Chapter 21:

The Glacier Systems and the Ice Age Presentation

The Anatomy of a Glacier

Alpine glaciers extend from the upper zone of accumulation downhill to the lower zone of ablation. The firn field, which occupies a cirque basin, is where snow accumulates to produce firn, the precursor to glacial ice. At the leading edge of the glacier debris is deposited along the sides of the glacier (lateral moraines) and at its furthest-most point (terminal moraine).

The Anatomy of a Glacier

Cross section of an alpine glacier.

The Anatomy of a Glacier

In this photograph the different forms of moraine can easily be identified. Where two glaciers merge, their lateral moraines form medial moraines, down the middle of the glacier.

Glacier Mass Balance

Glaciers may be described using a mass balance approach, where materials are added in the zone of accumulation and are removed in melting in the zone of ablation, or wastage. The two zones are said to separated by the equilibrium line. If there is more accumulation than ablation, the glacier advances, and if there is more ablation than accumulation, it retreats.

*See movie on glacier mass balance in the geodiscoveries section of your text’s website.

Glaciers

A glacier is any large natural accumulation of land ice affected by present or past motion. Where a great thickness of ice exists, the pressure on the ice at the bottom makes the ice lose its rigidity. This allows the ice mass to flow in response to gravity, slowly spreading out over a larger area or moving downhill.

Glaciers

Motion of glacial ice: Ice moves most rapidly on the glacier's surface at its midline. Movement is slowest near the bed, where the ice contacts bedrock or sediment.

Glaciers

A climber steps over a crevasse in Root Glacier.

Landforms Made by Alpine Glaciers

Mountains are eroded and shaped by glaciers, and after the glaciers melt, the remaining landforms are exposed to view.

Landforms Made by Alpine Glaciers

Landforms produced by alpine glaciers. (a) Before glaciation sets in, the region has smoothly rounded divides and narrow, V-shaped stream valleys. (b) After glaciation has been in progress for thousands of years, new erosional forms are developed. (c) With the disappearance of the ice, a system of glacial troughs is exposed.

Hanging Valleys

The drainage basins of river networks have graded “V”-shaped profiles where one stream flows into another.

Glacial Troughs and Fiords

Glacier flow constantly deepens and widens its rock channel, so that after the ice has finally melted, a deep, steep-walled glacial trough remains. When the floor of a trough open to the sea lies below sea level, the sea water enters as the ice front recedes, forming a deep, narrow estuary known as a fiord.

Glacial Troughs and Fiords

Development of a glacial trough. (a) During maximum glaciation, the U-shaped trough is filled by ice to the level of the small tributaries. (b) After glaciation, the trough floor may be occupied by a stream and lakes. (c) If the main stream is heavily loaded, it may fill the trough with alluvium. (d) Should the glacial trough have been deepened below sea level, it will be occupied by an arm of the sea, or fiord.

a).

c).

b).

d).

Landforms Made By Ice Sheets

The term glacial drift includes all varieties of rock debris deposited in close association with glaciers. Drift is of two major types: stratified drift and till. Where till forms a thin, more or less even cover, it is referred to as ground moraine.

Landforms Made By Ice Sheets

Marginal landforms of continental glaciers.

Landforms Made By Ice Sheets

Glacial Lake.

Greenland's Glaciers

The Greenland ice sheet occupies 7/8 of the surface of Greenland. At the coast, the exposed land is mountainous. The accumulation of snow that has built this ice sheet also contains a record of climate change in the form of different oxygen isotopes, and dates back thousands of years.

*See movie on glaciers in the geodiscoveries section of your text’s website.

Greenland's Glaciers

The Greenland ice sheet. Contours show elevations of the ice sheet surface.

Icebergs

Sea ice is the direct freezing of seawater. Where it completely covers the surface it is referred to as pack ice which may be broken up into individual patches (ice floes) separated by narrow strips of open water (leads).

*See movie on the calving of an iceberg in the geodiscoveries section of your text’s website.

Icebergs

Iceberg. San Rafael Glacial.

Last Ice Age

The last glaciation, known as the Wisconsin Glaciation, reached its maximum around 18,000 years ago. The current deglaciation began around 15,000 years ago during which the ice sheets retreated.

*See movie on the last glaciation in the geodiscoveries section of your text’s website.

This concludes the presentation on the Glacier Systems and the Ice Age.