Glacial Erosion

Moving glacier ice continuously grinds away at the high mountains of Alaska and Yukon to create the rugged beauty characteristic of the Alaska Range and the St. Elias and Selwyn Mountains. Glaciologists recognize two processes of erosion--abrasion and plucking.

The glacier ice itself cannot abrade rock. However, given a load of rock debris, the ice layer at the base of a glacier becomes as sandpaper. Much like a slow but steadily moving belt sander, the sliding base of the glacier can grind away at the valley floor on which the glacier rests.

Measurements of abrasion to a marble block fixed underneath a moving glacier in Iceland showed that 30 feet (10 meters) of ice motion cut away about one-eighth inch (3 mm) of the block. At that rate, the sliding of a debris-laden glacier would probably cut away a yard (1 meter) of rock in about 200 years.

A requirement for continuing abrasion is a steady supply of new abrasive material by downward transport of rock debris to the basal layer of the glacier. Also, the glacier ice must be thick enough to weigh strongly upon the surface being eroded.

The existence of water at the base of the glacier also seems extremely important. If the water is there under too high a pressure, it may lift up against the ice and prevent erosion. A key role of the water is to carry away ground-up rock flour particles. Unless that happens the base of the glacier becomes like a clogged up file and is unable to continue the erosion.

Plucking out of broken-up rocks from the glacier floor by moving ice that freezes to their surfaces obviously is a more irregular process and, for that reason, one hard to evaluate the speed of. The question of its effectiveness remains as one of the mysteries of the mountains.