 with the trenches, or did it start with the ridges? The new material coming up, or the old material sort of going down and dragging the plates with them. Well, I think it actually began with the heat, the convection driving up and creating the ridges. Well, then the Earth's lithosphere, the upper mantle and the crust, can accept an awful lot of compression. And through a few million years, even a thousand feet of plate can accept this compression.