* Crust: The thin, outermost layer, composed of solid rock.

* Mantle: The thickest layer, mostly solid rock but with a partially molten layer called the asthenosphere.

* Outer Core: Liquid iron and nickel.

* Inner Core: Solid iron and nickel due to immense pressure.

Some consider the lithosphere and asthenosphere to be separate layers, as they are distinct in terms of behavior and composition. The lithosphere is the rigid, outermost layer that includes the crust and the upper part of the mantle. The asthenosphere is the partially molten layer within the upper mantle.

So, depending on how you count, the Earth can be said to have four or six main layers.