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Looking Inside Earth Although the interior of Earth is only a few tens of miles beneath our feet, it is more difficult to reach than the surface of Pluto or even a nearby star! The deepest mines in the world are only three to four kilometers deep. The deepest well ever drilled only penetrates 12 kilometers into the interior. Since it is 6371 kilometers to the center of Earth, comparatively speaking, we have not even been able to get through the skin of the peach. Why haven't we gone any deeper? Even at a depth of 12 kilometers, the high pressures and temperatures of the surrounding rock cause the metal of even the strongest drills to quickly weaken and deform. At depths of 100 kilometers, rock flows like butter, and any hole we could form would quickly close. Yet we have learned a good deal about Earth's interior using indirect means. Earthquakes and nuclear explosions generate sound (seismic) waves that travel through the entire volume of Earth. By measuring the intensity and timing of seismic waves at many locations on the surface of Earth, we are able to determine the density of materials at different levels in the interior. (The technique is a very sophisticated version of tapping on a barrel or wall to see if there is something inside.) By measuring the properties of different materials under high pressure in the laboratory, we can make a good estimate of the composition of different parts of Earth's interior.
The large zone corresponding to the flesh of the peach is called the "mantle." It is made mostly of rocky minerals with names like olivine, pyroxene, quartz, and periclase. (Any good book on geology or mineralogy will tell you everything you ever wanted to know about these minerals.) Almost all rocks are made of silicon and oxygen mixed with other elements, such as calcium, aluminum, and iron. Mantle rock contains particularly large amounts of iron and magnesium. The mantle is also divided into two solid parts, the lower mantle and the upper mantle. This division in the mantle may be due to small differences in the amounts of iron and magnesium or to differences in the nearness of atoms to each other. (As an analog, think of a piece of Styrofoam. If you crush it, the particles inside move closer together because you have closed the tiny open spaces inside, but it is still made of Styrofoam.)
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From all these measurements, we
find Earth's interior is layered something like the inside of a peach. The "pit"
at the center of Earth is called the "core." It is made mostly of iron, some
nickel, and about 10- 15 percent of a less dense material, probably silicon, oxygen, or sulfur.
The core itself is made of two concentric pieces, a solid inner core and a liquid outer
core.