Geograpgy 1111 Lecture 23 Notes
Geograpgy 1111 Lecture 23 Notes GEOG 1111
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This 5 page Class Notes was uploaded by Bridget Notetaker on Monday October 10, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2015. Since its upload, it has received 8 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 10/10/16
Geography 1111 Lecture 23 Notes Plate Tectonics: A theory of global dynamics in which the lithosphere is believed to be broken into dozens of pieces or plates that move in response to convection and other forces in the upper mantle o History of the theory: Alfred Wegener, a German geologist and meteorologist, gave it serious consideration in 1912, and was the first scientist to suggest that continents were moving, and to supply some evidence for his idea in 1915 Evidence: similarities of fossil records, climatic records, and geologic records between distant continents, especially South America and Africa o Ex: the land dwelling, mammal-like reptile Lystrosaurus, whose fossils have been found in South America, Africa, Antarctica and Asia OR mountain belts of similar age, rock structure, fossils, and structural style appear on both sides of the Northern Atlantic, (Appalachians, Ireland, Scotland, and Norway) Wegener proposed that one landmass existed about 200-225 MYBP (Triassic-Jurassic Period) and called it "Urkontinent", German for "primal continent" and analogous to the Greek Pangaea o This Pangea began to split over the next 10-20 million years o By about 135 MYBP (Cretaceous Period) it had broken into 2 large pieces, Laurasia (basically present-day northern continents) and Gondwana (the southern continents His Theory of Continental Drift took a while to be fully accepted by the established scientific community, however, because he could not explain a mechanism for moving the plates, but near the end of his life, a Theory of Thermal Convection was developed (1928), which added a mechanism for moving the continents, which Wegener had not done 1940-1960's, Atlantic Ocean floor mapping using sonar enhanced our knowledge of tectonic processes by showing a truer image, than had previously been known, of the sea floor topography In 1960, Harry H. Hess and Robert S. Dietz proposed a theory of Sea-Floor Spreading Sea-floor spreading: is the idea that the movement of oceanic crustal material is the result or a mechanism of upwelling magma along a system of mid-oceanic ridges and the pull of gravity at the other end of the plate o The upwelling pushes the crust away from the ridge forming spreading zones, while pulling of the tectonic plate by gravity at the other end forms subduction zones o Because of this, the oldest oceanic crust (seafloor) is furthest from the ridge, and the youngest crust is at the ridge o To date, the oldest seafloor has been dated to ~200 MYBP o The discovery of geomagnetic reversals, or paleomagnetism, first proposed in the 1920s and later connected to seafloor spreading in 1963, helped support the theory of sea-floor spreading and plate tectonics Geomagnetic reversals (paleomagnetism) refers to the magnetic alignment of charged particles (especially iron material) in a rock which exhibit a symmetrical pattern as you move away from the mid-oceanic ridge o The polarity, the orientation of positive (+) and negative (-) particles to the Magnetic Poles of the Earth, changes as you move away from the mid-oceanic ridge o This occurs because as the upwelling magma cools forming new sea-floor (basalt), the charged particles of the iron in the magma are aligned according to the Earth’s magnetic alignment o Different sections of magma cool at different times and thus the polarity changes as the magnetic alignment of the Earth changes o Remember that the Earth’s magnetic alignment changes because of the movement of magma in the Outer Core which forms the Earth’s magnetic field o Paleomagnetism does not cause the plates to move, it only shows that they are moving o By 1980's the term Plate Tectonics was developed, which combined the ideas of sea-floor spreading, continental drift, paleomagnetism and other ideas Plate Tectonics is the theory covering crustal movements and the foundation of crustal tectonic processes to include: upwelling of magma, plate movement, subduction of crust, folding, faulting, warping, fracturing, earthquakes, and volcanic activity o Distribution of Plates: Currently at least 15-20 plates are recognized, with 8 major plates Major plates: Pacific, North American, South American, Eurasian, African, Australian, Indian, Antarctic Smaller plates: Nazca, Cocos, Caribbean, Arabian, Somalian, Scotia, Caroline, Fiji, China sub-plate, and Philippine plates are the largest of these 1 small plate is of interest to the U.S., that being the Juan de Fuca plate which lies between the Pacific and North American plates