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Transcript
Plate Tectonics Continental Drift A. B. Idea that individual continents could shift position on the globe Early Scientists 1. 2. 3. 4. Sir Frances Bacon- 1620 – noticed continents had similar coastlines Antonio Snider- 200 years later- did fossil correlations and similar coastlines Eduard Suess- 1885- said similar geological formations in the southern hemisphere could have been a single land mass called Gondwanaland Alfred Wegener- attempted to explain all of the above and proposed the Theory of Continental Drift in 1912. c. Wegener’s Hypothesis and support 1. Fossil evidence a. Paleontologist found fossil remains of both plants (seed fern- Glossopteris) and animal (reptile- Mesosaurs) b. fossils occurred over limited areas on several different continents (South America, India, Aftrica, Australia, and Antarctica) c. Wegener suggests organisms flourished when continents were together; continents split after fossils were entombed in rock. 2. Climate evidence a. many factors are used to determine an area’s climate; the most prevalent one is latitude b. equatorial regions- warmest; polar areascoldest c. sedimentary rocks preserve evidence of climatic conditions d. Wegener’s interpretation was that rocks indicating cold conditions were depositd when the continent was near a pole and vice versa for warm areas. Changes were the result of continent’s drifting in and out of different latitudes. 3. Jigsaw Puzzle a. Land masses looked as though they would fit together in one large continent (Pangaea) b. Laurasia- continent in northern hemisphere which includes Europe, Asia, and North America c. Gondwanaland- continent in the southern hemisphere which includes Australia, Antarctica, India, Africa and South America d. Ocean surrounding Pangaea was called Panthalassa e. Pangaea split 200-225 million years ago 4. This hypothesis did not gain much support from the scientific community for many years; however, this theory is considered quite logical and led to the theories of sea floor spreading and plate tectonics II. Sea Floor Spreading A. Proposed by Harry Hess and Robert Dietz in the mid-1960’s B. Suggests that the sea floor itself is moving as a result of convection currents 1. moves live a conveyor belt away from the crest of mid-oceanic ridges 2. moves as a rate of 1-6 cm/yr C. Driving force is convection currents (temperature and density) D. Support for Sea Floor Spreading 1. youngest sea floor should be at the crest of the ridge with the age of the sea floor getting progressively older as you move toward the continents (oldest rock on the ocean floor is approximately 200 million years old) 2. If mid-ocean ridges were spreading centers, they should be hot. They are! 3. Ocean floor should be denser as it moves away from the ridge so the ocean should be deeper farther from the spreading center. It is! 4. Must have subduction zones to balance the spreading. Evidence of this came from Wadati- Benioff zones. Zones of concentrated earthquakes (both subduction zones and spreading centers) 5. Lack of sediment at ridge. 6. Paleomagnetism (Vine – Matthews hypothesis) A. When magma solidifies, the metals align with the magnetic field (they point north). When they cool this is frozen in the rock like a magnetic fossil. Show new material being pushed away from ridges. B. Magnetic Anomalies or stripes at the mid-ocean ridge are mirror images of each side of the ridge. As magma rises and solidifies it records the magnetic field (which reverses once every 500,000 years). As the sea floor spreads the new rock at the ridge is split into two. A pattern of magnetic anomalies on one side of the ridge is a mirror on the other side. C. Rate of mothion can be determined by taking Distance from ridge/age of the rock D. Mantle plumes also tell us direction of plate movement (Hawaiian Islands). III. Plate Tectonics A. The surface of the Earth is actually composed of a series of rigid plates that are all moving relative to each other. B. Plate – a large mobile slab of rock that is part of the earth’s surface 1. includes all of the lithosphere (both oceanic and continental crust) 2. floats on top of the asthenosphere C. Lithosphere- is a rigid outer layer approximately 100 km thick. It includes rocks of the earth’s crust and uppermost portion of the mantle. a. made up of 6 large plates and approximately 20 small plates D. Asthenosphere- is a zone approximately 150 km thick and behaves plastically (solid that flows) because of extreme temperature. 1. This layer acts as a lubricating fluid allowing the plates to move. 2. Convection currents – the mantle close to the hot outer core heats, becomes less dense and rises. As it rises, it spreads and cools. In cooling it becomes denser and sinks. This carries the plates. The mantle loses heat at volcanic eruptions, hot spots, and by seawater. Mantle sinks at trenches. E. Subduction – the sliding of sea floor beneath a continent or island arc 1. If the plate is made up of mostly sea floor it can be subducted down into mantle forming an ocean trench 2. If the leading edge of the palte is made of continental rock, the plate will not subduct. (too bouyant) 3. The amount of new crust formed by sea floor spreading is essentially balanced by material consumed by subducting, folding or faulting of rocks when two plates collide. IV. Plate Boundaries A. Divergent boundaries – plates move apart <----> 1. Magma wells up from asthenosphere and new lithosphere is created 2. Ex. Mid Atlantic Ridge B. Transform boundaries – transform are moving horizontally past one another 1. Ex. San Andreas Fault C. Convergent boundaries – plates are moving toward each other 1. continent and continent results in folding, faulting, and mountains being formed – ex. Himalayas 2. ocean and ocean results in island arcs and trenches 3. Continental and oceanic results in subduction, deep sea trenches and folded mountain ranges on coast – ex. Andes Mountains V. Why do plates move? A. Convection cells – hot magma rises at spreading ridges; some escapes to form new lithosphere, but the rest flows outwrd and drags the overlying lithosphere with it B. Subduction zones – the weight of the dense, cold, down going slab of lithosphere is the subduction zone pulls the rest of the trailing plate