What is the Earth mainly composed of?
The bulk of Earth is made of the following elements: Iron, Oxygen, Silicon, Magnesium, Sulphur, Nickel, Calcium and Aluminium.
The abundance of elements of which the Earth is made of is different in different layers. Chemical Composition of Earth's crust is different from that of mantle or core. Details of the elemental abundances of core, mantle and crust are given in the presented in the following sources:
Some References :
Wikipedia article on Earth's Chemical Composition :
What Four Elements Make Up Almost 90% of the Earth? : http://classroom.synonym.com/four-elements-make-up-almost-90-earth-2592.html