What's at the center of the earth?
Dear Straight Dope:
What's at the earth's core? Molten lava? Rock? Hell? Someone told me no one knows for sure.
Making vacation plans, are we?
The inner core of the earth is a hot, dense solid sphere, composed primarily of iron, with some nickel. Its diameter is about 2400 km, about 19% of the earth's diameter. There is some evidence [see reference 1] that the core is in hexagonal crystalline form, possibly one giant crystal!
Surrounding the inner core is a layer of outer core 2300 km thick. The outer core is liquid, again  mostly iron (85%) with some nickel (5%) and some lighter element(s) (10%). The heat-induced roiling in this liquid iron layer is what creates the earth's magnetic field. Together the outer and inner core comprise 1/8 of the earth's volume, but 1/3 of its mass.
On top of the outer core is a layer called the mantle. It's composed mostly of silicate minerals containing magnesium, iron, aluminum, and calcium . The mantle is hot and malleable. It's 2900 km thick (45% of the earth's depth) and comprises 7/8 of the earth's volume and 2/3 of its mass.
On top of all that is the earth's crust. The crust is cool and thin and crispy--it tends to fracture in earthquakes. The crust is 30-50 km thick at the continents and about 10 km thick under the oceans.
How do I know all this? Well, I dug this really deep hole in my backyard, and ... aw, gee, even I don't have a shovel that big. Actually, how we know about all these layers is a fascinating mixture of data, laboratory research, and conjecture.
The composition of the crust is easy to determine--you just dig holes. The crust is composed of--I'm sure you saw this coming--rock. Granite and such.
However, digging holes doesn't get you much beyond the crust. The deepest man-made hole I'm aware of is in the Kola peninsula in Russia. It's about 12 km deep, nowhere near reaching mantle. Some holes are being dug in the ocean , where the crust is thin, and sometimes they find chunks of mantle in those holes. Volcanoes sometimes spew up chunks of mantle as well, so we do have samples of (upper) mantle to study in the laboratory.
We can also do lab experiments to see what happens to the earth's materials under enormous heat and pressure. That's how we know the olivine structure of magnesium-iron orthosilicate in the upper mantle, for example, converts to the spinel structure in the lower mantle [2,4].
As for the rest of the earth, we have to be a lot cleverer. First, we know the mass of the whole earth based on its gravitational influence on other objects. We know the radius of the earth by looking at the inside cover of our old college physics textbook. This gives us the overall density of the earth. Next, we assume that the solar system formed from one interstellar cloud of crud, which clumped together into the sun and planets and other objects; thus the earth must be made of the same general stuff as the rest of the solar system. Most of that cloud of crud formed the sun, and we know what the sun is composed of by looking at its spectral lines. Other interstellar crud is available here on earth in the form of meteorites. So now we can estimate the general abundances of materials we expect to find on earth. Anything we don't find in the crust or mantle (or in the atmosphere) is probably somewhere inside. The largest unaccounted-for element is iron, so that must be in the core.
But most of our evidence comes from . . . earthquakes! Earthquakes provide a kind of X-ray of the earth. Whenever there's a major tremor, waves propagate from the hypocenter all over the earth. There are two basic kinds of waves, S-waves and P-waves . S-waves, or shear waves, are waves in which the wave amplitude is perpendicular to the direction of the wave propagation, like waves on the surface of a lake, or light waves. P-waves, or pressure waves, have wave amplitude parallel to the direction of wave propagation, like sound waves, or shoving a pulse down a Slinky. The two types of waves have different speeds, and move at different speeds in materials of different density, just like light waves travel at different speeds through materials with different indexes of refraction. And, like light, earthquake waves reflect and refract at material boundaries. For the purpose of analyzing the earth's interior, this is the mother lode. Imagine, if you will, an earthquake. It sends out S- and P-waves in all directions. Some waves travel over the surface of the earth, some travel through the crust, some reflect off layer boundaries, like reflections from a pane of glass. The most famous boundary--between the crust and the mantle--was discovered  by Andrija Mohorovičić in 1909 after a big earthquake near Zagreb, Croatia. This Mohorovičić Discontinuity is also called the Moho for the obvious reason that scientists can't be trusted to spell "Mohorovičić" correctly twice in succession.
Earthquakes also give evidence that the outer core is liquid. S-waves are never detected in certain regions of the earth--a shadow zone. Since S-waves don't propagate through liquid, a liquid layer must be forming this shadow. The size of the liquid layer can be determined by the size of the shadow.
Thus, by measuring the speeds of S- and P-waves as they travel through the depths of the earth, and by measuring reflections of those waves off discontinuities, we can put together a density profile of our planet. This profile can be compared to meteorite analyses and the results of the pressure and temperature experiments I mentioned, and a fairly good picture of the earth's interior emerges. Alas for adventure lovers, the picture is strikingly different from the one described by Jules Verne in his 1864 scientific paper, Journey to the Center of the Earth.
 David Schneider, "A Spinning Crystal Ball," Scientific American (Oct 1996)
 William F. McDonough, "The Composition of the Earth" in Earthquake Thermodynamics and Phase Transformations in the Earth's Interior, edited by R. Teisseyre and E. Majewski (2000)
 Enrico Bonatti, "The Earth's Mantle below the Oceans", Scientific American (March 1994)
 Paul Henderson, Inorganic Geochemistry (1982)
 Charles F. Richter, Elementary Seismology (1958)
 Shawna Vogel, Naked Earth (1995)