Download Chapter 20

Chapter 19
Glacial Landforms and the Ice Age
This chapter examines the role of glacial ice as a denudation and landforming agent.
Glacial ice had a major impact on the landscapes of midlatitude and subarctic regions
during the past Ice Age and still covers many high latitude and high elevation areas of the
Earth.
 Glaciers are natural bodies of land ice that have, or have had in the past, the ability to
flow. They form where winter snowfall exceeds summer ablation over long periods of
time.
 Glaciers erode the land surface by plucking and abrasion. Eroded material is
incorporated into the glacier, transported, and eventually deposited when the ice melts.
 Alpine glaciers form in cirques in high mountain locations and often flow down preexisting stream valleys carving them into U-shaped glacial troughs.
 Snow builds up in the zone of accumulation found at the upper end of the glacier
where layers of snow undergo compaction and recrystalization to produce firn.
Beneath the surface of the glacier, the ice acts as a plastic substance and will flow
slowly. Glaciers also move by basal sliding.
 Glaciated landscapes tend to be very rugged. Freeze-thaw weathering and glacial
erosion produce cirques, arêtes, and horn peaks.
 The erosional capacity of glaciers also produces large quantities of depositional
materials. Piles of unsorted debris that form along the end and sides of glaciers are
moraines.
 Ice sheets are accumulations of ice that cover large areas and extend over major
topographic features. Greenland and Antarctica are sites of present-day ice sheets.
 Ice shelves are extensions of ice sheets that float on ocean water. Icebergs are pieces
of ice that break free from ice shelves and glaciers to float in the ocean.
 Continental ice sheets expand and contract during an ice age, causing alternating
periods of glaciation, deglaciation, and interglaciation.
 During the Late-Cenozoic Ice Age, extending over the past two to three million years,
continental ice sheets have grown and melted up to thirty times.
 Much of North America and Europe, as well as parts of Asia and South America, were
covered with ice during the most recent episode of ice sheet expansion, the Wisconsin
Glaciation.
 The erosive action of alpine glaciers and ice sheets produces grooved, scratched, and
polished surfaces on more resistant rock, and strips away regolith and weaker rock.
 Glacial drift refers to all those sediments that are deposited by glaciers. Unstratified
drift deposited directly from glaciers is called till. Those sediments derived from
glaciers, but modified by transportation by meltwater, are called stratified drift.
 Some of the more common landforms made up of till deposits are moraines, till
plains, and drumlins.
 Stratified drift features include outwash plains, which form where braided meltwater
streams issuing from glaciers deposit sediment over a wide area. Sediment deposited
by meltwater streams flowing in ice tunnels beneath a glacier form ridge-like eskers.
 Kames are stratified drift deposits that originate as deltas in meltwater lakes near
glacier margins.
 Three possible causes of the Late-Cenozoic Ice Age are:
 a change in continent positions due to plate tectonic activity
 an increase in the number and severity of volcanic eruptions
 a reduction in solar energy output
 Cycles of glaciation appear to be related to cyclic changes in the Earth’s axial tilt and
distance from the sun.
Global warming has the potential to change both ablation and snowfall on the Earth’s
ice sheets. The net effect of these changes on global sea level is uncertain.