Oceanic crust (e.g Nazca) subducts under the less dense continental crust (e.g South American)
The subduction process occurs due to the trench pull movement, this is when the desending plate has a gravitational mass which pulls down the rest of the plate.
Another process which occurs is plate drag, this is when convection currents in the mantle move the overlying plates with it.
Shallow,intermediate and deep focus earthquakes occur along the plate boundary as a result of friction, this is the benioff zone. These earthquakes can be up 700km deep.
The subducting Oceanic plate is saturated in its upper layers, this means when the plate subducts the melting point of the rocks in the upper mantle is reduced. The subducting plate,continental crust and the upper mantle start to melt at 800 degrees because of the present water.
Ryholitic and Andesitic magma is formed from the melted material.
This magma rises in large plutons, nearing the surface.
This creates explosive volcanoes such as composite and ryholitic domes.
When two oceanic plates meet, the denser/faster oceanic plate will subduct under the other oceanic plate.
As it subducts,melting will occur. This is because the oceanic plate has a saturated layer…