8.12: Glaciers

Can solid ice really move?

Yes! This is the Hubbard Glacier in Alaska. As the glacier meets the relatively warm seawater, it calves. Pieces of the glacier break off and fall into the water. These icebergs may be as large as a ten-story building.

Where Are The Glaciers?

Nearly all glacial ice is in the polar regions. The largest ice sheets are in Antarctica and Greenland. The remaining 1% of Earth's ice is found elsewhere, like in the mountains, for example. At higher altitudes, temperatures are colder. Snow is more likely to fall and not melt. Except Australia, every continent has glaciers in the high mountains.

Types of Glaciers

The types of glaciers are:

• Continental glaciers are large ice sheets that cover relatively flat ground. These glaciers flow outward from where the greatest amounts of snow and ice accumulate.
• Alpine (valley) glaciers flow downhill. Ice accumulates near the top of a mountain and then travels down existing valleys (Figure below).

An alpine glacier in the Swiss Alps. Where do you think the word "alpine" came from?

• Ice caps are large glaciers that cover a larger area than just a valley. An ice cap can bury an entire mountain range or region (Figure below). Glaciers come off of ice caps into valleys.

The continent of Antarctica is covered with an ice cap.

Glacial Growth

Glaciers are not static; they grow, they move, and they shrink. Currently, most glaciers are shrinking.

Formation

Glaciers grow when snowfall exceeds melting. Snow falls higher on the glacier. Over time, the snow converts to granular ice known as firn. As more snow and ice collect, the firn becomes denser. This is now glacial ice.

Glaciers form only on land because water is too warm. When a glacier flows into water, it usually breaks up into icebergs. Those icebergs eventually melt into the water.

Movement

How does an ice field become a glacier? In other words, why does the ice move? Whether an ice field moves depends on the amount of ice. It also depends on the steepness of the slope and the roughness of the ground surface. Ice moves where the pressure is so great that the ice needs to move away. Ice also slides at the bottom. Often the ground is lubricated by water that melted from the glacier above. Under some conditions, a glacier will move extremely fast. Under other conditions, the glacier may not move and will become an ice sheet.

Because the ice is moving, glaciers have cracks called crevasses (Figure below). There is a large crevasse at the top of an alpine glacier called a bergshrund. Below the bergshrund, the ice is moving downhill. Above the bergshrund, the ice is stuck to the mountain.

Crevasses in a glacier are the result of movement.

Shrinking

Glaciers are melting back in many locations around the world. When a glacier no longer moves, it is called an ice sheet. This usually happens when it is less than 0.1 square kilometers in area and 50 m thick.

In Glacier National Park (Figure below), many glaciers have become ice sheets. In 1850, the park had 150 glaciers. There are only about 25 today. The reason that there is so much melting is that summer temperatures have risen. Scientists estimate that the park will have no active glaciers as early as 2020.

This satellite image shows Grinnell Glacier, Swiftcurrent Glacier, and Gem Glacier in 2003. The outlines show the extent of the glaciers in 1950. Gem Glacier is only 0.020 km² (5 acres) in area. This makes it about one-fifth the size of the smallest active glaciers. Is Gem really a glacier?

Glaciers As A Resource

Melting glaciers in mountain regions provide an important source of water in the summer. Organisms and nearby human populations may depend on glacial melt water during long dry summers. People near the Andes in South America are just some of the groups that rely on glacial meltwater in the summer.

Summary

• Glaciers are ice that moves.
• Continental glaciers form in a central location with ice moving outward in all directions. Alpine glaciers form in high mountains and travel through valleys. Ice caps cover large areas.
• Because glaciers move, they have characteristic features like crevasses and bergshrunds.

Review

1. Compare and contrast alpine glaciers, continental glaciers, and ice caps.
2. Describe how an ice sheet could become a glacier.
3. How do glaciers serve as a water resource for people and organisms in the summertime?

Explore More

Use the resource below to answer the questions that follow.

1. What determines how fast a glacier moves?
2. How does a glacier move like a stream? Why?
3. Where are alpine glaciers and how can you tell that one was in an area?
4. What are the two continental glaciers that remain today? What did North America look like during the last Ice Age?
5. What happens in the zone of accumulation if more snow accumulates than ice melts? what happens if there is more melting than snowfall?