How Does a Glacier Move?

gla·cier

noun /ˈglāSHər/
glaciers, plural

  1. A slowly moving mass or river of ice formed by the accumulation and compaction of snow on mountains or near the poles

– Google Dictionary

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Bridge Glacier June 2014 from the air. It’s flowing, and we’re trying to measure how fast.

One of the most fundamental concepts (really, the fundamental concept) of glaciology is the idea of glacial flow. The fact that a large mass of ice can flow under its own weight is the defining quality that separates a glacier from a snowfield or the like. While the “river of ice” imagery often associated with glacier flow is not wrong, in reality, the factors that go into determining how a glacier moves is (slightly) more nuanced.

The most fundamental glacier flow mechanism is similar to the common image of a river flowing out to sea. A glacier follows the laws of gravity, and flows downhill under it’s own weight. This is called, in physical terms, internal (viscous) deformation. The difference between how a river flows, and how a glacier flows isn’t really in the mechanics, gravity is still acting on the substance, but is in the physical property known as viscosity. Viscosity is a measure of how ‘thick’ a substance is; for example, molasses has a higher viscosity than water, while peanut butter has an even higher viscosity than molasses. A glacier ice has a viscosity roughly 80 trillion times(!) greater than peanut butter. Needless to say, glacier flow by internal deformation is a very slow process.

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Bridge Glacier, with Bridge Peak in the background. Where the glacier bed is steeper, stresses are higher, resulting in faster flow.

What speeds up and slows down this process of internal deformation is a second, interrelated concept known as “stress”. Stress is a measure of the forces acting within/between particles in the ice. If the stress is higher, particles are more likely to move, and we should expect more deformation and faster glacier flow. As it turns out, the thickness of the ice, and the slope of the glacier are the main controls of stress in a glacier. Steeper slopes, or thicker ice increases stress, which in turn increases deformation and glacier velocity. This helps explain why steep glaciers tend to flow faster than small valley glaciers.

While deformation does a pretty good job at explaining how glaciers move in cases where the glacier is frozen to bedrock underneath the glacier, in most glaciers around the world, it is only one of the mechanisms moving the glacier. In some cases, the bottom of the glacier doesn’t connect directly to bedrock; instead it connects to a layer of clay, silt, rocks and sand. The incredible weight of the ice above, combined with water liquefying the debris, allows the glacier to ‘slide over’ and deform the debris, adding a second mechanism of movement.

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The deformation of the ice is more obvious at the close margin (where the drag from the valley walls is greater). The ‘warped’/bent crevasses show that the glacier is flowing faster in the middle than at the sides.

Finally, and perhaps most importantly, glaciers also move by a process known as basal sliding. In most glaciers outside of the Arctic and Antarctic, where temperatures are 0oC at the bottom of the ice (someone was feeling clever when they called it a “warm-based glacier”) water forms at the ice/bedrock interface. Water gets here either by melt-water flowing through a system of cavities and ‘rivers’ within and underneath the glacier, or by a process known as pressure melting, where the huge weight of the glacier allows ice near the bottom to melt at temperatures below 0oC.  If there’s enough water on the glacier bed, it can form a film that allows the ice to slide downslope. In many cases, basal sliding is responsible for most of the glacier movement we observe (up to 10-100 times the movement from internal deformation).

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Here the glacier transitions from ice that has some contact with the bedrock underneath (although there is certainly lots of water enabling sliding down there), to a floating terminus at the very left corner of the glacier. We would expect flow speeds to be higher where the glacier is floating.

Basal sliding is particularly interesting because it can be unpredictable. We understand very well the physics behind internal deformation, meaning that, with knowledge of the glacier slope, which we can observe, and an idea of the ice thickness, we can make a very solid prediction about how much movement we should see. However, when we throw a bunch of water in the base of the glacier, crazy things start to happen. Sometimes the water is stored, and released in one big surge, carrying the glacier with it, and sometimes it is released very regularly. In the case of my glacier of choice, Bridge Glacier, basal sliding is taken to a new level since the terminus is actually floating in a lake, while the upper reaches of the glacier are resting on bedrock. This creates an interesting problem, as the floating part is moving substantially faster than the grounded part. Something has to give, and often does, when large chunks, like the one below, separate from the main glacier, and float away.

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Large crevasse/crack that has penetrated all the way down the glacier, and only a couple of hours away from calving off completely. Picture is from the flight out – time-lapse cameras captured the event later that evening – we missed the event by 6 hours.

Further Reading:

Nice short explanation of glacier flow here at AntarcticGlaciers.org from a scientist working in Antarctica (mostly?)

National Snow & Ice Data Center has a nice explanation of why these fantastic lumps of ice move as well here

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