How do glaciers move? Glaciers are pulled downhill by gravity.
A glacier is a river of ice that is flowing downhill. A river is a body of water that flows from a high elevation to a low elevation, usually ending in the sea or a lake, under the force of gravity. A glacier does the same thing, but it is made of ice and not water. It can take hundreds of years for something to move from the top to the bottom of a glacier. A glacier is made up of ice crystals, snow, air, water, and other sediments and rocks it picks up along the way.
Glaciers start in an area where snow builds up. This is usually in an elevated area. As the snow builds up, the weight of the layers on top start to compact the layers underneath. Snow is 90% air, but it is surprisingly heavy. 1 cubic meter of snow weighs about 50 kg. The weight of the snow above crushes the snow below and squeezes the air out, compacting it. The snowflakes become grains of ice. In the top 40 to 120 m of the ice buildup, these grains of ice are called “firn”. Firn is the stage between snow and ice. The snowflakes are compressed and the air is gradually squeezed out. The firn grains get fused together by the pressure and become coarser and larger. At the lower end of the firn layer, the pores are sealed up and the remaining air below that remains as bubbles. The accumulation zone of a glacier can be hundreds of meters thick.
In some places, a buildup of snow may never become a glacier. If the summers are hot, the summer snow melt could equal the winter snowfall and not enough snow will build up to start a glacier. Or, if there is nowhere for the ice to go, there won’t be a glacier. However, if the snowfall exceeds the snow melt and there is an outlet for the accumulating snow, a glacier will form.
The area where the snow is building up is called the accumulation zone. As the ice accumulates it gets heavier and the ice on the top pushes the ice on the bottom. If there is an outlet in the place where the ice is accumulating, the lower ice will escape that way. This is the start of the glacier. As the glacier goes over the edge of the accumulation zone it starts to flow because it is pushed by the weight of the snow above it and because it is pulled by gravity. Once the ice has crossed that edge, gravity is the force that keeps it moving.
Glaciers are made of solid ice but behave in the same way as a fluid, in that they flow. Gravity pulls the glacier and it heads downhill following the natural lie of the land. There are three ways that a glacier can move. The first is through internal deformation. This is where the weight of the ice and the pull of gravity cause the ice crystals to slide over each other. This causes the glacier to crumple and move in layers as it is pulled downwards. The top often moves faster than the bottom. The second way is by sliding over a layer of water. This is called basal sliding. The friction with the ground or the heat of the Earth can melt the very bottom of the glacier, making a lubricating layer of water that the glacier can slide on. The third way is by pushing through soft ground. If the ground beneath the glacier doesn’t offer much resistance, the weight of the glacier will easily push through it.
The glacier doesn’t all move at the same speed. The center will move the fastest because the sides will be affected by friction. Because the ice doesn’t all move at the same speed, it is subject to a lot of stress. If the front part of the glacier starts to move faster than the upper parts, it will stretch the glacier and produce cracks, called crevasses. Some of these crevasses go deep into the glacier.
The glacier spreads out at the end as it clears valleys and obstructions. The very end of the glacier is called the terminus. This is where the ice has melted away to nothing, or breaks off if the glacier enters the sea. Sometimes, if the glacier enters the sea, the glacier can clave off as icebergs. And this is what I learned today.
Sources
https://byjus.com/physics/shearing-stress/
https://nsidc.org/learn/parts-cryosphere/glaciers/science-glaciers
https://www.nps.gov/articles/howglaciersmove.htm