Glaciation

• CATEGORY: Geological processes and formations

Glaciation is the effect of glaciers on the Earth’s surface, including erosion and the deposition of glacial sediments. Glaciers are related to a number of natural resources, helping to provide freshwater, rich soils, and deposits used for building materials.

Definition

The Dictionary of Geological Terms, published by the American Geological Institute (AGI)—now the American Geosciences Institute—defines glaciation as the “alteration of the Earth’s solid surface through erosion and deposition by glacier ice.” Approximately 69 percent of the Earth’s freshwater is stored in glaciers and ice caps. Glaciation has a profound effect on climate, just as climate strongly influences glaciation, and glaciers have important economic benefits. For example, water melted from glaciers is an important source of freshwater.

Overview

Glaciers begin above the snowline. Snow becomes compacted into granules, and as additional snow is added, weight and pressure lead to recrystallization in the form of dense glacial ice. Once the ice reaches sufficient thickness, the internal strength of the crystals is overcome by the weight of the ice, and the ice begins to flow in the form of a glacier. Glaciers can flow by internal deformation only, or by deformation in combination with basal sliding on a thin layer of meltwater. Recent research has highlighted the importance of subglacial hydrology, as pressurized meltwater beneath glaciers can significantly influence sliding speed and flow behavior. As glaciers flow, they erode the surface of the Earth, scouring it and plucking up boulders large and small. Glaciated valleys are distinctly U-shaped, as contrasted with the typical V-shape of river valleys.

Glacial scouring can create a number of landforms. These include small, steep-sided valleys called cirques and sharp ridges called arêtes. Three or more cirque valleys can leave land in a recognizable horn shape, such as the famous Matterhorn in the Pennine Alps. Smaller glaciers feed larger glaciers, much the same way that small rivers feed larger ones. Since the depth of scour is proportional to the mass of the glacier, smaller tributaries can leave forms known as hanging valleys isolated more than 100 meters (328 feet) above a steep-sided main valley.

Rock and sediment transported by glaciers form deposits such as glacial till and landforms including moraines and drumlins. As glaciers retreat, they leave their burden of rock behind. Erratics, boulders that have been carried great distances and then left behind as glaciers retreat, have been used since prehistoric times as construction material for homes and stone fences. Meltwater from glaciers can sort transported sand and gravel, forming long, sinuous eskers and landforms called kames. The finely graded sand and gravel is an important source of aggregate for the construction industry.

In some northern countries, meltwater from glaciers not only is used as a source of freshwater but also—where there is sufficient height and volume—can be used to generate hydroelectric power. At the same time, increased meltwater has heightened the risk of glacier-related hazards, including glacial lake outburst floods, which pose growing threats to downstream communities. Glaciation has other important economic benefits. The scouring effect of glaciers produces fine rock flour, which can be transported by wind and deposited as loess. Wind eventually transports and deposits the mineral-rich loess, helping to create some of the richest agricultural soils in the world.

Recent studies have documented accelerated glacier retreat worldwide, with mountain glaciers contributing increasingly to global sea-level rise. According to recent assessments, glacier mass loss now rivals that of the Greenland and Antarctic ice sheets as a driver of near-term sea-level change.

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