PLATE TECTONICS SCIENTIFIC METHOD Observations (Facts) Hypothesis (Interpretation incompletely tested) Testing (More critical observations) SCIENTIFIC THEORY (Tested and accepted interpretation). The foundations of our understanding 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 Earths 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 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 AfricaEurasiaAntarctica 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 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 THE BIG PICTURE Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean DeadHimalayas Dead, rotted and dismembered Appalachians, Urals (Yuck!) 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) THE BIG PICTURE Convergent Boundaries Continental Collision DRIVING MECHANISM? Convection Currents in Mantle Simple Convection Currents In Mantle PLATE TECTONICS THE SUPPORTING EVIDENCE Wegeners original evidence for Continental Drift 1913 Evidence compiled since 1950, especially paleomagnetism WEGENERS EVIDENCE 1. Distribution of Mountains & Earthquakes Circumpacific Ring of Fire Alpine-Himalayan 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 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. 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 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 Ma Modern oceans 50-0 Ma