The trenches reflect the bending of the ocean floor as the whole plate is driven down below the continents. And those plates we can trace down as far as 700kms on the basis of big earthquakes. Island arcs- islands in the middle of the oceans due to subduction of one oceanic plate underneath another one. (ex, japan and part of indonesia). We are also adding sea water and sediment with the basalt. When we melt all these together we produce a rock type called andesite (which lighter, lower density, and typical of continental crust: this is possibly how the first continent developed in the planet. We get recycling, deep earthquakes: vancouver is located in a tectonic setting that"s called cascadia subduction zone (midterm), rocky mountains- back arc thrust belts. If you were to drive out to calgary from here, you would be driving over the old canadian.