In the picture above, an oceanic plate and a continental plate are coming together. Because the oceanic is more dense, it sinks below the continental crust in to the mantle, causing it to melt, and when it melts, magma is formed. The magma then rises through the crust and rises to the surface, creating a volcano. When the plate sinks below the continental crust, it is called subduction. When this occurs, volcanoes, mountains and trenches are formed. Also, two continental plates can come together, and one will be pushed more upward, and the other one more downward. This mainly causes mountains. And last but not least, two oceanic plates can come together, basically doing the same thing as when two continental plates come together.