* Crust: The outermost layer, made of solid rock.

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

* Outer core: A liquid layer made mostly of iron and nickel.

* Inner core: A solid ball of iron and nickel.

P-waves are compressional waves, meaning they travel by squeezing and expanding the material they pass through. This allows them to travel through both solid and liquid materials, unlike secondary waves (S-waves), which can only travel through solids.