Continental Drift Seafloor Spreading Theory of Plate Tectonics

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Presentation transcript:

Continental Drift Seafloor Spreading Theory of Plate Tectonics

Continental Drift Evidence for Continental Drift Pangea: Alfred Wegener (1912) suggested that all the continents were joined together at some time in the past Continental Drift was suggested that this huge land mass (called Pangea) began to break apart about 200 mya Idea wasn’t accepted as he was unable to explain how they drifted apart Long after his death (1930) was there enough evidence to accept Continental Drift

Pangea Videos http://www.5min.com/Video/Learn-about-The-Continental-Drift---Plate-Tectonics-117505893 http://www.pond5.com/stock-footage/128542/pangea-earth-s-continents-separate.html

Clues to support Continental Drift Fossil clues: fossils found (Mesosaurus) in S America and Africa that were freshwater land animals that couldn’t have swum sea water to both sides, therefore lived together before the split Widespread Plants: Glossopteris fossil found on Africa, Australia, India, Antarctica, S America suggested that these plants thrived once when continents were attached

Clues to support Continental Drift Rock clues: similar rock structures found on different continents Climate Clues: glacier deposits found in tropical climates and tropical plants found on island in Arctic Ocean

How could continents drift? Wegener’s radical idea of continental drift provided no evidence as to how the continents plowed through higher density ocean floor material His proposal suggested it was caused by Earth’s rotational spin, but no evidence supported that Only after discovery of seafloor spreading was his idea re-introduced as explanation for continental drift

Seafloor Spreading Mapping the Ocean Floor: sound and radar waves were used to map the ocean floor in 1940’s and 50’s Noticed that in middle of oceans were huge mountain ridges This discovery caused scientists to ask why?

Seafloor Moves Harry Hess suggested that the seafloor moves Proposed that hot, less dense material below Earth’s crust rises toward the surface at mid ocean ridges , lows sideways carrying the seafloor away from te ridge in both directions

Evidence for Spreading Research ship Glomar Challenger gathered data in 1968 by drilling ocean floor Found younger rock are located at the mid-oceanic ridges and older rock found farther away Mid-oceanic ridges have molten rock, heat, exotic life-forms

Evidence for Spreading Magnetic clues: when molten rock cools, it cools with magnetic attractions to the north and south poles Found that magnetic north has changed several times in earth’s history and the magnetic poles are preserved in the rock at mid-oceanic ridges On each side of the mid-ocean ridges portrays these magnetic changes Magnetometer detects the magnetic field reversals on both side of ridges indicating that the ridges are moving apart when new rock is formed

Seafloor spreading and Magnetic reversals

Theory of Plate Tectonics Plate Movement: plate tectonics combine the knowledge of continental drift and seafloor spreading in the Theory of Plate Tectonics Earths crust and upper mantle are broke into plates These plates ride on top a plastic-like layer of the mantle much like a raft on water

Composition of Earth’s Plates Lithosphere is the outer layer of earth’s crust Made of less dense material than plastic-like layer below it (asthenosphere)

Plate Boundaries When plates move they interact with each other in many ways They can move toward, pull away or slide past each other. When plates interact, the result of their movement is seen at the plate boundaries

Plate Boundaries Plates Move Apart: Divergent boundaries are areas where plates move away from each other Mid-Atlantic Ridge is divergent boundary between North American Plate and the Eurasian and African Plate Great Rift Valley in eastern Africa is result of continental plate being pulled apart

Plate Boundaries Plates Move together: Convergent boundaries are areas where plates move together As crust is added in one area, crust disappears in other areas when seafloor cools, becomes denser and sinks (convergent boundaries) When oceanic crusts converge with less dense continental crust, the denser oceanic plate sinks under the continental plate in subduction zones Deep-sea trenches are associated with convergent boundaries

http://www.geocraft.com/WVFossils/subduction.html

Plate Boundaries As ocean plates sink under continental plates, they warm and melt and magma forms. This less dense magma rises to surfaces and form volcanoes Cascade Mt ranges are result of convergent boundaries and subduction zones A subduction zone can form from two oceanic plates that collide where the older, denser plate sinks (Mariana Islands are chain of volcanic islands ) When 2 continental plates collide, usually they crumple up forming mountain ranges (earthquakes common, but no volcanoes) (Himalayan Mts)

Plate Boundaries When plates slide past each other: transform boundaries are result where two plates slide past each other They move in opposite direction and usually earthquakes are associated with transform boundaries San Andreas fault is an example

Causes of Plate Tectonics Question about what was force that would cause plates to move were finally answered by convection currents Convection currents in earth’s crust are currents caused by warm less dense magma rising Some magma reaches the surface at ridges, some is moved away from rising magma and drags the plates with them. They cool and settle back to earths center where it moves and is heated up and cycle repeats

Causes of Plate Tectonics

Causes of Plate Tectonics

Features Caused by Plate Tectonics Interactions of plates build mountains, create ocean basins and cause volcanoes Earthquakes are associated with these interactions Tension forces result in earth crust being pulled apart and form normal faults Causes large blocks of crust to break, tilt, or slide down and large fault-block mountains form Rift valleys and mid-oceanic ridges are associated with divergent boundaries where tension forces pull crust apart and normal faults occur

Tension forces and Normal faults Rock layers above the fault move down when compared with rock layers below the fault

Compression Forces cause Reverse Faults Where plates meet, compression force cause massive folding and faulting into mountain ranges (Himalaya) by reverse faulting Rock layers above the fault surface is moved up relative to rock layers below the fault

Reverse and normal faulting Reverse fault: Compression forces Normal fault: Tension forces

Strike-slip Faults Caused at transform boundaries where 2 plates move past each other (usually in opposite directions, but can be where both plates are moving in same direction but at different speeds) These are called strike-slip faults

New technology and Plate tectonics With GPS and modern technology we have been able to record crustal movement on plates as low as 1 cm per year Pacific plate is moving at rate of 2.5 cm per year average