Additional Geology Review Study Guide for McGraw-Hill's ASVAB (page 2)
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.
Plate Tectonics and Continental Drift
For many years, scientists discussed the theory of continental drift—the idea that the continents, as we know them now, were situated in different locations in the past and even were once combined into one large continent called Pangaea. This theory was based on fossil evidence, climatic evidence, the shapes of the continents and the way they seem to "fit together" like pieces of a gigantic jigsaw puzzle, and the similarity of geologic deposits found on different continents. But it was unclear how continental drift could take place.
Later, scientists developed the theory of plate tectonics, which provides the "how" for this worldwide geologic process. The theory is based on the assumption that Earth's crust is divided into several plates that move around and change over time. The places where these plates bump into each other or meet are locations of very intense volcanic and earthquake activity and mountain building. The force behind this movement is sea-floor spreading, a process in which molten rock beneath the seabed moves upward and forms underwater ridges, then spreads laterally away from the ridges and along the ocean floor. Sea-floor spreading creates new oceanic crust that slowly moves away from the mid-ocean ridges.
Further research indicated that sea-floor spreading is caused by very large convection currents within Earth's mantle. These currents cause the magma to rise up in the ocean floor, to spread outward, and to become subducted (sink back down), creating deep oceanic trenches. This process causes the plates on Earth's crust to move about Earth's surface. Where an oceanic plate converges with (bumps against) a continental plate, volcanic activity is prevalent and may create mountain ranges or volcanic islands like those in the Caribbean Sea. When two continental plates converge, mountains are formed on the land. (The Himalayas in Asia are one example.) The areas of convergence undergo tremendous stress, which is often relieved by earthquakes along fault lines.
- Coats and Car Seats: A Lethal Combination?
- Kindergarten Sight Words List
- Child Development Theories
- Signs Your Child Might Have Asperger's Syndrome
- 10 Fun Activities for Children with Autism
- Social Cognitive Theory
- Why is Play Important? Social and Emotional Development, Physical Development, Creative Development
- GED Math Practice Test 1
- The Homework Debate
- Problems With Standardized Testing