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Plate Tectonicshttp://pubs.usgs.gov/gip/dynamic/preface.htmlhttp://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.htmlhttp://pubs.usgs.gov/publications/text/dynamic.htmlmodified fromhttp://www.cc.colorado.edu/Dept/GY/rweb/plates.htmlhttp://pubs.usgs.gov/publications/text/dynamic.htmlBasic Details of Plate Tectonics Three types of plate margins: divergent (spreading centers), convergent (subduction zones or orogenic belts), and transform margins (horizontal slip). The details of the different zones are dependent on the types of crust involved. 1) Divergent plate boundaries. In the zone of separation, basaltic magma rises to the surface. The rigid rocks on either side are stretched and form extensional zones dominated by normal faulting (hanging wall down). This is seen both in continental-continental divergence and oceanic-oceanic divergence and is referred to as a rift valley. Because the Earth is a sphere, divergent boundaries separate more the farther away from the axis of separation (Euhler pole). In order to accommodate the change, the divergent boundary is offset along transform faults. These transform faults leave scars called fracture zones.
2) Convergent plate boundaries. If the collision is between a continental-oceanic crust or oceanic-oceanic crust, the more dense plate will subduct below the less dense plate (e.g., oceanic below continental or old oceanic below young oceanic). The angle of subduction is related to the age of the plate; the older the subducting plate, the steeper the angle. Eventually, the subducting plate melts at about 80 km below the surface and the molten lava (typically andesitic) rises to the surface. Again, remember that the surface of the Earth is a sphere so the intersection of the rising magma and the surface forms a volcanic arc that is concave toward the subduction zone. If in the ocean, the volcanoes become an island arc (e.g., Aluetian Islands) and if on land, they form an arcuate belt (e.g., the Cascade Mountains). Many geographical features are associated with subduction zones, described later.
In continental-continental collisions, neither plate subducts. Rather, the convergent zone becomes a site of intense montain building (thrust or reverse faults, folds, metamorphism, etc.). Currently, this can be seen in the Himalaya where India is crashing into Asia. Ancient examples are found along most continental boundaries.
3) Transform plate boundaries. Transform boundaries occur when the two plates move past one another. This is primarily a function of equal density of the plates; however, it also occurs due to the direction of movement. That is, if the direction of movement of the two plates is parallel but opposite, the plates will neither subduct nor diverge. The boundary of movement is called the transform fault. In reality, it is rarely a singular fault but rather a zone. The San Andreas Fault system is a good example (transform margin between the Pacific and North American plates). Outlying the transform faults are records of past tectonic activity called "fracture zones."
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. In fact, in some places, such as Iceland, the mid-ocean ridge is raised above sea level! 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. Very old submerged volcanoes can become abyssal hills. 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. What drives the plates?
slab-pull - Cold, dense oceanic lithosphere is being subducted. It pulls the trailing lithosphere down with it. ridge-push - Elevated slabs on ocean ridges are pulled down due to gravity. mantle plumes- Convection currents occur within the mantle, causing partiallly molten rock to flow. Map showing current plate movement via satellite datahttp://sideshow.jpl.nasa.gov/mbh/series.htmlConvection current animationhttp://education.sdsc.edu/optiputer/flash/convection.htm
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Three basic models for mantle
convection are currently being evaluated.