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The lithosphere (earth’s crust and rigid upper mantle) is broken into about 12 major plates and several small ones Oceanic: under oceans Thin 3-4 miles thick Basalt Continental: “land” 20-30 miles thick granite The lithospheric plates interact with one another at plate boundaries (where two plates meet). Plates can: Converge or come together 2. Diverge or move apart 3. Slide past one another horizontally 1. Oceanic-continental When a oceanic plate collides with a continental plate the thinner more dense oceanic plate subducts; it is overridden by the thicker less dense continental plate. Features: volcanic mountains along the continental plate and a trench where the oceanic plate subducts. Example: Andes Mts. http://geology.com/nsta/convergent-boundary- oceanic-continental.gif Ocean-Ocean When two oceanic plates meet one of them subducts. Features: a trench, Volcanic island arc Example: Aleutian Trench and Islands, Mariana Trench and Islands http://geology.com/nsta/convergent-boundary- oceanic-oceanic.gif Continental-Continental When two continental plates collide both are less dense than the mantle so neither of them subducts. The colliding continental masses are folded and uplifted to form mountains. Features: tall, folded mountains Example: Himalayas Continental When continental lithospheric crust begins to separate a long, narrow depression called a rift valley forms. The rising magma puts pressure on the crust producing fractures Features: rift valley Example: East African Rift Valley Oceanic Two oceanic plates are separating and a rift (a fault- bounded valley) forms. New crust is created Features: A ridge of underwater mountains Example: Mid-ocean Ridge When two plates are not moving toward each other or apart but are sliding past one another horizontally. Features: a fault Example: San Andreas Fault Convection is the tranfer of energy through movement of a fluid. The mantle is solid but part of it the asthenosphere can flow – it is like a soft, pliable plastic Convection current is set up by the transfer of energy from the hot, lower mantle and the cooler, upper mantle. Ridge push – during formation of a ridge asthenosphere rises; the weight of the uplifted ridge pushes an oceanic plate toward the trench at the subduction zone. Slab pull – a sinking region of mantle convection current may suck an oceanic plate downward; weight of the subducting plate helps pull the trailing lithosphere into the subduction zone. Other Tectonic Mechanisms Hot Spots – volcanoes that occur due to unusually hot regions within the mantle Form along plate boundaries. Form at hot spots Volcano Formation Rising magma gathers in an area of weak rock called a magma chamber The magma breaks through a fissure or crack as it is forced to the surface. The gas-rich magma moves up a circular tube called a vent to the surface. Eruptions of lava and pyroclastic material build a volcanic mountain Volcanic Material Lava: molten rock – the composition of the magma determines the type of eruption. High viscosity lava erupts more violently than low viscosity lava. Gases: SO2, H2S, CO2, HCl, HF – can produce volcanic smog, acid rain and diminished daylight Pyroclastic material: ejected particles – from the size of fine dust to large boulders Cinders – lapilli 2-64mm Blocks - >64 mm Bombs – semimolten blocks Types of Volcanoes Shield: accumulation of fluid basaltic lava; low cone Cinder: accumulation of cinders Composite or stratovolcano: acculmulation of alternating layers of ash and lava Dangers Earthquakes Mudslides; lahar Pyroclastic flows Gases Pyroclastic falls Tephra falls back to earth due to gravity Pyroclastic flows Hot gas and rock flows downslope at high speed http://www.youtube.com/watch?v=yvG_N7eqMWk Lahar or mudflows Mud is created by tephra and water and flows down slope; can also contain debris http://www.youtube.com/watch?v=kznwnpNTB6k Lava flows Lava flows down slope Aa – rough textured lava pahoehoe – smoother flowing block-lava flows Aa pahoe pahoe ravioli Landslides Debris travels down slope at high speed