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Transcript
CHAPTER 2 Plate Tectonics and the Ocean Floor (I) Instructor: Dr. Dong at ECC Layered Earth Fig. 1.14 2 Fig. 1.15 3 Asthenosphere Upper mantle Plastic—deforms by flowing High viscosity—flows slowly 4 Plate tectonics explains: Global distribution of ◦ Volcanoes ◦ Earthquakes ◦ Faults ◦ Mountain belts ◦ Features of seafloor Evolution of continents and oceans Exe. 2-1 1. How many structure? layers in 2. What is the plate tectonic? the earth Hypothesis: Continental drift Wegener proposed one large continent (1912) ◦ Pangaea Surrounded by single large ocean ◦ Panthalassa About 200 million years ago Fig. 2.2 Evidence for continental drift Puzzle-like fit of continents ◦ Edward Bullard fit continents at 2000m water depth Fig. 2.3 Evidence for continental drift Matching sequences of rocks and mountain chains ◦ Similar age, rock types, structures Figure 2.4a Figure 2.4b Evidence for continental drift Glacial ages and other climate evidence ◦ Ancient glaciation in modern tropical regions ◦ Direction of glacial flow Distribution of organisms ◦ Same land animals distributed in different continents (e.g., South America and Africa) Fig. 2.5 Fig. 2.6 Objections to continental drift Continents cannot “plow” through ocean crust Gravitational forces associated with tides too small Exe. 2-2 1. List evidences to support the continental drift 2. Why was the continental drift a hypothesis? Evidence for plate tectonics Earth’s magnetic field Paleomagnetism ◦ Magnetic alignment (N or S) ◦ Magnetic inclination (magnetic dip) Latitude Fig. 2.7 Apparent polar wandering Fig. 2.8 Pangaea Magnetic polarity reversals Fig. 2.9 Exe. 02-03 How is earth’s magnetic field used for the tectonic plate? Sea floor spreading Harry Hess (1962) Mid-ocean ridge site of new ocean crust Oceanic trench site of crust destruction Fig. 2.10 (subduction) Evidence to support sea floor spreading 1. Parallel magnetic anomalies record changes in Earth’s magnetic polarity as sea floor created Fig. 2.11 2. Age of ocean floor increases away mid-ocean ridge Fig. 2.12 from crest of 3. Global distribution of earthquakes Fig. 2.13 Evidence to support sea floor spreading Parallel magnetic anomalies record changes in Earth’s magnetic polarity as sea floor created Age of ocean floor increases away from crest of mid-ocean ridge Most large earthquakes occur along plate margins Fig. 2.10 Plate tectonics theory Lithospheric plates “float” on the asthenosphere Large scale geologic features occur at plate boundaries Two major tectonic forces ◦ Slab pull ◦ Slab suction Exe. 02-04 List evidences to support plate tectonic theory.
