How does the thickness of the oceanic crust and continental crust compare?

Oceanic crust is about 6km thick and is made up mainly of an igneous rock called basalt. Continental crust is about 50km thick, is made up of granite and is less dense that oceanic crust.

Thus when an oceanic tectonic plate collides with a continental tectonic plate, the more dense oceanic plate subducts under the continental plate. The oceanic crust is forced down into the mantle where it begins to melt. Magma rises through the other plate and solidifies into granitic continental crust. Hence, at convergent plate boundaries oceanic crust is destroyed and continental crust is created.

Also of note is that we get trenches and island volcanoes formed when an oceanic plate collides with a less dense (but thicker) continental plate.