The Way the Earth Works Plate Tectonics Plate Tectonics The paradigm of how the Earth works Earth s outer shell is broken into rigid plates that move A case study of scientific revolution It allows scientists to predict geologic events and Reconstruct the geologic past Tectonic theory evolved in the 1960s Previous research provided a strong foundation Wegener 1915 evidence supporting continental drift Hess Dietz 1968 the sea floor spreading hypothesis By 1968 evidence for tectonics was overwhelming Sea floor spreading continental drift plate tectonics Plate tectonic theory provides a unified mechanism explaining Igneous sedimentary and metamorphic rocks The distribution of earthquakes and volcanoes The origin of continents and ocean basins The distributions of fossil plants and animals The genesis and destruction of mountain chains Continental drift Lithosphere Tectonic plates are fragments of lithosphere Lithosphere is made of BOTH crust and the upper mantle The lithosphere is in motion over the asthenosphere Buoyancy Lithosphere bends elastically when loaded Asthenosphere flows plastically when loaded First described by Archimedes more than 2 2 Ka Floating solids displace water equal to their mass An iceberg sinks until the mass of water it displaces is equal to the total mass of the iceberg This concept applies to lithospheric plates 2 types of lithosphere Continental 150km thick Granitic crust 35 40 km thick Lighter less dense More buoyant floats higher Oceanic 7 to 100 km thick Basaltic crust 7 10 km thick Heavier more dense Less buoyant sinks lower Plate boundaries Lithosphere is fragmented into 20 tectonic plates Plates move continuously at a rate of 1 to 15 cm yr Plates interact along their boundaries Global earthquakes Outline lithospheric plate Specific types of earthquakes occur at different types of plate boundaries Plate boundaries Location on Earth where tectonic plates meet Identified by concentrations of earthquakes Associated with many other dynamic phenomena Plate interiors are almost earthquake free Continental margins Where land meets the ocean Margins near plate boundaries are active Margins far from plate boundaries are passive Passive margin continental crust thins seaward Transitions into oceanic crust Traps eroded sediment Develops into the continental shelf Plate boundaries 3 types Divergent normal faults e g mid ocean ridge Transform strike slips faults e g San Andreas fault Convergent reverse or thrust faults e g the Andes Divergent Tectonic plates move apart also called spreading boundary Convergent tectonic plates move together also called convergent margin subduction zone consuming boundary trench Transform tectonic plates slide sideways also called transform fault Divergent boundaries Sea floor spreading causes plates to move apart Magma wells up to fill the gap Magma cools adding material to each plate Example the Red Sea Mid Ocean Ridges Linear mountain ranges in Earth s ocean basin Example the Mid Atlantic Ridge Sea floor spreading opens the axial rift valley Rising asthenosphere melts forming mafic magma Pooled magma solidifies into oceanic crustal rock Pillow basalt magma quenched at the sea floor Dikes preserved magma conduits Gabbro deeper magma Ocean Crustal Age Oceanic lithosphere New crust is closer to the ridge older crust farther away Oldest oceanic crust is found at the far edge of the basin The hot asthenosphere is at the base of the MOR Aging ocean crust moves away from this heat Cooling increasing in density and sinking Accumulating increasing thicknesses of sediment Convergent boundaries Lithospheric plates move toward one another One plate dives back into the mantle Subduction Subduction recycles oceanic lithosphere Subduction is balanced by sea floor spreading Earth maintains a constant circumference Sediments scraped off subducting plates are smeared and welded onto the overriding plates These contorted sediments can be pushed above sea Volcanic arc volcanic belt on an overriding plate Subduction Old oceanic lithosphere is more dense than mantle Bent down the leading edge sinks like an anchor Fate of subducted plates Subduction Features The lower the mantle may be a plate graveyard Subduction is associated with unique features Accretionary prisms Volcanic arcs Back arc basins Accretionary prisms deformed sediment wedges The descending plate partially melts at 150km depth Magmas burn upward fueling volcanic eruptions A curved earth volcanic belts are curved Arc Type Continental crust continental arc Oceanic island arc Back arc basin a marginal sea behind an arc Forms between an island arc and a continent Offshore subduction traps a piece of oceanic curst or Stretching creates a new spreading ridge Types of convergent boundaries Ocean Ocean convergence Ocean continent convergence Continent Continent convergence Transforms boundaries Lithospheric slides past not created or destroyed Characterized by Earthquakes Absence of volcanism Oceanic transforms The MOR axis is offset by transform faults Transforms bear strong evidence of sea floor spreading Abundant earthquakes common between ridge segments Earthquakes vanish past ridge segment overlaps Lithospheric plates slide past each other Many transform faults offset spreading ridge segments Some transforms cut through continental crust Continental transforms chop continental crust Ex The San Andreas Fault
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