Geologists of the early part of the last century recognized that Earth history must be many millions of years, not thousands as advocated by theologians. With the discovery of radioactivity and nuclear reactions, we obtained a heat source to allow the sun to be billions of years old, and a clock by which we can measure the age of the rocks. The Earth is about 6370 km in radius. The outermost layer of the Earth is called the crust . Oceanic crust is the layer below the deep ocean basins.
Continental crust is the layer that forms the continents.
The lithosphere is generally considered to be the upper 100 km of the Earth. Note that the lithosphere includes the crust, but also includes part of the upper mantle. The region of the mantle below the lithosphere deforms plastically, although solid, and is termed the aesthenosphere . Note that the aesthenosphere includes most of the mantle, but excludes the uppermost part of the mantle and the crust. Convection in the solid mantle is believed responsible for motion of the surface plates.