lec12_Weathering_and_Soils
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Weathering and Soils The Rotting Earth • • • • Physical weathering processes Chemical weathering processes Factors affecting weathering Soil: The residue of weathering • Why isn’t the Earth covered with impact craters like the Moon? • Both planetary bodies have experienced the same frequency of impact events over geologic time The Moon Earth has evolved a hydrosphere and the water cycle Continually induces degradation of its surface Hydrosphere + Plate tectonics erases evidence Acid rain disfigured limestone statue Meteor Crater, AZ 50,000 years ago Importance of Weathering: • produces soil • produces some important mineral deposits (Al, Fe) • Essential part of the rock cycle • Weathering of silicate minerals removes CO2 from atmosphere and influences global climate Bauxite (Al) deposit Soil profile, Cape Cod, Ma Essential Part of Rock Cycle weathering Small rock fragments Na+, Cl-, ions Transport Wind, water Ice, gravity Deposition of sediment Burial & Lithification Sedimentary Rock Weathering The mechanical breakdown and chemical alteration of rock and sediment when exposed to the atmosphere Weathering The mechanical breakdown and chemical alteration of rock and sediment when exposed to the atmosphere There are two (interrelated) main types of weathering Physical weathering Chemical weathering Physical Weathering Breakdown of rock – by physical processes – involves no change in chemical composition – rock is simply broken down into small fragments Physical Weathering Breakdown of rock – by physical processes – involves no change in chemical composition – rock is simply broken down into small fragments Both Physical and Chemical Weathering are facilitated by JOINTS, FRACTURES, and CRACKS in rocks Joints- fractures in rocks along which no movement has taken place Rocks break at weak spots when they are twisted, squeezed, or stretched by tectonic forces. Such forces form joints. Joints- fractures in rocks along which no movement has taken place Rocks adjust to removal of overlying rock by expanding upward cracks Removal of the weight of overlying rocks releases stress on the buried rock causes joints to open, allowing water, air, and microscopic life to enter. Columnar Joints Basalt Sheet Joints in massive granite, Yosemite National Park Physical Weathering • Frost Wedging Water in crack Freezing water expands in cracks and wedges the rock apart Ice expands by 9% increases pressure on walls of the crack Frost Wedging- produced Granite blocks from bedrock (Mt. Whitney) Physical Weathering Crystal Growth • Water moving slowly through fractured rocks contains ions, which may precipitate out of solution to form salts. • force exerted by salt crystals growing can be very large break-up of rocks. Weathering in Antarctica by Crystal Growth Effective in desert regions, both hot and cold Physical Weathering Effect of Heat Heat Spalling Root Wedging Physical Weathering: Small effects acting over long geologic time Chemical Weathering Pressure Magma Igneous rock Temperature Chemical Weathering Pressure Magma Igneous rock Low T Low P High H2O High O2 High P High T Surface Condition Temperature Chemical Weathering Pressure Magma Igneous rock Low T Low P High H2O High O2 Surface Condition High P High T Formation of new minerals stable at Earth’s Surface Temperature Chemical Weathering Pressure Igneous Rocks + Acids Sedimentary Rocks + Salty Oceans Low T Low P High H2O High O2 Magma Igneous rock High P High T Surface Condition Temperature Chemical Weathering Main Agent water solutions that behave as weak acids Main Chemical Processes Hydrolysis, oxidation, dissolution, leaching Main Products New minerals, ions in solution Hydrolysis of Feldspar Carbon Dioxide + Rain Becomes Acid (H2CO3) Dissolves Feldspar (H+) Leaves ----Clay Carries away---Ions Silica, K+ Other Common Chemical Weathering Reactions Oxidation of Fe silicatesFe-oxides Monument Valley Other Common Chemical Weathering Reactions • Dissolution All of mineral is completely dissolved Limestone (CaCO3)+ Carbonic Acid Ca ions +bicarbonate Other Common Chemical Weathering Reactions • Oxidation Fe-oxides • Dissolution All of mineral is completely dissolved Limestone (CaCO3)+ Carbonic Acid Ca ions +bicarbonate • Leaching Ions are removed by dissolution in water K-feldspar+waterK ions Exfoliation and Spheroidal Weathering • concentric shells of rock may break from the outside of an outcrop or a boulder, a process known as exfoliation. • Exfoliation is caused by differential stresses within a rock that result mainly from chemical weathering. Spheroidal weathering produces, by such progressive decomposition, rounded boulders. The effects of surface area The effects of surface area The effects of surface area • r 1 cm3 of rock (surface area= 6cm2) clay mineral, 40 million cm2 The effects of surface area • r Physical/Chemical weathering increases surface area More physical/chemical weathering Factors that Influence Weathering • Rock type (composition) High quartz content resists chemical weathering Think of beach sand Granite, high quartz Marble, calcite Tombstones, same location, different rock types Factors that Influence Weathering • Texture/Structure – Massive or jointed rock – Differences in weathering rates Differential Weathering Mudstone layers weather fast Factors that Influence Weathering • Role of Time Marble tombstones Same location, 1970, 1870 Factors that Influence Weathering • Climate Moisture and heat promote chemical reactions Warm/moist climates chemical weathering is more intense Cold/dry climates chemical weathering is slow The effects of climate • A Tale of Two Egyptian Obelisks Granite, 1500 BC One stayed in Egypt One went to NYC in 1880 Wetter climate Importance of climate Climate and Weathering Products of Weathering Soils are one of the most important natural resources. • Soils support plants • Basis for the terrestrial food chain • Soils store organic carbon Rates of Soil Formation • Typical (to make 1 m thick soil layer) Midwest ~10,000 years Tropics ~100’s -1000 years Human perspective non renewable resource Soil Erosion • Natural process ACCELERATED by human activity • Soil erosion rates ~ few years • Global topsoil depletion ~ 7% per decade Leading Causes of Soil Erosion Deforestation Overgrazing Farming practices urbanization Final Thought Global food production 25 billion tons of topsoil is lost per year 90 million new people per year Soil erosion from deforestation
