14.S: Summary

Summary

Glaciers form when average annual snowfall exceeds melting and snow compresses into glacial ice. There are three types of glaciers, alpine or valley glaciers that occupy valleys, ice sheets that cover continental areas, and ice caps that cover smaller areas usually at higher elevations. As the ice accumulates, it begins to flow downslope and outward under its own weight. Glacial ice is divided into two zones, the upper rigid or brittle zone where the ice cracks into crevasses and the lower plastic zone where under the pressure of overlying ice, the ice bends and flows exhibiting ductile behavior. Rock material that falls onto the ice by mass wasting or is plucked and carried by the ice is called moraine and acts as grinding agents against the bedrock creating significant erosion.

Glaciers have a budget of income and expense. The zone of income for the glacier is called the zone of accumulation, where snow is converted into firn then ice by compression and recrystallization, and the zone of expense called the zone of ablation, where ice melts or sublimes away. The line separating these two zones latest in the year is the equilibrium or firn line and can be seen on the glacier separating bare ice from snow-covered ice. If the glacial budget is balanced, even though the ice continues to flow downslope, the end or terminus of the glacier remains in a stable position. If income is greater than expense, the position of the terminus moves downslope. If expense is greater than income, a circumstance now affecting glaciers and ice sheets worldwide due to global warming, the terminus recedes. If this situation continues, the glaciers will disappear. An average of cooler summers affects the stability or growth of glaciers more than higher snowfall. As the Greenland and Antarctic ice sheets flow seaward, the edges calve off forming icebergs.

Glaciers create two kinds of landforms, erosional and depositional. Alpine glaciers carve U-shaped valleys, and moraine carried in the ice polishes and grooves or striates the bedrock. Other landscape features produced by erosion include horns, arête ridges, cirques, hanging valleys, cols, and truncated spurs. Cirques may contain eroded basins that are occupied by post glacial lakes called tarns. Depositional features result from deposits left by retreating ice called drift. These include till, and moraine deposits (terminal, recessional, lateral, medial, and ground), eskers, kames, kettles and kettle lakes, erratics, and drumlins. A series of recessional moraines in glaciated valleys may create basins that are later filled with water to become paternoster lakes. Glacial meltwater carries fine-grained sediment onto the outwash plain. Lakes containing glacial meltwater are milky in color from suspended finely ground rock flour. Ice Age climate was more humid and precipitation that did not become glacial ice filled regional depressions to become pluvial lakes. Examples of pluvial lakes include Lake Missoula, which dammed behind an ice sheet lobe, and Lake Bonneville in Utah whose shoreline remnants can be seen on mountainsides. Repeated breaching of the ice lobe allowed Lake Missoula to rapidly drain causing massive floods that scoured the Channeled Scablands of Idaho, Washington, and Oregon.