Glaciers 5:Glaciers, Deserts, and Wind

Types of Glaciers 5

A glacier appears to be motionless, but it's not. Sit beside a glacier for an hour and you may hear a sporadic chorus of creaks, cracks, and groans as gravity pulls the mass of ice slowly downhill. Just like running water, groundwater, wind, and waves, glaciers are dynamic agents of erosion. They accumulate, transport, and deposit sediment. Thus, glaciers are an important part of the rock cycle.

Types of Glaciers 2

A glacier is a thick ice mass that moves slowly over the land surface. Today, glaciers still cover nearly 10 percent of Earth's land area. In these regions they continue to sculpt the landscape.

Types of Glaciers 1

As recently as 15,000 years ago—the blink of an eye in geologic history—up to 30 percent of Earth's land was covered by glacial ice. At that time, Earth was coming out of an ice age—a period of time when much of Earth's land is covered in glaciers. Sheets of ice that were thousands of meters thick shaped places like Cape Cod, Long Island, the Great Lakes, and the fjords of Norway.

How Glaciers Move 2

Basal slip is the second cause of glacial movement. Due to gravity, the entire ice mass actually slips and slides downhill along the ground. The upper 50 meters of a glacier is not under enough pressure to have plastic flow. The surface of the glacier behaves differently than the ice below. This uppermost zone of a glacier is brittle, and it is referred to as the zone of fracture. This brittle topmost ice piggybacks a ride on the flowing ice below.

Crevasses are gaping cracks that form in the upper part of a glacier. Why do crevasses form?

Brittle ice breaks as the glacier crosses rough terrain.

Rates of Glacial Movement

Different glaciers move at different speeds. Some flow so slowly that trees and other vegetation grow in the debris on their surface. Other glaciers can advance several meters per day. Some glaciers alternate between periods of rapid movement and periods of no movement whatsoever.

Glaciers of the Ice Age 1

During the recent ice age, continental ice sheets and alpine glaciers covered a lot more land than they do today. People once thought that glacial deposits had drifted in on icebergs or that they swept across the landscape in a catastrophic flood. However, scientific field investigations during the nineteenth century provided convincing evidence that an extensive ice age explained these deposits and many other features.

Budget of a Glacier 3

Glaciers also lose ice when large pieces break off their fronts in a process called calving. Calving creates icebergs where glaciers meet the ocean. Because icebergs are just slightly less dense than seawater, they float low in the water. Only about 10 percent of their mass is visible above the surface. The Greenland Ice Sheet calves thousands of icebergs each year. Many drift southward into the North Atlantic where they are navigational hazards.

Why are glaciers an important part of the rock cycle?

Glaciers are dynamic agents of erosion that accumulate, transport, and deposit sediment. Thus, glaciers are an important part of the rock cycle.

Budget of a Glacier 1

Glaciers form where more snow falls in winter than can melt during the summer. They constantly gain and lose ice. Snow accumulates, and ice forms at the head of the glacier in the zone of accumulation. At the top, new snowfall thickens the glacier and promotes movement.

Types of Glaciers 3

Glaciers originate on land in places where more snow falls each winter than melts each summer. The snowline is the lowest elevation in a particular area that remains covered in snow all year. At the poles, the snowline occurs at sea level. Closer to the equator, the snowline is near the top of tall mountains.

Ice Sheets 1

Ice sheets are enormous ice masses that flow in all directions from one or more centers and cover everything but the highest land. Ice sheets are sometimes called continental ice sheets because they cover large regions where the climate is extremely cold. They are huge compared to valley glaciers. Ice sheets covered much of North America during the recent ice age.

Budget of a Glacier 5

If a glacier gains ice at the same rate as ice melts or calves off, the front or terminus of the glacier remains stationary. Whether the front of a glacier advances, retreats or remains stationary, the ice within the glacier continues to flow forward. In the case of a receding glacier, the ice still flows forward, but not rapidly enough to offset wastage.

Types of Glaciers 4

Instead of completely melting away, snow above the snowline accumulates and compacts. The compressed snow first recrystallizes into coarse grains of ice. Further pressure from added snow above changes the coarse grains into interlocking crystals of glacial ice.

Glacial deposits are part of the land that is now Canada and the northern United States. How did these deposits form?

