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Grade 8 Science Unit: 07 Lesson: 01 Theory Development Abraham Ortelius Lived 1527–1598 Dutch Map Maker and Geographer 1596 – Ortelius suggested that North and South America were connected to Eurasia and Africa, but have drifted apart by natural events such as flooding and earthquakes. He believed that the continents fit together like pieces of a jigsaw puzzle. The continental shelves, located underneath the surface of the ocean, fit together especially close. http://commons.wikimedia.org/wiki/File:Abraham_Ortelius.jpg Alfred Lothar Wegener Lived 1880–1930 German Meteorologist and Geophysist Pangaea Theory – Wegener claimed that the continents had formed a single landmass, which split apart. He was not the first to suggest that the continents had been one, but was the first to present evidence from several fields. http://commons.wikimedia.org/wiki/File:Alfred_Wegener.jpg ©2012, TESCCC 1912 - Continental Drift Theory – Wegener studied South America and Africa, which are separated by a large ocean and found that each had similar glacial deposits, which would indicate that the continents once had a very different climate. He also found that fossils of tropical plants were located on an Arctic island. Additionally, fossil remains of a reptile, similar to a crocodile, were found on South America and Africa. This indicated that the two continents must have touched at some point because the reptile would have been unable to swim across the ocean. Wegener’s theory was initially rejected because he did not have geological proof that would explain how the continents had been separated. 06/13/13 page 1 of 4 Grade 8 Science Unit: 07 Lesson: 01 Arthur Holmes Lived 1890–1965 British Geologist http://en.wikipedia.org/wiki/File:Arthurholmesin1912.jpg 1929 - Convection Currents Theory – As Wegener’s theory was being dismissed because he did not have sufficient geological proof to explain why the continents moved, Holmes introduced his theory on thermal convection in the mantle. His theory explained that the molten material in the mantle moved in a circular current. As the magma heated, it would rise up to the crust, then cool off, and sink again. Much like the material in a lava lamp. When the magma reaches the Earth’s crust, sometimes it breaks through and pushes the continents in opposite directions. This theory was not widely accepted at the time. Harry Hess Lived 1906–1969 American Geologist 1959 – The theory of Sea Floor Spreading explains that the sea floor is moving away from a central point, which causes the continents to move with it. The theory is well-accepted now. It is caused by convection currents in the molten, very weak upper mantle, or asthenosphere. http://en.wikipedia.org/wiki/File:Hess.gif ©2012, TESCCC This geological mechanism was the missing piece of Alfred Wegener’s Continental Drift Theory. Hess explained that magma could push up between two plates, then cool and expand, pushing the plates beside it. This explained how North and South America could have been pushed west and Eurasia and Africa could have been pushed east, away from the Americas. It is called Sea Floor Spreading because the ocean is getting wider, but the coastlines of the land masses do not change dramatically. 06/13/13 page 2 of 4 Grade 8 Science Unit: 07 Lesson: 01 Frederick John Vine Lives 1939– British Marine Geologist and Geophysicist 1962 – Vine-Morley-Matthews’ hypothesis was further evidence that built on Harry Hess’ sea floor spreading theory. Vine, along with Lawrence W. Morley and Drummond Matthews, developed the theory that the sea floor was spreading equally away from the mid-ocean ridge. Their idea was that the magnetic poles of the Earth have shifted from North to South and back again several times. As magma pushes through the Earth’s surface, before it cools, the magnetic minerals, such as iron, would turn in the direction of the poles. If the Earth’s polarity changed, then the minerals would change direction. This would create magnetic stripes in the rocks of the sea floor. If the sea floor was spreading, as Hess described, then the magnetic stripes would be symmetrical on both sides of the mid-ocean ridge. The Glomar Challenger 1968 http://en.wikipedia.org/wiki/File:GlomarChallengerBW.JPG ©2012, TESCCC The Glomar Challenger was a scientific drilling ship that set out across the Atlantic Ocean from South America towards Africa. Its job was to drill into the ocean floor and take samples of rock so that scientists could determine the age of the rocks. What they found helped to solidify the theory of sea floor spreading. The rock samples closest to South America and Africa were the same age and much older than the age of the sea floor closer to the mid-ocean ridge. This meant that new crust forms in the middle of the ocean and the older crust was at one time close to the mid-ocean ridge, placing South America and Africa together. 06/13/13 page 3 of 4 Grade 8 Science Unit: 07 Lesson: 01 Dan McKenzie Lives 1942– English Geophysicist 1968 - Current Theory of Plate Tectonics – A combination of two earlier ideas: continental drift and sea floor spreading. This theory was formed as new information was learned about the nature of the ocean floor, Earth's ancient magnetism patterns, the location of volcanoes and earthquakes, the flow of heat from Earth's interior, and the worldwide distribution of plant and animal fossils. McKenzie wrote a paper summing up the currently known information. Earth's outermost layer, the lithosphere, is broken into seven large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist. The plates are all moving in different directions and at different speeds, from 2 cm to 10 cm per year in relationship to each other. The main features of plate tectonics are: • The Earth's surface is covered by a series of crustal plates. • The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. ©2012, TESCCC • Convection currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is from radioactivity deep in the Earth’s mantle. 06/13/13 page 4 of 4