emerge from the confining walls of mountain valleys. Here the advancing ice spreads out to form a broad sheet. The size of individual piedmont glaciers varies greatly. Among the largest is the broad Malaspina Glacier along the coast of southern Alaska. It covers more than 5000 square kilometers of the flat coastal plain at the foot of the lofty St. Elias Range.

This map of a portion of North America shows the present day coastline compared to the coastline that existed during the last ice age and the coastline that would exist if present ice sheets in Greenland and Antarctica melted

Because of Earth’s water being in constant motion, the same water is evaporated from the oceans into the atmosphere, precipitated upon the land, and carried by rivers and streams back to the sea. However, when precipitation falls at high elevations or high latitudes, the water may not immediately make its way toward the sea. Instead, it may become part of a glacier. Although the ice will eventually melt and continue its path to the sea, it can be stored as glacial ice for many tens, hundreds, or even thousands of years. For example, data collected from Greenland showed that portions of its glacier are more than 25,000 years old. According to the U.S. Geological Survey, only slightly more than two percent of the world’s water is accounted for by glaciers. However, this small number may be misleading when the actual amounts of water are considered. About 80 percent of the world’s ice and nearly two-thirds of the earth’s fresh water are represented by Antarctica’s ice sheet, which covers an area almost one and one-half times that of the United States. If this ice melted, sea levels would rise about 60-70 meters, and the ocean would inundate many densely populated areas along the coast. Putting this into perspective, if Antarctica’s ice sheet were melted, it could feed the Mississippi River for more than 50,000 years! It would also take care of all United States rivers for almost 17,000 years and the Amazon River for about 5,000 years. This ice sheet could even provide water for all of the world’s rivers for 750 years. The quantity of ice on earth today is truly significant. However, present glaciers occupy only slightly more than one-third the area they did in the very recent geologic past. This period of extensive glaciations is often called the Ice Age. Describing the formation of glacial ice is relatively simple. Snow is the raw material from which glacial ice originates. Therefore, glaciers form in areas where more snow falls in winter than melts during the summer. Before a glacier is created, snow must be converted into glacial ice. When temperatures remain below freezing following a snowfall, the fluffy accumulation of delicate crystals soon changes. As air infiltrates the spaces between the crystals, the extremities of the crystals evaporate and the water vapor condenses near the center of the crystals. In this manner snowflakes become smaller, thicker, and more spherical, and the large spaces disappear. By this process air is forced out and what was once light, fluffy snow is recrystallized snow is called firn and is commonly found making up old snow banks in the winter. As more snow is added, the pressure on the lower layers increases, compacting the ice grains at depth. Once the thickness of ice and snow exceeds 50 meters, the weight is sufficient to fuse firn into a solid mass of interlocking ice crystals, which then makes up glacial ice. Movement of ice sheets happens when an ice sheet moves down slope in a number of directions from a central area of high altitude and is not restricted to a channel or valley. The ice sheet must expand because of the constant accumulation of ice and snow. Ice sheets do not move as quickly as alpine glaciers because there is less slope and more mass involved. Ice sheets move mostly by plastic flow. Mountain ranges are completely buried by the ice sheet at the South Pole, which is greater than 3,000 meters thick. Glaciers can move more than 15 meters a day. The larger volumes of ice on steeper slopes move more quickly than the ice on the more gentle slopes farther down the valley. These dynamics allow a glacier to replenish the ice that is lost in the zone of wastage. Glaciers in temperate zones tend to move the most quickly because the ice along the base of the glacier can melt and lubricate the surface. Other factors that affect the velocity of a glacier include the roughness of the rock surface (friction), the amount of melt water, and the weight of the glacier. A valley glacier has various components of flow. First, the entire glacier moves as a single mass over the underlying rock surface. The pressure from the weight of the glacier generates a layer of water that helps the ice glacier move down slope. This process is called basal sliding. In addition to basal sliding, which slowly moves the glacier down slope as a unit, plastic flow causes glacial ice buried underneath more than about 50 meters to move like a slow-moving, plastic stream. The central and upper portions of a glacier, as do those portions of a stream, flow more quickly than those near the bottom and sides, where friction between the ice and valley walls slows down the flow. In general, the rate of plastic flow is greater than the rate of basal sliding. Above a depth of about 50 meters, the weight of the overlying ice is not sufficient to cause plastic flow. This more rigid upper zone, which is called the zone of fracture, is carried along the top of the plastic flow piggyback style. Sometimes the zone of fracture moves faster than the underlying plastic flow. When this happens, especially down a steep slope, the surface breaks into a series of deep fissures called crevasses. Crevasses also result where a valley glacier curves because the ice flows faster toward the outside of the curve than the inside. A steep, rapid descent may result in an icefall, a piled-up mass of splintered ice blocks from a series of rapidly formed crevasses.

