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Plate Tectonics Evolution of Continents, Ocean Basins, Mountains Continental Margins Plate Margins - Where Stuff Happens….. Divergent Convergent Transform Relative Movements of Plates a) Divergent, (b) Convergent-continent/ocean basin, © transform, (d) convergent-continent/continent Plate Boundaries • Seismic activity • Volcanic activity • Mountain building Ocean-Ocean Divergent Margin • Mid Ocean Ridges - long, sinuous ridge system, central rift valley • New ocean crust • Active volcanism • Shallow earthquakes < 100 km • Rounded “Pillow” basalts • Ridge offset by transform faults Ocean-Ocean Divergent Margin Lava fountains (10 m high) spouting from eruptive fissures during the October 1980 eruption of Krafla Volcano. • MOR cools by: • Conduction • Convection - cold sea water circulates • Hydro-thermal springs - >350°C • Circulating waters rich in H2S, SiO2, Mn, CO2, H2, CH4, K, Ca, etc. • Precipitate - form black / white smokers • Adaptations to Vents • Hydrogen-sulfide- (H2S) oxidizing bacteria - base of food chain Pillow Basalts • Hot magma cools quickly in cold ocean water ContinentContinent Divergent Margin • ACTIVE - Red Sea began ~10 Ma • INACTIVE - East African Rift Zone; parts up to 20 Ma • Example of stable triple junction - 3 breaks. One arm fails http://earthview.sdsu.edu/trees/tecqest.htm Continent-Continent Divergent Margin • Rio Grande Trough (USA) and African Rift Valley • - < 100 km deep • Volcanism - basaltic and rhyolitic Oldoinyo Lengai, active volcano in the East African Rift Zone Seafloor Spreading at Mid-Ocean Ridges • Sediment accumulation increases away from ridge - “pelagic rain” • Depth of crust increases away from ridge • Age of crust increases away from ridge Sea Floor Age (less than 200 Ma) How Do Divergent Margins Develop? DOMING • High heat flow, thermal expansion of crust - few km high / 1000’s km across. • High, dry climate RIFTING • Normal faults accommodate stretch • NO ocean crust • Possible formation of triple junction • High, dry climate: non-marine sandstones, conglomerates in basin • East African Rift Zone DRIFTING • Crust thins and initial intrusion of basalts as new seafloor • Shallow ocean; high evaporation • Later “normal” marine sediment • Drainage reverses in to the sea. • Gulf of California (5 Ma) • Red Sea (10 Ma) Convergent Boundaries • Plate Boundaries where plates are moving together ….. or converging • • • • High seismicity to depths of 700 km Volcanism and/or metamorphism Areas of continental accretion / growth Subduction of crust / mountain building • Types: – Ocean to ocean and ocean to continent – Continent to continent Types of Convergence Ocean-Ocean, Ocean-Continent, and Continent-Continent Seismic Activity Along Descending Plate Ocean - Continent Collision • Oceanic crust more dense than continental crust • • • • • Continental margin / Volcanic Arc Deep trench on ocean side; Fore arc attached to continental crust Volcanoes - mountain range parallel to / 300-400 km from trench High volcanic activity- Andesitic High seismic activity; earthquake foci 0 to 700 km deep • W margin of North / South America – Andes and Cascades Components: • Oceanic plate • Trench • Fore-arc ridge (melange) • Fore-arc basin • Continental plate with volcanic mountain range (e.g. Andes) The Ring of Fire- Volcanics associated with Pacific plate boundaries Mountain Building-e.g. the Andes Ocean to Ocean Collision • Two plates of same thickness (? Age/temperature) Which one wins? • Deep trench on “ocean” side • Arcuate line of islands ~100 Km land ward from trench - Island Arc • Zone is long and narrow (1000’s Km x 200-300 Km) • High volcanic activity- Andesitic • High seismic activity; earthquake foci 0 to 700 km deep • • • • Aleutian Trench Marianas Trench (10,860 m deep) Tonga Trench (10,800 m deep) Lesser Antilles Components: • • • • Oceanic plate Trench Fore-arc ridge (melange) Fore-arc basin, Island arc Back-arc basin (down-going plate melts / accom. of spreading) Skinner and Porter, 2000 Continent to Continent Collision • • • • • Two pieces of continental crust of equal - light density Neither subducts; mountain building, continental accretion Australia/India and Eurasia - Himalayas Africa and Europe - Alps (early Mesozoic) Africa and North America – Appalachians (Paleozoic) Building Continental Material Precambrian Shields