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Theory of Plate Tectonics Plate Tectonics The theory that explains how large pieces of the Earth’s outermost layer move and change shape Explains how the lithosphere moves around on top of the asthenosphere How do we know the plates exist? Earthquake and Volcano Zones Ocean floor features (Trenches and Mid-Oceanic ridges) Plate Boundary Features Lithospheric Plates 7 Major PlatesPacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic 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 Drift Supporting Continental Movement of the Pacific Plate (Hawaiian island chain formed over a hotspot) Fossil Evidence Pangaea Breakup How do the Plates Move? Sea Floor Spreading rift valley mid-ocean ridges newer less dense crust older more dense crust lithospheric plate magma asthenosphere Sea Floor Spreading Occurs at a rift valley that is volcanically active, forming new oceanic crust on each side of the valley. This creates a MidOcean Ridge. Mid-Ocean Ridge- A system of underwater mountains that have a deep crack (rift valley) running through the center The new crust pushes the older crust away from the mid-ocean ridge, moving the ocean floor & the continents Divergent Boundary Formation Evidence of Sea Floor Spreading Rock samples are younger closer to a midocean ridge Magnetic stripes in oceanic crust match on both sides of a mid-ocean ridge Seafloor Spreading and Magnetization 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 Mantle Melting and Plate Tectonics Trenches of the World When Continents Collide Continental–Continental (No Subduction) Formation of the Himalaya Mountains Convergent Margins: India-Asia Collision II 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 Oceanic-Continental Collision Oceanic sinks because it is more dense Creates mountains on the continents and volcanoes Oceanic-Oceanic Collision The older oceanic plate will sink because it is more dense Creates volcanoes and island arcs Continental-Continental Collision No subduction occurs because both plates have the same density Creates mountain ranges Tectonic Settings and Volcanic Activity Terrane Formation What Causes the Plates to Move? Convection Currents in mantle Ridge Push Slab Pull Lava Lamp Convection Convection and Tectonics
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