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Earthquakes Converging Boundaries: Three Types • ______Lithosphere Convergence (Subduction)

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Earthquakes Converging Boundaries: Three Types • ______Lithosphere Convergence (Subduction) Plate Tectonics The Major Plates on Earth Plate Tectonics Plate boundaries can be seen as undersea mountain chains (Mid Ocean Ridges—____________ plate boundaries), deep sea trenches (___________ boundaries), or major faults that primarily separate sections of mid-ocean ridges (________________ plate boundaries). Isostatic Equilibrium • The lithosphere is composed of continental and ocean crust and the uppermost solid mantle. • The asthenosphere is composed of the upper mantle and is at or near the melting temperature for upper mantle rock. • Lithosphere “floats” on a partially melted asthenosphere, similar to a raft floating on water. The _____________ is in _________________with the _________________________. Isostatic Equilibrium • When a load, such as an ice sheet, is placed on the lithosphere, it will isostatically depress the lithosphere relative to the asthenosphere. • When the ice melts the lithosphere __________________ __________________. Isostatic Equilibrium Note that the thin (7-10 km), dense (_____ gm/cm3) ocean lithosphere “floats” lower than the continental lithosphere, which is less dense (_____gm/cm3) and thicker (40-70 km). What Drives Plate Tectonics? • Heat escaping from core creates convection currents in the asthenosphere • Where currents rise, plates are pulled apart (____________) • The heat and material rising here causes a “_________________,” which forces the plates away from each other What Drives Plate Tectonics? • Where currents sink, plates are pushed together (_______________), and usually one is pushed down into the asthenosphere (___________________) • The weight of the subducting slab creates a “_________________,” which helps the plate descend further. Types of Plate Boundaries •Divergent Boundaries: plates move away from each other Mid-Ocean Ridges • Convergent Boundaries: plates move towards each other Subduction zones • Transform Boundaries: plates slide past one another (Faults like San Andreas) • Hot Spots: Not usually at plate boundaries: volcanic areas, often in the middle of plates (__________ is an exception) Oreo Tectonics Divergent Plate Boundaries • Rising magma from below pushes plates apart • New oceanic crust is formed • Eruptions are all basaltic (non-explosive) • Undersea mountain chain is created called a ____________________ • Process widens existing oceans (i.e. Atlantic ocean), or creates new ones (Red Sea, Great Rift Valley in Africa) Pillow Basalts • Divergent margins often first form under continental crust because of the insulating properties. • Eventually, the upwelling basaltic magma will completely melt through the overlying continental crust, a new ocean basin will form, and the two segments of continental crust will rift apart. • The ocean floor becomes wider as the new ocean lithosphere is produced at the divergent margin (referred to as sea floor spreading). Divergent Plate Boundaries Divergence splits up continents and makes new oceans Divergent Plate Boundaries • Divergence beneath Africa • The Red Sea is a newly forming ocean • The rift valleys in Africa are splitting it apart •Basalt Flows •The East African rift zone represents a continental rift. The topography of East Africa is highest in Africa because of heating the base of the continent by upwelling basaltic magma. The Red Sea is an incipient ocean basin. Note the dark basalt flows seen along the margins of the Red Sea in the above satellite image. Hydrothermal Vents at Divergent Boundaries Hydrothermal Vents Hydrothermal Deposits Hydrothermal circulation leaches metals from rocks, then concentrates and deposits them in Volcanogenic _____________________ ________________(VMS) Ore minerals and metals found at VMS’s: Ore mineral Metal Sphalerite (ZnS) _____ Chalcopyrite (CuFeS2) _____ Galena (PbS) _____ We know that the Earth is not increasing in size due to the creation of new ocean lithosphere along seafloor spreading zones; thus it stands to reason that the ocean lithosphere must be consumed along convergent boundaries. Convergent Plate Boundaries • Sinking convection currents in asthenosphere brings plates together • Denser, thinner plate is subducted beneath the other plate • Subducted plate releases water-rich fluids, melting portions of the overlying mantle and crust, producing explosive volcanoes on upper plate. • Deep trenches are formed by bending of down-going slab • Very large earthquakes Converging Boundaries: Three types • ____________________ lithosphere convergence (subduction). Examples: Japan and Phillipines • ____________________ lithosphere convergence (subduction). Examples: Andes