Physical Science II
Physical Science II SCI 3120
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Mrs. Clemens Smitham
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This 4 page Class Notes was uploaded by Mrs. Clemens Smitham on Monday October 5, 2015. The Class Notes belongs to SCI 3120 at Clayton State University taught by Staff in Fall. Since its upload, it has received 23 views. For similar materials see /class/219552/sci-3120-clayton-state-university in Science at Clayton State University.
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Date Created: 10/05/15
Chapter 33 Our Restless Planet The theom of continental drift Until recently it was thought that the crust of the Earth was solid and stable in position It was obvious that there were wrinkles in the crust in the form of mountain ranges and some movement in the form of earthquakes but it was thought that these phenomena were the result of contraction of the crust as the Earth slowly cooled and solidified Many people had observed however the strange coincidence in the shapes of the coastlines of the eastern and western sides of the Atlantic Ocean In the early 20th century Alfred Wegener studied this situation He found that the above water land masses of South America and Africa seemed to fit like pieces of a jigsaw puzzle and that the continental shelves fit even more closely He found that fossils of ancient plants and animals seemed to match up both sides of the Atlantic This evidence led him to propose that the continents of Africa Europe North America and South America were millions of years ago part of one larger land mass called Pangaea which has since split up Such an arrangement would explain the strange glacial evidence found at various places on the present continents If the continents were where they are today glaciers must have crossed the equator If the continents have moved the glaciers could all have been confined to much higher latitudes In 1915 Wegener published his hypothesis which came to be called continental drift Although Wegener showed much evidence to back up his idea that the continents had moved he could not provide an adequate explanation of m they moved This situation was similar to the status of evolution before Charles Darwin Other scientists has recognized that life forms had changed drastically over great periods of time but it was Darwin who provided the first explanation of m life changes through the process of natural selection Without a mechanism to explain continental drift most scientists dismissed the idea It was not until the 1950 s that new evidence brought renewed interest in continental drift The field of paleomagnetism is concerned with the Earth s magnetic field over long periods of time Scientists were able to find the location of the North Pole during past eras They discovered that the location changed drastically over time Either the magnetic field had drifted or the continents on which the pole was located had drifted The evidence favored the latter After WWII scientists began studying the ocean oor with the new generation of submarines developed during the war These submarines could dive deep enough to map the ocean oor in the midAtlantic There they mapped the huge mountain range extending North to South throughout the entire ocean Mountain ranges were also mapped in the Pacific and Indian Oceans Geologists were able to witness the actual building process of these mountain ranges when they saw lava being extruded through the crust forming new lithosphere An American geologist H H Hess proposed a hypothesis called sea oor spreading which summarized the observations this way convection currents in the upper mantle asthenosphere create upwellings that eventually push between plates in the lithosphere and harden to form basaltic sea oor and underwater mountains The sea oor is pushed apart which widens the Atlantic Ocean thus providing the mechanism for continental drift that Wegener lacked Sea oor spreading also explains the deep underwater trenches found near some continents As the sea oor spreads it is pushed downward when it encounters continental crust This sea oor is eventually melted into magma which circulates in the mantle Evidence for sea oor spreading is found in the fact that the rocks in the ocean oor are magnetized differently in regions at different distances from an oceanic vent As new material is extruded from the vent it is magnetized by the prevailing magnetic field DIAGRAM When the magnetic field varies as it has done many times the new material is magnetized differently creating alternating strips of magnetic material with the youngest material the vent and the oldest material farther away Theog of plate tectonics The concepts of a differentiated Earth continental drift and sea oor spreading are closely related to the theog of plate tectonics This theory says that the lithosphere is broken into 8 large plates and several smaller ones that oat on the asthenosphere and move with its convection currents The movement of these plates explains a great deal of what we know about geology Three types of plate boundaries Moving lithospheric plates interact in several different ways If plates are moving away from each other we say they have divergent boundaries As plates move away from each other magma may be extruded in the form of lava and new lithosphere may be built This is true in several locations on the ocean floor If magma wells up under a continental landmass it may form an upwarping that may produce volcanic activity or a spreading center may form where the crust becomes thinner and produces a rift valley If the rift valley is near the ocean it may fill in with water and become a linear sea If plates are moving toward each other they are said to have convergent boundaries The collisions of plates may be put into 3 different categories 1 Each plate has an oceanic leading edge one plate will be pushed under the other creating a subduction zone The subduction zone is usually marked by the presence of a deep underwater trench such as the Marianas Trench in the western Pacific The subducted plate will gradually melt forming magma that rises to the surface of the crust and may solidify into underwater mountains If the mountains grow tall enough they will form a chain of islands called a volcanic island arc like the Aleutians near Alaska or the islands of Japan or Philippines 2 One plate has an oceanic leading edge and the other has a continental leading edge Again subduction takes place with melting of the subducted oceanic plate The rising magma appears on the continental plate in the fOI H l of a mountain range such as the Andes Mountains formed as the Nazca Plate is subducted under the South American plate In the US this process has produced the Cascade Range which includes the volcanic mountains of Ranier Shasta and St Helens 3 Each plate has a continental leading edge this interaction does not produce as much subduction as the other convergences Instead the collision is a headon collision Landmasses buckle and produce nonvolcanic mountain ranges such as the Himalayas which are still growing as the IndoAustralian plate collides with the Eurasian Plate When plates move horizontally relative to each other they are neither diverging nor converging They are said to have transformfault boundaries No new lithosphere is created at these boundaries Instead they scrap against each other as they move The boundaries will sometimes stick together by friction The surrounding area will be subjected to tremendous stress as the plates try to move When the plates finally slip the resulting movement is an earthquake In 1906 the Pacific plate suddenly moved about 6 meters north along a 434 km section of the fault This movement produced a deadly earthquake in San Francisco In 1989 a similar occurrence took place A theog that explains much We see that the theory of plate tectonics eXplains a great number of things about our Earth 1 Why are mountain ranges where they are They are near convergent plate boundaries Underwater mountain ranges are near divergent boundaries Where does metamorphism take place In subduction zones Where does basalt come from Sea floor spreading Where does granite come from When the basaltic sea floor is subducted the basalt melts It then undergoes fractional crystallization because it does not crystallize all at once The fractional crystallization raises the silica content and produces granite when the magma finally hardens as a pluton or underground intrusion Where do sedimentary rocks come from When mountains grow as a result of plate collisions they weather and the clastic sediments collect in layers which are eventually compacted and cemented 6 Why do earthquakes occur Movement of plates rubbing against each other Where do volcanoes occur Usually at plate margins AWN U l
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