FEATURE ARTICLE
The Eurasian Arctic During the Last Ice Age
A vast ice sheet once covered the Barents Sea. Its sudden disappearance 100 centuries ago provides a lesson about western Antarctica today
Anders Elverhøi, Martin Siegert, Julian Dowdeswell, John-Inge Svendsen
Glaciology 101
Ice is, of course, a solid, but it deforms very slowly when a large stress is applied—such as the stress induced in an ice sheet by its own great weight. This deformation causes a parcel of ice within a glacier to move slowly over time. Also, a piece of ice on the surface of an ice sheet is buried by subsequent snowfalls, which cause it to move downward into the ice sheet at a significant velocity with respect to the deformation. Overall the motion tends to be down in the middle and out to the sides.
More specifically, the flow at the center of an ice sheet radiates from the ice divide, the place where there is no lateral movement on the surface. As the ice moves away from the ice divide, its lateral velocity increases from an initial value of perhaps a few meters per year. Nearer their margins, ice sheets are effectively "drained" by fast-flowing rivers of ice, known as ice streams. The velocity of an ice stream is typically several hundred meters per year. These streams flow quickly because water at the base reduces friction, allowing the ice to slide across the underlying ground, with internal deformation making only a small contribution to the total velocity.
Broadly speaking, the ice continues to flow faster and faster until it reaches its demise in one of two ways. The ice sheet may terminate on land (stopping because the ice at the surface melts as fast as it is supplied) or it can terminate at sea. Where an ice sheet flows into the ocean intact and becomes afloat, it forms an ice shelf. Such an ice shelf loses mass by "calving" icebergs from its edge and by melting at the bottom.
As they flow over land, ice sheets erode and entrain sediments at their bases. They can transport this rocky material great distances before ultimately depositing it at their margins. This is why in front of any glacier you will see moraines, piles of sediment resembling building rubble. After glaciers and ice sheets melt away, moraines are left behind, providing a geological marker of the extent of the ice in the distant past. The problem of reconstructing the boundaries of an ancient ice sheet would thus appear to be simple—just map the location of the terminal moraines. In reality, the situation is more complicated because in some areas terminal moraines are either absent or are now hidden below sea level. Often several moraines from different glacial advances are jumbled together, making the relevant one difficult to distinguish.
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