Based on a tremendous number of analyses of earthquakes, carried out globally for nearly a century, the following basic structure of the interior of the earth has emerged. Below a thin crust up to 35 miles thick, lies the mantle, some 1800 miles thick; whereas the deepest zone, the core constitutes the last 2200 miles to the center of the earth.

Separating the crust and the mantle is a boundary called the Mohorovicic discontinuity. Below the Moho or M discontinuity (as it is often abbreviated) lies the mantle. It comprises over 80% of the earth's volume and nearly 70% of its mass. The uppermost portion of the mantle is solid and rigid, part of the lithosphere, with a density near 3.5 g/cm³, and is primarily composed of iron and magnesium silicates (silicates are a group of materials rich in the elements silicon and oxygen). As the temperatures rise with depth, there is a zone in the upper mantle where materials begin to melt, the asthenosphere. Beyond this partially molten zone, the mantle is mostly solid because the higher pressures prevent melting. The lower mantle is composed of oxides (combinations rich in oxygen) rather than silicates, and densities approach 6 g/cm³. The upper and lower mantles are separated by a transition zone. Another major discontinuity, the Gutenberg-Weicher (G/W) discontinuity, separates the mantle from the core. 

It is generally accepted that the core is composed of two zones, both of nickel/iron composition. The high temperatures of the outer core (estimated to be near 2500C) keep it liquid. Again, as was the case for the mantle, even though the inner core is hotter than the outer core, the pressure also rises and prevents the inner core from melting. Therefore, the inner core is solid. The circulation of the outer core coupled with the earth's rotation generates the major part of the earth's magnetic field. Some 16% of the earth in volume, the core accounts for over 30% (32.4) of its mass because of its high density which ranges from 10 to 13 g/cm³ and averages near 11 g/cm³. 

In sum, based on seismic information, the overall structure of the earth consists of three basic layers, the crust, mantle and core, greatly differing in composition and density. These layers represent relict features of an early mobile earth that density zoned itself. The lighter materials rose to the top to form the crust, and the heavier iron settled to the center to form the core, while intermediate materials formed the mantle.