Plate Boundaries

Oceanic-Oceanic Convergent Plate MarginThe slower or older/colder oceanic plate is carried downwards at the trench, into an area of higher temperature. The geothermal gradient increases with depth, so as the plate is subducted the temperature increases. partial melting of the basaltic oceanic plate begins as the lower temperature minerals begin to melt the melted minerals separate and being less dense than the surrounding oceanic crust, begin to rise as magma with an intermediate composition the magma erupts at the surface to form volcanoes which erupt intermediate and mafic lava such as andesite and basalt the volcanoes create an arc of volcanic islands, reflecting the curving shape of the convergent plate boundary beneath the rocks of the island arc are metamorphosed by the increase in temperature and pressure. Some of these rocks are metamorphosed sediment scraped off the descending plate Oceanic-Continental Convergent Plate MarginAll the processes operating at oceanic-oceanic plate margins are the same here. However, the presence of continental crust makes it more complicated. the magma rising from the subducting oceanic plate is at a high enough temperature to cause partial melting of some of the continental crust that passes through. This means that some of the silicic magma formed may reach the surface to form explosive rhyolitic volcanoes and some will mix with the rising mafic magma to create large volumes of intermediate magma that form the enormus strato volcanoes of the Andes. large batholiths will also be formed from melted continental crust and intruded deep in the core of the fold mountains compression of the continental crust and the sediments scraped off the subducting plate will form fold mountain chains. These are intensely folded and faulted, creating fold mountains with major thrusts, nappes and overfolds. The continental crust shortens laterally and therefore extends vertically as high mountains with deeo roots, adding to any uplit caused by the oceanic plate forcing its way under the continental plate. many of the rocks in the deeper part of the mountain chain will be regionally metamorphosed by the heat and pressure. Gneiss and migmatite in the core of the mountains grade outwards  through schist and slate segments of the former oceanic crust may be broken off at the top of the subduction zone, trapping an ophiolite suite within the fold mountains. Conservative Plate MarginsIn some places plates slide laterally past each other. The San Andreas Fault, along with its many associated faults, is the best known example. There is no subduction and therefore earthquakes which occur are shallow-focus. There are no volcanoes or other features to mark the plate boundary. The movement of one plate alongside the other is unremittins, so California will continue to suffer severe earthquakes well into the future.The difficulty when trying to predict this movement is that parts of the fault are creeping, making many small movements, while the other sections are stuck. In these areas there are seismic gaps where no earthquakes have occurred to release stress. The frictional strength is overcome as stress builds up and the rocks suddenly move to produce a major earthquake. The San Andreas Fault zone has seismically active sections where small earthquakes are frequent, and locked sections where there is no known recent movement. The strain builds up over many years in the locked sections, until there is a sudden break. The last big earthquake on a locked section was at San Francisco in 1906. It registered 8.6 on the Richter scale. Many parts of the city were completely flattened and approximately 700 people were killed. This part of the fault has not moved since. If the fault had locked for 100 years then 5m of sudden movement would be possible. (5cm x 100 years)

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