What causes continental lithosphere to become thinner and weaken?

Continental litosphere is made of relatively light weight minerals. It has a low density. Oceanic litosphetre is composed of a heavier minerals. Tensions and heat flow in the asthenosphere (beneath the litosphere) waekens the floating solid plate causing it to break apart. Thinning of the continental litosphere is result of hydration weakening. It is a special consequence of plate tectonics. By hydration weakening the basal portions of litosphere can be transformed into convecting asthenosphere.