Chapter 6: Erosion & Deposition

6.1 Changing the Earth’s Surface Erosion: the process by which weathered rock and soil particles are moved from one place to another. Deposition: the process by which sediments are laid down in new locations. The 5(five) agents of erosion are: gravity, wind, running water, glaciers, and waves.

6.2 Gravity Gravity is an agent of erosion that pulls rocks and soil down slopes. The downhill movement of sediments caused by gravity is mass wasting. Mass wasting can happen quickly or slowly. Examples of rapid mass wasting: landslide, mudflow, slump. Examples of slow mass wasting: earthflow & creep.

6-3 Wind Wind is the most active agent of erosion in deserts, plowed fields, and on beaches. Wind erodes the Earth’s surface in 2 ways: 1.) deflation:wind erosion when loose materials such as clay, silt, and sand are removed from the land. 2.) abrasion: the wearing away of a substance by solid particles carried by wind.

6-3 Factors for erosion by wind: size of the particles, speed of the wind, length of time the wind blows, the resistance of the rocks exposed to the wind. Sand dunes: mounds of sand deposited by the wind. They vary in size and shape. Loess: accumulations of fine particles of sand and silt deposited by the wind.

6-4 Running Water Running water is the major cause of erosion. Forms of running water include: rivers, streams, and runoff. Runoff is water that flows over the Earth’s surface, usually after a rainfall or spring thaw. Runoff flows into streams and rivers.

6-4 Runoff & Erosion 3 things can happen when rain falls on the Earth’s surface: 1.) the rain can evaporate, 2.) it can sink into the ground, 3.) it can flow over the land surface as runoff. Runoff can pick up and carry particles of clay, sand, and gravel. As the water and sediments move downhill, they cut into the soil and form tiny grooves called rills. As erosion continues, the rills become wider and deeper; eventually gullies form. Gullies act as channels for runoff.

6-4 Several factors affect the amount of runoff: 1.) the amount of rainfall in an area, 2.) the amount of plant growth in an area(soil absorbs some water), 3.) the shape of the land(steep slopes have greatest amount of runoff),

Streams & Erosion Streams are important agents of erosion because they carry large amounts of sediments. The soil particles and rock materials carried by a stream are called the stream’s load. Load is the amount of sediment carried by a stream. Sediments in a stream’s load are transported in different ways: they can be pushed or rolled downstream, if they are lighter they can be picked up and carried along by the force of running water, or they can dissolve in stream water.

Development of a river system Rills form gullies, gullies form streams and streams form rivers. Rivers are important agents of erosion because they affect a large area. The network of rills, gullies, streams & rivers in an area is a drainage system. The small streams flow into larger streams called tributaries. A tributary is a large stream or small river that flows into an area’s main river.

The area drained by a main river and its channels is a drainage basin. The land that separates one drainage basin from another is a divide. One of the largest divides is the Continental Divide in Colorado. The Continental Divide is a continuous line that runs north and south the length of North America. West of the divide, all water flows into the Pacific Ocean, east of it the water flows into the Atlantic Ocean.

Life Cycle of a River Rivers can be immature or mature. An immature river is a young river that is in an early stage of development. A mature river is a river that has been developing for many thousands of years.

Deposits by rivers Streams and rivers can carry large amounts of sediments. These sediments can be deposited in different locations. These deposits constantly change surrounding land areas. Sediments are usually deposited on a river bank where it bends or curves because the speed of a river decreases at a bend. Rivers tend to erode material on the outside of the curve and deposit it on the inside.

Oxbow lakes are formed when the deposited sediments block up the ends of the meander. An oxbow lake is a small lake and is separated from a river. Alluvial fans are formed when all the sediments the river is carrying are dropped. The sediments spread out from the river channel in a fanlike shape. Deltas: large amounts of sediments deposited at the mouth of a large river that flows into a lake or ocean.

On both sides of a mature river or stream flat areas called flood plains are formed. Flood plains have fertile soil. For example, the flood plains on either side of the Mississippi River are very fertile areas due to flooding here. Sediments deposited on a flood plain are made of fine particles. The larger particles settle first and are deposited along the sides of the river. These larger particles build up to form ridgelike deposits called levees.

6-5 Glaciers A glacier is a large mass of moving ice and snow. It is formed where there are many large snowfalls and the temperatures remain very cold. A glacier is one of the most powerful agents of erosion. Glacial ice erodes by abrasion and by plucking away at the rock below it. Rocky Mts. were once covered by glaciers, and the Great Lakes were formed by glaciers.

6-5 Deposits by Glaciers