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Geology LibreTexts

9.3: Mass Movement - Weathering by Gravity and Water

In chapter 5 you read about sink holes, one form of weathering by water and gravity. Sink holes form what is known as Karst Topography, which has the effect of making the landscape appear pockmarked. Karst topography results from the chemical weathering of limestone underground, and the action of gravity forcing the overlying earth to fill in the void. You might want to look for examples of topographic maps of karst topography in the central United States (one example of water-gravity weathering) or other places in the world.

Another, more common example of weathering by water and gravity is mass movement or landslides. Mass movement occurs when soil is saturated with water, which makes it heavy, and the force of gravity overcomes the resistance of the slope.

Finally, the alluvial fan, is an example of weathering and movement of material by water and gravity. Alluvial fans occur in desert landscapes which see very little water year-round. Alluvial fans form at the mouths of mountain canyons that lead to valleys. When rains come to the usually dry mountains, they form brief, torrential rivers which flow out of the canyons and into the valleys carrying with them rocks and debris. The larger objects are deposited close to the canyon mouth, but the torrent of water flowing out of the canyon usually has enough momentum to carry small particles several miles out from the canyon mouth. The materials are deposited in a 'fan' formation, leading to the name. Try to find a picture of an alluvial fan on the internet or your library.

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