Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
THE WORK OF ICE
GLACIERS AND GLACIAL LANDFORMS
At high altitudes, glaciers form
when temperatures are low
and heavy snowfall occurs.
These long, narrow glaciers
confined between rock walls
are Alpine Valley Glaciers.
- A glacier is a body of ice that formed on land by the compaction and recrystallization of snow. A glacier moves outward under its own weight. Alpine valley glaciers occupy previously existing river valleys. Continental glaciers (ice sheets) advance from a central region of accumulation and cover the landscape.
- Ice accumulates when there is a global lowering of temperature and when winter snowfall exceeds summer ablation (melting and evaporation). As snow is buried, it changes to firn (layers of granular partially compacted snow) and then ice. Glacier ice moves by a combination of plastic internal flow and basal sliding. Glacier ice travels faster in the middle of the glacier than on the sides and bottom. The sides and bottom move more slowly due to friction with the adjacent rock. The upper part of a glacier is brittle and rides upon the lower plastic layer.
- A glacier budget includes a zone of accumulation above the snow line and a zone of ablation below the snow line. Glaciers always move forward but if ablation is greater than accumulation, a glacier has a negative budget and recedes. If accumulation is greater than ablation, a glacier advances.
- Glaciers erode the landscape by plucking (the lifting away of bedrock as the ice moves forward), and abrasion (the rock enclosed in the ice scrapes and grinds the underlying rock). Abrasion produces striations (long scratches) in the bedrock.
- Glaciers transport moraine (rock debris carried and deposited by ice) beneath, within, and on the surface of the moving ice (lateral and medial moraine). This rock debris is eventually deposited as terminal moraine at the end of the glacier. Terminal moraines form natural dams and trap streams. These streams then form long ribbon or finger lakes.
- Mountainous regions are dramatically changed by alpine valley glaciers. Glaciated landscapes, shaped by frost wedging as well as plucking and abrasion tend to be angular. Hollows on the mountainsides become deepened into bowl-shaped depressions called cirques. Steep jagged ridges called aretes, and pyramidal peaks called horns form where several cirque backwalls meet.
- Alpine valley glaciers widen, deepen, and straighten valleys into U-Shaped troughs, leaving tributary hanging valleys above the steep truncated spurs of the valley walls. Glaciers flowing down to the sea break off and form icebergs. After the ice age ends, sea level rises and drowns the troughs forming fiords.
On the diagram below showing an alpine glaciated region, identify the arete, cirque, fiord, hanging valley, horn, truncated spurs, and U-Shaped glacial trough.
- Ice sheets (Continental glaciers) exist today in Greenland and Antarctica and may be 3000-4000 meters thick (approximately 10,000-12,000 feet). During the Pleistocene epoch (beginning 2 million years ago and ending 10-15,000 years ago), ice sheets covered much of Europe and North America as far south as the Ohio and Missouri rivers.
- Ice sheets erode the landscape by gouging and grooving the underlying bedrock with the rock debris they carry. Rock knobs formed beneath the ice, show evidence of both plucking and abrasion. An ice-scoured plain such as the Canadian Shield consists of these knobs and rock cut basin lakes.
- The rock debris picked up by ice sheets is eventually deposited directly as till (a mixture of unsorted rock fragments and clay) or laid down and sorted by water (stratified drift).
- Glacial till may form extensive till plains or recessional or terminal moraines. Some till may be molded into streamlined hills called drumlins which point in the direction of ice movement. Till at the edge of the ice sheet may contain blocks of ice which later melt forming hollows called kettles or kettle lakes.
- Outwash plains of stratified sand and gravel are built by meltwaters emerging from the edges of an ice sheet. Narrow winding ridges of stratified drift known as eskers are built by glacial streams moving in tunnels beneath the ice. Marginal lakes may accumulate layers of silt and clay called varves.
- Define a glacier and distinguish between alpine valley glaciers and continental glaciers.
- Describe the preconditions for the formation of glacier ice. Describe firn. How do glaciers move?
- Describe the concept of a glacier budget and the relationship between the zone of accumulation and zone of ablation in the advance and receding of glaciers.
- Explain two ways in which glaciers erode.
- Describe the ways in which glaciers transport and deposit rock debris.
- Explain how alpine valley glaciers change mountainous regions. Define cirque, arete, and horn.
- Describe how alpine valley glaciers change pre-existing river valleys. Define U-Shaped trough, hanging valleys, truncated spurs, icebergs, and fiords.
- Name two existing ice sheets. Describe the length of the ice age and when it ended. Describe the extent of ice sheets in North America. How far south did they advance?
- Describe the erosional work of ice sheets.
- Describe the two types of glacial deposit. Account for the differences in structure and composition.
- Describe the landforms created by till.
- Describe the landforms left behind by stratified drift.
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Created by Rita D. Haberlin, Geography Professor
Peralta Community College District
333 East Eighth Street
Oakland, CA 94606
This Geography Site Is Maintained By Patricia A. Kulda Last Update August 20, 2010