In geology, a crust
is the outermost shell of a rocky planet, like Earth (as opposed to a gaseous planet, like Jupiter). The crust of the Earth consists of various rock composites and other material. It is about three times thicker under the continents than it is under the oceans, and the oceanic crust is made up of different materials and denser rock. The crust makes up less than 1% of Earth's volume.
The Earth's core is at the true center of our planet. No one will ever drill deep enough to see it. (And no one would want to; the temperature of the inner core is close to that of the sun — almost 10,000 degrees Fahrenheit!) But seismic measurements prove that the earth's core is divided into two parts, a solid inner core with a diameter of about 1,500 miles — about 70% of the size of the Moon — and a liquid outer core with a diameter of about 4,200 miles. The inner core was discovered in 1936 by Danish seismologist Inge Lehmann and is believed to be made up of about 80% iron (by comparison, only about 4% of the Earth's crust is made up of iron). Recent discoveries suggest that the innermost part of the core may contain gold and platinum, too.
Why does the core have so much more iron than the crust? As the Earth was forming about 4.5 billion years ago, the extreme temperatures would have caused all metals to melt, and denser substances like iron sank toward the center of our world while lighter material would have floated to the crust.
The Earth's crust and core are separated by the mantle. Both the continental and oceanic crust "floats" on the mantle. About 84% of the Earth's volume resides in the mantle. While volcanoes erupt lava from here, most of the mantle is actually solid rock.
How far into the earth have we explored? As of this writing, scientists have only been able to drill about 7.5 miles into the earth's crust, but Japanese scientists are making huge advances in drilling and they should break through the mantle within a few years. Once there, these scientists hope to find that primordial microbes are still living under the same conditions as those of the early Earth. This, they hope, could provide new insight into the origin of life.