Erosion and Deposition by Ice

What You Will Learn• 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 IceGlaciers form in areas so cold that snow stays on the ground year-round. In polar regions and at high elevations, snow piles up year after year. Over time, the weight of the snow on top causes the deep-packed 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 GlaciersAlpine 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 GlaciersNot 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 MoveWhen 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 Figure 2. In some places, the ice sheet was several kilometers thick!

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 GlaciersContinental 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 GlaciersTypes of Glacial DepositsAs 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 DepositsUnsorted 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 DriftWhen 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.

Figure 6 Kettle lakes form in outwash plains and are common in states such as Minnesota.

 Explain the difference between a till deposit and stratified drift.