Plate Tectonics
Sir Francis Bacon, 1620, noted that the continental coasts on opposites sides of the Atlantic fit together like puzzle pieces. Could North and South America once have been joined to Europe and Africa? Fossil evidence from Western Africa and Eastern South America supports this assertion (Mesosaurus, Glossopteris)
1915 - German geophysicist Alfred Wegener Tried to explain climate patterns indicated by fossils Suggested all continents might have been a single super continent Pangaea Said that Pangaea began to break apart 180 million years ago
Although rejected, Wegener s idea did not die 1940 - Hugo Benioff plotted locations of deep earthquakes at edge of Pacific (Pacific Ring of Fire) Seismographs revealed that earthquakes and volcanoes do not occur at random - but in zones Other support: Radiometric dating - oceanic crust much younger than Earth Echo sounders used to map sea floor (ocean ridges match shorelines)
1960 - Harry Hess suggests that new seafloor develops at ocean ridges and spreads outward with trenches along edges of ocean basins, powered by convection currents in the mantle Hypothesis: Seafloor Spreading! 94% of energy comes from radioactive atoms breaking down releasing energy, must be areas of cooling near trenches!
Seafloor Spreading - A Key Idea Seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move. (right) The Mid-Atlantic Ridge conforms to the shape of the adjacent continents. The inset shows the central rift.
Evidence for Seafloor Spreading 1963 - Frederick Vine & Drummond Matthews Magnetic patterns in ocean floor were mapped Noticed that there were magnetic patterns on either side of an ocean ridge Crust is moving like a large conveyer belt away from ocean ridges! http://www.wwnorton.com/college/geo/egeo/f lash/2_3.swf
1965 - Sir Edward Bullard used a computer to compare coastlines at the continental shelf edge
Theory which describes the movement of crustal plates on the mantle Plate Tectonics! Work of John Tuzo Wilson - lithospheric plates floating on the asthenosphere
Types of Plate Boundaries Some plates move fast ~ (16 cm/year) and some slowly ~ (2-3 cm/year) All plate motions can cause earthquakes at plate edges (mostly ridges & trenches)!
Plate Boundaries
1. Transform - locations where crustal plates move past one another (ex. San Andreas Fault) Translation at transform boundaries causes shear.
2. Divergent - boundaries where plates move apart; new ocean floor is created, spreading rates can differ at ridges (slower, steep slopes) and rises (faster, gentler slopes) Divergent Oceanic Crust - the Mid-Atlantic Ridge, East Pacific Rise Divergent Continental Crust - the Rift Valley of East Africa Extension of divergent boundaries causes splitting and rifting.
http://www.wwnorton.com/college/geo/egeo/flash/2_7.swf
3. Convergent Plate Boundaries - regions where plates are pushing together, trenches have the largest earthquake magnitudes, can be further classified as: Island Arc Trench Systems - subduction of oceanic crusts, lithosphere is heated, rises back to surface as volcanoes (islands), ex. Japanese Islands Continental Arc Trench Systems - oceanic plate will subduct under continental plate, volcanoes will form on continents (ex. Peru-Chile Trench & Andes) Continental Mountain Systems - two continents collide & sedimentary rock is folded to form mountains (ex. Himalayas)
http://www.wwnorton.com/college/geo/egeo/flash/2_6.swf
Evidence for Plate Tectonics: Ancient Life & Climates (fossils) Coral reefs in eastern North America, western Europe, Alps, & Himalayas rocks that are 350 myo Ocean Sediments - sediments are younger & much thinner than should be (must be recycled over time) Continent Fit
Paleomagnetism: most igneous rocks contain magnetite - becomes permanently fixed as rock cools (mini-compass) 1.Strips of alternating magnetic polarity at spreading regions
2. Apparent Polar Wandering: plate movement causes the apparent position of the magnetic poles to have shifted The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.
Hot Spots: surface expression of plumes of magma (ex. Hawaii, Iceland, Yellowstone) 6% of energy for plate tectonics comes from hot material (magma) rising up from the core volcanic island chain can form when a plate passes over a hot spot and a stationary mantle plume.
show Exploring the Earth video clip
Website for plate tectonics worksheet http://geoinformatics.sdsc.edu/doe/student /tectonics/continental_drift.html