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Continental drift and seafloor spreading
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- 2. Continental Drift is themovement of the Earth's continents relative to each other, thus appearing to "drift" across the ocean bed it was first put forward by Abraham Ortelius in 1596 and fully developed by Alfred Wegener in 1912
- 3. Wegener was convincedthat all of Earth’s continents were once part of an enormous, single landmass called Pangaea. the theory states that Earth’s outermost layers is broken into 7 large, rigid Places called Plates: the African, North and South American, Eurasian, Australian, Antarctic, and Pacific Plate. (Arabian, Nanza, and Philippines plates are the minor plates)
- 4.
- 5. Pangaea existed about240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Laurasia (300-200 million years ago) and Gondwana (300 – 180 million years ago)
- 7. What is PlateTectonics?
- 8. If you lookat a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.
- 9. The Earth’s crustis divided into 15 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.
- 10.
- 11. Who discovered plate tectonicsand published the first paper on it?
- 12. By 1915, afterhaving published a first article in 1912, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans. In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius, Antonio Snider- Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few)
- 13. What are tectonicplates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle
- 14. The plates arearound 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium).
- 15. TYPES OF PLATEMOVEMENT Divergence Convergence Lateral Slipping
- 16. . Divergent Plate Movement: SeafloorSpreading Seafloor spreading is the movement of two oceanic plates away from each other (at a divergent plate boundary), which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a a mid-ocean ridge. Where the oceanic plates are moving away from each other is called a zone of divergence. Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960's.
- 18. A modern mapof the eastern Pacific and Atlantic Oceans. Darker blue indicates deeper seas. A mid- ocean ridge can be seen running through the center of the Atlantic Ocean. Deep sea trenches are found along the west coast of Central and South America and in the mid-Atlantic east of the southern tip of South America. Isolated mountains and flat featureless regions can also be spotted.
- 19. Convergent Plate Movement: Whentwo plates collide (at a convergent plate boundary), some crust is destroyed in the impact and the plates become smaller. The results differ, depending upon what types of plates are involved.
- 20. a. Oceanic Plateand Continental Plate - When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction.
- 21. Subduction zones circlethe Pacific Ocean, forming the Ring of Fire.
- 22. b. Two OceanicPlates Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes.
- 23. North of thecontiguous U.S. lies Canada, and north of Canada lies Alaska. A line of volcanoes, known as the Aleutian Islands, is the result of ocean-ocean convergence.
- 24. c. Two continentalplates Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges.
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- 26. Transform boundaries where platesslide passed each other. The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created.
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- 29. What is Isostasy? Ifyou were to take a large rubber ball and place something heavy on top of it, such as a bowling ball, what do you suppose would happen to the rubber ball? The weight of the bowling ball will push the skin of the rubber ball inward, creating a dent. Scientists observe the same phenomena with the Earth’s crust. When the Earth’s crust is pushed down, creating a small dent, we refer to this as isostasy.
- 30. Greek ísos"equal", stásis "standstill" Isostasy is a geophysical phenomenon describing the force of gravity acting on crustal materials of various densities (mass per unit volume) that affects the relative floatation of crustal plates. It specifically describes the naturally occurring balance of mass in Earth's crust .
- 33. depth of compensation In thetheory of isostasy, a mass above sea level is supported below sea level, and there is thus a certain depth at which the total weight per unit area is equal all around the Earth;
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- 35. John Henry Pratt(1809–1871) proposed his own hypothesis stating that the mountain ranges (low density masses) extend higher above sea level than other masses of greater density. Pratt's hypothesis rests on his explanation that the low density of mountain ranges resulted from expansion of crust that was heated and kept its volume, but at a loss in density.
- 36. Clarence Edward Dutton (1841–1912),an American seismologist and geologist, also studied the tendency of Earth's crustal layers to seek equilibrium. He is credited with naming this phenomenon "isostasy."
- 37. A third hypothesis, eventuallydeveloped by Finnish scientist Weikko Aleksanteri Heiskanen (1895– 1971) was a compromise between the Airy and Pratt models. John Fillmore Hayford John William Bowie
- 38. Airy hypothesis thatmountains have "roots" which extend down into the mantle. Therefore, elevation is proportional to the depth of the underlying "root".
- 40. Pratt hypothesis Pratt hypothesized thatelevation is inversely proportional to density. Therefore, the higher the mountain, the lower is its density
Editor's Notes
- #7 Scientists did not accept Wegener’s theory of continental drift. One of the elements lacking in the theory was the mechanism for how it works—why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. (It doesn't.)
- #9 If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle…..the shape of Africa and South America are a good example. This is because they DID used to fit together
- #10 The theory of plate tectonics (meaning "plate structure") was developed in the 1960's. This theory explains the movement of the Earth's plates (which has since been documented scientifically) and also explains the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and many other geologic phenomenon.
- #13 However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930.
- #14 Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep.
- #16 At the boundaries of the plates, various deformations occur as the plates interact; they separate from one another (seafloor spreading), collide (forming mountain ranges), slip past one another (subduction zones, in which plates undergo destruction and remelting), and slip laterally.
- #17 In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading
- #23 When a convergent boundary occurs between two oceanic plates one of those plates will subduct beneath the other. Normally the older plate will subduct because of its higher density. The subducting plate is heated as it is forced deeper into the mantle and at a depth of about 100 miles (150 km) the plate begins to melt. Magma chambers are produced as a result of this melting and the magma is lower in density than the surrounding rock material. It begins ascending by melting and fracturing its way throught the overlying rock material. Magma chambers that reach the surface break through to form a volcanic eruption cone. In the early stages of this type of boundary the cones will be deep beneath the ocean surface but later grow to be higher than sea level. This produces an island chain. With continued development the islands grow larger, merge and an elongate landmass is created.
- #32 Isostasy takes place on the Earth wherever a large amount of weight is present. This weight might be due to a large mountain, ice from an ice age, or even from manmade structures, such as the weight from large manmade lakes.
- #34 In the theory of isostasy, a mass above sea level is supported below sea level, and there is thus a certain depth at which the total weight per unit area is equal all around the Earth; this is known as the depth of compensation. The depth of compensation was taken to be 113 km (70 miles) according to the Hayford-Bowie concept, named for American geodesists John Fillmore Hayford and William Bowie. Owing to changing tectonic environments, however, perfect isostasy is approached but rarely attained, and some regions, such as oceanic trenches and high plateaus, are not isostatically compensated.
- #37 American geologist and pioneer seismologist who developed and named the principle of isostasy. According to this principle, the level of the Earth’s crust is determined by its density; lighter material rises, forming continents, mountains, and plateaus, and heavier material sinks, forming basins and ocean floors.
- #38 Advanced by American geodesists John Fillmore Hayford (1868–1925) and John William Bowie (1872–1940), geodesists, or specialists in geodesy, are mathematicians who study the size, shape, and measurement of Earth and of Earth forces The Heiskanen hypothesis, developed by Finnish geodesist Weikko Aleksanteri Heiskanen, is an intermediate, or compromise, hypothesis between Airy’s and Pratt’s. This hypothesis says that approximately two-thirds of the topography is compensated by the root formation (the Airy model) and one-third by Earth’s crust above the boundary between the crust and the substratum (the Pratt model).
- #39 The Airy hypothesis says that Earth’s crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy, an English mathematician and astronomer, assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater.
- #41 The Pratt hypothesis, developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth’s crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled.