plate tectonics. ► tectonics : bending and breaking of the lithosphere ► plate tectonic theory ...

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Lithosphere : 15 plates See pp

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Plate Tectonics Tectonics : bending and breaking of the lithosphere Plate tectonic theory explains volcanism, seismic activity, continental movement, folding and faulting Lithosphere : 15 plates See pp Lithospheric plates rest on soft, plastic asthenosphere. Allows plates to move away from, towards and against one another. Plate boundaries Oceanic and continental crust at boundaries Types of Plate Boundaries: 1. Divergent (Spreading) 2.Convergent3.Transform 1. Divergent (Spreading) Boundaries: plates pull apart a) at oceanic/oceanic crust boundaries (most common) * mid-oceanic ridges (sea-floor spreading) on ocean floor b) at continental/continental crust boundaries * rift valleys on land a) oceanic/oceanic crust boundary P 426 P 429 a. oceanic/oceanic crust boundary sea-floor spreading: as plates beneath oceans spread, magma wells up from mantle and solidifies as new ocean floor resulting ridge of igneous rock: mid-oceanic ridge (axial rift) (See undersea topography pp. 430 431) b) continental/continental crust boundary cause continental rupture rift valleys form narrow sea may form new oceans may form Examples: East African Rift Valley, Iceland rift valley, Red Sea East African Rift Valley Iceland 2. Convergent Boundaries a) at oceanic/continental crust boundary b) at continental/continental crust boundary c) at oceanic/oceanic crust boundary a) oceanic/continental Oceanic crust is thinner and denser; it plunges into the soft asthenosphere beneath continent in a process called subduction. Ocean floor trench forms at subduction zone On land, a chain of volcanic mountains parallels the subduction zone Earthquakes examples: Andes, Cascade Range Andes Pacific Ring of Fire b) continental/continental Plates collide; crustal rocks fold, break, become fused in a suture mountain chains Example: Himalayas c) oceanic/oceanic Subduction of one plate beneath another submarine trench and island arc (chain of volcanic islands) Example: Aleutians Aleutian Trench 3. Transform Boundaries 2 plates move past one another in opposite directions laterally; plates stick as they move; tremendous strain builds up and is released in earthquakes most transform boundaries occur along mid-oceanic ridges, parallel to direction of plate movement Example: San Andreas Fault Hot Spots (Mantle Plumes) Do NOT occur at plate boundaries Individual spots of upwelling molten rock; randomly distributed produce plumes of magma which pierce lithosphere and create a volcano; as plate moves, chain of volcanic islands develops sea mounts (guyots) :volcanoes which do not surface above sea level Examples of hot spots: Hawaiian Islands, Yellowstone, Iceland