Erosion and Deposition by Glaciers

Lesson Objectives

  • Describe how continental and valley glaciers form.
  • Explain how glaciers cause erosion.
  • Identify landforms deposited by glaciers.

Vocabulary

  • continental glacier
  • glacial till
  • glacier
  • moraine
  • plucking
  • valley glacier

Introduction

Glaciers
are masses of flowing ice. Today, they cover only about 10 percent of
Earth’s surface. They are getting smaller and smaller as Earth’s
temperature rises. But just 12,000 years ago, glaciers dipped as far
south as Chicago and New York City. Much of Europe was also covered with
glaciers at that time.

Glaciers erode and leave behind telltale
landforms. These landforms are like clues. They show the direction a
glacier flowed and how far it advanced. Did glaciers leave clues where
you live? Would you know what to look for?

How Glaciers Form

Glaciers
form when more snow falls than melts each year. Over many years, layer
upon layer of snow compacts and turns to ice. There are two different
types of glaciers: continental glaciers and valley glaciers. Each type
forms some unique features through erosion and deposition. An example of
each type is pictured in Figure below .

  • A continental glacier
    is spread out over a huge area. It may cover most of a continent.
    Today, continental glaciers cover most of Greenland and Antarctica. In
    the past, they were much more extensive.
  • A valley glacier is long and narrow. Valley glaciers form in mountains and flow downhill through mountain river valleys.

(A)
The continent of Antarctica is covered with a continental glacier. (B) A
valley glacier in the Canadian Rockies. (C) The surface of a valley
glacier.

Erosion by Glaciers

Like flowing
water, flowing ice erodes the land and deposits the material elsewhere.
Glaciers cause erosion in two main ways: plucking and abrasion. Both are
illustrated at this link:

[Insert a link to an animation showing how glaciers cause erosion by plucking and erosion.]

  • Plucking
    is the process by which rocks and other sediments are picked up by a
    glacier. They freeze to the bottom of the glacier and are carried away
    by the flowing ice.
  • Abrasion is the process in which a glacier
    scrapes underlying rock. The sediments and rocks frozen in the ice at
    the bottom and sides of a glacier act like sandpaper. They wear away
    rock. They may also leave scratches and grooves that show the direction
    the glacier moved.

Erosion by Valley Glaciers

Valley glaciers form several unique features through erosion. You can see some of them in Figure below and at this link:

[Insert a link to a video that shows features eroded by valley glaciers.]

  • As
    a valley glacier flows through a V-shaped river valley, it scrapes away
    the sides of the valley. It carves a U-shaped valley with nearly
    vertical walls. A line called the trimline shows the highest level the
    glacier reached.
  • A cirque is a rounded hollow carved in the
    side of a mountain by a glacier. The highest cliff of a cirque is called
    the headwall.
  • An arête is a jagged ridge that remains when
    cirques form on opposite sides of a mountain. A low spot in an arête is
    called a col.
  • A horn is a sharp peak that is left behind when glaciers erode all sides of a mountain.

Features Eroded by Valley Glaciers. Erosion by valley glaciers forms the unique features shown here.

Deposition by Glaciers

Glaciers
deposit their sediment when they melt. They drop and leave behind
whatever was once frozen in their ice. It’s usually a mixture of
particles and rocks of all sizes, called glacial till. Water from the melting ice may form lakes or other water features. Figure below shows some of the landforms glaciers deposit when they melt. You can see an animation of glacial deposition at this link:

[Insert a link to an animation showing how landforms are deposited by glaciers.]

  • Moraine
    is sediment deposited by a glacier. A ground moraine is a thick layer
    of sediments left behind by a retreating glacier. An end moraine is a
    low ridge of sediments deposited at the end of the glacier. It marks the
    greatest distance the glacier advanced.
  • A drumlin is a long,
    low hill of sediments deposited by a glacier. Drumlins often occur in
    groups called drumlin fields. The narrow end of each drumlin points in
    the direction the glacier was moving when it dropped the sediments.
  • An esker is a winding ridge of sand deposited by a stream of meltwater. Such streams flow underneath a retreating glacier.
  • A
    kettle lake occurs where a chunk of ice was left behind in the
    sediments of a retreating glacier. When the ice melted, it left a
    depression. The meltwater filled it to form a lake.

Take a look at the glacial deposits. How far did the glacier in the diagram advance before it started retreating?

Lesson Summary

  • Glaciers
    are masses of flowing ice. Continental glaciers are huge. They may
    spread out over much of a continent. Valley glaciers are long and
    narrow. They form in mountains and flow through mountain river valleys.
  • Glaciers
    cause erosion by plucking and abrasion. Valley glaciers form several
    unique features through erosion, including cirques, arêtes, and horns.
  • Glaciers deposit their sediment when they melt. Landforms deposited by glaciers include drumlins, kettle lakes, and eskers.

Lesson Review Questions

Recall

1. What is a glacier?

2. Describe how glaciers form.

3. Identify the two main ways glaciers cause erosion.

4. Name and describe three unique features eroded by valley glaciers.

5. What is glacial till?

Apply Concepts

6. Create a lesson to teach younger students how a kettle lake forms. Outline your lesson.

Think Critically

7. Compare and contrast valley and continental glaciers and how they change Earth’s surface.

8.
Areas once covered by glaciers may have large boulders called erratics,
like the one in the photo below. Infer why erratics typically consist
of a different type of rock than the bedrock where they are found.

Points to Consider

So far in this chapter, you’ve read how moving water, air, and ice
shape Earth’s surface. Water and ice move because of gravity.

  • Do you think gravity can erode and deposit sediment without the help of water or ice?
  • How might gravity alone shape Earth’s surface?

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