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What is weathering? Rocks on the Earth’s surface undergoing changes in appearance and composition What are the 2 types of Weathering? 1. Mechanicalphysically changing rock into smaller pieces without changing its composition 2. Chemicallybreakdown of rock by changing its chemical composition Types of Mechanical Weathering 1. Exfoliation (unloading)- process where sheets of rock peel or flake away Example of exfoliation: Types of Mechanical Weathering weathering 2. Ice wedge- occurs when water seeps into rocks and freezes Ice wedging Types of Mechanical Weathering weathering 3. Biological activity- the activity of organisms, including plants, burrowing animals, and humans that weather away rock Weathering by plants Weathering by sandmartins nesting in cliff Types of Mechanical Weathering weathering 4. Abrasion- collision of rock with one another resulting in breaking and wearing away. • Abrasion takes place in many environments: – fast-moving streams – beaches subject to storm waves – desert environments with high winds – beneath glaciers that are loaded with fragments of rock. Water-carried particles eating away at the rock Storm waves have eaten away at the shoreline The ocean waves have eroded this beach leaving behind a cliff as the soil was washed away Observe the effects of mechanical weathering. Types of Chemical Weathering 1. Hydrolysis: Change in composition of minerals when they react with water Feldspar combines with H2O to form a common clay called kaolin Types of Chemical Weathering 2. Carbonation – When some minerals come in contact with carbonic acid, they form a new product Stalactites caused by carbonation Limestone is eaten away by acid and deposits as it drips from ceiling of cavern Types of Chemical Weathering 3. Oxidation – When metallic elements combine with oxygen Oxidation = rust! Types of Chemical Weathering 4. Acid Precipitation “acid rain” Types of Chemical Weathering 5. Plant acids – plants secrete acids that erode away the rock Lichens and mosses grow on rocks and secrete weak acids that dissolve the surface What 4 things affect the rate of weathering? 1. Rock composition • Igneous and metamorphic rocks don’t weather easily • Sedimentary rocks do 2. Amount of exposure • More exposure it receives, faster it will weather • Amount of time and amount of surface area exposed is also important 3. Climate • Slow in very hot / very cold climates • Fairly rapid in warm, humid climates Cleopatra’s needle after only one century in New York City 4. Topography • Elevation or slope of surface where rock is located affects rate of weathering Regolith • A layer of weathered rock fragments • covers much of the Earth’s surface Regolith Soil • part of the regolith that supports plant growth • Complex mixture of minerals, water, gases, and remains of plants and animals What is Soil Composition? • has 4 major components: –mineral matter –organic matter (humas) –water –air Composition of Soil: What is Humus? • Dark, organic material, remains of animals and plants Humus: • What are the Soil Textures? Texture refers to the proportions of different particle sizes. - Sand (large size) - Silt - Clay (small size) • Loam (a mixture of all three sizes) is best suited for plant life. CLAY SILT SAND Soil Texture What are the most important factors in soil formation? (1) parent material •source of the mineral matter in soil Residual soil—parent material is the bedrock below the soil • Transported soil—has been carried from parent material and deposited elsewhere • (2) Time • • Important in all geologic processes The longer a soil has been forming, the thicker it becomes. (3) Climate • Greatest effect on soil formation (4) Organisms • Organisms influence the soil's physical and chemical properties. • Furnish organic matter to soil (5) Slope •Steep slopes often have poorly developed soils. • Direction the slope is facing influences soil formation (affects soil temperature and moisture) Thinner on slope because water erodes soil & deposits it down the slope Tends to be thicker here (deposited) What is a Soil profile? • A cross-section in which the layers of the soil and bedrock can be seen • Each layer is called a horizon. • In fully developed residual soil, there are three horizons A horizon (topsoil) • mixture of organic matter and small rock particles B horizon (subsoil) •contains minerals and clay C horizon (regolith) •partially weathered bedrock Bedrock • Solid, unweathered rock that lies beneath regolith Bedrock What role does Climate play in soil formation? • Climate is most important factor influencing soil formation What is a Temperate climate? • Temperatures range from cool to warm • Rain fall is not excessive • Two types of soil are found dependent on amount of rainfall: What are the soil types of temperate climates? 1. Pedalfer – soil formed if an area receives 65 cm or more of rain per year (clay, quartz, iron) -found in eastern United States 2. Pedocal – soil formed if rainfall is less then 65 cm a year (calcium carbonate) -found in western United States Pedalfer Pedocal What is the soil formation in tropic areas? • forms laterites (thick and infertile soil) *rain always is washing away the A horizon but constant rotting vegetation covers the B Horizon Laterite What is soil formation of a desert? • Forms soils from mechanical weathering • Soil is thin and mostly of regolith What is erosion? • erosion is the process by which the products of weathering are transported • agents of erosion are gravity, wind, glaciers, water, ocean waves, currents streams, groundwater What is Soil Erosion? 1. occurs world wide and is normally a slow process 2. ordinarily, new residual soil forms about as fast as the existing soil erodes 3. However, unwise use of the land and unusual climatic conditions can upset this natural balance How is erosion accelerated? 1. unwise farming and ranching methods increase soil erosion • one example is clearing of trees,small plants, and animal overgrazing • another example is furrows plowed in land When soil is washed away by rainfall, furrows become larger forming gullies (gravity and water at work) •Sheet erosion is another type of soil erosion that strips away parallel layers of topsoil •may occur when continuous rainfall evenly washes away the topsoil •can also be caused by wind during unusually dry periods Severe soil erosion from wind in the 1930's was a result of drought coupled with unsound farming practices. The photo on the right though, was taken at 10:00 AM, March 13, 1991 in downtown Topeka, Kansas, looking into the sun. There was so much soil in the air it darkened the sky. So the problem is not just something of the past. constant erosion reduces the fertility of the soil by removing the A horizon, which is rich in humus (unable to grow crops) What is soil conservation? • Methods of protecting the soil by reducing soil erosion and loss of soil nutrients. 1. contour plowing: soil is plowed in circular bands that follow the shape of the land (prevents water from flowing directly down slope) 2. strip cropping: crops are planted in alternate bands •Contour cropping of corn, alfalfa and wheat. Contour farming (planting crops along the slope of the land) and contour strip cropping (alternating different crops in the same field) reduces soil erosion. Traps soil on the field. 3. terracing: construction of step-like ridges that follow the contours of a sloped field During heavy rain storms terraces catch water and lead it slowly off the field. Slowing down the speed of flowing water reduces soil erosion. 4. crop rotation: farmers plant one type of crop one year and a different type of crop the next year <> With the development site cleared of all vegetation, area is now prone to soil losses in excess of 70 tons per acre. Gravity and Erosion • Gravity, through its downward pull, causes rock fragments to move down inclines • mass movement is the movement of fragments down a slope (rapid or slow) rockfall: the fall of rock from a steep cliff •is the most rapid •can be from tiny fragments to large boulders Figure 21 Rockslide The scar on the side of this mountain in northwestern Wyoming was made by an enormous rockslide that happened more than 75years ago.The debris in the slide formed a dam 70 m high across the Gros Ventre River. landslide: sudden movement of masses of loose rock and soil down the slope of a hill Slumgullion landslide Hinsdale County, Colorado 700 years old and still moving mudflow: the rapid movement of a large mass of mud slump: downhill movement of a large block of soil under the influence of gravity A slump is a type of slope failure which involves the rotational movement of soil or rock creep: extremely slow downhill movement of weathered rock material erosion of mountains: weathering and erosion wear down rugged peaks to rounded peaks and gentle slopes Rocky Mountains Appalachian Mountains Examine a landscape formed by erosion.