The Core

Shadow zones. The behavior of P and S seismic waves has been used to identify the presence of the core. When P waves originate from an earthquake and encounter the core, they are refracted inward. This refraction creates two areas on the opposite side of the earth where P waves are not detected. Called P-wave shadow zones, these are the intervals on the surface between the last unrefracted P wave and the first refracted P wave (Figure 1 ). Knowing how P waves behave allows the location and shape of the core to be estimated from P-wave data.

Figure 1

P‐Wave Shadow Zone

S waves cannot penetrate the region of the core at all, creating an extensive S‐wave shadow zone across about half of the earth's surface (Figure 1). Since S waves can pass through only solid material, it is very likely that at least the outermost core is liquid or molten.

Figure 2
S‐Wave Shadow Zone

Combining this information with a more detailed analysis of P waves traveling through the core, geophysicists think the core has two parts: a solid inner core and a liquid outer core.
Other data. A combination of seismic data and studies of the speed of the earth's rotation on its own axis and around the sun can also help determine the composition of the core. The average density of the earth is about 5.5 g/cm3; the density of crustal and mantle rocks varies from about 2.7 to 5.5 g/cm3. Since the crust and mantle combined make up over three‐quarters of the earth, the core must have a density ranging from about 10 to 13 g/cm3. This kind of density can be achieved by having a composition of mostly iron mixed with small amounts of oxygen, sulfur, or silica. Some meteorite fragments that have been found approach this composition and are thought to represent primitive pieces of our solar system. The fact that the earth has a strong magnetic field also suggests its core is metallic.

A prominent transition boundary separates the core from the mantle. Up to 200 kilometers (120 miles) thick, the boundary is marked by different densities and temperatures. It may represent the lower limit of mantle convection, where colder, higher‐density material such as subducted plates are reassimilated into the mantle. It may also be the upper limit for convection in the core, where hotter, lower‐density material rises from the center of the earth, cools, and sinks again.