UNDERSTANDING THE EARTH'S LITHOSPHERE AND PLATE TECTONICS The lithosphere is the solid, outermost shell of the Earth, consisting of the crust and the upper part of the mantle. It varies in thickness, about 50 km beneath the oceans and 70 to100 km beneath continents. This rigid layer is not continuous, it’s broken into tectonic plates that float atop the asthenosphere, a hot, semi-fluid layer of the upper mantle. These tectonic plates move, and their interactions are responsible for earthquakes,