The Earth’s Interior

The average density of the earth as a whole is twice that of the crust. Evidently the earth cannot be hollow (as was once thought) or even like the crust but must consist of extremely dense materials. What are they likely to be? Are they solid or liquid? Is the interior a uniform mass or does it have a structure of some sort? Is it hot or cold? Answers exist for these questions which, though based on indirect arguments and leaving many details unsettled, nevertheless fit together into a reasonably complete picture of the inside of our planet.

Interior Structure

Earthquake P and S waves do not travel in straight lines within the earth. There are two reasons for this. The first is that the speeds of both kinds of waves increase with depth, so that their paths are somewhat curved owing to refraction. The second reason is more spectacular: there are layers of materials having different properties within the earth. When an earthquake wave traveling in one layer reaches the boundary, or discontinuity, that separates it from another layer in which its speed is different, refraction also occurs. Now, however, the refracted wave shows an abrupt change in direction, unlike the more gradual change due to speed variations within each layer.

Let us suppose that an earthquake occurs somewhere. We consult the various seismological observatories and find that most though not all of them have recorded P waves from this event. Curiously, the stations that did not detect any P waves all lie along a band from 103° to 143° (11,400 to 15,900 km) distant from the earthquake. We would find, if we consulted the records of other earthquakes, that no matter where they took place, similar shadow zones existed. This is the clue that confirmed an early suspicion that the earth's interior is made up of concentric layers.

The earth is divided into a central core and a surrounding mantle. P waves leaving the earthquake are able to go directly through the mantle only to a limited region slightly larger than a hemisphere. Those P waves that impinge upon the core are bent sharply toward the center of the earth, and, when they emerge, they are 4,500 km or more away from those P waves that just barely cleared the core. From an accurate analysis of the available data, it was found that the mantle is 2,900 km (1,800 mi) thick, which means that the core has a radius of 3,470 km (2,160 mi), over half the earth's total radius. However, the core constitutes less than 20 percent of the earth's volume.

Supporting the above finding and giving further important information about the nature of the core is the behaviour of the S waves, which require a solid medium for their passage. The transverse motion involved in such waves requires that each particle of the medium in which they occur drag with it adjacent particles, which is impossible in a liquid where each particle is not firmly attached to its neighbours. The back-and-forth motion involved in P waves, on the other hand, simply requires that each particle exert a push on the next one, which can happen as easily in a liquid as in a solid. Since it is found that S waves cannot get through the core at all, the conclusion follows that the core is a liquid. A liquid core not only accounts for the absence of S waves but also for the marked changes in the velocity of P waves when they enter and leave the core.

The division of the earth into a core and mantle was first suggested in 1906 by R. D. Oldham to explain the presence of shadow zones. Later very sensitive seismographs came into use which detected faint traces of P waves in the shadow zones, which should not have been able to get there at all. In 1936 Inge Lehmann proposed that within the liquid core there is a smaller solid inner core that can bend some of the P waves passing through it so that they reach the shadow zones. Subsequent research confirmed this notion and the inner core is now believed to have a radius of 1,250 km (780 mi).

From observations first made on a 1909 earthquake it became clear that there is a distinct difference between the surface regions of the earth and the underlying mantle. The line of demarcation is known as the Mohorovicic discontinuity, after its discoverer. Under the oceans it is seldom much more than 5 km thick; under the continents it averages about 35 km, and it may reach 70 km under some mountain ranges!

Rocks

There is hardly any limit to the variety of rocks on the earth's surface. We find coarse-grained rocks and fine-grained rocks, light rocks and heavy rocks, soft rocks and hard rocks, rocks of all sizes, shapes, and colors. But close study reveals that there is order in this diversity, and a straightforward scheme for classifying rocks has been developed which simplifies the problem of understanding their origins and properties.