The document describes the four main layers of Earth: crust, mantle, outer core, and inner core. The crust is the thinnest layer and is divided into continental and oceanic crust. Below the crust is the mantle, which makes up over 80% of Earth's volume and is composed of iron-rich rock. Within the mantle are the lithosphere and asthenosphere layers. The outer core is liquid, while the inner core is solid due to extreme pressure. Seismic waves have helped scientists determine the composition of Earth's layers.