Evidence that support the concept of a layered structure of the earth comes from several sources.
First, rock samples taken from great depths do have higher concentrations of Iron and Magnesium in them than average crustal rocs. And second, when the mass and size of the Earth are measured, the resulting density figure is 5.5 gm/cm3. This is about double the density of rocks found in continental crust. Again, the heavier, denser part of the Earth should be in the deep interior. Further evidence for the internal structure of the Earth comes from the high temperature and pressures known to prevail at deeper levels.
But the most convincing body of evidence is derived from the analysis of earthquakes which generate pulses of energy called Seismic Waves that can pass through the entire Earth.
Nature of Seismic Waves:
Seismic Waves take time to travel through the Earth. In general terms, the speed of an earthquake wave is proportional to the density of the material through which it travels. The denser the material, the faster the speed is. Seismic waves change direction under certain circumstances. When a seismic wave travelling through a less dense material reaches a place where the density becomes much greater, it may be bounced back; this is seismic reflection. If the contrast in densities between the adjacent layers is less severe, the wave may be bent rather than being reflected. Here, seismic refraction changes the course of the seismic wave. Within a given layer, the speed of seismic waves generally increases with depth.
Figure: When seismic waves travel through the interior of the earth, several things happen. When they reach a plane where the rock material becomes much denser, they may be reflected back. If the contrast in rock density is less, they may be refracted. Their velocities are also also affected. Speeds would be less in the layer marked X and greater in layer Y.
Types of Seismic Waves:
Seismic waves behave differently as they propagate through the Earth. One type of wave travels along the surface of the crust, controlled by the elasticity of the strata, and is termed a Surface/L-waves.
Two other types of wave travel through the interior of the earth and are referred to as body waves. The body waves are of two kinds, P-waves and S-waves. The P-waves are compressional waves, sometimes called ”Push waves”. As they propagate, they move material in their path parallel to the direction of the movement. They even travel through material in the liquid state, although their impact is then much reduced. The S-waves are called ”shear” or ”shake” waves. These waves move objects at right angles to their direction of motion. They do not propagate through liquid material.
Thus, depending on the nature of the layers through they pass, seismic waves are speeded up or slowed down or bent, and in some cases, stopped altogether. These observable changes in seismic wave motions enable seismologists to probe the Earth’s interior.
Proof of Earth’s Internal Layers:
If an earthquake occurs at 0 degree, P as well as S-waves are recorded by seismographs anywhere to 103 degrees from its source. Then, from 103 and 142 degrees, neither P nor S waves are recorded. But from 142 to 182 degrees , P-waves reappear. From this evidence, it is concluded that the earth possesses a liquid that begins about 2900 km below the surface. At the contact between this liquid layer and the layer above it, S-waves cease to propagate and P-waves are refracted.
But some P-waves that arrive on the far side of the Earth, between 142 and 182 degrees, have not been refracted once or twice but for times. Moreover, their speed has increased. This means that the P-waves that reach the seismographs located antipodally to the earthquake source (i.e, on the exact opposite point of the spherical earth), must have traveled through a very dense mass inside the liquid layer. Confirmation of the existence of such a dense mass at the core of the times of these reflected P-waves, it is concluded that the earth has solid inner core.
Examine the path of the P and S-waves as they travel through the Earth in the diagram below.
Note the curved wave paths indicating gradual increases in density and seismic wave velocity with depth. Also note the sharp refraction (bending of waves) at the discontinuities or boundaries between layers. Also note the shadow zones, there is a large S-wave shadow zone labelled no direct S-waves extending across the side of the globe opposite from the epicenter from 105 degree. S-waves can not travel through the molten (liquid) outer core. There is a smaller P-waves shadow zone, seen on the both sides, from 105 to 140 degree. The P-waves shadow zone makes a ring around the globe. Major layers of the Earth were detected before 1950.
On the basis of seismic and other evidence, therefore, the interior earth is believed to have four layer; a solid inner core, a liquid outer core, a solid lower mantle, and a partially molten upper mantle. On top of all this lies the crus, still very thin and in places, active and unstable.
Refraction and reflection of S- and P-waves are explained through the following diagram very clear.