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PLATE TECTONICS INTERNAL STRUCTURE CRUST Thin, cool, brittle 5-40 miles thick CORE Outer part liquid Inner part solid Down to 4000 miles 6700oC MANTLE Hot, plastic Down to1800 miles Evidence from analysis of earthquake waves DENSITIES ATMOSPHERE < 1 g/cm3 CRUST 2.7-3.0 g/cm3 HYDROSPHERE 1g/cm3 6700oC CORE 11-14 g/cm3 MANTLE 3.3-5.6 g/cm3 Average density 5.5 g/cm3 THEORY OF PLATE TECTONICS 1968 Continental Drift –Hypothesis first proposed by Alfred Wegener in 1915 Sea Floor Spreading –Hypothesis first proposed by Harry Hess in 1962 Plate Tectonics 1968 – combination of these two hypotheses BASIC PROPOSITIONS Earth’s crust is divided into rigid plates that float buoyantly on the denser plastic mantle Elevation of the crust depends on density (rock type and temperature) & thickness Plates move laterally over surface generating unstable earthquake belts where they interact Isostacy Recall that the crust (density 2.7-3.0 g/cm3) floats on the upper mantle (density 3.3 g/cm3) with elevation dependent on: Density (Rock type & Temperature) Thickness Earthquake Foci STABLE & UNSTABLE CRUST Stable Plates Granitic Crust – Continental Craton Basaltic Crust – Abyssal Plains Unstable Plate Boundaries Divergent – Rift Zones (Midocean Ridges) Convergent – Subduction Zones Shear Zones – Transform Faults Eight Major Plates North America South America Africa Eurasia Antarctica Consist of both granitic & basaltic crust Australia – India Two granitic continents plus basaltic crust Pacific- all basaltic crust Nazca – all basaltic crust A World Cruise A Note Lithosphere Rigid outer layer of Earth down to depth of about 60 mls. Includes crust and uppermost mantle. Forms plates, generates earthquakes due to faulting Asthenosphere Hot plastic zone in upper mantle (60-400 mls) which forms base of lithospheric plates No earthquakes generated PLATE TECTONICS & THE EVOLUTION OF OCEANS & CONTINENTS Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Red Sea The Red Sea Rift Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Red Sea Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Red Sea Atlantic Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Red Sea Atlantic Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Red Sea Atlantic Pacific Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Red Sea Atlantic Pacific Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Red Sea Atlantic Pacific Mediterranean Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Dead Red Sea Atlantic Pacific Mediterranean Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Dead Red Sea Atlantic Pacific Mediterranean Himalayas Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Dead Dead, rotted and dismembered (Yuck!) Red Sea Atlantic Pacific Mediterranean Himalayas Evolution of Ocean Basins according to Dr. Chowns Baby Ocean Adolescent Mature Senile Dead Dead, rotted and dismembered (Yuck!) Red Sea Atlantic Pacific Mediterranean Himalayas Urals, Appalachians Age, Temperature & Depth of Oceans Hot rock is less dense than cold rock Density affects ocean depth Age and depth increase away from Ridges Young, hot basalt forms high Mid-Ocean Ridges Old, cold basaltic crust is subducted in trenches Heat Flow from Pacific Sea-floor Three types of convergent boundary Basalt – Basalt (Ocean-Ocean) Island Arc-Trench (Japan, Marianas) Basalt-Granite (Ocean-Continent) Mountain Chain-Trench (Andes) Granite-Granite (Continent-Continent) Mountain Chain (Himalayas) Convergent Boundaries Continental Collision OPHIOLITES Usually oceanic crust is subducted butSometimes a slab of upper mantle and ocean crust gets caught up in continental collisions (obducted) and preserved in the mountains Examples Cyprus, Alps, Appalachians DRIVING MECHANISM? CONVECTION CURRENTS IN MANTLE Driven by internal heat resulting from radioactivity Analogy of convection in a pot of soup Estimates of temperature made from analysis of Earthquake waves Exact details unknown because it is difficult to collect data from the mantle Simple Convection Currents In Mantle More Complex Models PLATE TECTONICS THE SUPPORTING EVIDENCE Wegener’s original evidence for Continental Drift 1913 Evidence compiled since 1950, especially paleomagnetism WEGENER’S EVIDENCE 1. Distribution of Mountains & Earthquakes Himalayan AlpineCircumpacific Ring of Fire 2. Remarkable fit between continental margins (especially across Atlantic) 3. Continuation of old mountains from one continent to another (eg. Appalachians) 4. Similarities in rock layers and history between continents that were formerly contiguous (eg. North America & Europe, Africa & South America) 5. Similarity in fossils between continents ( eg. Mesosaurus) 6. Peculiarities in distribution of modern organisms Distribution of Marsupials 200 Ma 7. Paleoclimatic Evidence (eg. Ancient Glacial Deposits) EVIDENCE SINCE 1950 1. Direct measurements of plate movement by laser, satellite and GPS 2. Tracking of hot spots like Hawaii The Hawaiian Hot Spot Hot Spots and Island Chains Movement of plates over hot spots in mantle produces chains of volcanoes. As old volcanoes move away from hot spots they cool and subside to form atolls or guyots Eg. Hawaiian chain & Emperor seamounts 3.Tracking of continents by position relative to the magnetic poles PALEOMAGNETISM Modern Magnetic Pole Two modern continents with magnetic rocks aligned to magnetic poles Modern Magnetic Pole Pole positions inferred from ancient magnetic rocks Ancient Magnetic Pole Continents repositioned so that ancient pole matches modern pole position Apparent Polar Wandering 4. Proof of spreading rates from record of magnetic reversals in seafloor basalts Magnetic stripes produced by sea floor spreading Age of the Ocean Floor THE BIG PICTURE Evolution of the Oceans 200 Ma Breakup of Pangaea 200-100 Ma Modern oceans 50-0 Ma