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Chapter 4 – PLATE TECTONICS A Revolution in Geology • Introduction • Continental drift • Proposed by Alfred Wegener • Pangaea • Puzzle piece argument • S. America & Africa Matching Geology • Matching rocks • Rock ages • Radiometric dating • Rock sequence correlation • Continuous mountain chains Matching Geology • Glacial evidence – Striations & direction of ice movement Matching Geology • Matching fossils • Fossil record matches • Glossopteris: ancient tree with large seeds found on several continents • Mesosaurus: small reptile found in Brazil & S. Africa Apparent Polar Wandering Paths • Paleomagnetism – Study of Earth’s past polarity – Apparent polar wandering • Magnetic poles moved Seafloor Spreading • Missing clue: – Seafloor spreading • Seafloor splits & moves apart along midocean ridge • Forms new oceanic crust along ridge The Plate Tectonic Model • Plate tectonics – Movement & interactions of large fragments (i.e. plates) of Earth’s lithosphere (i.e. crust & upper mantle) – Asthenosphere • “Weak sphere” due to hot temperatures – Isostasy • Relationship between lithosphere & asthenosphere • Balance of gravity & buoyancy Types of plate margins • Divergent boundary – Plates move away from each another – Creates crust Types of plate margins • Convergent boundary – Plates come together (i.e. collide) – Destroys crust Types of convergent plate margins • “Subduction” zone – 1 plate plunges into mantle beneath another plate • 1 or 2 ocean plates involved • Ocean plates = higher density (heavier) due to composition (basalt) – Continents = granite (less dense) – Ocean trenches & volcanic arcs • “Collision” zone – 2 continental plates collide • Form mountains Types of plate margins • Transform boundary – 2 plates slide past each other – No creation or destruction of crust Volcanoes, earthquakes& plate margins Search for a mechanism • Earth’s internal heat – Conduction • Slow release of heat – Convection • Hot material moves from hotter to colder regions, looses its heat, & repeats cycle • Convection cells The tectonic cycle • Tectonic cycle – Movements/ interactions where rocks are cycled from mantle to crust & back – Includes earthquakes, volcanism, & plate motion; driven by mantle convection – Seafloor recycles itself every 200 million years – Plate tectonics affects everything Hawaiian Islands • Mantle plumes – Localized hot areas not along plate boundaries – Move with plate – Eventually become inactive – Hot spot trail • Hawaiian islands Determining Plate Velocity • Velocity = distance traveled / travel time – Usually expressed in cm / year OR km / million years • Calculating plate velocities – Measure distance (usually in km) – Convert distance to cm (x 100,000) – Divide by travel time (millions of years) • Hint: for travel times, be sure you divide by correct # • I.e. 2.1 million years = 2,100,000 years
