Glaciers are large ice masses that form on land areas that are cold enough and have enough snowfall to sustain them year after year. They form wherever the winter accumulation of snow and ice exceeds the summer ablation (also called wastage). Ablation (wastage) is the loss of snow and ice by melting and by sublimation to gas (direct change from ice to water vapor, without melting). Accumulation commonly occurs in snowfields—regions of permanent snow cover (Figure 1) – Glacial processes.
Zones of glaciation
Glaciers can be divided into two zones, accumulation and ablation (Figure 1). As snow and ice accumulate in and beneath snowfields of the zone of accumulation, they become compacted and highly recrystallized under their own weight. The ice mass then begins to slide and flow downslope like a very viscous (thick) fluid.
If you slowly squeeze a small piece of ice in the jaws of a vise or pair of pliers, then you can observe how it flows. In nature, glacial ice formed in the zone of accumulation flows and slides downhill into the zone of ablation, where it melts or sublimes (undergoes sublimation) faster than new ice can form.
The snowline is the boundary between the zones of accumulation and ablation. The bottom end of the glacier is the terminus.
It helps to understand a glacier by viewing it as a river of ice. The “headwater” is the zone of accumulation, and the “river mouth” is the terminus. Like a river, glaciers erode (wear away) rocks, transport their load (tons of rock debris), and deposit their load “downstream” (down-glacier).
The downslope movement and extreme weight of glaciers cause them to abrade and erode (wear away) rock materials that they encounter. They also pluck rock material by freezing around it and ripping it from bedrock. The rock debris is then incorporated into the glacial ice and transported many kilometers by the glacier. The debris also gives glacial ice extra abrasive power. As the heavy rock-filled ice moves over the land, it scrapes surfaces like a giant sheet of sandpaper.
Rock debris falling from valley walls commonly accumulates on the surface of a moving glacier and is transported downslope. Thus, glaciers transport huge quantities of sediment, not only in but also on the ice.
When a glacier melts, it appears to retreat up the valley from which it flowed. This is called a glacial retreat, even though the ice is simply melting back (rather than moving back up the hill). As melting occurs, deposits of rocky gravel, sand, silt, and clay accumulate where there once was ice.
These deposits collectively are called drift. Drift that accumulates directly from the melting ice is unstratified (unsorted by size) and is called till.
However, drift that is transported by the meltwater becomes more rounded, sorted by size, layered, and is called stratified drift. Wind also can transport the sand, silt, and clay particles from drift. This wind-transported sediment can form dunes or loess deposits (wind-deposited, unstratified accumulations of clayey silt).
There are five main kinds of glaciers based on their size and form.
– Cirque glaciers—small, semicircular to triangular glaciers that form on the sides of mountains. If they form at the head (up-hill end) of a valley and grow large enough, then they evolve into valley glaciers.
– Valley glaciers—long glaciers that originate at cirques and flow downstream valleys in the mountains.
– Piedmont glaciers—mergers of two or more valley glaciers at the foot (break in slope) of a mountain range.
– Ice sheet – a vast, pancake-shaped ice mound that covers a large portion of a continent and flows independent of the topographic features beneath it. The Antarctic Ice Sheet (covering the entire continent of Antarctica) and the Greenland Ice Sheet (covering Greenland) are modern examples.
– Ice cap – a dome-shaped mass of ice and snow that covers a flat plateau, island, or peaks at the summit of a mountain range and flows outward in all directions from the thickest part of the cap. It is much smaller than an ice sheet.
Cirques, valley glaciers, and piedmont glaciers tend to modify mountainous regions of continents, where climatic conditions are sufficient for them to form. Such regions are said to be under the influence of “mountain glaciation” (Figure 1). Ice sheets cover large parts of continents, or even entire continents, which are then said to be under the influence of
Mountain glaciation is characterized by cirque glaciers, valley glaciers, and piedmont glaciers. Poorly developed mountain glaciation involves only cirques, but the best-developed mountain glaciation involves all three types. In some cases, valley and piedmont glaciers are so well developed that only the highest peaks and ridges extend above the ice.
Mountain glaciation also is called alpine glaciation, because it is the type seen in Europe’s Alps. Figure 2. shows a region with mountain glaciation. Note the extensive snowfield in the zone of accumulation. Snowline is the elevation above which there is permanent snow cover.
Also, note that there are many cracks or fissures in the glacial ice of Figure 1. At the upper end of the glacier is the large bergschrund (German, “mountain crack”) that separates the flowing ice from the relatively immobile portion of the snowfield. The other cracks are called crevasses—open fissures that form when the velocity of ice flow is variable (such as at bends in valleys).
Transverse crevasses are perpendicular to the flow direction, and longitudinal crevasses are aligned parallel with the direction of flow.
Figure 2. shows the results of mountain glaciation after the glaciers have completely melted. Notice the characteristic landforms, water bodies, and sedimentary deposits.
For your convenience, distinctive features of glacial lands are summarized in three figures:
• Erosional features in Fig. 3.
• Depositional features in Fig. 4.
• Water bodies in Fig. 5.
Note that some features are identical in mountain glaciation and continental glaciation, but others are unique to one or the other. Study the descriptions in these three figures and compare them with the visuals in Figures 1. and 2.
During the Pleistocene Epoch, or “Ice Age,” that ended 11,700 years ago, thick ice sheets covered most of Canada, large parts of Alaska, and the northern contiguous United States. These continental glaciers produced a variety of characteristic landforms (Figure 6, Figure 7).
Recognizing and interpreting these landforms is important in conducting work such as regional soil analyses, studies of surface drainage and water supply, and exploration for sources of sand, gravel, and minerals. The thousands of lakes in the Precambrian Shield area of Canada also are a legacy of this continental glaciation, as are the fertile soils of the north-central United States and south-central Canada.
Adapted by AGI; NAGT R. M. Busch; Illustrated by D. Tasa