Can solid ice really move?
Yes! Ice that moves downslope is called a "glacier." Glaciers move extremely slowly along the land surface. They may survive for thousands of years.
Where are the Glaciers?
Nearly all glacial ice, 99%, is contained in ice sheets in the polar regions, particularly Antarctica and Greenland.
Glaciers often form in the mountains because higher altitudes are colder and more likely to have snow that falls and collects. Every continent, except Australia, hosts at least some 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 from where the snow and ice accumulates through mountains along existing valleys.
- Ice caps are large glaciers that cover a larger area than just a valley, possibly an entire mountain range or region. Glaciers come off of ice caps into valleys.
The Greenland ice cap covers the entire landmass.
Glaciers grow when more snow falls near the top of the glacier, in the zone of accumulation , than is melted from lower down in the glacier, in the zone of ablation . These two zones are separated by the equilibrium line.
Snow falls and over time converts to granular ice known as firn. Eventually, as more snow and ice collect, the firn becomes denser and converts to glacial ice.
Water is too warm for a glacier to form, so they form only on land. A glacier may run out from land into water, but it usually breaks up into icebergs that eventually melt into the water.
Whether an ice field moves or not depends on the amount of ice in the field, the steepness of the slope and the roughness of the ground surface. Ice moves where the pressure is so great that it undergoes plastic flow. Ice also slides at the bottom, often lubricated by water that has melted and travels between the ground and the ice.
The speed of a glacier ranges from extremely fast, where conditions are favorable, to nearly zero.
Because the ice is moving, glaciers have crevasses , where cracks form in the ice as a result of movement. The large crevasse at the top of an alpine glacier where ice that is moving is separated from ice that is stuck to the mountain above is called a bergschrund .
Crevasses in a glacier are the result of movement.
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 km2 in area and 50 m thick.
Glacier National Park
Many of the glaciers in Glacier National Park have shrunk and are no longer active. Summer temperatures have risen rapidly in this part of the country and so the rate of melting has picked up. Whereas Glacier National Park had 150 glaciers in 1850, there are only about 25 today. Recent estimates are 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 with an outline of the extent of the glaciers as they were in 1950. Although it continues to be classified as a glacier, Gem Glacier is only 0.020 km 2 (5 acres) in area, only one-fifth the size of the smallest active glaciers.
Glaciers as a Resource
- Glaciers are ice that moves because the amount of snow and ice that collects in the zone of accumulation exceeds the amount that melts off in the zone of ablation.
- Continental glaciers form in a central location with ice moving outward in all directions. Alpine glaciers form in high mountains and travel through valleys.
- Because glaciers move, they have characteristic features like crevasses and bergschrunds.
Use the resource below to answer the questions that follow.
- Yosemite Nature Notes: Glaciers at http://youtu.be/mgnzSTY5zRg (8:35)
- Where are glaciers found in Yosemite National Park?
- What is the largest glacier in Yosemite National Park?
- What is the environment on and around a glacier like?
- What are the dangers on glaciers?
- What is a crevasse? What creates it?
- What is a glacier?
- Describe the bergschrund.
- What is the challenge with protecting glaciers in Yosemite National Park?
- Compare and contrast alpine glaciers, continental glaciers, and ice caps.
- With a glacier that is melting back, what is happening in the zone of accumulation and the zone of ablation? What is happening to the equilibrium line?
- How do glaciers serve as a water resource for people and organisms in the summertime?