Earth's Interior

We've only directly sampled a very small part of Earth's interior. Much of what we know about the interior is the result of analysis of surface rocks and rocks transported to the surface during volcanic eruptions, our understanding of the origin of elements and composition of the solar system, and by geophysical imaging of the interior, often using seismic waves. We've drilled no deeper that about 10 km, and only once did we go that deep. To a good approximation, Earth is a radially stratified planet - a result of it's warmth and its graviational field.

Earth's Composition

Earth's radius is about 6,371 km (about 3,960 mi). The thickness of the continental crust ranges from about 10 to 70 km and averages about 40 km. Beneath the oceans, the crust is about 7 km thick. The radius of the core is about 3,486 km (2,228 mi). The inner core radius is about 1,217 km (756 mi); a little more than two-thirds of the radius of the Moon. 

Earth's mass is approximately 5.973x1024 kg, and its mean density is 5,515 kg per cubic meter. The crust accounts for less than one-half of one percent of the mass of the planet. The mantle accounts for about 84% of Earth's volume but the core contains almost 70% of the planet's mass. The principal elements of the different regions of the planet are listed in the diagram above - many other elements are present, but those listed are the most common.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.