Convergent boundaries are where one tectonic plate subducts, or is forced underneath another tectonic plate. Depending on the type of lithosphere, different geological features can occur at these different locations. First, we will take a look at what happens when a oceanic and a continental plate collide. Here we see where the Juan de Fuca plate is subducting under the North American plate, just offshore of Northweastern United States and parts of Canada. You can see again there are lots of earthquakes and volcanoes; these are very active spots on planet Earth.
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The image from Google Earth to the left shows the age of the Juan de Fuca plate to be very young (10 million years). Because of this the subduction angle is relatively shallow. There is a perfectly good explanation for this.
To understand subduction angles and to determine which plates will subduct under which ones we need to understand density. Being that the oceanic lithosphere is made of basalt, which has a density of roughly 3.0 g/cm cubed. Continental lithosphere has a density of roughly 2.7 g/cm cubed. When a material is less dense than another material, it will float and the more dense material will sink. The age of the sea floor affects density as well, the younger the sea floor the more residual heat it holds. The hotter a material is the less dense it is relative to it being more dense at a cooler temperature. This is why the Juan de Fuca Plate has a more shallow subduction angle compared to some other plates. Notice how the earthquakes are still relatively shallow. We will look a t a deeper subduction angle to see the depths of the earthquakes at these regions. This is where the Pacific plate subducts under the South American plate, just west of Chili.
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As you can see the sea floor is slightly older than at the Juan de Fuca site. This causes the lithosphere to be cooler and more dense thus subducting at a steeper angle here. The earthquakes happen at deeper levels and can be much stronger.
Also notice at these sites the abundance of volcanoes. As oceanic lithosphere subducts beneath another plate, the residual moister causes partial melting of some of the mantel. As these volumes of partially melted rock creep up towards the surface they build mountains and when this melted rock called magma reaches the surface and erupts we have an active volcano. This is how the Andes mountains were formed and what caused the eruption of Mt. St. Helen in May of 1980.