in northwestern U.S./southwestern Canada How to tell a plate boundary? o This was originally done by mapping the distribution of volcanic and earthquake activity observed/recorded across the Earth’s surface The majority of this distribution is quite linear in shape and often yields the location of plate boundaries EX: “Pacific Ring of Fire” which is a pattern of volcanoes on the Western side of both North and South America, then the eastern side of Asia down to New Zealand (also a zone of high earthquake activity o Mid-Atlantic Ridge and other spreading zones also map as linear features of high earthquake and volcanic activity Types of Plate Boundaries or Interactions: o Divergent plate boundaries: areas where the plates are moving apart from each other These areas are under tensional stress and formed by the upwelling of magma (ridge push) and the pull of gravity (slab pull) They are areas of abundant earthquake activity and relatively mild volcanic activity Seafloor spreading along mid-oceanic ridges is a prime example Ex: Along Mid-Atlantic Ridge, or the formation of the Red Sea as the Arabian plate spreads away from the African plate A spreading center (divergent zone) can also exist on a continent where it may form rift zones or valleys Ex: Great Rift Valley of East Africa, where the Somalian plate is pulling away (eastward) from the African plate o Convergent plate boundaries: areas where plates are moving toward each other These areas are under compressional stress and formed by gravity pulling the opposite end from a spreading ridge of a plate down toward the center of the Earth (slab pull) This causes the plate to collide with another plate, forming what is known as a subduction zone There are 3 basic types of convergent plate boundary: Continental-Oceanic convergent plate boundaries are areas where oceanic crustal material (which is of higher density material) and continental crustal material (which is of lower density material) converge The oceanic crust is subducted (pushed/pulled) under the continental crustal material and eventually remelted in the Asthenosphere This forms a deep underwater canyon or trench on the seafloor at the area of subduction along the plate boundary At the same time, continental crustal material is compressed, forming folded mountains, fault-block mountains and volcanic mountains as magma reaches the surface through cracks and weak points These are areas of numerous earthquake and volcanic activity Ex: Nazca and South American plates converge forming the Peru Trench and the Andes mountains (on the S. A. plate); also the Pacific northwest of the U.S. at the convergence of the Juan de Fuca and North American plates Oceanic-Oceanic convergent plate boundaries are areas where oceanic crustal material of one plate collides with oceanic crustal material of a second plate One plate will be subducted under the other and this is usually the faster moving plate or the one with more force moving it Deep trenches are formed at these boundaries and they are areas of high volcanic and earthquake activity The formation of a volcanic island archipelago (chain) is often common These are formed on the plate that is not subducted Ex: Philippine islands, Japanese islands, and the Aleutian Islands off of Alaska Continental-Continental convergent plate boundaries are areas where continental crustal material of one plate collides with continental crustal material of a second plate There is little or no subduction of either continental plate, but subduction of oceanic material earlier may occur These are areas of great compression and mountain building, both folded and fault-block mountains, often on both continental plates and this numerous earthquakes, but little or no volcanism Ex: The Appalachians formed some 250-300 MYBP when the North American, African and Eurasian plates converged, and the Himalayas currently being formed by the Indian plate converging with the Eurasian plate o Transform or Lateral plate boundaries These areas undergo shear stress as the plates slide laterally past one another They are areas of high earthquake activity and some crustal deformation (creation of hills and small mountains), but no subduction or volcanism Ex: San Andreas Fault Zone in Southern California What causes the plates to move? o Near the end of Wegener’s life, a Theory of Thermal Convection was developed (1928), which added a mechanism for moving the continents, which Wegener had not done This theory suggested that the continents were driven by huge convective cells within the asthenosphere o More recently another theory is that gravity pulls down on the leading, subducting edge of a plate, which drags the rest of the plate with it o Most likely plates move as a combination of these two actions OR a combination of the two ideas
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