The Earth can be divided into four main layers: the solid crust on the outside, the mantle, the outer core and the inner core. Out of them, the crust is the thinnest layer of the Earth. The thinnest parts are oceanic crust, while the thicker parts are continental crust. The crust is not one rigid layer, but is broken into fifteen tectonic plates, all in relative movement one to the other. The crust is where rocks interact with the hydrosphere and more importantly, the atmosphere.