Plate Tectonics Earth Science Chapter 9

Continental Drift  scientific theory proposing the slow, steady movement of Earth’s continents  Alfred Wegener: the mind behind the theory, proposed in 1915 –Published “The Origin of Continents and Oceans”

Wegener’s Continental Drift Hypothesis  Supercontinent called “PANGAEA” began breaking apart about 200 million years ago –Continents “drifted” to present positions –Continents “broke” through the ocean crust

Evidence Used by Wegener  Fit of South America and Africa  Matching Fossils  Matching Rock Structures  Ancient Climates

Main Objection to Continental Drift Wegener’s theory did not provide a mechanism for the movement of continents

Plate Tectonics  Theory proposed in the 1960’s that replaced the continental drift theory  Could provide an explanation for the movement of continents

Plate Tectonics  Involved Earth’s Rigid Outer Shell, the Lithosphere –Consists of about 20 slabs (plates)  Plates are moving slowly  Largest Plate is the Pacific plate  Plates are mostly beneath the ocean

Asthenosphere  The partially molten, hot, flowing layer of the mantle  Found beneath the lithosphere  Allows for motion of lithosphere

Lithosphere Asthenosphere

Plate Boundaries  Where tectonic plates meet and interact  Some boundaries converge, diverge or move horizontally  Associated with: –Seismic activity (earthquakes) –Volcanism –Mountain building

Divergent (Spreading) Boundaries  Plate tectonics that MOVE AWAY from each other  Most exist along oceanic ridge crests, in a rift or valley  Process initiates Seafloor Spreading –Forms fractures (openings) on the ridge crests –Fracture fills with magma—creates new seafloor

Divergent (Spreading) Boundary

When Divergent Boundaries are on continents, Rift Valleys form Rift Valley: long narrow depression

Convergent (Together) Boundary  3 types of Convergent Boundary –Oceanic-Continental Boundary –Oceanic-Oceanic Boundary –Continental-Continental Boundary

Oceanic-Continental Boundary  Forms a subduction zone with a deep- ocean trench  Dense ocean plate subducts beneath continent  Results in volcanic mountains  Examples: –Andes South America –Sierra Nevadas California

Oceanic-Oceanic Boundary  1 ocean plate subducts beneath another  Creates deep sea trenches  Often forms volcanoes on the ocean floor  Island arc forms as volcanoes emerge  Examples: –Aleutian Trench & Islands –Mariana Trench and Islands

Continental-Continental Boundary  1 continental plate pulled into subduction zone, but not forced into mantle  Can produce mountains  Examples: –Himalayas (India and Asia) –Other possibilities  Alps  Appalachians

Transform Plate Boundary  2 Plates slide past one another –No new crust is created –No crust is destroyed, but deformed or fractured –Associated with long faults  Transform faults –Most often in the ocean: 1 land example San Andreas Fault in California

Transform Plate Boundaries

Evidence to Support Plate Tectonics  1. Paleomagnetism: study of Earth’s magnetism –Ancient Magnetism preserved in rocks –Paleomagnetic records show  Earth’s Magnetic Field Reversals –Recorded in the Seafloor Rocks –Confirms seafloor spreading

Evidence to Support Plate Tectonics  2. Earthquake Patterns –Associated with Plate Boundaries –Deep-focused Earthquakes along trenches provide a method for tracking the plate’s descent

Evidence to Support Plate Tectonics  3. Ocean Rock Age –Youngest are near ocean ridges –Oldest at a distance from ocean ridges –Ocean basins are geologically young  4. Ocean Sediment –Thicker further from o. ridge

Driving Mechanisms of Plate Tectonics  No one model explains all plate motions  Earth’s heat is the driving force  Several models have been proposed –Convection currents in mantle –Slab-pull and slab-push model

Slab pull-push  Slab pull and slab push –Descending oceanic crust pulls the plate  Convergent boundary –Elevated ridge system pushes the plate  Divergent boundary

Volcanism at convergent boundaries Volcanism at convergent boundaries  Most, 80% volcanoes occur at convergent plate boundaries  Oceanic-oceanic  volcanic islands  Oceanic- Continental  volcanic mountains The island of Bora Bora Mount Shasta in the Sierra Nevada ’ s of Oregon

Volcanism at Convergent boundaries  Plates come together at convergent boundaries  creates a subduction zone  plate descends into the mantle and melts into magma  comes back up through the boundary  forms volcanoes when it reaches the surface.

Volcanism at divergent boundaries Volcanism at divergent boundaries  15% of volcanoes are found at divergent boundaries  Magma is forced upward into the fractures and faults that form as the plate separates  Iceland is an example of this volcanism, its part of the mid- Atlantic ridge

Hot Spots 10. 5% of volcanoes are hot spots. 11. These volcanoes are found far away from any plate boundary. 12. Unusually hot regions of the earths mantle where high temperature plumes of mantle material rise toward the surface

Hot Spots Plume melts rock  melt straight through the mantle and crust  become volcanoes on earth ’ s surface Plume melts rock  melt straight through the mantle and crust  become volcanoes on earth ’ s surface  A Hot Spot!!!  nk/bcpid /bctid nk/bcpid /bctid nk/bcpid /bctid