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The Rock Cycle, Isostasy, and the Dynamics of the Lithosphere Siccar Point, Scotland Rock - aggregates of minerals Rock cycle - movement and transformations of rock in the lithosphere What is the ultimate origin of rock in the lithosphere? Molten magma from the Earth’s mantle. Basic geology on Mars would be easy to learn - almost all of the rocks Mars has volcanism. are basalt. Mars does not have: • Plate Tectonics • Oceans • Water-rich atmosphere The Rock Cycle Cooled from molten rock Igneous rock Magma / Lava Igneous Rocks (from the L. ingnis – fire) • rocks that have crystallized (solidified) directly from a molten state. • Molten rock - magma / lava • Composed of randomly interlocked crystals of silicate minerals. Plate Tectonic Setting Dry mantle (basalt, gabbro) Physical Geology - Rock Cycle and Isostasy Wet mantle (andesite) Continental crust (rhyolite, granite) The Rock Cycle Gravel Sand Mud Dissolved ions Cooled from molten rock Igneous rock uplift and weathering Magma / Lava Weathering / Erosion • the physical and chemical breakdown of rock. • occurs at the Earth’s surface. Physical Geology - Rock Cycle and Isostasy Weathering does several very important things: • Breaks down rock to make sediments. • Forms soils. • Forms economic mineral deposits. • Removes CO2 from the Earth’s atmosphere. Physical Geology - Weathering Mechanical Weathering physical disintegration of rock • jointing • sheeting • wedging Physical Geology - Weathering Vertical jointing - tectonic stress + gravity Failed geology 1C Passed geology 1C Physical Geology - Weathering Lysefiord, Norway Sheeting of Granite bedrock Enchanted Rock, Texas Physical Geology - Rock Cycle and Isostasy Ice and Frost Wedging • caused by freeze - thaw cycles. • water expands when it freezes. • ice has 9% more volume than liquid water. Physical Geology - Weathering Chemical Weathering The chemical reaction of minerals with air and water. • hydrolysis (acid attack) • oxidation • dissolution (leaching) Physical Geology - Weathering CO2 + H2O = H+ + HCO3- Chemical Weathering H+ + feldspar = Ca++ clay + silica to the ocean HCO3-, Ca+, silica Minerals prone to chemical weathering • feldspars • pyroxenes • amphiboles • micas Common in Igneous Rocks Stable mineral products of chemical weathering • quartz • clay minerals (e.g. kaolinite) • iron oxides (e.g. hematite) • carbonate (calcite) Common in Sedimentary Rocks Physical Geology - Weathering Sedimentary Rocks (from the L. sedimentum– to settle) • Composed of layers of mineral grains. • Grains produced by the weathering of other rock. • Layers form at the Earth’s surface. • Layers preserve fossils and other information about past environments. Alluvial fan depositional environment, Death Valley, California River System depositional environment, New Zealand Fossils are organic remains buried in sediments and preserved in sedimentary rock. These Cape Buffalo carcasses are on their way to becomming fossils. Ankylosaur skeleton being excavated from sandstone, Mongolia. Ankylosaur skeleton being excavated from sandstone, Mongolia. Fossil Fuels are found in Sedimentary Rock Coal - carbon derived from plant matter Plants Peat Lignite Tar, oil, natural gas hydrocarbon from marine algae Coal Fossil Fuels oil coal The Rock Cycle Uplift and Erosion burial and subsidence Redeposition Deposited in layers on the surface. Sedimentary Rock Cooled from molten rock uplift and erosion Igneous rock Magma / Lava Metamorphic Rocks (from the G. meta– to change, and morphos– form, shape) • Rocks altered by heat and pressure. • Changes texture and mineral composition. • Usually happens when rock is buried beneath mountain ranges during plate collisions. Sedimentary Rock 3000 ATM 200° C Low Grade Metamorphic Rock High Grade Metamorphic Rock Migmatite 600° C Magma Physical Geology - Rock Cycle and Isostasy Progressive Metamorphism Phyllite Slate Schist Low Grade High Grade The Rock Cycle Uplift and Erosion burial and subsidence Redeposition Deposited in layers on the surface. Changed by heat and pressure. Sedimentary Rock Uplift and Erosion Cooled from molten rock uplift and erosion Igneous rock Magma / Lava Metamorphic Rock Burial and subsidence melting Uplift - the movement of the crust upward. Subsidence - the movement of the crust downward. The crust can move because it is floating on the asthenosphere. What causes the lithosphere to float higher or lower on the underlying asthenosphere? changes in density of the crust. changes in thickness of the crust. What changes the density of rock? • Heating - rock expands, becomes less dense. • Cooling - rock contracts, becomes more dense. • Changes in density cause the lithosphere to float higher or lower on the asthenosphere. What changes the thickness of the crust? • Plate collisions • Shortening and thickening of the crust in the collision zone. • Thrusting of one plate on top of another. • Erosion • Thinning of the crust exposed above sea level. Isostasy - equilibrium position of lithosphere “floating” on asthenosphere. continental lithosphere oceanic lithosphere oceanic lithosphere Asthenosphere Physical Geology - Rock Cycle and Isostasy Isostacy continental lithosphere oceanic lithosphere Rock at earth’s surface. Asthenosphere oceanic lithosphere Isostacy Crust is “loaded” with rock, increasing its thickness. Surface rock is buried. Loaded crust subsides. Loaded crust subsides. Loaded crust subsides. Loaded crust subsides. Loaded crust subsides. New isostatic equilibrium is achieved. Surface rock is now deep in the Earth’s interior. Surface topography erodes. Crust rebounds to new isostatic equilibrium. Erosion Rebound Erosion Rebound Erosion Rebound Erosion Rebound Erosion Rebound Erosion Rebound Erosion Rebound Erosion Once deeply buried rock has been uplifted back to the surface Rebound The Rock Cycle Uplift and Erosion burial and subsidence Redeposition Deposited in layers on the surface. Changed by heat and pressure. Sedimentary Rock Uplift and Erosion Cooled from molten rock uplift and erosion Igneous rock Magma / Lava Metamorphic Rock Burial and subsidence melting
