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PLATE TECTONICS. Theory of Lithospheric Plate Motion. Continental Drift Theory. Proposed by Alfred Wegener, early 1900s, Germany Suggested that Earth’s continents have split and rejoined many times during Earth’s history.
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PLATE TECTONICS Theory of Lithospheric Plate Motion (c) Vicki Drake, 2011
Continental Drift Theory • Proposed by Alfred Wegener, early 1900s, Germany • Suggested that Earth’s continents have split and rejoined many times during Earth’s history. • Last joining was approximately 200 million years ago with the formation of “Pangaea” (all land) with Europe, North America, Africa and South America joined together. (c) Vicki Drake, 2011
PANGAEA (c) Vicki Drake, 2011
Evidence for Plate Tectonics • “Fit” of continents, especially along edges of continental shelves • Similar fossils found on continents separated by oceans • Dinosaurs of similar species found widely separated continents • Indicates that continents were joined • Mammals differentiated on separate continents • Indicates that continents were separated • “Belts” of mountain ranges with similar geology and ages • Mountains in North and South America connect to mountains in Africa • Age of ocean floor and magnetic reversals • Moving from mid-ocean ridges on the sea floor, ocean sediments grow older (youngest closest to the ridge) • Oldest age of any ocean floor dates only to approximately 180 million years ago • Moving from mid-ocean ridges on sea floor, the magnetism of specific rocks making up the deep ocean floor, indicate reversals of normal magnetism (i.e, north and south magnetic poles ‘flip’ positions) • Glacial ‘scour’ as large glaciers moved across continents, ‘scratching’ the surface. • Pattern discernible when continents pushed back to Pangaea formation
Magnetic Reversals and Age of Ocean Crust Mid-Ocean Ridge Edges of Continents (c) Vicki Drake, 2011
What is the Theory of Plate Tectonics? • The Earth’s crust and upper mantle are bonded together into the Lithosphere • Lithosphere composed of Earth’s crust and upper rigid Mantle bonded together • Lithosphere is broken into many moving slabs/sections (‘plates’). • Plates are sliding on softer, hotter layer of mantle just below Lithosphere: Asthenosphere • Theory of Plate Tectonics explains the motion and interaction of lithospheric plates (c) Vicki Drake, 2011
Map of tectonic plates and relative motions (c) Vicki Drake, 2011
What’s Causing the Plates to Move? • Many hypotheses have been presented to explain the motion of the lithosphere/tectonic plates • Wegener thought some kind of centrifugal forces broke up Pangaea and drove the continents apart • Scientists today postulate that strong convection currents within the mantle are moving the lithosphere around • Shallow Cell Convection: only the upper mantle (the asthenosphere) is fluid enough to move the plates • Deep Cell Convection: The entire mantle (from the outer core boundary to the asthenosphere) is hot enough to produce strong convection cells • Two-tiered Convection: The asthenosphere has its own convection cells, driven in part by larger convection cells in the lower mantle (c) Vicki Drake, 2011
Deep Convection cell hypothesis to explain movement of lithospheric plates (c) Vicki Drake, 2011
Three Basic Types of Plate Boundaries • Convergent Boundaries: • two lithospheric/tectonic plates are on a collision course • Divergent Boundaries: • two lithospheric/tectonic plates are splitting into two or more pieces and are moving away from each other in opposite directions • Transform Boundaries: • two lithospheric/tectonic plates are sliding past each other, horizontally, in opposite directions (neither colliding nor splitting apart) (c) Vicki Drake, 2011
TYPES OF PLATE BOUNDARIES (c) Vicki Drake, 2011
CONVERGENT BOUNDARIES • When two lithospheric plates are moving towards one another – they form a “Convergent Boundary” • One of two interactions are the result of a Convergent Boundary • Subduction Zone • Collision Zone (c) Vicki Drake, 2011
Subduction Zone: Convergent Boundary • In a subduction zone, one lithospheric plate will be pushed beneath another (‘subducted’). • The non-subducted plate will ride over the subducted plate. • This is a ‘destructive’ boundary, as lithosphere is destroyed and consumed back into the mantle • Subducted plate subjected to extreme heat with melting and formation of magma occurring • Volcanic arcs (either on land or the ocean floor) • Earthquakes • Deep trenches on ocean floor (c) Vicki Drake, 2011
Subduction Zone: Oceanic to Oceanic • When two oceanic plates enter a subduction zone, the older, colder and more dense of the two will be subducted. • A chain of volcanic islands on the ocean floor will develop. • Philippines • Japan • Indonesia • Marianas (c) Vicki Drake, 2011
(c) Vicki Drake, 2011 Convergent Plate Boundaries: oceanic-oceanic subduction zone
Subduction Zone: Oceanic to Continental • When an oceanic plate and continental plate enter a subduction zone, the oceanic plate will be subducted. • Chains of volcanic mountains will form on the continental plate. • Cascade Range (CA to WA), North America • Andes, S. America (c) Vicki Drake, 2011
Pacific Plate subducting beneath North American Plate; producing Cascade Volcanoes extending between northern California through Washington Pacific Plate subducting beneath South American Plate; producing volcanoes in the Andes Mountains (c) Vicki Drake, 2011
Pacific Ring of Fire:volcanoes and earthquakes found along the subducting edges of the Pacific Plate (c) Vicki Drake, 2011
Collision Zone: Continental to Continental • When two continental plates collide, neither will be subducted. • Continental plates are too thick and buoyant for subduction. • Great folded and faulted mountains will result from the collision, along with earthquakes • Himalaya Mtns, Asia • Swiss Alps, Europe • Appalachians, North America • Atlas Mtns, NW Africa (c) Vicki Drake, 2011
COLLISION ZONE: EUROPE (c) Vicki Drake, 2011
INDIA COLLIDING WITH ASIA India’s collision with Asia pushed up the Himalaya Mountains and the Tibetan Plateau India is still pushing up the Himalaya Mountains even today! (c) Vicki Drake, 2011
Earthquakes and Collision Zones (c) Vicki Drake, 2011
DIVERGENT BOUNDARY • When a lithospheric plate is split into two or more pieces, a Divergent Boundary is formed • “Sea-floor spreading” is a type of divergent boundary in which new ocean floor is formed • This is a ‘constructive’ boundary where new lithosphere is created • First identified by Dr. Harry Hess, Princeton • Rift Valleys are another type of divergent boundary where the plate has not completely split apart. • East African Rift Valley • Region of Eastern Africa extending from Mozambique to Ethiopia is being separated from the rest of continental Africa (c) Vicki Drake, 2011
MID-ATLANTIC RIDGE: Sea floor spreading zone South America Africa Atlantic Ocean Basin (c) Vicki Drake, 2011
East African Rift Valley (c) Vicki Drake, 2011
TRANSFORM BOUNDARY • When two lithospheric plates slide past each other in opposite directions, this is a Transform Boundary (also known as a Strike-slip boundary) • This is a ‘conservative’ boundary – crust is neither created or destroyed • San Andreas Fault, North America • Plate boundary between the Pacific Plate and North American plate • There will be no subduction, no volcanoes • There will be earthquakes and horizontal displacement of surface features (c) Vicki Drake, 2011
Transform Boundary – two tectonic plates slip past each other, parallel, but in opposite relative directions (c) Vicki Drake, 2011
San Andreas Fault, western North America San Andreas Fault through Carrizo Plain (west of Bakersfield) (c) Vicki Drake, 2011
