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Plate Tectonics What is Plate Tectonics? • The Earth’s crust and upper mantle are broken into sections called plates • Plates move around on top of the mantle like rafts The Crust • Outermost layer • 5 – 100 km thick • Made of Oxygen, Silicon, The Mantle • Layer of Earth between the crust and the core • Contains most of the Earth’s mass • Has more magnesium and less aluminum and silicon than the crust • Is denser than the crust The Core • Below the mantle and to the center of the Earth • Believed to be mostly Iron, smaller amounts of Nickel, almost no Oxygen, Silicon, Aluminum, or Magnesium Earth’s Layers The Earth's rocky outer crust solidified billions of years ago, soon after the Earth formed. This crust is not a solid shell; it is broken up into huge, thick plates that drift atop the soft, underlying mantle. Tectonic plates What is the Lithosphere? • The crust and part of the upper mantle = lithosphere –100 km thick –Less dense than the material below it so it “floats” What is the Asthenoshere? • The plastic layer below the lithosphere = asthenosphere • The plates of the lithosphere float on the asthenosphere 2 Types of Plates • Ocean plates - plates below the oceans • Continental plates - plates below the continents How Plates Move Plates move because there is slow movement of hot, softened mantle that lies below the rigid plates http://pubs.usgs.gov/gip/dynamic/unanswered.html Plate Boundaries Divergent Boundaries • Boundary between two plates that are moving apart or rifting • RIFTING causes SEAFLOOR SPREADING Divergent Boundaries Two tectonic plates separate (move apart) Most happen on the sea floor Sea-floor Spreading! Features of Divergent Boundaries • Mid-ocean ridges • rift valleys • fissure volcanoes Sea floor spreading • Large continents begin to crack and split apart • The gaps fill with water • Small seas become oceans • The mid ocean ridge continues to produce new crust Sea Floor Spreading Sea Floor Spreading • Mid Ocean Ridges – underwater mountain chains that run through the Earth’s Basins • Magma rises to the surface and solidifies and new crust forms • Older Crust is pushed farther away from the ridge Divergent Boundary – Arabian and African Plates Plates move away from each other Convergent Boundaries • Boundaries between two plates that are colliding • There are 3 types… Type 1 • Oceanic plate colliding with a less dense continental plate • Subduction Zone: where the more dense plate slides under the less dense plate • VOLCANOES occur at subduction zones Convergent Boundaries Two tectonic plates collide Continental-Oceanic Collisions Oceanic-Oceanic Collisions Continental-Continental Collisions Convergent Boundaries • Continental-Oceanic Collisions: – Continental plate collides with oceanic plate – The denser oceanic plate sinks (subducts) beneath the less-dense continental crust Convergent Boundaries Continental-Oceanic Collisions: Subduction may cause a chain of volcanoes to form along the plate boundary Convergent Boundaries • Oceanic-Oceanic Collisions: – Oceanic plate collides with oceanic plate – The denser of the 2 oceanic plates sinks (subducts) Convergent Boundary – Indian and Eurasian Plates Andes Mountains, South America Type 2 • Ocean plate colliding with another ocean plate • The more dense plate slides under the less dense plate creating a subduction zone called a TRENCH Aleutian Islands, Alaska Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: –a place where folded and thrust faulted mountains form. Transform Fault Boundaries • Boundary between two plates that are sliding past each other • EARTHQUAKES along faults Transform Boundaries Two tectonic plates slide past one another Building tension Releases tension in a sudden, often violent, jerk called an EARTHQUAKE!! Transform Boundaries • The San Andreas fault is an example of a transform boundary ◦ The Pacific plate (west of fault) is moving northwest ◦ The North American plate (east of fault) is moving southeast Pacific Plate San Andreas Fault, CA A fault is a break in a rock across which there is observable movement. When the break occurred an earthquake was generated, whether or not anyone was there to detect it. Sometimes faults die at depth and do not break the surface. Sometimes they do such as for the Armenia 1988 earthquake. Normal Fault Normal Fault Reverse Fault Reverse Fault Strike-Slip Fault Strike-Slip Fault Strike-Slip Fault Strike Slip Fault Causes of Plate Tectonics Convection Currents • Hot magma in the Earth moves toward the surface, cools, get denser, and then sinks again with the pull of gravity • Creates convection currents in asthenosphere beneath the plates that cause the plates to move. Continents Collide • Eventually when plates move together the continental crust collides • The heat and pressure make metamorphic rock • It also pushes and folds the rocks into high mountains • The Himalayas rise to 8848m and are still growing today So the Earth must be growing? • Dense heavy oceanic crust can be subducted below less denser continental crust. • The friction melts rock • This magma rises through the crust to form new volcanoes • This is happening in South America (The Andes)
