The Formation and Movement of Glaciers
How Snow Becomes Glacial Ice and What Causes It to Flow
Nov 4, 2009
Glaciers are moving sheets of ice that exist year-round. Glaciers cover approximately 10% of the Earth’s surface, eroding and shaping the underlying rock. Several specific environmental factors must be in place in order for these ice rivers to form and begin flowing.
How Glaciers Form
In Earth: Portrait of a Planet (W.W. Norton & Company, 2001), Steven Marshak lists three conditions that must be met in order for glaciers to form. First, the local climate must be cold enough to prevent full melt of accumulated winter snow. This type of climate generally exists in the polar regions and high elevations. Second, large accumulations of snow must exist in the area. Third, the slope of the rock surface must be gentle enough to keep snow from being lost by avalanche (generally less than a 30% grade) while simultaneously being protected enough that snow doesn’t blow away.
According to Marshak, glacial ice develops in a multi-step process where pre-existing ice recrystallizes in the solid state, which means that molecules in the ice rearrange to form new crystals without first melting into water:
- Older snow becomes buried by newer snow. Snow contains about 90% air due to its ice crystal structure.
- Over time, the pointed ends of individual snowflakes take on a blunted shape, causing the snow to pack together more tightly.
- The weight of the overlying snow increases pressure on the buried snow, causing a pressure solution to form. This occurs because increased pressure triggers melting of the contact points where snowflakes meet, a process called pressure melting. Pressure melting turns the snow into firn, which is a packed granular material that contains only about 25% air.
- Pressure melting continues to melt firn crystals at their contact points. This produces miniscule amounts of water, which then recrystallizes in the spaces between the firn crystals. As this process is repeated over time, the firn forms a solid mass of interlocking ice crystals that contains only about 20% air.
The entire process of turning snow into glacial ice can take anywhere from decades to millennia depending on the amount of snowfall in the region.
How Glaciers Move
Just as with regular rivers, the ultimate cause of glacier flow is gravity. Glaciers can move from 10 to 3000 meters each year, depending on the steepness of the slope and whether water is present at the glacier’s base.
Marshak describes two main types of glacier movement. In basal sliding, pressure from the weight of the glacier causes water to melt at its base. The water, or slurry if the water mixes with underlying sediments, reduces friction between the glacier and its substrate, or underlying rock, allowing it to flow.
With internal flow, the mass of ice changes shape internally over time without breaking or completely melting. One method of internal flow occurs when individual ice crystals change shape by stretching and rotating over time. An alternate method of internal flow occurs when water forms very thin films on the surface of ice crystals, allowing them to slide past each other. Marshak points out that internal flow generally takes place in the coldest polar regions where a glacier’s base in frozen to its substrate.
Conditions Must be Right to Form a Glacier and Induce Flow
Several environmental factors must be in place in order to form a glacier. The climate must produce sufficiently cold temperatures and amounts of snow. Then physics takes over, as snowflakes become glacial ice crystals through pressure melting. Pressure melting is then responsible for the presence of water either at the base of a glacier or between ice crystals, allowing the force of gravity to take over to make the ice sheet move. It is this flow that produces the remarkable landscapes of glacial regions.
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