Geology Reference
In-Depth Information
The rate of glacier flow is slow compared to the flow of water: Large ice
sheets may flow only a few feet or meters per year, while alpine glaciers in warmer
regions may flow a couple hundred meters in a year.
Occasionally, a glacier experiences a
surge
when its flow increases greatly for a short
period of time, such as 100 feet per day for several months. Scientists think a surge oc-
curs when the water underneath the glacier builds up and leads to a sudden increase in
basal sliding.
Eroding at a Snail's Pace: Landforms
Created by Glacial Erosion
In Chapter 12, I explain how water carries rocks and sediments in streams. The power of
a stream to carry earth materials is called its
competence.
Because it is solid, ice has a
very high competence for carrying rocks and sediment. This fact makes glaciers a major
mover of earth materials — able to carry large amounts of sediment and very large
boulders over great distances.
Today, glaciers are limited to a relatively small portion of the earth's surface, but in the
past when glaciers were much more extensive they shaped the landscape by moving
rocks and sediment. In this section, I describe the different ways glacial ice removes
(erodes) earth materials and the features of the landscape created by glacial erosion.
Plucking and abrading along the way
Glaciers remove rocks and sediment in two ways:
Plucking:
Plucking (or
quarrying
) occurs when ice moves across a surface of bed-
rock and lifts large blocks of rock up into the ice flow. The ice plucks the rocks by
catching the edge of a previously created fracture (likely created by water seeping
into cracks and freezing) and applies enough force to break off a chunk of rock.
The rock is then carried in the ice flow until it is dropped or reaches the
terminus
(end) of the glacier and melts out.
Abrading:
When a glacier carries rocks and sediments, some of these materials
scrape against the surface of bedrock that it flows over. This scraping is called
ab-
