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Earth Sciences

Plate tectonicsContinental crust is silicarich lower density and has a thicker crust while oceanic crust is iron and magnesiumrich with a higher density then continental crustContinental crust is 7 km thick largely composed of high silica content material like quartz and feldspar which gave it its light density the ocean crust is more dense because it contains more iron and magnesium rich materialsPlates moves away from each other at divergent boundaries most commonly at midocean spreading centers continental rift zones are associated with divergent boundaries oceanic and continental spreading occur at divergent boundariesPlates move toward each other at convergent plate boundaries which result in subduction zones in cases where one or both of the plates are oceanic lithosphere When two continental plates collide neither side is dense enough to be subducted into the asthenosphere so the two sides may often crumple into a thick mass of lowdensity continental material This type of convergent margin is where the largest mountain ranges on Earth are built produces stratovolcanoes island arc majority of volcanoes are at convergent boundariesIf earth generates new oceanic crust at boundaries where plates pull away from each other then it must destroy old oceanic crust somewhere else It swallows old oceanic crust at convergent plate boundariesAs the plates collide Earth solves its dilemma as one plate slides beneath the other and dives into the hot interior of the planet The plate that sinks is the denser of the two the one with oceanic crust on its outer surface It absorbs heat as it sinks into the much hotter rock beneath until it finally loses its identity as cold lithosphere at a depth of several hundred kmContinentContinent Collision where two continental plates collide either plate sinks High mountains such as Mount Everest are pushed up in fits and starts accompanied by large earthquakes Along a transform fault earthquakes can only occur on the segment thats connected to the spreading ridge partBecause the ridges are spreading zones the plates move away from the ridges Studies showed that the earthquakes only occurred in the part of the transform fault that connected the ridge segments The parts of the transform fault that extended beyond the ridge are inactive fracture zones where earthquakes do not occur The section of the fault between the offset ends of the spreading ridge shows significant relative movement Spreading occurs in the opposite direction from the two offset ridge sectionsSubduction zones occur in cases where one of both of the plates are oceanic lithosphereWhere an oceanic plate sinks in a subduction zone a line of picturesque stratovolcanoes rises inland from the trench The process begins at the oceanic spreading ridge where fractures open in the ocean floorEarthquakesIn order to measure and record earthquakes scientists use a seismographThere are different types of seismographs including shortperiod seismographs which are used to record local earthquakes also there are longperiod seismographs which are used to record distant earthquakes longperiod ones record weaker waves at distance with greater accuracy also there are broadband seismographs which are used to record both local and distant earthquakes but cannot record strong earthquakes in direct vicinity of the seismograph lastly there is strongmotion seismographs which record local and very strong earthquakesP waves primary or compressional are the first to arrive and these waves shake back and forth along the direction of wave travel they travel 8 kmsecond through the upper mantle and 56 kmsecond through the continental crust
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