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Glaciers are part of the water cycle. Water evaporated from the oceans is moved over land by winds. As the moist air is pushed up over mountain ranges, the air cools, the water vapor condenses (or more commonly crystallizes) and falls as snow. Over lowlands the snow often melts and falls as rain; over mountains it often reaches the ground, still as snow. There it accumulates, year after year, until it recrystallizes under its own weight to ice. If ice were like almost every other common crystalline material, it would lie where it forms until it eroded or melted away. It doesn't. Instead, if it is thick enough or on a steep enough slope, ice deforms under its own weight and flows downhill. A glacier is defined as a permanent mass of snow and ice which flows. |
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Glaciers may be permanent, but the ice within them is gained in the accumulation zone and lost in the ablation zone, after flowing much of the length of the glacier. As the ice moves inexorably down hill, the end (or terminus) of the glacier may move up- (due to an excess of ablation over accumulation) or down- (if an excess of accumulation over ablation) hill. This variability summarizes the mass budget of the glacier. The balance of mass (accumulation - ablation) is driven by the variables of weather and climate. Accumulation is dominantly a phenomenon of precipitation (mass addition), but ablation reflects melt (energy availability). Thus, the occurrence of glaciers on most of the Earth's surface is a function of the balance between winter precipitation and summer melting. |
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The generic term "glaciers" includes ice sheets (continental-scale ice masses which are not constrained by topography), ice caps (sub-continental in scale but still unconstrained), and many types of glaciers (which are topographically constrained). Glaciers (and ice sheets) can also be classified based on their surface climate (polar, temperate, or maritime) or on the conditions at their bed (wet, warm, or melting vs. dry, cold, or freezing. |
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Regardless of their (debated) efficiency as erosive agents, glaciers are undeniably effective at transporting debris. Although ice is sufficiently deformable that it flows, it is also sufficiently rigid that debris cannot settle through ice as debris does through air or water. Thus, debris introduced to a glacier's surface, by whatever means, is usually transported to the glacier's terminus. If introduced near the head of the glacier it may be covered with snow. It will be carried within the glacier towards the terminus. There, melting of ice from the glacier surface exposes more and more debris, explaining the "dirty" look of a glacier's terminus (see "Mass Budget", above). Glacial debris commonly consist of material of all sizes, described as "poorly sorted". |
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Glaciers carry any material that is added to the glacier from above, from the side, or from below. Although much of the debris carried within the glacier is dropped onto it, some is generated by glacial erosion - the removal of material from it place by direct glacial action. Glaciers erode through plucking as shown here, in which particles are removed from the bed by freezing onto the bed; by abrasion (see below), by thrusting of the bed in front of or into the glacier ("bulldozing"), and by subglacial stream activity. Glacial erosional landforms reflect the dominant process or processes which shaped them. |
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Clasts plucked from the glacier's bed are frequently dragged along near the bed. There, they are ground against the bed, grooving, striating, and polishing the bed and grinding both the clasts and the fragments of bed to silt-sized fragments. That silt will later constitute part of a deposit of till, or most of the mineral material in glacier meltwater runoff. If the base of the ice is at the melting point of ice (which is commonly the case because of the thickness of insulating ice overhead), continued basal melting draws additional clasts to the bed, thus making abrasion an effective long-term process. |
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Continued erosion, first under snow (nivation) then under ice, forms a arcuate depression, often described as "armchair-shaped", called a cirque. The process is a positive feedback loop, in which the formation of a nivation hollow helps collect more snow, to form a cirque, to collect more ice, to deepen the cirque, and so on. Cirques are obvious from the air, from the ground, and on topographic maps because of their broad floors (the armchair seat - often occupied by a scour-basin lake, or tarn) and steep headwalls and sidewalls (the back and arms of the armchair). |
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Cirques enlarge both backward and downward into a mountain mass or plateau. As they do, so do adjacent cirques, thus reducing the width of the ridge separating the cirques. As the cirque walls touch, they do so along a knife-edge ridge, or arete. A mountain range which has been heavily glaciated in the past is often distinguishable by the network of such ridges which define the divides between adjacent stream drainages. Although we tend to associate New England more with areal scour than local erosion, the Knife Edge on Maine's Katahdin is a noteworthy arete. |
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As cirques continue to enlarge, the space separating the backwalls of three or more cirques becomes smaller and smaller. What started as a plateau remnant may be reduced to a flat-topped summit, then to a jagged peak or horn. While cirques are backwearing their separating aretes are downwasting, thus the horns become more isolated. The classic final stage of this process is a peak like the Matterhorn, which has no true aretes, only steep ridges. Because the sidewalls have been lowered there is less protection for snow and ice from the summer sun, thus the rate of glacial erosion should be reduced. |
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The ice collected in a cirque must flow downhill until it reaches a sufficiently warm climate to melt. As it flows it modifies its valley, changing a preexisting V-shaped river valley into a glacial trough. Troughs are often described as "U-shaped", but their sidewalls are shaped by landslide and rockfall, thus usually have a measurable slope. Only rarely, as in Yosemite National Park, are the trough walls sufficiently vertical to be accurately described as a "U". Troughs are deepened by glacial erosion and may be locally overdeepened, so that melting reveals a basin to be filled with meltwater and stream runoff. |
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A fiord is a special case of a trough in which the base of the trough lies below sea level, thus the trough is inundated by seawater. Only the upper trough walls remain above the water, thus their steepness is accentuated. When a glacier reaches the ocean it floats (see any ice cube!) if the water depth is greater than about 90% of the thickness of the glacier. Because a glacier generally thins towards its terminus, a tidewater glacier is likely to have a deep, scoured basin where it first encounters water (but is thick) and a shallow threshold near its maximum extent (where it is shallow, thus floats in shallow water). There are many spectacular examples of fiords worldwide, such as Milford Sound, NZ. |
