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Sea-Floor Spreading, Subduction,& Plate Boundaries Lecture 21 1. Continental “Fit” 2. Similar Rocks, Ages 3. Similar

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Sea-Floor Spreading, Subduction,& Plate Boundaries Lecture 21 1. Continental “Fit” 2. Similar Rocks, Ages 3. Similar Sea-Floor Spreading, Subduction,& Plate Boundaries Lecture 21 Prop: Test 3 Invitations Alfred Wegener’s Evidence for Continental Drift 1. Continental “Fit” 2. Similar Rocks, Ages 3. Similar Fossils 4. Widespread Glaciation Pangaea = Gondwanaland + Laurasia Source: http://vishnu.glg.nau. edu/rcb/Penn.jpg 1 Alfred Wegener’s Evidence for Continental Drifts Was NOT Accepted in USA. Problem: No Mechanism for Continents to Plow Through Oceanic Crust Answer lies on & in the Sea Floor - Ch. 18 + 19 in Text WW II http://www.geocities.com/Pentagon/2560/bowph1.jpg http://www.spe.sony.com/movies/da sboot/ Submarine Science Mapping of Ocean Basins http://members.microdsi.net/shirschm/Welcome.index.html Seismic & Sonar Study Reveals Sea-Floor Topography Geophysicists’s Answers Life’s Important Questions Spongebob Theme Song Lyrics Are ya ready kids? Aye Aye Captain! I CAN'T HEAR YOU! AYE AYE C APTAIN! ohhhhhh!!!! Who lives in a pineapple under the sea? SPONGEB OB SQUAR EPANTS! Absorbant and yellow and poreous is he SPONGEB OB SQUAR EPANTS! His nautical nonsense be somethin you wish SPONGEB OB SQUAR EPANTS! Then drop on the deck and flop like a fish! SPONGEB OB SQUAR EPANTS! Ready? SPONGEB OB SQUAR EPANTS! SPONGEB OB SQUAR EPANTS! SPONGEB OB SQUAR EPANTS! SPONGEYB OB SQUAR EPAAAANTS 2 Know these mega-scale Landforms • Continental Shelf • Continental Slope • Continental Rise • Abyssal Plain • Mid-Ocean Ridge (~ Mid-Ocean Rise) • Trench Know differences between Oceanic Crust and Continental Crust Ocean Basin Bathymetry See Figures 18.5 to 18.16 in Plummer & Others 10th ed. Ocean Basin Bathymetry See Figures 18.5, 18.15 in Plummer & Others 10th ed. 3 Cold War More Submarine Science Magnetic Anomalies in Ocean Basin Deep-Sea Magnetic Anomalies Deep Sea Drilling Project (DSDP) Studied Geology of Ocean Basins Continental Record = Incomplete Deep Ocean Basin Record Was Thought to be More Complete Glomar Challenger Harry Hess 4 Magnetic Anomalies: Atlantic Ocean South of Iceland New Ocean Crust Created at Mid-Ocean Ridges: Spread Centers DSDP Surprise Finding: Ocean Basins are <200,000,000 y Old Much Younger than the Continents Very Young Sediments at the Mid Ocean Ridges Age of the Ocean Basins 5 Birth of an Ocean Basin? Spreading at the Red Sea: Break-Up of Asia and Africa Note a “Failed” Arm of Rifting in East Africa Break-Up of a Continent and Origin of an Ocean Basin Problem: If New Crust is Created at Mid- Ocean Ridges, Where is Old Crust Consumed to Balance the New Crust? 6 Where is Old Crust Consumed to Balance the New Crust? Answer Lies in Deep-Focus Earthquakes. Earthquakes Occur in Brittle (Elastic) Lithosphere Lithosphere 70 - 100 km Asthenosphere Distribution of All Earthquakes (Deep and Shallow Focus) 7 Subduction Zones: Ocean-Ocean Margin Subduction Zones: Ocean-Continent Margin Deep-Focus Quakes Only in Subduction Zones 8 Isotherms in Subduction Zones 400o C 600o C Japan Trench & Volcanic Arc Back-Arc and Fore-Arc “Basins” See Figure 19.35 in Plummer & Others 9th ed. 9 Subduction Zones & Volcanoes Spread Centers Feed Subduction Zones Missing Mechanisms for Continental Drift: Sea Floor Spreading & Subduction Continents are Passive “Boats” 10 How Are Mountains Built? First a Brief Review of Sea Floor Spreading Himalayas Break-Up of a Continent and Origin of an Ocean Basin Missing Mechanisms for Continental Drift: Sea Floor Spreading & Subduction Continents are Passive “Boats” 11 Spread Centers Feed Subduction Zones Subduction Zones: Ocean-Ocean Margin Subduction Zones: Ocean-Continent Margin 12 Orogeny (Mountain Building) in the Himalayas Continental Lithosphere Will NOT Flush Down Subduction Zones Orogeny in Himalayas 13 Orogeny in Himalayas Orogeny in the Himalayas Orogeny - Mountain Building - Sediment Supply 14 Orogeny - Mountain Building - Glaciation Plate Margins: • Spread Centers (Mid-Ocean Ridges) • Subduction Zones (Trenches) • Transform Faults Plate Margins: 15 Transform Faults & Fracture Zones Transform Faults San Andreas Fault: Right-Lateral Transform Fault Left-Lateral Offset!! Why?: Fault is Produced by Offset, vs. Offset Produced by Fault 16 Transform Faults: Three Causal Scenarios Plate Motion on a Sphere Plate Motion on a Sphere Requires Transform Faults Because of Different Rates of Motion 17 The Plates The Plates: Pacific-Centered View (Distorted by Mercator Map Projection) What Drives Plate Tectonics? 1. Convection Cells in Mantle 2. Sinking of Cool, Dense Plates 3. Mantle Plumes - Hot Spots 18 Convection Cells Convection Cells in Mantle 19 Mantle Plumes Mantle Plumes Under Continents 20 • Known “Hot Spots” Hawaiian Islands Mantle Plumes - Hot Spots • Numbers = Age of Rock in Millions of Years 21 Sinking of Cool, Dense Plates Seamount Volcanoes Surrounded by Fringing Coral Reefs Atolls: Submerged Volcanos Support Reefs Only in Warm Clear Seas Tuamoto Islands NASA 22.
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