There are two main types of seismic waves that travel through the earth when an earthquake occurs. One type of wave is called the Compressional wave or P wave; the other is called the S wave or Shear wave.

Now, Shear wave cannot go through a liquid since a liquid offers no resistance to shear; if you pour a liquid from one bottle to another different shape bottle, the liquid rapidly assumes the form of the container! This means that the liquid offers NO RESISTANCE to shear.

So, the outer metallic core of the earth is liquid, then when P and S waves come in ONLY the P wave goes through and then they travel through the inner core. As the P waves pass from outer core to inner core, some of the P wave energy gets converted into shear wave energy and then when the shear wave comes out of inner core, some of ITS energy gets converted back to a new P wave. SOUNDS COMPLICATED? Well, yes it is, but by carefully measuring seismograms, geophysicists determine the density of the material in Earth all the way to the center! And the density of the inner core is consistent with SOLID iron not liquid iron!

Many hundreds of years ago, Issac Newton was one of the first scientists to theorize about the structure of the earth. Specifically, Newton determined that the density of the earth was more than twice the density of the rocks we find at the surface of the earth. How could this be possible? This would only be the case if the interior of the earth was even more dense that the rocks we find on the surface of the earth. Therefore, the center of the earth must be solid and very dense. This observation has been confirmed by measuring the radiating energy of earthquakes; when a powerful earthquake shakes the earth at a certain point, some of the energy radiates as seismic waves, which can be detected around the planet. If we measure the waves on the exact opposite side of the globe from the point that the earthquake originated, the first waves to arrive will have travelled not around the surface of the earth, but right through its center. These waves are very different from the waves that later arrive that travelled through the outer, liquid core of the earth. Only certain types of waves can pass through solids, and so when these waves arrive, we can prove that they must have gone through a solid when they passed through the center of the earth. The earth’s core must therefore be solid.

The second reason that we know the earth has a solid core is that the earth has a magnetic field. A magnetic field is created by magnetic material. Although the solid iron in the inner core is likely too hot to be magnetic, the movement of the molten iron in the outer core over the solid inner core is likely responsible for this magnetic field. Once again, this serves as evidence that the earth’s inner core is solid.

Seismic waves traveling through the earth refract for the same reason that light refracts when entering different substances (e.g. air, water). The inner core has a different density from the outer core, and this difference in density tells us that it is solid.