Of the 92 naturally occurring elements, the Earth's geosphere -- the solid part of the Earth made up of the core, the mantle and the crust -- is primarily composed of only four. These four are iron, oxygen, silicon and magnesium. These elements make up more than 90 percent of the Earth's mass.
The vast majority of the Earth's supply of iron is found in the core and the mantle. The solid inner core is almost entirely composed of iron, while the liquid outer core is an alloy of iron and nickel, with small amounts of lighter elements. The mantle is composed of iron-magnesium silicates, and the crust is composed of roughly 5 percent iron. All together, iron makes up an estimated 35 percent of the Earth's mass.
Oxygen, the second most abundant element on Earth, is found mainly in the crust. Although oxygen is most often thought of as an atmospheric gas, it is also one of the primary components of the silicate minerals that make up the vast majority of the Earth's crust. Oxygen itself makes up approximately 46.6 percent of the crust and 30 percent of the mass of the entire Earth.
Silicon is found in compounds in both the mantle and the crust, making it the third most abundant element. In the mantle, it combines with iron and magnesium and, in the crust, with oxygen in the form of silicate minerals. These minerals make up common compounds such as quartz, mica and talc, as well as rare stones such as emeralds and opals. All together, silicon accounts for approximately 15 percent of the Earth's mass.
Since the majority of the Earth's volume is taken up by its liquid mantle, it makes sense that magnesium, one of the mantle's key components, is the fourth most abundant element on Earth. Magnesium is also found in the Earth's crust in compounds such as dolomite, talc and magnesium carbonate. Magnesium ranks close behind silicon in mass percentage, making up an estimated 13 percent of the Earth.
A Matter of Definition
The abundance of Earth-forming elements is a geological question and as such is governed by the geological definition of the Earth. This definition includes only the solid geosphere composed of the crust, mantle and core. It does not take into consideration the composition of the atmosphere, the hydrosphere (the Earth's water systems) or the biosphere (the Earth's living systems). These Earth systems must be considered separately in regard to their elemental composition.