Earth's solid mass is composed of iron, oxygen, silicon, magnesium, sulfur, nickel, calcium and aluminum. These materials, according to Learner.org, are spread unequally among the planet's three main shells: its core, its mantle and its crust (from deepest to outermost).
According to Space.com, Earth's core is made up primarily of iron at 89 percent and nickel at 6 percent. This layer is subdivided into two sections. The inner core is the solid ball at the Earth's center. It has a diameter of about 750 miles and lies 3,200 to 3,960 miles below the surface. The liquid outer core surrounds the inner core and is about 1,400 miles thick.
Enveloping the outer core is the mantle, which is divided into two shells. The lower mantle mainly consists of magnesium oxide and silicates, while the upper mantle is primarily composed of silicates and aluminum oxide. The thickness of the mantle is about 1,800 miles. The mantle comprises about 84 percent of the Earth's mass by volume and about 67 percent by mass.
Earth's crust is the hard, rigid outer layer and is made up of several plates that variously interlock, overlap and interact while floating on the upper mantle. The crust consists of several elements: 32 percent iron, 30 percent oxygen, 15 percent silicon, 14 percent magnesium, 3 percent sulfur, 2 percent nickel and trace amounts of calcium, aluminum and other elements. The thickness of the crust varies from about 3 miles under the oceans to about 20 miles under the continents.