Additional Geology Review Study Guide for McGraw-Hill's ASVAB

Weathering and Erosion

Weathering and erosion are two processes by which nature works to flatten Earth's surface. Erosion involves the removal and transport of Earth's materials by natural agents, such as glaciers, wind, water, earthquakes, volcanoes, tornadoes, hurricanes, mud flows, and avalanches. Weathering is the breakup of rocks that are exposed to the atmosphere. Weathering includes both mechanical and chemical processes.

Causes   Weathering and erosion take place as a result of various causes. The major ones are given here.

Freeze/Thaw Cycle Water collects in cracks in rocks. When the water freezes and expands, the crack grows larger, and eventually the rock breaks up into smaller pieces.

Wet/Dry Cycle This process occurs mostly in rocks that contain clay (i.e., shale and mud stone) when they are repeatedly wetted and dried. Clay expands when it is wet and contracts when it is dry. Eventually this process breaks up the rock into smaller parts.

Plants   Plants can grow in the cracks in rocks. As their roots expand, the rocks can eventually break up. Mosses and lichens often grow on rocks and help to break the rock into smaller pieces through chemical means.

Sheet Jointing   When rock layers are worn away, the pressure on the rocks below them is relieved. Those underlying rocks move up and expand. Joints or breaks in the rock occur. After some period of time, sheets of rock are loosened and fall away in a process called exfoliation. This typically occurs with domes of granite.

Wind   Wind can cause larger rocks to break up into smaller particles. This happens in places where the winds are particularly strong and persistent. If winds are strong enough to carry particles like sand, larger rocks can be scoured.

Water   Erosion takes place in water, such as streams where rocks are carried downstream, bumping into each other and making smaller particles. Strong rivers can carry very large boulders, which crash and smash, creating smaller pieces. Waves crashing on beaches cause erosion of the shoreline.

Chemical Processes   When water is mixed with substances that form an acid, it can erode rock. Limestone is particularly vulnerable to this process, as seen in the formation of caves and caverns.

Glaciers   Glaciers occur when the temperatures are so cold for so long that snow does not have a chance to completely melt. The snow accumulates and forms sheets of compressed snow or ice called glaciers. There are two categories of glaciers: continental glaciers and mountain or alpine glaciers.

Continental glaciers were prevalent during the Ice Ages, when massive sheets of ice covered much of the land. They spread from the poles down across the continents. At one time a large part of what is now the United States was covered by ice.

Mountain glaciers form high in the mountains and spread downward as a result of gravity.

Each type of glacier causes massive erosion through the scraping, plucking, scouring, and pushing of land material. Each produces distinctive land forms.

Continental glaciers act as sort of a conveyer belt, carrying rock and debris forward. When the glacier stops for a while and melts at its front, debris is dropped, forming a moraine. Continental glaciers have dammed up rivers to form lakes and have actually changed the direction of a river's flow. Alpine glaciers have distinct features as well. They scour mountains to form cirques (bowl-shaped depressions), block alpine rivers to form hanging valleys, and create the sharp peaks and characteristics shapes of arêtes and horns. Horns, like the Matter-horn in Switzerland, are pyramidal peaks that form when several cirques carve a mountain on three or more sides.