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Plate Tectonics 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 438-439 ► 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.Convergent 3.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