I put up this link in g+ and it has created quite a stir. Oceanic underplating is a fascinating topic, and one that has not received enough attention. You can only really pick them up with very detailed seismic surveys, either using an array, or direct geophysics.
Why is it important? It's because of the large carbon cycle (not the little one) as outlined in Carbon1, carbon2, and carbon3.
Now when plates collide they can do a few things. If they are both floaties (silicas, continental crust) then they just smoosh up into mountain ranges. Island arc complexes plate onto existing continents, as happened in the NA east. If it is old oceanic crust, it has become more dense than the hot mantle, and plunges (causing plate pull). This is the case up in Alaska. The plate recycles, as in it melts and puts out volcanoes.
Oceanic crust is the great carbon store of the planet. Most everything ends up at the bottom of the ocean, or ancient shallow seas with huge limestone deposits. All of this gets shoved down.
It it interesting if that oceanic crust is not so old and dense, as with California. Then it doesn't want to go down, and it plates in huge stacks of carbon and water. We are lucky that it stores so much.
Maybe not so lucky if all the continents decide to jam together in a super continent. (We've got a while to go before the next big party). Then what happens? Simple physics tells us we've got a thermal blanket, and the crust gets hot. Lots of volcanoes, but all that carbon stacking is now melting and spewing out carbon and water. It's the Mesozoic! Hot, high carbon and water vapour, and the dinos rule.