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PLATE TECTONICS Objectives • Explain how paleomagnetism provided the definitive evidence for continental drift. • Define seafloor spreading. • Define theory of plate tectonics. • Describe the different types of plate margins. • Explain the role of mantle convection in plate tectonics. A Revolution in Geology • Continental drift • Proposed by Alfred Wegener • Slow, lateral movement of continents along Earth’s surface • The puzzle piece argument • Pangaea • Continental shelf • Continental slope Matching Geology • Matching rocks • What is the “true” edge of a continent? Matching Geology • Matching rocks • How well do the continents fit? Matching Geology • Matching rocks • How well do the ages match? Matching Geology Matching Geology • Matching rocks • What evidence did the glaciers leave? Matching Geology • Matching fossils • What evidence did the glaciers leave? • Glossopteris: ancient tree with large seeds found on several continents • Mesosaurus: small reptile found in Brazil and S. Africa Matching Geology Apparent Polar Wandering Paths • Paleomagnetism – Apparent polar wandering • Magnetic poles wandered – Used to indicate America and Europe were a single continent Apparent Polar Wandering Paths Seafloor Spreading • The missing clue – Seafloor spreading • The process by which the seafloor splits and moves apart along a midocean ridge • New oceanic crust forms along the ridge – Global Positioning Systems (GPS) valuable tool The Plate Tectonic Model • Plate tectonic – The movement and interactions of large fragments of Earth’s lithosphere (i.e. plates) – Below lithosphere- asthenosphere • Weak due to hot temperature (near melting point) – Relationship between lithosphere and asthenosphere is isostasy • Fault – A fracture in Earth’s crust along which movement has occurred The Plate Tectonic Model Types of plate margins • Divergent margins – A boundary along which two plates move apart from one another Types of plate margins • Convergent margin – A boundary along which two plates come together Types of plate margins • Subduction zone – A boundary along which one lithosphere plate plunges into the mantle beneath another plate • “Collision zone” between continents – Deep oceanic trenches – Arcs of volcanoes Types of plate margins • Transform vault – A fracture in the lithosphere where two plates slide past each other Earthquake and plate margins • Earthquake – Tectonic movement produce pressure and friction. – Friction is overcome, the block slips and pent up energy releases with a huge “snap” – Focus • Where earthquake begins – Epicenter • Point on earth’s center directly over the focus The search for a mechanism • Earth’s internal heat – Conduction • Slow release of heat – Convection • A form of heat transfer in which hot material circulates from hotter to colder regions, looses its heat, and then repeats the cycle The tectonic cycle • Tectonic cycle – Movements/ interactions by which rocks are cycled from the mantle to the crust and back – Includes earthquakes, volcanism, and plate motion, driven by convection in the mantle – As a result, seafloor recycles itself every 200 million years The tectonic cycle Hawaiian Islands Critical Thinking • What are some of the important questions about plate tectonics that remain unanswered today? • Why do geologists call plate tectonics a “unifying theory”? • As Africa slowly separated from Arabia, and the sea began to enter the rift, what kind of sediment would you expect to be deposited?