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Chapter 2: Plate Tectonics Tectonic Plates of Earth Pangaea Pangaea: Pan = all Gaea = Earth Panthalassa Thalassa = sea AKA Tethys Sea Continental Drift Progressive breakup of Pangaea into modern continents Similar to sea ice Wegner’s Evidence Present-day shorelines fit like puzzle pieces Better fit from continental shelves Fossil Evidence Identical fossils present in S. Amer. & Africa e.g. Mesosaurus e.g. Glossopteris e.g. Marsupials Geologic Evidence Identical rocks on different continents e.g. 2.2Ga igneous rocks in Brazil & Africa Similar Mountain Ranges e.g. Appalachian Mts ~ Caledonian Mts Mesozoic Supergroup Identical package of rocks & fossils found in S.Amer., Africa, Australia & Antarctica Paleoclimatic Evidence Paleo = ancient Climate = weather conditions Glacially transported sediments Glacial striations Rejection of Continental Drift Hypothesis No evidence of continents “breaking through” oceanic crust Tidal forces necessary would halt Earth’s rotation Danish scientists found no astronomical evidence of drift from 1927-1948 Earth’s Magnetic Field Similar to bar magnet Magnetic materials align themselves to magnetic field N-S Orientation & Dip Magnetic orientation has 2 dimensions North-South Dip angle (Inclination) Curie point (T) Fossil Magnetism / Paleomagnetism Magnetic Inclination Magnetization = degrees from N pole Magnetization + Latitude = 90° e.g. Lavas from Puerto Rico show 75 ° from N pole & Puerto Rico = 15 ° from equator Apparent Polar Wander Seafloor Spreading Paleomagnetism Progression of Seafloor Spreading Plate Boundaries Plate Boundaries Corresponds to Earthquakes & Volcanoes Plate Boundaries Three Types Divergent Convergent AKA Rift AKA Subduction Transform AKA Strike-slip Divergent Boundaries AKA Spreading Centers AKA Rifts Largest mountain chains Plates move apart due to eruption of lava New lava = new oceanic crust Oldest oceanic crust 180Ma Pillow Basalts Form when lava extruded under water Immediately outer layer freezes New material pushes through like toothpaste Continental Rifting Rifts also can form in continental settings Linear depressions Lakes, valleys, etc. Asthenosphere thins due to tension e.g. East Africa Rift Zone, Mt. Kilimanjaro Continental Rifting (con’t) If tension continues, eventually continental rift develops into oceanic spreading e.g. Red Sea, Sea of Cortez Convergent Boundaries Old oceanic crust dense & heavy Heavy vs. light => subduction AKA destructive margins Large earthquake & explosive volcanoes Melting triggered at ~100km depth Oceanic-Oceanic Convergence e.g. Virgin Islands, Japan, Philippines Oceanic-Continental e.g. Andes, MesoAmerica, Italy Continental-Continental e.g. India Hot Spots Caused by mantle plumes Plumes do not move, plates do Bend at 40Ma Major change in plate motion Relative Plate Motions Relative to Hot Spots & other plates Measure motions with Paleomagnetism Forces Driving Plate Motions Convection of Mantle Upwelling Mantle Ridgepush Slab Suction AKA Slabpull Layer-Cake Model Two zones of convection, above & below ~660km Explains why mid-ocean ridge basalt different than hot spot basalt Whole Mantle Convection Cold oceanic crust descend to bottom of mantle, “stirring” it Hot plumes rise from core-mantle boundary Bring “primitive” mantle to surface Not popular b/c complete mixing in 100s Ma Deep Layer Model Heat from Earth’s interior causes two layers to shrink & swell Similar to lava lamp Small amt of material rises to surface to create hot spots Little seismic evidence to support this model Importance of Plate Tectonics First theory to provide comprehensive view & explain: Earth’s major surficial processes Geologic distribution of earthquakes, volcanoes & mountain ranges Distribution of mineral resources & ancient organisms