Mountains and Cascade Mountains. • ____________________ lithosphere collision (no subduction). Examples: Himalaya Mountains and European Alps. Convergent Plate Boundaries Ocean-Ocean Collisions • Older, colder, denser ocean plate gets subducted and melts • Deep trench is formed by bending of down-going plate • Chain of volcanic islands is formed on upper plate = _______________________ • All islands are approximately the same age • Volcanoes are explosive due to sediments (silica-rich) which get subducted with ocean plate • Volcanoes are tall and pointy Ocean-Ocean convergence. Formation of an island arc (Japan). Ocean-Ocean Collisions • Examples include the volcanic islands in the Western Pacific • Island chains are usually ____________________ • Mineral-rich hydrothermal deposits may occur Accretionary wedge: a deformed pile of sediments from the subducting slab. Carribean Island Arc • “__________________” forms when lots of sediment on subducting plate cannot be forced down, so it piles up in front of the upper plate • Accretionary wedges are non-volcanic (mostly folded and faulted sediment) Ocean-Continent Collisions • Ocean crust gets subducted and melts beneath continent • Explosive volcanoes form on continental plate (andesite: viscous and full of “volatiles” – aka gases and water- rich fluids) • Volcanoes are Mt. Shasta approximately the same age • Volcanoes are tall and pointy • Trench is formed off the coast Ocean-Continent Collisions • ______________________________________ • Andes in S. America • Hydrothermal mineral deposits often occur (gold in CA) Columbia Plateau Puget Cascades Olympic or Lowland Coastal Mts. Structure of a subduction zone. Note that the geographic features relevant to the Cascadia subduction zone are noted in red. Mt. Rainier, Washington. Andesitic volcano. Andesite is a volcanic rock formed at subduction zones. Note the two sizes of crystals related to two-staged cooling (_______________) Ocean-Continent Collisions • What about the Olympics and the mountains on Vancouver Island? • Olympics are non- volcanic, and marine fossils are found in its tallest peaks • Sedimentary layers are extremely folded • _________________ (like Barbados) See Slab Maps at http://earthquake.usgs.gov/research/data/slab/ See Interactive Plate Tectonics Maps at http://www.iris.edu/hq/programs/education_and_outreach/animations/interactive Continent-Continent Collisions • Continental collisions are preceded by oceanic- continental convergence. • As the continental lithosphere arrives at the subduction zone, it cannot be subducted because of its low density. • The “collision” causes upwarping and deformation of the ocean floor and ocean lithosphere. Even segments of the upper mantle can be squeezed to the surface along the suture zone (where the two continents are stitched together). Continent-Continent Collisions • Continental crust is too thick and not dense enough to subduct, so very tall mountains are produced • Mountains are non-volcanic • Himalayas are still rising to this day • Older examples: Alps Appalachians Uplift of the Himalayas is an ongoing result of collisional tectonics between the __________________________________Plates. What evidence do you think geologist used to determine the timing of plate motion of the Indian sub-continent relative to the collision with Eurasia? Paleomagnetic signature preserved in the Fe-bearing minerals in igneous rocks of various ages. European Alps have formed from the collision of the African Plate with the Eurasian Plate. The Mediterranean Sea will disappear over time. Formation of Appalachian Mountains resulted from the collision of the _________________ Plates with North American Plate prior to _________ million years ago. Formation of the Ural Mountains related to collision tectonics _____________________Ma ago. Ural Mountains Transform Plate Boundaries Fig.14–4c • Transform faults occur where tectonic plates move _____________________ directions. • Usually this occurs between different segments of mid-ocean ridges • Plates slide past each other • No volcanic activity occurs along transform faults • Earthquakes are common Transform Plate Boundaries Transform boundaries typically form along mid-ocean rift zones, where the ___________________differs and the rigid ocean crust is offset along the transform fault. Transform Plate Boundaries • Most transform faults are located on the ocean floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. • The _________________fault zone in California is a rare transform fault on land. • It connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge to the north. Transform Plate Boundaries The San Andreas fault connects two diverging plate boundaries, and is one of the only transform faults cutting through continental crust (most are under the ocean) Bay Area S. Cal
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