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Plate Tectonics and Sea Floor Geography-Lecture Notes Plate Tectonic theory, formalized in the 1960's by J. Tuzo Wilson, explains the past and present configurations of the continents and the basins. The theory was a combination of realizing that continents had moved away from one another ("continental drift" by Wegener in 1912) and that new sea floor was growing in the ocean basins (sea floor spreading recognized independently by Robert Dietz (1961) and Harry Hess (1962) and being subducted back into the Earth in the deep trenches (Kiyoo Wadati, 1935 and Hugo Benioff, 1940's). Plate Tectonics states that the outer layer of the Earth is made up of many plates of both oceanic and continental crust that move about the surface crashing into and sliding past one another. New plates are formed in one place and are subducted back into the Earth in another, thus the Earth is not increasing in radius. Main Plates as given in the text: Antarctic Plate Also: Juan de Fuca Plate (our local little sliver) Oceanic crust is primarily composed of silica-poor, iron-rich rocks (such as basalt) and has a density of about 2.9 g/cm3. Typically, oceanic crust is about 11 kilometers thick. Continental crust is composed of silica-rich rocks (such as granite) and is less dense (2.7 g/cm3). The thickness of continental crust varies but can be as much as 70 kilometers thick, and averages 35 kilometers thick There are three basic types of boundaries between plates: separation or divergent; collisional or convergent; and "slide-past" or transform. Divergent margins are where two plates are moving away from one another. The space between the plates is the site of formation of iron-rich magma, called basalt. Other features of divergent margins include normal fault-bound deep basins or rift valleys and an increase in heat flow. Convergent margins are where plates crash together. The crash can result in one plate diving below another plate or both plates crunching up against each other. The result is a function of the contrast in thickness and density of the plates involved. Rarely, the thinner, denser plate can override the thicker, less dense plate. This is called obduction. As can be expected, these margins are the sites of very large earthquakes. Also, if one plate is diving under another plate, it eventually melts and the rising magma forms an arc of volcanoes. Transform margins are where two plates slide past one another. Many earthquakes are associated with the sliding so it is not a "smooth" process. Where associated with divergent margins, the transform faults terminate in fracture zones where there are no relative movements. Plate tectonics is driven primarily by convective forces within the mantle. These convective currents may originate at or within the core of the Earth where heat anomalies cause the mantle material to heat and rise. Eventually, this flowing material cools and descends back toward the core. Gravity also plays a role in driving the motions by pulling the older, denser oceanic crust into the mantle. However, we are very far from realizing just what causes the motions and how the various factors interplay. Proof of Plate Tectonics comes from: 1. Recreating the fit of the continents-both through shape and similar features, such as fossils and unique rocks. 2. Recognition of mirror-image magnetic stripes on the sea-floor abounding mid-ocean ridges. 3. Location and chemistry of arc volcanoes. 4. Global earthquake plots, particularly deep earthquakes at the subduction zones. 5. Traces of hot spots-areas where molten magma rises to the Earthís surface independently of the motion of the plate. Therefore, if the plate is moving over a "hot spot" the result is a linear line of volcanics which becomes younger toward the hot spot. Geographical features associated with Plate Tectonics: Mid-ocean ridges. Long mountain chains on the sea-floor that are elevated relative to the surrounding ocean floor. These areas are also much hotter than the surrounding sea floor due to the increase in volcanic activity. Trenches. Deep, arcuate features, typically at the borders of the oceans where oceanic crust meets continental crust. The trenches are filled up with mixed-up sediments that have been scraped off of the plates that are colliding into each other. Trenches also occur where one oceanic plate is diving below another oceanic plate. Fracture zones. Lie outboard of transform faults where the fault ends and so the same plate borders both sides of the "fault." The fracture zones record sites of past faulting activity. Continental margin. Because of the density difference between continental and oceanic crust, a particular geometry develops where the two types of crust meet. Starting from the continent, there is first a broad, flat zone called the "continental shelf." Then, near the end of continental crust, the angle increases and the area is called the "continental slope." Further out, at the actual border between the two crusts, the slope decreases, thus the "continental rise." Mid-Plate volcanoes. A broad term to explain the many volcanoes found far away from the spreading center, or mid-ocean ridge. The volcanoes formed either due to hot spots (such as the Hawaiian chain), or actually formed at the spreading center but were carried away along with the plate. Over time, the volcanoes stop accreting new material and sink below sea level as the oceanic crust cools. Sea mounts are volcanoes below sea level, and guyots are volcanoes below sea level in which the top has been planed off. Island or volcanic arcs. Found adjacent to trenches. Site where the rising magma from the subducting plate reaches the surface. These chains are arcuate owing to the spherical geometery of the Earth. Typically, these volcanoes have a mixed lithology between continental and oceanic crust (andesite). The closest volcano of this type is Mt. Shasta. Other famous island arcs include the Aluetians, Japan, and the Andes. Other sea-floor sites of interest: Submarine canyons. Deep canyons eroded into the continental margins. Still much debate as to how these canyons form. Although most may have begun at the end of subaerial (above the ocean) land canyons, all show signs of being eroded underwater. Canyons serve as conduits for the transport of sediment from the continental shelves into the deep sea. These sediments may move as dense "turbidity flows." Submarine canyons can be larger than the largest land canyons! Hydrothermal vents. Found in many areas of the sea floor near spreading areas, and even in Lake Biakal! Vents are small chimney in which super-heated water emerges from underground. The water is not steam due to the high pressure of the overlying water column. Most, but not all, vents release waters that are rich in minerals. These vents are sites for bizare animal/plant communities. Abyssal
plains. Broad, featureless areas of the deep ocean floor. The
smooth surface is caused by the underlying topography being smothered
by thick deposits of sediment. Typical depths are 3,700-5,500 meters
below sea level. http://gldss7.cr.usgs.gov/ http://triton.ori.u-tokyo.ac.jp/~nakanisi/lineation.html http://geosun1.sjsu.edu/~dreed/onset/exer5/cookxs.html http://www.geo.cornell.edu/geology/cap/ADSW_report/Fig_1.html http://vulcan.wr.usgs.gov/Imgs/Gif/Cascades/Cascades_eruptions.gif http://www.agu.org/sci_soc/trehu.html http://obs.er.usgs.gov/mendo94.htm http://www.geol.ucsb.edu/%7Eatwater/Animations/Animations-FR.html http://www.geo.cornell.edu/geology/GalapagosWWW/GalapagosGeology.html http://newport.pmel.noaa.gov/axial98.html http://bonita.mbnms.nos.noaa.gov/sitechar/geol2.html http://walrus.wr.usgs.gov/docs/outreach/seamap/LAshelf.html http://newport.pmel.noaa.gov/gis/brwn97leg3.html http://www.pmel.noaa.gov/vents/objectives.html http://www-ocean.tamu.edu/~wormuth/hydrothermalvents.html
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