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Chapter 5 & 16: Surface Processes (Weathering, Erosion, & Sedimentary Rocks) Add section detail briefings 1.Mechanical— physical disaggregation of the rock or mineral into particles that are then subjected to erosion and transport by wind, water and glacier. 2.Chemical—the disintegration of the rock or mineral into its constituent cations and anions. These ions constitute the dissolved loads of rivers, lakes, soil waters, groundwaters, rainwater, and the salt of the sea. Mechanical weathering—what goes up must come down. Freeze/thaw opens up cracks Chemical weathering Chemical weathering Granite is made up of several minerals that decay at different rates. Feldspar Magnetite Biotite Quartz The dissolution progresses, and the rock weakens and disintegrates. Cracks form along crystal boundaries. 2 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering… 1 cm …than small rocks do, so smaller rocks weather more quickly. Chemical weathering uses CO2 (g) from the atmosphere Reduced weathering rate… Lower temperatures and decreases in atmospheric CO2 reduce weathering. Lowered CO2 leads to climate cooling. Variability in atmospheric carbon dioxide leads to variability in the rate of weathering. Weathering reduces CO2 in atmosphere as CO2 HCO3–. …leads to an increase in atmospheric CO2,… …which leads to climate warming, which increases weathering. Chemical weathering of silicate rocks affects climate and climate affects the weathering of rocks—feedback loop Weathering of silicates removes carbon dioxide from the atmosphere. CO2 release from volcanism CO2 uptake by silicate weathering Carbon dioxide is added to the atmosphere during volcanic eruptions. Carbonic acid forms when CO2 and H2O molecules combine. Carbonic acid ionizes to form hydrogen (H+) and bicarbonate ions. NaAlSi3O8 Feldspar CaAl2Si2O8 KAlSi3O8 Hydrogen ions (H+) react with feldspar. + Kaolinite clay Al2Si2O5(OH)4 …makes shales Pyroxene dissolves, releasing silica, Mg And ferrous iron. Olivine (Fe,Mg)SiO4 Pyroxene (Fe, Mg)SiO3 Soluble Mg2+ to the ocean Silica Ferrous iron Ferrous iron is oxidized, forming ferric iron. Ferric iron Ferric iron precipitates a solid, iron oxide (rust). Iron oxide (hematite) Fe2O3 …which makes deserts red Add section detail briefings The salt of the sea—product of chemical weathering of continental igneous minerals Na and Ca from plagioclase K from K-feldspar (orthoclase) H4SiO4 is excess Si not needed to form clay minerals Mg from mafic minerals (amphiboles, pyroxenes and olivines) The sedimentary stages of the rock cycle Weathering breaks down rocks. Erosion carries away particles. Glacier Sedimentary rocks Metamorphic rocks Plutons Transportation moves particles downhill. Delta Desert Playa lake Deposition occurs when particles settle out or precipitate. Burial occurs as layers of sediment accumulate. Diagenesis lithifies the sediment to make sedimentary rocks. Sedimentary Bedding 1 Click video to begin playing Classification of Sedimentary Rocks Clastic—disaggregated rocks consisting of minerals and rock fragments 2. Chemical—minerals precipitated from seawater and freshwaters. They formed where they are found. 3. Biological/Bioclastic—minerals precipitated from seawater and freshwaters through the action of organisms (shells and skeletons) that may have been subjected to transport (as clasts) and redeposition. 1. Add section detail briefings Click here to type in text Deep sea carbonate deposition since the early Jurassic (microscopic plankton with calcite shells)—forams and coccoliths. Add section detail briefings A rift develops as the ancient continent stretches and thins. Rift valley Faulted valleys Heating of lithosphere Volcanics and nonmarine sediments Continental crust Continental lithosphere Asthenosphere Seafloor spreading begins. Transportation Subsidence Former position of lithosphere Evaporites, deltaic sediments, and carbonates are deposited. Carbonate platform Deposition Burial and diagenesis Former position of lithosphere These deposits are then buried and undergo diagenesis. Thermal sag basin Continental margin Continental crust sags from weight of sediments and cooling of lithosphere Abyssal plain SEDIMENTARY ENVIRONMENTS Geographic location and plate tectonic setting Transport agent and medium Organic processes and organisms that modify