Weathering and Erosion


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Ch 12Weathering and Erosion : 

Ch 12Weathering and Erosion Mechanical Weathering physically breaks down rocks Chemical Weathering chemically breaks down rocks

Mechanical Weathering : 

Mechanical Weathering Agents of weathering, Ice, running water, wind, plants and animals, gravity, forces within the rock (exfoliation)

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Ice Wedging Water seeps into cracks and freezes, expanding by 10% forcing the rock apart

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Organic Activity Burrowing animals expose rocks, Plant roots work into cracks Abrasion Rocks hitting rocks, either by falling or being washed down stream, wind blowing sand

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Chemical Weathering Oxygen, Carbon Dioxide, water, acids alter chemical structure of rock

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Hydrolysis Process of water altering the structure of rock. May lead to leaching, where water dissolves minerals and carries them to a lower layer of rock

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Carbonation CO2 from air dissolves in water forming Carbonic Acid H2CO3 which speeds of dissolving of calcite. Sink holes, Caves. Oxidation Minerals react with O2. Iron Oxide.

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Acid precipitation Rain water is naturally acidic, pollution increases acidity, speeding up weathering Plant Acids Plants produce acids to change soil, speeds up weathering. Pine trees.

Rates of Weathering : 

Rates of Weathering Rates depend on many factors Composition, Exposure, climate and topography

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Rock Composition Igneous and Metamorphic tend to be resistant Quartz very resistant Limestone resists mechanical, subject to chemical weathering Shales and sandstones break up mechanically

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Amount of exposure The more surface area exposed, the faster the weathering process. Several small blocks weather faster than one large block

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Climate Water and freezing temperatures speed up weathering. Weathering is very slow in hot, dry climates Example Cleopatra’s Needle Topography Elevation and Slope

Weathering and Soil : 

Weathering and Soil Bedrock weathers into regolith, broken up rock, which joins humus, organic matter, to form soil

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Soil Composition Clay Less than .002mm Silt from .002 mm to .05mm Sand .05mm to 2mm

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Clay forms from feldspar, Al. Sandy soils form from Granites and other quartz bearing rocks Silt carried by water Transported soils, soils Carried to another location

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Soil Profile Horizons are layers in soil A Horizon Topsoil B Horizon subsoil, clay leached minerals C Horizon partially weathered bedrock regolith

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Soil and Climate Laterites, tropical with thin A and thick B horizons eroded quickly, dense vegetation replaces soil quickly

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Arctic and desert Soils thin, rocky because only mechanical weathering occurring

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Soil Erosion Unwise use by humans such as overgrazing or plowing can accelerate erosion, Gullying and sheet erosion Soil Conservation Contour plowing, strip cropping, crop rotation and terracing

Erosion : 

Erosion Definition: movement of weathered rock Agents are water, glaciers, wind, gravity

Mass Movement : 

Mass Movement Mass movement is the movement, due to gravity of earth materials Mass Movement, can be rapid or slow

Rapid Mass Movement : 

Rapid Mass Movement Rock fall, rock from cliff Landslide soil and rock, usually occurs in steep terrain Mudflow, mud move by massive rainfall or volcanic action Slump, a large piece of hillside move en masse saturated conditions


SLOW MASS MOVEMENTS Less dramatic, more soil and rock moved Soliflucation, Artic or alpine, Top soil is saturated, subsoil is frozen, top flows

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Creep The most effective form of mass movement, very slow movement of entire hillside. Factors such as slope, soil, vegetation and animals all contribute.

Erosions and landforms : 

Erosions and landforms Landforms result from the interaction between crust movement and erosion

Major and Minor Landforms : 

Major and Minor Landforms Major landforms are Mountains, Plains and Plateaus Minor landforms include hills, valleys and dunes

Erosion of Mountains : 

Erosion of Mountains During the early formation of mountains, (young mountains) they are uplifted faster than they are eroded, tops are sharp and rocky and valleys are deep and narrow.

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As the mountain ages the tops weather into rounded shapes and valleys become U shaped

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In old age mountains are turned into low, almost featureless areas near sea level called peneplains (almost flat). Peneplains are usually rolling hills, knobs of tougher rock (granite) are called monadnocks.

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Plain flat land form not generally high above sea level, there are exceptions

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Plateau is a high flat landform. Young plateaus (Colorado) have deep stream valleys separating the high flat areas. Older plateaus erode into rugged hills and valleys (New York).

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Climate and composition of rock affect how plateaus will erode.

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Dry areas such as the SW U.S. will form Mesas, which are smaller, flat-topped landforms. Mesas erode into buttes, which have a narrow top, locally we have hoodoos, which are even smaller. END

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