How does a glacier move?

How it works

Above the snow line, more snow falls each year than melts. New snow presses down on old snow, squeezing it tighter until it slowly turns into hard, dense ice. When the ice becomes very thick, it behaves like a giant lump of very stiff dough and starts to flow downhill under its own weight.

Glaciers move very slowly. Many creep just a few dozen metres a year, too slowly to see, though some steep, snowy glaciers move much faster. Their surface can crack into deep splits called crevasses, which are very dangerous, so people should only go onto a glacier with a professional guide.

Rocks frozen into the bottom of a glacier turn it into giant sandpaper that scrapes the valley as the ice moves. Rivers cut narrow V-shaped valleys, but glaciers grind out valleys with flat floors and steep sides, shaped like a U. When the glacier melts away, the U-shaped valley is left behind.

Where to see it in New Zealand

On the South Island's West Coast, Franz Josef Glacier / Kā Roimata o Hine Hukatere and Fox Glacier flow down from the Southern Alps into valleys edged with rainforest. Milford Sound / Piopiotahi in Fiordland is a U-shaped valley that was carved by ice and later flooded by the sea.

Try it together

Put some sand and small pebbles in a plastic cup, add water and freeze it. Pop out the ice block, hold it with a glove and push it firmly across playdough or damp sand. What scratches and grooves does it leave? That is how glaciers scrape valleys.