Theory of Plate Tectonics

How do we know the plates exist?  Earthquake and Volcano Zones  Ocean floor features (Trenches and Mid-Oceanic ridges)

Lithospheric Plates  7 major Plates (continental crust, oceanic crust) The Earth is made up of a dozen or so major plates and several minor plates. Tectonic plates are constantly on the move. The fastest plate races along at 15 centimeters (6 inches) per year while the slowest plates crawl at less than 2.5 centimeters (1 inch) per year.

Lithospheric Plate Cross Section

How do we know the plates move?  Evidence Supporting Continental Drift  Movement of the Pacific Plate (Hawaiian island chain formed over a hotspot)

Continental Drift & Pangea

Fossil Evidence

How do the Plates Move?

Sea Floor Spreading mid-ocean ridges rift valley lithospheric plate asthenosphere magma newer less dense crustolder more dense crust

Sea Floor Spreading  A rift valley is volcanically active forming new oceanic crust on each side of the valley creating Mid-Ocean Ridges.  The new crust pushes the older crust away from the mid-ocean ridge, moving the ocean floor & the continents  Ex: Mid-Atlantic Ridge

Divergent Boundary Formation Divergent Boundary Formation

Evidence of Sea Floor Spreading  Rock samples are younger closer to a mid- ocean ridge  Magnetic stripes in oceanic crust match on both sides of a mid-ocean ridge

Transform faults  Gives mid-ocean ridges their shape

San Andreas Fault  Transform faults can become plate boundaries (sliding past each other)

What Destroys Crust? Subduction Zones  Occurs when oceanic crust collides with oceanic or continental crust  The older, more dense, oceanic crust plunges back into the mantle at a deep ocean trench.  Forms deep ocean trenches & volcanic mountains or island arcs.

Oceanic - Continental Subduction Oceanic - Oceanic Subduction

Trenches of the World

When Continents Collide Continental–Continental (No Subduction)

Formation of the Himalaya Mountains

Three Plate Boundaries  Divergent –plates move apart; sea floor spreading  Convergent –Plates collide together; subduction  Transform (strike-slip) –Plates slide past each other; San Andreas fault

Motion at Plate Boundaries Motion at Plate Boundaries

Tectonic Settings and Volcanic Activity Tectonic Settings and Volcanic Activity

Plate Boundaries

What Causes the Plates to Move?  Convection Currents in mantle  Ridge Push  Slab Pull

Convection and Tectonics Convection and Tectonics