105_06-weathering-soil-sed_rx_part_1 - Weathering, Soil,...

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Unformatted text preview: Weathering, Soil, and Sedimentary Rocks Weathering and Erosion Chapter 6 Weathering and Erosion WEATHERING: The mechanical fragmentation (disintegration) and chemical alteration (decomposition) of rock - involves little or no movement or removal of weathered material. Weathering and Erosion EROSION: The picking up or physical removal of weathered rock particles TRANSPORTATION: the movement of eroded particles by an agent such as wind, wind, water or ice. Weathering processes continue continue during transportation. How are Earth Materials Altered? Why Weathering?? • Obtain equilibrium with a new environment • Rocks were formed under high pT conditions • Turn into something stable under our conditions: clay minerals (dirt) Differential Differential weathering and erosion – Structural and chemical differences in rock can produce spectacular formations 1 Delicate Arch – Arches NP, UT - Summer 2000 Delicate Arch Arches NP, UT Summer 2000 Hole in the Wall Arch Arches NP, UT Spire Arches NP, UT Landscape Arch - Arches NP, UT - Summer 2000 2 Medicine Hat – Near Medicine Hat, UT - Summer 2000 Medicine Hat – Near Medicine Hat, UT - Summer 2000 Medicine Hat – Near Medicine Hat, UT - Summer 2000 Hoodoos Bryce Canyon NP, UT Weathering and Erosion Physical Physical (mechanical) weathering (chewing) – Disintegration of rocks and minerals by physical processes Chemical Chemical weathering (melting) – Chemical alteration or decomposition of rocks and minerals. Results from exposure to water and atmospheric gases. Hoodoos Bryce Canyon NP, UT Summer 1999 Both Both work together! 3 Mechanical Weathering Physical Physical forces break rocks into smaller pieces that retain the chemical composition of the parent material Increases Increases surface area Mechanical Weathering Increases Increases surface area Mechanical Weathering Increases Increases surface area Mechanical Weathering Frost Frost wedging – Freezing water expands 9%, cracks rocks Frost wedging 4 Mechanical Weathering Exfoliation Exfoliation (Pressure release) – Occurs on exposed surfaces of plutons. – Plutons form as magma cools within the crust, but can be exposed at the surface by uplift and erosion. – Removal of overlying rock releases pressure within the pluton, and the rock expands and cracks parallel to outer surface. Mechanical Weathering Cracks are called sheet joints sheet Peeling off of these layers is called exfoliation exfoliation Exfoliation Exfoliation 5 Enchanted Rock Texas Stone Mountain, Georgia Mechanical Weathering Abrasion Abrasion – Wearing of rocks in rivers, waterfalls, beaches Mechanical Weathering Thermal Thermal expansion and contraction When exposed to the same heat, quartz expands about 3 times as much as feldspar! Mechanical Weathering Animals Animals – Crush – Digest rocks Mechanical Weathering Plants Plants – Crush – Dissolve rocks (chemical) 6 Chemical weathering • Changes in the chemical composition of minerals and rocks that are unstable at the Earth’s surface – Solution (acids) – Oxidation – Hydrolysis (feldspars) August 2005 - Yosemite NP, CA Chemical Weathering Solution: Solution: Limestone weathered fresh Chemical weathering of limestone tombstone 7 Chemical Weathering H2O + CO2 H2CO3 H+ + HCO-3 Chemical Weathering Oxidation Oxidation – Rust forms when oxygen reacts with minerals which contain iron 4Fe+3 + 3O2 → 2Fe2O3 Fe2O3 = hematite! CaCO3 + H+ + HCO3- Ca2+ + 2HCO3- H2CO3 = carbonic acid HCO-3 = bicarbonate ion CaCO3 = calcite 8 ...
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