How Do We Know What the Earth's Interior is Like?

Ever wonder how we know what the inside of the Earth is like? You were told in school that the inner core is solid iron and nickel, the outer core was liquid iron and nickel, the mantle was partially melted rock, and the crust was solid rock. Did you just accept this and move on? Or did you wonder, "How do they know that? If they can't drill that far, aren't they just guessing?" No, scientists are not guessing. Here is the science explained. The science teacher is in the house.

Scientists have determined what the interior of the earth is like through analyzing seismic or earthquake waves. When earthquakes occur, there are three different types of waves that are sent out from the focus of the earthquake: an p-wave, an s-wave, and an 1-wave. A p-wave is a compression wave that travels down through the earth. Picture a slinky. If you were to show a p-wave using a slinky, you would stretch out the slinky and push the slinky in so that the springs would be getting closer together and farther apart. P -waves can travel through solids and liquids. They are the fastest moving earthquake wave. When the p wave enters a rock of greater density, it speeds up; when it enters a rock of lower density (or liquid) it slows down.

An s-wave is a shear wave. It also travels through the earth's interior. Picture the slinky again. If you were demonstrating a s-wave using a slinky, wave the slinky up and down so that you would create a typical wave. S-waves are slower moving than a p-wave, but also more destructive earthquake waves. S-waves only travel through solids.

An l -wave is a surface wave which, as the name suggests, travels on the surface of the earth. These are the earthquake waves that we think of when earthquakes occur. Picture that slinky one more time. If you were to demonstrate an l-wave with a slinky, you would move it like a jump rope. It moves out from the epicenter of the earthquake like a ripple on a pond. L-waves are earthquake waves that travel through solids, liquids, and gases (that is why you can hear a rumble during an earthquake).

When an earthquake occurs, seismic stations all over the world are receiving the earthquake wave data. They are detecting the p-waves, s-waves and l- waves. Although faint, seismic stations thousands of miles away are able to detect these waves. However, when studying these waves, scientists noticed a "shadow zone" where s-waves were not able to be detected after an earthquake (see picture). The p-waves and l-waves would be detected at the seismic station, but the s-waves would not be detected. Scientists deduced that, because s-waves cannot travel through liquids, a section of the earth's interior must be liquid as well. Performing some simple mathematics on the size of the s - wave shadow zone and the diameter of the earth, scientists were able to tell exactly where and how big that liquid area was, without seeing it.

The scientists had also noticed that, after an earthquake, the p-waves would not follow a straight line when they were traveling through the earth. They would bend or refract, as the traveled through the earth, as indicated by the seismic stations. This refraction of earthquake waves is due to the fact that the waves slow down or speed up when entering a medium of a different density. By following the waves' path, they were able to determine the density of the material that the wave traveled through, and therefore able to tell if the layer was solid, liquid, or partially melted. Through these techniques and the indication that our earth has a magnetic field, scientists were able to determine the composition and nature of the interior of earth, without having ever been there.