How does fallen snow turn into a river of ice that flows?

A glacier forms wherever more snow falls than melts away, year after year, for many years. The snow never gets a chance to disappear, so it piles up, and the pile slowly turns into ice that flows downhill under its own weight. That is why people call glaciers rivers of ice: they really do move, only very slowly.

The change from snow to ice is a slow squeeze. Under the weight of everything that falls on top of them, the crystals are squeezed together and bond where they touch, pushing the air out or trapping it in bubbles. Over time the snow becomes a grainy stuff called firn, which looks like wet sugar. Over a period of years the firn is crushed further until it becomes solid glacier ice.

Then comes the surprise. Ice feels brittle, yet a thick enough pile of it behaves like very stiff putty. Ice needs to be at least 30 metres thick to start flowing, and once it is about 50 metres thick, the layers inside slip past one another, and the whole mass deforms and creeps across the landscape. Some glaciers also slide on a thin film of meltwater at their base.

The top of the glacier doesn't bend this way. The upper 50 metres or so are rigid and ride along on the slowly flowing ice below, so when the glacier stretches over a bump in the valley floor the surface simply cracks. Those cracks are crevasses, the hazard every glacier traveller dreads.

For a long time nobody agreed how a solid could flow. In the 1840s James Forbes came up with the essentially correct explanation, but it took decades more for the idea to be fully accepted.