Frost Wedging

By Michael Dallin

Frost wedging is a form of mechanical weathering (that is, weathering that involves physical, rather than chemical change). Frost wedging is caused by the repeated freeze-thaw cycle of water in extreme climates.

Most rocks have small cracks in them, called joints (or, tectonic joints). When it rains, rainwater seeps into these joints. As the day cools and temperatures at night drop below freezing, the water inside the joints freezes.

As water freezes into ice, it expands. You can test this at home -- simply fill a bottle halfway with water, and draw a line on the bottle where the water level is. Put the bottle in a freezer (but don't put a cap on it!). Come back a few hours later, after the water is frozen. As the water froze, it expanded beyond the mark you placed on the bottle.

The expanding ice places pressure on the joints in the rock. Finally, when the pressure is too much, the joint expands. In some cases, the rock will split, though this usually happens after repeated freeze and thaws. As new water is added during the warmer days, more ice is created at night, wedging the joints apart further.

The pictures in this article are from the summit ridge of Ypsilon Mountain, perhaps one of the best spots in the park to see the effects of frost wedging. You can clearly see in these pictures how the rocks were split apart into plates -- a sure sign of frost wedging at work.

In heavily glaciated areas, rock ridges usually see an ample amount of frost wedging. Aretes (sharp ridges between glaciated valleys) are usually made sharp by frost wedging.

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