Evidence for the Internal Structure

The evidence that has allowed scientists to build this model of the Earth’s interior has come from many sources. The Earth’s spherical shape, its mean (or average) radius, and its mass, can be used to determine the average density of the Earth, 5.51 g/cm³.

Density is a measure of mass per unit volume and is usually given in grams per cubic centimeter, written g/cml This calculated density is considerably greater than the average density of the Earth’s crust found by direct measurement, which is approximately 2.7 g/cm³. The high average density value requires that the material below the crust have a much greater density.

Because the Earth wobbles only very slightly as it rotates and the acceleration due to gravity over the Earth’s surface is quite uniform, the Earth’s mass must be distributed uniformly about the Earth’s center as a series of concentric layers. Gravity, density, and the Earth’s dimensions enable us to calculate the pressures within the Earth and also the temperatures that can be reached under these pressures, and, because there is a magnetic field around the Earth, the Earth’s core must include materials that produce magnetic fields.

Another clue to the Earth’s structure is furnished by meteorites that occasionally hit the Earth and are considered to be the remains of unknown planets. More than half of the meteorites that have been found are “stony” silicate or rocky lumps, another large group is made mainly of iron, nickel, and other metals, and a few are “stony-iron” with metal inclusions. Radiometric dating of meteorites gives a maximum age of 4-6 billion years, the same as the age of the solar system. These fragments allow us to directly analyze the density, chemistry, and mineralogy of the nickel-iron cores and stony shells of bodies that we believe to have a composition similar to that of the Earth.

A major tool used to understand the Earth’s interior is the seismic wave. A seismic wave is an underground Earth shock wave, or vibration, that is produced by an earthquake or underground explosion. All over the surface of the Earth geologists and geophysicists monitor recording stations that measure the type, strength, and arrival time of seismic waves caused by earthquakes, volcanic eruptions, landslides, and deliberately caused detonations.

There are two basic kinds of seismic waves: surface waves travel along the surface of the Earth, and body waves travel through the Earth’s interior. Most of the information we have about the Earth’s interior comes from the study of body waves. There are two kinds of body wave. These are P-waves, or primary waves (so called because they travel at the highest velocity and are the first to arrive at a distant station), and S-waves, or secondary waves (so called because they travel more slowly than P-waves and are the second waves to arrive at a station).

P-waves and S-waves produce different types of motion in the material they pass through. P-waves, also known as compressional waves, alternately compress and stretch the material they pass through, causing an oscillation in the same direction as they move (similar to sound waves). P-waves will travel through all three states of matter: solid, liquid, and gas. S-waves, also known as shear waves, oscillate at right angles to their direction of motion (similar to a plucked string).

S-waves propagate only through solids, so they cannot pass through the liquid outer core. When seismic waves encounter boundaries, some of their energy can convert from compressional to shear energy or vice versa. Thus S-waves do pass through the inner core when some of the energy of P-waves passing through the liquid outer core convert to S-waves at the outer core-inner core boundary. These two types of waves are shown in figure 2.2.

Fig. 2.2. Waves passing through a solid are of two types: primary, or compressional waves, called P-waves (top), and secondary, or shear waves, called S-waves (bottom)

As the seismic waves move through one Earth layer and into another, their speeds of travel change and the waves bend, or refract, as shown in figure 2.3 a, b. The paths taken by the seismic waves as they pass through the Earth provide information about the dimensions, structure, and physical properties of each of the internal layers. Because of the paths followed by the S-waves (shown in fig. 2.3b) we know that the outer core is liquid.

The P-waves (fig. 2.3a) are refracted, but not stopped, by changes in density as they pass through the layers. In both diagrams (fig. 2.3a and 2.3b), shadow zones are formed where waves are absent. Measurements of travel time and wave direction confirm that there are abrupt changes of speed and direction at certain depths corresponding to the interior layers. If the interior of the Earth had uniform properties, the waves would follow straight lines and their speed would not change, as shown in figure 2.3c.

Fig. 2.3. Movement of seismic waves through the Earth, (a) Refraction of P-waves and shadow zones produced by the Earth’s interior structure, (b) Refraction of S-waves and shadow zones produced by the Earth’s interior structure, (c) No refraction and no shadow zones occur in an Earth with a uniform structure

Changes in the speed of P-waves as they pass around and through the layers of the Earth are used to develop three-dimensional images of its structure. The P-waves define location, size, and physical state of mantle features. Repeated observations of seismic waves from different locations provide a picture of the processes occurring in this area of the mantle. Changes in the behavior of seismic waves at deeper mantle depths provide data about the physical properties of these areas. The new data indicate that the core is not smooth but has major peaks and valleys over its surface.

These features extend as much as 11 km (7 mi) above and below the mean surface of the outer core. It is thought that the regions above a peak are areas where the mantle has excess heat and mantle material rises toward the crust, drawing the core upward. Cooler, denser, and more viscous mantle material sinks to cause depressions in the core’s surface. It is likely that these peaks and valleys last only as long as intakes a rising plume to lose its excess heat and sink back toward the core, perhaps a hundred million years.

Data from an increasingly densely spaced and sophisticated array of seismic-recording stations and the computer capacity to analyze the travel time of the thousands of seismic waves caused by the world’s earthquakes are used to produce three-dimensional maps of the interior of the Earth. This process, known as seismic tomography, is giving us a more detailed description of the Earth’s interior layers and is demonstrating that these layers are less homogeneous than had been thought.

Three-dimensional tomographic images of the mantle indicate that there are masses of material suspended here that are unlike their surroundings. These shadowy images are considered solid slabs of lithosphere that have sunk into the mantle, slowly melting at their edges. They may sink to great depths, approaching the outer core of the Earth. As these slabs are gradually heated they become less dense, rising again, remelting, and forming pools of molten rock, or magma, that feed regions of volcanism. Higher gravity values are also known to be associated with these sinking zones.

Seismic tomography is combined with model studies to try to understand how plumes of mantle material form. Using layered fluids of different densities and viscosities, researchers build rising plume models with a lower-density fluid underlying a more dense fluid. The structure of these model plumes, their distribution, and the speeds at which they ascend are controlled by the differences in density and viscosity of both liquids and the thickness of the overlying layer. Continuing research will eventually give us a more complete understanding of the processes at work in the Earth’s interior and a better window into the heart of our planet.