Plate Tectonics.docx

7 Pages

Earth and Ocean Sciences
Course Code
EOSC 116
Nancy Gallini

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Plate Tectonics: Grew out of the Theory of Continental Drift, observation by Alfred Wegener in 1915 that some of the continental land masses seemed to fit together, and later drifted apart - Plate move across the surface of earth at seemingly random directions and at different rate Significance of Plate tectonics: 1. Almost all earthquakes, volcanoes occur where lithospheric plates are interacting with each other 2. Most of main mineral and hydrocarbon resources occur in specific tectonic settings 3. Plate tectonic process are almost entirely responsible for what happens at the earth surface ex: size and shape of oceans, nature and distribution of landforms and general climatic conditions Paleogeography: study of evolution of landforms through time  Subaerial: areas exposed above sea level  Submarine: covered by very shallow seas Plate Tectonics: basic concepts  Oceanic crust is thinner but denser than continental crust  3 ways of interaction o Divergent boundaries: rift apart  Ex: red sea o Convergent: move towards  When one lithospheric plate pushed down another it is being subducted  When oceanic crust is pushed down another plate, large volumes of magma are produced, magma rises to shallow levels in the crust of the overriding plate, and form volcanoes, volcanoes form a linear or more arcuate line (an arch)  If the overriding plate is oceanic, than isolated volcanic island(oceanic arc) will be formed  If overriding plate is continental, the plate will be thicker and less dense than oceanic plate, so it will be mainly suberial, the arc formed is called continental or Andean-type  If continental plate subducted another continental plate, a mountain such as Humalayan forms o Transform: sliding along  Ex: San Adreas fault  Mantle Plumes (hotspots) o Hotspots from very deep in the mantle produce large volumes of magma that rise to the surface and erupt  Ex: Hawaiian Islands Mantle Convection Model -is that it is believed that heat from deep in the earth cause large scale convection to occur within the mantle, and caused individual plates to be dragged passively along on top of convection cells Ridge Push Model  Alternative model is for mid ocean spreading ridges: that newly formed oceanic crust is thermally uplifited, and that oceanic plates are sliding off this uplifited zone Slab Pull Model  When oceanic lithosphere is subducted to considerable depth, the elevated heat and pressure converts the rocks that make up normal oceanic crust, the oceanic crust materla basalt is less dense than the underlying mantle, and therefore floats on it. These are transformed into rock eclogite, which is much denser than the surrounding mantle, it then forms a huge weight on the lower edge of the subducting plate, and as eclogite sinks down into mantle, it pulls the rest of the plate behind it Current views of plate motion  Believed to be combination of slab pull and ridge push Plate Motions in Earth’s history  Magnetic anomalies on the sea floor o Critical tool for plate reconstruction is magnetic striping on the ocean floor o Magnetic stripes form at spreading ridge, the earth is affected by magnetic field, which changes orientation by 180 every few to tens of million years o We can compare the magnetic field that is locked into the rock and when the earth’s magnetic field is normal, there will be normal magnetism and vise versa o However we can only use magnetic stripes to about 180-190 million years  Paleomagnetism o Use to determine past movements of rocks in N-S Direction, the volcanic lava that flows onto the earth’s surface near the equator will preserve the magnetic lines of forces that are parallel to the top and bottom surfaces of the flow  Hotspot tracks o Chains of vaolcanoes produced when a lithospheric plate moves over a fixed mantle plume Volcanic Activity and Orogeny  As oceanic crust being subeucted, water is driven off the down going plate, and water passes up into the overlying wedge of mantle material where it causes the mantle material to melt. This rises buoyantly and building volcanoes. This produce chain of isolated volcanoes, theses volcanoes are called composite cones or stratovolcanoes The principle of Isostasy  Isostasy: state of gravitational equilibrium or balance between the earth’s lithosphere and asthenosphere  If the crust along a convergent margin becomes thicker, average desnity is decreased, it will cause the crust to float even higher and create a continuous mountain belt Plutons, batholiths, and mountain ranges  Not all of the magmas are erupted, most of it ends up cooling and crystalizing, called plutons. A group of adjoining plutons is referred as batholith. This emplacement of plutons inflates the crust with relatively low density rock masses, and so according to isotasy, it will create a continuous mountain belt. o Ex: Andes Moutain belt, coast range batholith Mountain rangers are ephemeral  Mountain doesn’t last very long because materials will being to erode off by rivers, streams and glaciers Mountain ranges can be long-lived  Although the mountains continually being eroded, the continued subduction of oceanic crust from the west continues to produce low density magmas that rise, inflate, and thicken the curst from below Glacial rebound  Allows portions of the curst to be uplifited when large sheet of ice is removed Crustal Extension and Orogeny  Fault is simply an inclined fracture or crack in the earth’s surface along which some movement has occurred  Normal fault is when inclined fault, the hanging wall block is down dropped with respect to the block below the fault foot wall block, the hanging wall block is always the one leaning onto the other, this type of faulting produces a typical pattern of parallel uplifted blocks called horsts and downdropped blocks called grabens, this creates narrow mountain belts  Ex: basin and range province Crustal compression and Orogeny  3 way to build mountain is by shortening/compressing parts of the crust, it will cause the hanging wall block to move up with respect to the footwall block.  Ex: sevier mountains and Canadian rockies Crust that is being shortened and compressed can be thickening by both reverse faulting and by folding Terrane: Crustal block or fragment that preserves a distinctive geologic history that is different from surrounding areas and that is usually bounded by faults Accreted Terranes: terranes that become attached to a continent as a result of tectonic process Cordilleran belt  Most of it comprises magmatic arc terrance Evidence for origins of exotic terrance  Faunal associations: o Ex:everal of the accreted terranes in the Canadian Cordillera contained Jurassic ammonite faunas that were typical of faunas formed far to the south of their present location  Late Paleozoic macrofossils o Using fossils o Ex: fossil in the Cache Creek terrane transported into Canadian cordillera from somewhere closer to equator Plate tectonics and paleogeography reconstruction  Early to middle Triassic (~240 Ma) o Oceanic crust was subducted under the western margin of N.American, producing volcanic arc  Early Jurassic (~180 Ma) o Large terrane called Wrangeilla beginning to move towards the edge of the continent  Middle Jurrasic(~160 Ma) o Southern end of Wrageilla collide with the edge of N.America  Late Jurrasic(~145 Ma) o Wrangeilla continue to collide with N.America, the oceanic plate Farallon plate converging obliquely to the south , so Wrangeila slide woutwards  Early Cretaceous (~125 Ma) o Farallon plate is almost orthogonally to the coast, Wrangeilla began to push against margin, start to create Canadian rocky.  Mid-Cretaceous (~105 Ma) o Farallon plate was converging obliquely to the north, so Wrangeilla begin to pushed slightly north, mountain build and rocky was being formed by crustal thickening  Late Cretaceous (~85 Ma) o Farallon plate split into southern and northern part (Kula plate), Kula plate moving parallel to cost, and farallon p
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