At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. Thus, plates and volcanic activity were considerably more active than they are today; the Earth's crust was not only thinner than is today, but probably broken up into many more tectonic plates, with numerous hot spots, rift valleys, and transform faults. The existence of plate tectonics in this eon is disputed: it is an active area of modern research.