This picture shows the vertical cross section through a glacier to illustrate the ice movement described above. Below about 50 meters, ice behaves plastically and flows. Notice that the rate of movement is slowest at the base of the glacier where frictional drag is greatest. The speed of glacial displacement is partly determined by friction. Friction makes the ice at the bottom of the glacier move more slowly than the upper portion. In alpine glaciers, friction is also generated at the valley's side walls, which slows the edges relative to the center. This was confirmed by experiments in the 19th century, in which stakes were planted in a line across an alpine glacier, and as time passed, those in the center moved farther. Mean speeds vary greatly. There may be no motion in stagnant areas, such as in parts of Alaska, where trees can establish themselves on surface sediment deposits. In other cases, they can move as fast as 20–30 meters per day, or 2–3 meters per day on Byrd Glacier, the largest glacier in Antarctica. Velocity increases with increasing slope, increasing thickness, increasing snowfall, increasing longitudinal confinement, increasing basal temperature, increasing melt water production and reduced bed hardness. A few glaciers have periods of very rapid advancement called surges. These glaciers exhibit normal movement until suddenly they accelerate then return to their previous state. During these surges, the glacier may reach velocities far greater than normal speed. These surges may be caused by failure of the underlying bedrock, the ponding of melt water at the base of the glacier, perhaps delivered from a supraglacial lake, or the simple accumulation of mass beyond a critical "tipping point". In glaciated areas where the glacier moves faster than one kilometer per year, glacial earthquakes occur. These are large scale tremblers that have seismic magnitudes as high as 6.1 on the Richter Scale.
The number of glacial earthquakes in Greenland shows a peak every year in July, August and September, and the number is increasing over time. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year. This increase in the numbers of glacial earthquakes in Greenland may be a response to global warming. Seismic waves are also generated by the Whillans Ice Stream, a large, fast-moving river of ice pouring from the West Antarctic Ice Sheet into the Ross Ice Shelf. Two bursts of seismic waves are released every day, each one equivalent to a magnitude 7 earthquake, and are seemingly related to the tidal action of the Ross Sea. During each event a 96 by 193 kilometer region of the glacier moves as much as 0.67 meters over about 25 minutes, remains still for 12 hours, and then moves another half meter. The seismic waves are recorded at seismographs around Antarctica, and even as far away as Australia, a distance of more than 6,400 kilometers. Because the motion takes place over such a long period of time, it cannot be felt by scientists standing on the moving glacier.
As a glacier moves, particularly a warm glacier, it causes erosion of the underlying surface. Material from underlying bedrock or sediment is picked up by the glacier and 'held' in the ice as it moves. Material falling onto the surface (often the result of freeze-thaw activity, or frost shattering, on the surrounding rock walls) is also transported, and often finds its way down through crevasses to the base of the glacier. Material held within the glacier is called englacial moraine. It is this material trapped in the ice that allows the glacier to erode its surroundings. A crevasse into which rock debris is falling from the glacier's surface.
With its load of abrasive rock fragments, the base of the glacier acts like a belt sander, scraping across the rock, eroding it, producing characteristic erosion features, and creating a supply of material that leads eventually to the formation of depositional features as well. This scraping process is called abrasion. Occasionally, a moving glacier may become stuck on its bed. This occurs when for some reason a reduction in pressure causes liquid water to freeze, attaching the moving ice to the bedrock. As the ice continues to move an immense pulling force is applied to the attached rock which may then fracture and be plucked from its position. The range of size of particles that can be carried by a glacier is tremendous, ranging from very fine, ground up rock termed rock flour to massive boulders. Rock flour is so prevalent in the melt water of some glaciers that the water is unsafe to drink. This is especially so in areas where the rocks contain high concentrations of mica, such as the Garnet Mica Schist of Svartissen. At the other end of the scale, a glacier can carry rocks hundreds of meters in diameter. These are usually from sources on the valley walls and fall onto the glacier rather than being plucked out of the underlying bedrock. Large rock being moved by an Alpine glacier. The person on the right is almost 6 feet tall!
