Convergent Plate boundaries

Convergent Plate Boundaries form when two plates collide together. There are three types:

Oceanic-continental convergence
Oceanic-oceanic convergence
Continental-continental convergence

How do plates subduct?

A flat-lying oceanic plate won’t subduct. And old oceanic lithosphere is more dense than mantle. When the two plates are pushed together, a crumple zone forms in the middle. Eventually one side will get pushed under the other and subduction begins.

The subducting plate descends at an average of 45°. This plate descent is revealed by Wadati-Benioff Zone. Shallow focus earthquakes occur in the upper 20 km of the Earth's crust, intermediate ones between 20 and 300 km, and deep focus earthquakes from 300 to 660 km. The location of the intermediate earthquakes coincides with the transition zone in the mantle. Quakes cease below 660 km. Why? Scientists really aren't sure. It could be that both the plate and the mantle are the same temperature, so there is less friction, and therefore less earthquakes. It could be that the plate starts to melt into the mantle at this depth. Or, it could be that the plate "pinches off" at this depth. There is seismic evidence for the lower mantle as a "plate graveyard". Clearly, more research is needed!

Subduction Zones Produce Volcanic Arcs. A curved Earth dictates that volcanic belts are curved. The arc type created depends on overriding plate.

Ocean + Continental = Continental Arc

Ocean + Ocean = Oceanic or Island Arc






Accretionary prisms

Accretionary prisms are deformed sediment wedges that form as sediments scraped off subducting plates are smeared and welded onto the overriding plates. Older plates have more sediment and therefore will produce a large accretionary prism; younger plates have less sediments and therefore have small or sometimes no accretionary prism.





Magma Formation

When the plate subducts, one of the by-products is magma generation. The geothermal gradient, pressure, friction, to name a few, all work together to begin to melt the top of the plate. At a depth of around 200 km all of the conditions are just right for this to occur. The melt rises up through the overriding plate, incorporating bits of the rock into the melt at it ascends, to the surface and form a volcano.

 

Continental-continental convergence

Recall that oceanic crust initially forms wedged inbetween continental crust. So, sometimes subducting oceanic crust has some continental crust attached to it. But continental crust can't subduct. So what happens?

Subduction changes to collision. Subduction consumes ocean basins. And ocean closure ends in continental collision






An example of this is India and Asia. Seventy million years ago (or so) India was an island continent, much like Australia is today. However, the ocean basin to the north of it was being subducted beneath what is now Tibet at a fast pace. Around 20 million years ago India slammed into Tibet and began to form the Himalayas. India is still pushing into Asia and causing the Himalayas to continue to rise today.