The rocks in the asthenosphere move in a fluid manner because of the high temperatures and pressures found there. Tectonic plates are able to float upon the fluid asthenosphere because they are made of rigid lithosphere. The earth's solid surface is about 40 percent continental crust. Continental crust is much older, thicker and less dense than oceanic crust. B Oceanic Crust Oceanic crust makes up the other 60 percent of the earth's solid surface.
Oceanic crust is, in general, thin and dense.
This production of crust does not increase the physical size of the earth, so the material produced at mid-ocean ridges must be recycled, or consumed, somewhere else. Geologists believe it is recycled back into the earth in areas called subduction zones, where one plate sinks underneath another and the crust of the sinking plate melts back down into the earth. Oceanic crust is continually recycled so that its age is generally not greater than 200 million years. It is composed of a top layer of sediment, a middle layer of rock called basalt, and a bottom layer of rock called gabbro. Both basalt and gabbro are dark-colored igneous, or volcanic, rocks.