How to measure the speed of a glacier

Slow movements of glaciers are always fascinating when filmed in a time-lapse sequence. This video, for example, captures a series of 436 frames taken at Columbia Glacier near Valdez, Alaska, between May and September of 2007. The time-lapse was taken as part of the ongoing Extreme Ice Survey, an ambitious project to capture global warming-induced glacial retreat in the act.

Some months ago, PBS channel showed a documentary called “Descent into the Ice”, in which a group of researches measured the speed of a glacier at the Alps. Once inside the glacier, 350 feet thick, they put a bicycle wheel mounted on a cantilever. As the glacier slides down the mountain, it spins the wheel. It moves two feet per day, or about an inch an hour. By speeding up his video record, we can see five month's worth of ice flow: