I What Is Plate Tectonics a Lithosphere floating on asthenosphere Plate Tectonics i Density differences between oceanic and continental crust reflected in their comparative buoyancy 1 Reflects Archimedes Principle of Buoyancy II Components of Plate Tectonic System a 12 major plates at least 8 microplates i Oceanic crust usually mixed with continental crust ii Boundaries can follow continental margins but some do not b Active vs passive margins c Plate boundaries i Delineated by earthquake belts d Plate interiors are relatively earthquake free III Types of Plate Boundaries a Divergent i 2 plates move apart ii As plates spread apart new lithosphere from underlying magma source fills the space created iii Occurs over narrow band of mid ocean ridge b Convergent i One plate slips below another ii One plate bends and slips into the asthenosphere producing a subduction zone iii Only oceanic crust can be subducted iv Wadati Benioff Zone where earthquakes occur in a subduction zone v Sinking continues to core mantle boundary and plates melt vi Typical features of convergent boundaries 1 Trench 2 Accretionary prism 3 Volcanic arc a Stratovolcanoes c Transform i Plates slip laterally past each other ii Fracture zones along mid ocean ridge result in linked segments 1 Movement on either side of a fracture zone close to the mid ocean ridge can be opposing iii Most transform faults 1 Relatively short located in oceanic crust 2 Exception San Andreas Fault 3 Earthquakes d Special Features i Triple junctions three plates intersect ii Hot spots 1 Produce shield volcanoes usually in the middle of plates 2 Sourced by mantle plumes 3 Fixed can be used to trace plate motion iii Seamount 1 Submerged hot spot island iv Guyot 1 Flat topped seamount IV Life Cycle of Plate Boundaries a Continental Rifting i Lithosphere stretched horizontally 1 Faults develop 2 Causes crustal thinning 3 Hot asthenosphere arises 4 Melting occurs molten rock erupts ii New mid ocean ridge may develop 1 Sea floor spreading begins i Continental lithosphere collides with continental lithosphere or an island arc b Continental Collision V What Drives Plate Tectonics a Ridge push b Slab pull i Due to increased elevation of mid ocean ridges i Due to greater density of subducting oceanic lithosphere