Melting glaciers left the deposits as they retreated during the last ice age.

Glaciers of the Ice Age 2

The Northern Hemisphere had twice the ice of the Southern Hemisphere. The Southern Hemisphere has far less land, so glaciation was mostly confined to Antarctica. By contrast, North America and Eurasia have plenty of land where the ice sheets could spread.

Budget of a Glacier 2

The area of the glacier beyond the snowline is called the zone of wastage. The glacier loses ice—and any new snow—to melting.

Ice Sheets 2

The figure shows the two remaining ice sheets, which combined cover almost 10 percent of Earth's land area. One ice sheet covers about 80 percent of Greenland. It averages nearly 1,500 meters thick, and in places it rises to 3,000 meters above the island's surface.

Budget of a Glacier 4

The foot of a glacier can advance, retreat, or remain in place. Which course it follows depends on the glacier's budget. The glacial budget is the balance or lack of balance between accumulation at the upper end of a glacier and loss, or wastage, at the lower end. If more ice accumulates at the glacier head than melts or calves at the glacier foot, then the glacier advances. The glacier retreats when it loses ice faster than it gains ice.

Glaciers of the Ice Age 5

The formation and growth of ice sheets triggered changes in climates beyond the glacial margins. Regions that are arid today became cooler and wetter. This change in climate resulted in the formation of lakes in such areas as the Basin and Range region of Nevada and Utah. One of these lakes was ancient Lake Bonneville, which covered much of western Utah. The Great Salt Lake is all that remains of this glacial lake.

Ice Sheets 3

The huge Antarctic Ice Sheet in the Southern Hemisphere is nearly 4,300 meters thick in places. This glacier accounts for 80 percent of the world's ice, and it holds nearly two-thirds of Earth's fresh water. If it melted, sea level could rise 60 to 70 meters, and many coastal cities would flood.

Glaciers of the Ice Age 4

The ice sheets greatly affected the drainage patterns over large regions. For example, before glaciation, the Missouri River flowed northward toward Hudson Bay in Canada. The Mississippi River flowed through central Illinois. Furthermore, the Great Lakes did not exist. Their locations were marked by lowlands with rivers that flowed toward the east. During the recent ice age, glacial erosion transformed these lowlands into wide, deep basins that filled with water and eventually became the Great Lakes.

How Glaciers Move 1

The movement of glaciers is referred to as flow. Glacial flow happens two ways: plastic flow and basal slip. Plastic flow involves movement within the ice. Under high enough pressure, the normally brittle ice begins to distort and change shape—a property known as plasticity. The weight of overlying ice exerts this pressure on the ice below, causing it to flow. Plastic flow begins at about 50 meters below the glacier surface.

Glaciers of the Ice Age 3

The recent ice age began two to three million years ago. Many of the major glacial episodes occurred during the Pleistocene epoch when wooly mammoths and saber-toothed cats roamed the landscape. To some people the Pleistocene is synonymous with the recent ice age, but it actually began before this epoch on the geologic time scale.

How Glaciers Move 3

The zone of fracture experiences tension when the glacier moves over irregular terrain. This tension results in gaping cracks called crevasses. Crevasses can be 50 meters deep. They are often hidden by snow and make travel across glaciers dangerous.

How are the glaciers of Antarctica different from valley glaciers?

They form a huge mass that covers the entire continent.

How would you contrast advancing and retreating glaciers?

Though both types flow and carry debris, advancing glaciers accumulate ice faster than ice melts; retreating glaciers melt faster than ice accumulates.

Valley Glaciers

Thousands of small glaciers exist in high mountains worldwide. Unlike fast-flowing mountain streams, glaciers advance only a few centimeters to meters each day. Valley glaciers are ice masses that slowly advance down valleys that were originally occupied by streams. A valley glacier is a stream of ice that flows between steep rock walls from a place near the top of the mountain valley. Like rivers, valley glaciers can be long or short, wide or narrow, single, or with branching tributaries.

Where in a glacier does plastic flow occur?

about 50 meters below the glacial surface

How do valley glaciers flow?

as a stream of ice down a mountain valley

Compared to a valley glacier, an ice sheet _____.

is much larger

Which change to Earth's surface was not caused by the most recent ice age?

the separation of Antarctica from other continents