How did scientists determine earth has layers?

Scientists can tell by observing the seismic waves that are recorded all over the surface of the earth from distant earthquakes. The seismic waves are reflected (bounced off) layers of different density, and they are refracted (bent) when they enter layers of different density. read more

Geologists have known for about 100 years that the Earth is composed of four layers; the Crust, the Mantle, the Outer Core, and the Inner Core . Scientists still argue about the makeup of these layers and exactly how each layer interact with the other layers. read more