sediments Sedimentary environment Sediments deposited Desert Turbidity currents Climate CONTINENTAL ENVIRONMENTS Geographic location and plate tectonic setting 4 Glacier 1 Lake 3 Desert Lake 2 Rivers Transport agent and medium Organic processes and organisms that modify sediments Sedimentary environment Sediments deposited Desert Turbidity currents Climate SHORELINE ENVIRONMENTS Geographic location and plate tectonic setting 4 Glacier 1 Lake 3 Desert Lake 2 Rivers Transport agent and medium Organic processes and organisms that modify sediments Sedimentary environment Sediments deposited 7 Tidal Flat Desert 5 Delta 6 Beach Turbidity currents Climate MARINE ENVIRONMENTS Geographic location and plate tectonic setting 4 Glacier 1 Lake 3 Desert Lake 2 Rivers Transport agent and medium Organic processes and organisms that modify sediments Climate Sedimentary environment Sediments deposited 7 Tidal Flat Desert 5 Delta 9 6 Beach Turbidity currents 8 Deep sea 11 Continental margin/slope 10 Organic reef Click here to type in text Cycle above Floodplain: mud and silt 1m Deep channel: coarse-grained sediments, large-scale crossbedding Photograph of Interpretive drawing alluvial cycle section Cycle below Increasing grain size Shallow channel: fine-grained sand, small-scale cross-bedding One cycle Diagenesis Sediments are buried, compacted, and lithified at shallow depths Diagenesis …or subducted, where they are subjected to higher pressure and heat. Diagenesis Compaction by burial squeezes out water. Diagenesis Cementation Precipitation or addition of new minerals cements sediment particles. Lithification Different sediments result in different sedimentary rocks. Fine Mud Shale Pressure Sand Silt and siltstone, mudstone and shale, clay and claystone Sandstone Pressure Gravel Conglomerate Pressure Course Organic Matter Pressure Heat to 90° - 120° C Oil and gas Heat to 90° - 120° C Coal Sandstones that reveal information about transport processes and depositional setting Arkose: feldspar-rich 1 mm Alluvial fan Lithic sandstone: rock fragment-rich 1 mm Delta Quartz arenite: pure quartz Beach Graywacke: matrix-rich 1 mm Deep-sea fan 1 mm Atolls Carbonate platforms are built by reefbuilding organisms that precipitate calcium carbonate as calcite aragonite. Coral reef Light Lagoon Open ocean Within the lagoon, growth of these organisms is rapid, and sediment forms quickly,... …whereas outside the reef, sedimentation is much slower. Light If the sea level rises, the reef continues to grow toward the light at sea level… …and lagoon sedimentation outpaces sedimentation in the open ocean. Eventually, a carbonate platform Grows, with steep sides. Inorganic carbonate also precipitates Out of the supersaturated lagoon Water and adds to the platform sedimentation. Light Carbonate platform Add section detail briefings A short geological history of the Mediterranean Sea During the Miocene epoch (5-20 million years ago), the Mediterranean Sea became a shallow evaporite basin. Freshwater inflow (small) Evaporation France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals Evaporite sediments Salt water entered through a narrow channel. Freshwater inflow (small) Evaporation France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals Evaporite sediments Salt water entered through a narrow channel. Evaporation removed more water… Freshwater inflow (small) Evaporation France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals Evaporite sediments Salt water entered through a narrow channel. Evaporation removed more water… Freshwater inflow (small) Evaporation …than was replaced by freshwater inflow. France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals Evaporite sediments Salt water entered through a narrow channel. Evaporation removed more water… Freshwater inflow (small) Evaporation …than was replaced by freshwater inflow. France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals As the basin became more saline, gypsum and halite precipitated… Evaporite sediments Salt water entered through a narrow channel. Evaporation removed more water… Freshwater inflow (small) Evaporation …than was replaced by freshwater inflow. France Italy Spain MEDITERRANEAN SEA Salt water from open ocean Gypsum and halite crystals As the basin became more saline, gypsum and halite precipitated… Evaporite sediments …forming evaporite sediments.