Changing Earth’s Surface

Erosion is the process by which natural forces move weathered rock and soil from one place to another. Gravity, running water, glaciers, waves, and wind all cause erosion. The material moved by erosion is sediment.

Deposition When the agents of erosion lay down sediment, deposition occurs. Deposition changes the shape of the land. Weathering, erosion, and deposition act together in a cycle that wears down and builds up Earth’s surface. Erosion and deposition are at work everywhere on Earth. Erosion and deposition are never-ending.

The following image was created from DEMs (Digital Elevation Model) for the following 1:24,000 scale topographic quadrangles: Telescope Peak, Hanaupah Canyon, and Badwater, California. To the left is the Panamint Mountain Range. To the right is Death Valley. Elevation spans from 3368 to -83meters and generally decreases from left to right. The blue line represents an elevation of 0 meters. Large alluvial fans extending from a number of mountain valleys to the floor of Death Valley can be seen in the right side of the image. The sediments that make up these depositional features came from the weathering and erosion of bedrock in the mountains located on the left side of the image. (This image was created with MacDEM software).

Gravity Gravity pulls everything toward the center of Earth. Gravity is the force that moves rock and other materials downhill. Gravity causes mass movement, any one of several processes that move sediment downhill.

Example of Gravity Erosion

Types of Mass Movement The different types of mass movement include landslides, mudslides, slump, and creep. Mass movement can be rapid or slow.

Landslides The most destructive type of mass movement is a landslide, which occurs when rock and soil slide quickly down a steep slope. Some landslides contain huge masses of rock, while others may contain only a small amount of rock and soil.

Mudflow A mudflow is the rapid downhill movement of a mixture of water, rock, and soil. The amount of water in a mudflow can be as high as 60 percent. Mudflows often occur after heavy rains in a normally dry area. In clay soils with a high water content, mudflows may occur even on very gentle slopes. An earthquake can trigger both mudflows and landslides.

Slumps In the type of mass movement known as a slump, a mass of rock and soil suddenly slips down in one large mass. It looks as if someone pulled the bottom out from under part of the slope. A slump often occurs when water soaks the base of a mass of soil that is rich in clay.

Creeps is the very slow downhill movement of rock and soil. It occurs most often on gentle slopes. Creep often results from the freezing and thawing of water in cracked layers of rock beneath the soil. Creep is so slow that you can barely notice it, but you can see its effects in objects such as telephone poles, gravestones, and fence posts. Creep may tilt these objects at spooky angles.

Dust Bowl Wind erosion was the cause of soil loss on the Great Plains in the 1930s. By 1930, almost all of the Great Plains had been turned into farms or ranches. Plowing removed the grass from the Great Plains and exposed the soil. In times of drought, the topsoil quickly dried out, turned to dust, and blew away. Wind blew the soil east in great, black clouds. The problem was most serious in the southern Plains states. This area was called the Dust Bowl. The Dust Bowl helped people appreciate the value of soil.

Soil Conservation Soil conservation is the management of soil to prevent its destruction. Conservation plowing disturbs the soil and its plant cover as little as possible. Dead weeds and stalks of the previous year’s crop are left in the ground to help return soil nutrients, retain moisture, and hold soil in place. In crop rotation, every year different crops are planted in the field.

Questions to Answer What processes wear down and build up Earth’s surface? What causes the different types of mass movement?