Why is continental crust thickest at collision boundaries?
There are 3 types of destructive plate margins (convergent boundaries). They are Continental-Continental, Continental-Oceanic and Oceanic-Oceanic. Now you are interested in the Continental margins. When two plates are moving towards eachother, the denser plate subducts beneath the other, and then begins to partially melt at depth, with increased heat. Oceanic Plates are denser than continental plates. The oceanic plate is therefore thrusted underneath the more bouyant, less dense continental plate. The water off the subducted plates comes off at depth, and lowers the melting points of the overlying mantle wedge, leading to partial melting of the mantle as well as the subducted plate. This leads to the generation of andesitic magma, as the melted rocks assimilate. The magma then rises to the surface because it is less dense, and hot, and well up underneath the continental plate, overtime, volcanoes will form, when the pressure becomes too great for the magma to stay beneath the surface
