There are two kinds of crust that make up the plates. The thick but relatively light continental plate, and the thinner but much denser oceanic plate. The oceanic plate is the stuff that fills in the gaps when plates spread apart. Some plates are all oceanic, while others are continental, with bits of oceanic plate hanging off the edges.Originally Posted by Porklet
Now, all oceanic plate is covered with water except where really tall mountains stick up out of it. Continental plate is mostly above water, but around the edges it drops off a bit and is covered with water. This frill of water covered continent is called a continental shelf. Sometimes a larger area, possibly reaching far inland, is flooded, and that's a continental sea.
At Oceanic-Continental convergent boundaries (where they are coming together) the oceanic plate "subducts" underneath and the ocean floor gets scraped up along the front edge of the continent. There isn't really much of a shelf in this case. The west coast of North America between southern BC and northern California (the Cascadia Subduction Zone) is a clear example of this. It has multiple parallel chains of mountains, frequent earthquakes, occasional MASSIVE ones, and volcanoes mixed in with the mountains.
At Oceanic-Oceanic convergent boundaries, you get similar subduction, but instead of mountains, you get a deep trench, and then a chain of volcanoes behind that which may reach up to form islands. Earthquakes are the same as for subduction under a continent: frequent, and occasionally huge.
Continental-Continental boundaries result in both being forced up into HUGE mountains. Without the subduction, you don't get the volcanoes or the really big earthquakes, though you still get the smaller quakes.
Then there are the crazy complicated parts like the Mediterranean basin or the west edge of the Pacific.
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