Although the surfaces of glaciers can seem to consist of nothing but rock debris, most of the debris in a mature glacier is concentrated near its base. In fact, basal ice may consist of only one part ice to every nine of sediment. The reason that the surfaces often seem so well coated in moraine is two fold. The surface is the obvious place for debris from the valley walls to accumulate, and the surface is subjected to insulation and melting. As the surface warms up, ice melts and runs off via a network of surface streams. Much of the larger intraglacial debris that drops out of the melting ice remains on the surface for a period of time. Some of this material will find its way down through crevasses and holes to aid the erosion of the bedrock, the rest will be covered by winter snows, possibly to appear again in another summer melt. During the summer, the effects of melting can produce interesting features. Small dark rocks can warm up faster than the surrounding ice, and melt their way into the surface of the glacier, where as larger rocks, especially light colored ones, may be slow to warm up and thus protect the ice below them. This leads to the formation of bumps and tables with a rocky surface and an ice column beneath. Running water is also very common on and within glaciers. Snow and ice that melts on the surface of the ice, or is generated by melting at the base, commonly forms extensive channels on, within, or beneath the glacier. This flowing water also transports a considerable amount of sediment to the snout of the glacier. Near the glacier snout debris also tends to move upwards along shear planes within the ice; curved surfaces which are due to the faster movement of ice above each plane relative to the ice below the plane, reversing the general burying of debris.
Glacial deposits are capable of acquiring and transporting a huge load of debris as they slowly advance across land. An advancing ice sheet carries an abundance of rock that was plucked from the underlying bedrock; only a small amount is carried on the surface from mass wasting. The rock/sediment load of alpine glaciers, on the other hand, comes mostly from rocks that have fallen onto the glacier from the valley walls. The various unsorted rock debris and sediment that is carried or later deposited by a glacier is called till. Till particles typically range from clay-sized to boulder-sized but can sometimes weigh up to thousands of tons. Boulders that have been carried a considerable distance and then deposited by a glacier are called erratics. Erratics can be a key to determining the direction of movement if the original source of the boulder can be located. Features left by valley glaciers and ice sheets are also intriguing. Moraines are deposits of till that are left behind when a glacier recedes or that are carried on top of alpine glaciers. Lateral moraines consist of rock debris and sediment that have worked loose from the walls beside a valley glacier and have built up in ridges along the sides of the glacier. Medial moraines are long ridges of till that result when lateral moraines join as two tributary glaciers merge to form a single glacier. As more tributary glaciers join the main body of ice, a series of roughly parallel medial moraines develop on the surface of main glacier. An extensive pile of till called an end moraine can build up at the front of the glacier and is typically crescent shaped. Two kinds of end moraines are recognized: terminal and recessional moraines. A terminal moraine is the ridge of till that marks the farthest advance of the glacier before it started to recede. A recessional moraine is one that develops at the front of the receding glacier; a series of recessional moraines mark the path of a retreating glacier. A thin, widespread layer of till deposited across the surface as an ice sheet melts is called a ground moraine. Ground moraine material can sometimes be reshaped by subsequent glaciers into streamlined hills called drumlins, long, narrow, rounded ridges of till whose long axes parallel the direction the glacier traveled. As a glacier melts, till is released from the ice into the flowing water. The sediments deposited by glacial melt water are called outwash. Since they have been transported by running water, the outwash deposits are braided, sorted, and layered. The broad front of outwash associated with an ice sheet is called an outwash plain; if it is from an alpine glacier it is called a valley train. Kames are steep-sided mounds of stratified till that was deposited by melt water in depressions or openings in the ice or as short-lived deltas or fans at the mouths of melt water streams. The rapid build-up of sediments can bury isolated blocks of ice. When the ice melts, the resultant depression is called a kettle. Kettle lakes, common in the upper Midwest of the United States, are bodies of water that occupy kettles. Eskers are long, winding ridges of outwash that were deposited in streams flowing through ice caves and tunnels at the base of the glacier. Generally well sorted and cross-bedded, esker sands and gravels eventually choke off the waterway. The great volume of melt water often results in the formation of glacial lakes between the end moraines and the retreating glacier front. The sediments that form at the bottom of the lake consist of fine-grained silt and clay that have an alternating light-dark layering. Finally, a varve consists of one light-colored bed and one dark-colored bed that represent a single year's deposition. The light-colored layer is mostly silt that was deposited rapidly during the summer months; the dark layer consists of clay and organic material that formed during the winter. The age of a glacial lake can be determined from the number of varves that have formed on the lake bottom. In conclusion, glaciers are interesting because they shaped Michigan and they are responsible for the creation of huge water areas that we all know today.

Bibliography

1. “The Earth” textbook, third edition by Tarbuck Lutgens, published in 1990
2. http://www.dcnr.state.pa.us/topogeo/field/glacial/index.htm
3. http://www.isgs.illinois.edu/research/glacial-geo.shtml
4. http://www.jsu.edu/dept/geography/mhill/phylabtwo/lab11/depositsf.html