Lecture 7 Triple Junctions Hot spots Places where 3 plate boundaries coincide Triple junctions migrate and change across time Volcanic plumes independent of tectonic plates Mafic magmas derived from the lower mantle Tattoo overriding plates with volcanoes Volcanoes perforate overriding plates Make volcanoes that drift off plume Volcano goes extinct and erodes Subsidence creates a seamount guyot Supports seafloor spreading Seamounts islands age away from originating hot spot Age change marks direction of overriding plate motion Tectonic boundaries evolve Oceanic plates Created at MOR spreading centers Destroyed at subduction zones Continental plates Torn apart at rifts Joined during collision Continental Rifting Continental lithosphere can break apart Lithosphere stretches and thins Brittle upper crust faults Ductile lower part flows Asthenosphere melts Melts erupt leading to Sea floor spreading Example East Africa Rift Plate Collision Subduction consumes ocean basins Ocean closure ends in continental collision Buoyant continental crust will not subduct Subduction ends mountains are smashed upward Plate tectonic collision may involve Two continents A continent and an island arc Collision sutures the convergent plate boundary Driving Mechanisms Modern Old plates are dragged atop a convecting mantle convection Ridge push elevated MOR pushes adjacent lithosphere Slab pull gravity pulls a subducting plate downward Convection in the asthenosphere adds or subtracts Plate Velocities Absolute plate velocities may be mapped by Plotting plate motion relative to a fixed spot in the mantle Measuring volcano ages distance along a hot spot track Plate vectors are determined by GPS measurements Knowledge of plate motion is now accurate and precise The Dynamic Planet Plate Tectonics the key to understanding geology Mantle is transferred to the surface and back down again The interior and surface of Earth are in constant motion Explains earthquakes volcanoes and continental drift Earth s surface changes continuously Earth looked different in the past Earth will look different in the future
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