It depends on the layer.

Let’s start with the crust, the most abundant element in the crust is the Oxygen, Silicon, Aluminum, Calcium, Iron, Potassium, Magnesium, Titanium, Hydrogen, Sodium, and others.

Mantle##rarr##Silicon, Oxygen, Iron, Magnesium

Outer core##rarr## Liquid Iron and Nickel

Inner core##rarr## Solid Iron and Nickel

http://www.livescience.com/50233-rock-layer-deep-inside-earth.html