Introduction to The Planet Within the Earth
If you were to go on a journey to the center of the Earth, you would first have to dig through 8 to 40 kilometers (5 to 25 miles) of rocky crust, the “skin” of the Earth. If you were able to accomplish that feat, you would then have to find a way to get through the mantle, another rocky layer–this one about 2,900 kilometers (1,800 miles) thick. As you passed through this layer, you would need some sort of magical suit of armor to survive the intense pressures and infernal temperatures that grow greater as you move deeper. The next layer down, the outer core, reaches a temperature of about 6100 °C (11,000 °F). This layer consists mainly of melted iron and nickel, flowing and swirling around in a soup some 2,250 kilometers (1,400 miles) thick. After miraculously surviving a passage through the outer core, you would come to the inner core, a sphere of iron at the center of the Earth. The inner core is 2,600 kilometers (1,600 miles) wide. Though temperatures here are even greater than in the outer core, the incredible pressure at this depth forces the inner core's iron to remain solid.
A Startling Discovery
Of course, a human could not really survive a journey to the center of the Earth. The trip described above is just a fantasy. It is inspired, however, not only by Jules Verne, the author of the novel A Journey to the Center of the Earth, but also by the work of scientists. Geophysicists have discovered much about the composition of the Earth's interior by using instruments such as seismographs, which amplify and record vibrations within the planet. In July 1996, geophysicists announced a new discovery about the center of the Earth–the inner core rotates faster than the rest of the planet. It is virtually a planet unto itself.
This discovery was made by geophysicists Xiaodong Song and Paul G. Richards, both of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. Song and Richards were inspired to do their research by work that other scientists–geophysicists Gary A. Glatzmaier of Los Alamos National Laboratory in Los Alamos, New Mexico, and Paul H. Roberts of the University of California at Los Angeles–did in 1995. Glatzmaier and Roberts used a highly sophisticated computer simulation to conclude that the inner core rotates faster than the rest of the Earth. Song and Richards tested and confirmed this conclusion by analyzing almost 30 years of data from seismographs.
The Nature of Seismic Waves
The vibrations detected by seismographs are known as seismic waves. Created by literally Earth-shaking events, such as earthquakes and nuclear explosions, they pass from one end of the planet to the other, and they move faster going northward or southward than when going eastward or westward. That is because as the waves pass through the center of the Earth, they encounter a feature of the inner core called the axis of anisotropy. Anisotropy is a variation in physical properties in different directions of measurement. The axis of anisotropy within the Earth refers to the direction in which the iron crystals of the inner core are pointing. These crystals are oriented at a 10° angle to the north-south axis around which the Earth spins. Therefore, the axis of anisotropy is said to be oriented at a 10° angle to the Earth's axis. The more closely the path of seismic waves matches the axis of anisotropy, the faster the waves travel. If the inner core is in fact rotating faster than the mantle and crust, the axis of anisotropy should, over time, change its orientation with respect to any particular point on the Earth's surface. And if this sort of wobbling is occurring, seismic waves moving from a particular source at one end of the planet to a particular site at the other end of the planet should move faster or slower over time.
Song and Richards found that seismic waves from earthquakes in the South Sandwich Islands, between South America and Antarctica, reached a seismic station at College, Alaska, one-third of a second faster in 1995 than in 1967. They explained this discrepancy by concluding that the axis of anistropy pointed more toward the seismic station in 1995 than in 1967, and therefore that the inner core must have rotated faster than overlying layers of the Earth. Their data indicated that the inner core rotates eastward about 1° per year faster than the mantle and crust. In other words, the inner core makes one complete revolution in relation to the Earth's surface every 360 years.
In December 1996, geophysicists Wei-jia Su and Adam M. Dziewonski, both of Harvard University in Cambridge, Massachusetts, and Raymond Jeanloz of the University of California at Berkeley announced the results of their own study of seismic waves. They used methods similar to those used by Song and Richards, but they analyzed data from about 2,000 different seismic stations collected over a 29-year period. They found, as Song and Richards did, that the inner core is spinning eastward faster than the rest of the planet. But their data indicated that the core's rate of rotation is about 3° per year faster than that of the mantle and crust.
Explaining the Core's Differential Rotation
Why does the inner core rotate at a different rate than the rest of the planet? Or, as scientists ask, why does it display differential rotation? Su, Dziewonski, and Jeanloz suggested that the differential rotation could be driven by either gravitational or electromagnetic forces. They noted that scientists have learned that the rotation of the Earth's mantle and crust is slowing down over time due to friction caused by the gravitational pull of the moon and sun. The result of the deceleration of the mantle and crust is that the length of a day on Earth is increasing by about two thousandths of a second every 100 years. Perhaps the deceleration of the inner core simply lags behind that of the mantle and crust, because the fluid of the outer core acts as a buffer. The other possible explanation offered by Su, Dziewonski, and Jeanloz is that physical forces involving the Earth's magnetism may be causing the differential rotation.
The computer simulation of Glatzmaier and Roberts provided a detailed explanation of how magnetism may be driving the rotation rate of the inner core. The simulation indicated that melted iron in the outer core is flowing eastward, carrying a magnetic field that is dragging the inner core along–much as the rotating armature of an electric motor causes its central shaft to turn. This theory implied that, over time, the rotation rate of the inner core may become faster or slower, depending on the movements of the fluid in the outer core. The gravity theory of Su, Dziewonski, and Jeanloz implied only a gradual deceleration of the inner core. Therefore, continued monitoring of seismic waves was expected to eventually reveal which, if either, of these theories is correct.
Earth's Core and Its Magnetic Field
As scientists gained a better understanding of the core of the Earth, they were also learning more about the origin and workings of the planet's magnetic field. Near each of the Earth's geographic poles (the North and South poles) is a magnetic pole. The magnetic poles act like the ends of a bar magnet. The north magnetic pole, for example, attracts the north pole of a compass needle. The magnetic field extends far into space, where it surrounds the Earth and attracts electrically charged particles, such as electrons and protons. Since the 1950's, scientists have explained the Earth's magnetic field with the geodynamo theory. According to this theory, the magnetic field is generated by the motion of the melted iron in the outer core. The metallic fluid is stirred as some of the iron solidifies and sinks toward the inner core and as lighter elements present in the outer core, such as oxygen and sulfur, rise toward the mantle. An electric current forms in the stirred fluid, and the movement of this current produces the magnetic field.
The computer simulation by Glatzmaier and Roberts supported much of the geodynamo theory. It also added a new twist to the theory by finding that the rotation of the inner core may cause additional disruption of the fluid motion in the outer core, which in turn would increase the magnetic force generated.
One indication that Glatzmaier's and Roberts's simulation was accurate was that it produced a reversal of the magnetic field. Studies of the alignment of crystals in ancient volcanic rocks (which oriented themselves relative to the Earth's magnetic field when the rocks solidified) have shown that the magnetic poles periodically trade places. Using more than 2,000 hours of supercomputer time to simulate about 40,000 years of real time, Glatzmaier and Roberts detected one reversal of the magnetic field–approximately what scientists could expect for that length of time.
Perhaps it is not exactly what Jules Verne had in mind, but the center of the Earth has begun to reveal some long-buried secrets to us. We will never actually see the innermost parts of our planet, but learning about them is nonetheless a fascinating journey.