Here's why:

* Crust: This is the outermost layer and is composed of relatively light materials like granite and basalt. It's the thinnest layer, with an average thickness of around 30 km.

* Mantle: The layer beneath the crust is primarily composed of silicate minerals and is denser than the crust.

* Outer Core: This layer is composed mostly of liquid iron and nickel, which are much denser than the crust and mantle.

* Inner Core: This is the Earth's solid center and is the densest layer, composed primarily of iron and nickel under immense pressure.