Erosion and Deposition by Ice
What You Will Learn
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Explain the difference between alpine glaciers and continental glaciers.
Describe two ways in which glaciers move.
Identify five landscape features formed by alpine glaciers.
Identify four types of moraines.
Can you imagine an ice cube that is the size of a football stadium?
Well, glaciers can be even bigger than that.
A glacier is an enormous mass of moving ice. Because glaciers are
very heavy and have the ability to move across the Earth’s surface,
they are capable of eroding, moving, and depositing large amounts of
rock materials. And while you will never see a glacier chilling a punch
bowl, you might one day visit some of the spectacular landscapes
carved by glacial activity!
Glaciers—Rivers of Ice
Glaciers form in areas so cold that snow stays on the ground yearround. In polar regions and at high elevations, snow piles up year after
year. Over time, the weight of the snow on top causes the deeppacked snow to become ice crystals. These ice crystals eventually form
a giant ice mass. Because glaciers are so massive, the pull of gravity
causes them to flow slowly, like “rivers of ice.” In this section, you will
learn about two main types of glaciers, alpine and continental.
Alpine Glaciers
Alpine glaciers form in mountainous areas. One common type of alpine
glacier is a valley glacier. Valley glaciers form in valleys originally
created by stream erosion. As these glaciers slowly flow downhill, they
widen and straighten the valleys into broad U shapes as shown
in Figure 1.
Figure 1 Alpine glaciers start as snowfields in mountainous areas.
Where do alpine glaciers form?
Continental Glaciers
Not all glaciers are true “rivers of ice.” In fact, some glaciers spread
across entire continents. These glaciers, called continental glaciers, are
huge, continuous masses of ice. The largest continental glacier in the
world covers almost all of Antarctica. This ice sheet is approximately
one and a half times the size of the United States. It is so thick—more
than 4,000 m in places—that it buries everything but the highest
mountain peaks.
Glaciers on the Move
When enough ice builds up on a slope, the ice begins to move
downhill. Thick glaciers move faster than thin glaciers, and the steeper
the slope is, the faster the glaciers will move. Glaciers move in two
ways: by sliding and by flowing. A glacier slides when its weight
causes the ice at the bottom of the glacier to melt. As the water from
a melting ice cube causes the ice cube to travel across a table, the
water from the melting ice causes a glacier to move forward. A glacier
also flows slowly as ice crystals within the glacier slip over each other.
Think of placing a deck of cards on a table and then tilting the table.
The top cards will slide farther than the lower cards. Similarly, the
upper part of the glacier flows faster than the base.
Glacier movement is affected by climate. As the Earth cools, glaciers
grow. About 10,000 years ago, a continental glacier covered most of
North America, as shown in
several kilometers thick!
Figure 2.
In some places, the ice sheet was
Figure 2 Eleven U.S. states were covered by ice during the last glacial
ice period. Because much of the Earth’s water was frozen in glaciers,
sea levels fell. Blue lines show the coastline at that time.
Landforms Carved by Glaciers
Continental glaciers and alpine glaciers produce landscapes that are
very different from one another. Continental glaciers smooth the
landscape by scraping and eroding features that existed before the ice
appeared. Alpine glaciers carve out rugged features in the mountain
rocks through which they flow. Figure 3 shows the very different
landscapes that each type of glacier produces.
Figure 3
Landscapes Created by Glaciers
Alpine glaciers, such as those in the Rocky Mountains and the Alps,
carve out large amounts of rock material and create spectacular
landforms.Figure 4 shows the kinds of landscape features that are
sculpted by alpine glaciers.
Figure 4 Landscape Features Carved by Alpine Glaciers
Types of Glacial Deposits
As a glacier melts, it drops all the material it is carrying. Glacial
drift is the general term used to describe all material carried and
deposited by glaciers. Glacial drift is divided into two main
types, till and stratified drift.
Till Deposits
Unsorted rock material that is deposited directly by the ice when it
melts is called till. Unsorted means that the till is made up of rock
material of different sizes—from large boulders to fine sediment. When
the glacier melts, the unsorted material is deposited on the surface of
the ground.
The most common till deposits are moraines. Moraines generally form
ridges along the edges of glaciers. Moraines are produced when
glaciers carry material to the front of and along the sides of the ice. As
the ice melts, the sediment and rock it is carrying are dropped, which
forms different types of moraines. The various types of moraines are
shown inFigure 5.
Figure 5 Types of Moraines
Stratified Drift
When a glacier melts, streams form that carry rock material away from
the shrinking glacier. A glacial deposit that is sorted into layers based
on the size of the rock material is called stratified drift. Streams
carry sorted material and deposit it in front of the glacier in a broad
area called an outwash plain. Sometimes, a block of ice is left in the
outwash plain when a glacier retreats. As the ice melts, sediment
builds up around the block of ice, and a depression called
a kettle forms. Kettles commonly fill with water to form lakes or
ponds, as Figure 6 shows.
Kettle lakes form in outwash plains and are common in states
such as Minnesota.
Figure 6
Explain the difference between a till deposit and
stratified drift.