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There is a common preconception that ice fills valleys like ice cream fills a cone - to overflowing! Actually, ice fills valleys like a stream does - only up to the level it needs to continue to flow downhill. In most glacial valleys, a trimline separates areas affected by ice (below) and by mass wasting (landslide and rockfall) above. Although ice may appear solid (it breaks when you hit it with a hammer), across geological time (years or more) it acts like a fluid. Most mountain areas glaciated in the past experienced only a thin cover of ice at most on summit plateaus, and a finite thickness (less than full valley depth) of ice in the valleys. |
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Where ice covers the entire landscape, as under the present Antarctic Ice Sheet and Greenland Ice Cap, it may erode a broad area rather than locally deepen a valley. Under these circumstances, glacial erosion is more a function of the resistance of the underlying rock than of the power of the glacier. Soft, weathered, and faulted or jointed bedrock may be preferentially eroded, leaving behind a deglaciated landscape cross-hatched by valleys along joints or ridges along resistant bedrock units. Such landscapes are common in areas scoured by ice sheets 20,000 years ago, like northern Canada and Scandinavia. |
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So what happens to some of the debris eroded in the formation of cirques, troughs, and scoured regions? It is termed till, and is deposited under or around the glacier terminus. Till is characterized by a wide range of particle sizes, from "fragments" over a kilometer in length to ones measured in microns (clay). Till is deposited either under the glacier (lodgement till) from being forced into the glacier bed and having the ice deform around it, or at the glacier terminus (meltout till) from the ice melting away. Till is among the most common material at the ground surface, and is often recognized by the large boulders of rock not present locally, termed erratics. |
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Where glacially deposited material (till) is deposited in amounts thick enough to form recognizeable landforms, they are called moraines. Most moraines are formed at the terminus of a glacier (terminal moraines) or along its margins (lateral moraines). Other till given the name "moraine" is thick deposits with little shape, termed ground moraine, and material transported in stripes along a glacier, termed medial moraine. Medial moraines are the material which was at the edges of glacier sources which came together, trapping marginal material in the middle of the glacier. As the glacier melts, the medial material is widely distributed, thus rarely forms a distinctive landform. |
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Moraines are among the clearest evidence of the past extent and limits of glaciation. Continental ice sheet moraines are often subtle features, but mountain glacier moraines are prominent ridges which outline the former glacier termini. Multiple former ice advances can be recognized from nested moraines (one within another). Older, outer moraines are often deeply eroded by streams and rounded and smoothed by local erosion and deposition by wind and rain. Younger, inner moraines are frequently steeper and sharper, and have closed depressions, some holding lakes or ponds, which have not yet been filled in. |
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Not all of the erosion, transportation, and deposition associated with glaciation is performed by glaciers. In their lower reaches glaciers lose ice to melting, and meltwater becomes an important component of the glacial system. Some meltwater drains off of the surface of the glacier in intricate, seasonal channels eroded into the ice; the rest drains into and under the glacier. Subglacial meltwater escapes from the glacier into the glacier bed, as a film between the ice and the bed, through a network of small cracks and passages, and/or through large tunnels. In most cases the meltwater carries glacial debris with it, and in some cases it erodes the bed as well. |
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When subglacial meltwater drains through tunnels, it defines stream systems similar to surface streams. Those subglacial streams may deposit sediment just as a surface stream would, but confined to the tunnel. The result is a ridge of gravel termed an esker, often sinuous (snake-like), which wanders across a formerly glaciated region. The gravel in an esker may have been transported and even eroded by the glacier, but the subglacial stream sorts the fine material out and carries it away. Thus, eskers are well-drained and make great (if winding) roadbeds. They are commonly mined for their gravel. |
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Meltwater which escapes to the margin of a glacier will often deposit part of its load between the glacier and the valley wall. When the glacier retreats, the former stream bed is left perched above the deglaciated valley floor. This distinctive landform is termed a terrace, specifically, a kame terrace. ["Kame" is a lot easier to say than "deposited by water in contact with ice"!] Kame terraces are distinctive because of their flat surfaces (compared to the rolling moraines often found above them), steep slope into the valley, and composition of stream gravel rather than till. |
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Once debris-laden meltwater has left the glacier, it may still be considered glacial. The load, especially of large particles, and the fluctuating discharge of glacier-fed streams combines with the reduction of slope from the glacier margin to the valley floor to cause deposition of glacial outwash. Outwash coats vast areas just beyond glacier and ice-sheet margins, as well as covering valley floors inside the glacier margin as the glacier retreats. It may form a broad plain or be cut into by the postglacial stream to form terraces. Where outwash streams enter lakes or the ocean, deltas may form and thick sediments accumulate. These are often considered to be glacial sediments, although hundreds of kilometers from the actual glacial margin. |
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The magnitude of ice age ice sheets affected the globe in many ways. The ice sheets stored enough snow on the continents to lower global sea level (the ultimate source and sink of that snow) by about 150 meters. Sea levels from that time were submerged as the melting ice sheets refilled the ocean basins. The weight of the ice sheets depressed the land, however, to the extent that the rising of the land after melting of the ice sheets exceeded the rising of the water in many coastal regions. Raised beaches are depositional evidence of that process. Similar beaches are now being formed along gentle, relatively low energy coasts like the Atlantic and Gulf barrier islands of the United States. |
All thumbnails, photos, and annotated photos copyright W. W. Locke,1998; all rights reserved.
