Plate Tectonics Plate Tectonics Tectonics bending and breaking

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