Earth's Tectonic Plates

The Earth is made up of several layers. The top-most layer is called the lithosphere, which is composed of the crust and uppermost part of Earth's mantle. Tectonic plates are made up of Earth's lithosphere and "float" on top of the asthenosphere. The entire surface of the Earth is composed of tectonic plates.

There are two kinds of crust: ocean and continental. Ocean crust is dense, as it is made up of basaltic rocks. Continental crust is made up of granite rocks, which are less dense than basaltic rocks. Therefore, oceanic plates are more dense than continental plates.