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Earth's Dynamic Interior

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In Jules Verne's classic novel Journey to the Center of the Earth, professor Otto Lidenbrock leads an expedition to Earth's core. The explorers make their way through underground forests, witness lightning storms, and survive dangerous encounters with dinosaurs.

Today, scientists know that Earth's interior is nothing like what Verne imagined. But it is no wonder that Verne came up with such fantastical ideas about what might be below the planet's surface. Humans have never been able to explore Earth beyond its outermost crust, and no machine has been created that can probe more deeply. As a result, human understanding of Earth's interior is based almost entirely on indirect measures and educated guesswork.

Scientists have surmised that Earth's interior is a place of almost unbelievable heat and pressure. At its very core, Earth's iron heart is squeezed by a pressure perhaps 4 million times that felt by someone standing on the planet's surface. The temperature there is thought to hover at about 10,800° F (6,000° C).

The only direct measures scientists have come from the world's deepest mines and boreholes. These reach several miles downward into crust. Temperatures in these holes rise an average of 1 F degree per 100 feet (2 C degrees per 100 meters) of depth below the surface. Yet mines and boreholes constitute mere pinpricks in a planet the size of Earth. Indeed, Earth's diameter at the equator is roughly 7,900 miles (12,710 kilometers).

Still, seismologists have learned much about Earth's deep interior by studying seismic waves. Researchers also gather clues about the planet's interior structure by studying other factors. They examine the rotation of Earth, the tides, and variations in the gravitational field. Seismologists try to reproduce in the laboratory the conditions that are believed to exist in the interior of the planet.

Seismic Waves

Many important clues to the physical nature of the planet's interior come from the study of seismic waves, powerful pulses of energy produced by earthquakes and underground-nuclear-test explosions.

Each year, a number of earthquakes occur around the globe. Each quake releases a tremendous amount of energy. This energy travels from the focus, or source, of the disturbance in the form of waves through all parts of Earth, including its very deepest regions. When the seismic waves emerge again at distant points on the surface, their motion is recorded by instruments called seismographs. Scientists around the world keep thousands of seismographs in constant operation to record all such seismic waves.

The seismograph needle traces the movements of the ground under our feet on records called seismograms. If an earthquake occurs in New Zealand, for example, an observer in England can watch the seismograph needle trace the pattern of the arriving waves some 20 minutes later. Similarly, scientists have calculated the travel times of seismic waves produced by nuclear-test explosions.

If Earth were perfectly homogeneous (uniform in substance throughout), seismic waves would spread through it evenly, in all directions and at a constant speed. But this is far from the case.

The Nature of Seismic Waves

Two types of seismic waves are important for geophysical study: compressional, or P, waves (primary waves) and shear, or S, waves (secondary waves). P and S waves penetrate deep into Earth, and travel at different rates through different materials and conditions, their speed largely dependent on the density and elasticity of the material through which they pass. When passing through a homogeneous layer of material, seismic waves tend to increase speed with depth. This is because pressure increases with depth, and pressure squeezes any substance into a more compact, elastic material.

P waves travel through both solid and fluid parts of Earth's interior. In the rocks near the surface, they move at about 3 miles (5 kilometers) per second. They reach their top speed of 9 miles (14.5 kilometers) per second at a depth of about 1,735 miles (2,800 kilometers).

S waves traveling in solid regions move at about two-thirds the speed of P waves in the same region. S waves do not travel at all through fluid regions of Earth.

When S or P waves reach a boundary between two layers of Earth, their speed and direction change. Their path may be refracted, or bent, or entirely reflected, so that they turn upward, back toward the surface. With an understanding of these dynamics, seismologists can use the speed and intensity of seismic waves to chart the interior of Earth, and thereby divide it into distinct layers of different material in a variety of physical states.

In a way, seismic waves are like medical X rays. They allow the seismologist to get an indirect look under the planet's skin. Just as medical X rays pass through bone differently than they pass through soft tissue, seismic waves pass through the solid matter within the planet in a manner that is measurably different than the way in which they pass through fluid, or molten, material.

Admittedly, it is much more difficult to interpret seismograms than X-ray photographs. As a physician examines an X-ray image, he or she sees a picture bearing a definite resemblance to the human body. A seismogram offers only an intricate pattern of wavy lines. To decipher these, geophysicists must use sophisticated formulas and theorems from mathematics and physics.

Anatomy of a Planet

The evidence obtained from seismograms indicates that the interior of Earth consists of two main regions: a central core and a mantle surrounding this core. Each of these regions is subdivided. The core consists of the inner core and outer core; the mantle consists of the lower mantle (also called the mesosphere) and upper mantle (or asthenosphere). Capping it all is Earth's crust, or lithosphere; it is a layer of rigid rocks.

Boundaries

Scientists have also discovered natural separations, or boundaries, between Earth's interior regions.

The boundary closest to the surface is the known as the Mohorovičić discontinuity, or Moho for short. It separates Earth's crust from the underlying mantle. The Moho is found about 20 miles (32 kilometers) below the surface of continents and about 5 miles (8 kilometers) below the main ocean floors. Beneath mountain ranges, the Moho can bulge downward to about 40 miles (64 kilometers). The Moho is named for Croatian seismologist Andrija Mohorovičić, who obtained the first convincing evidence that the interior of the planet was composed of distinct layers. In 1909, Mohorovičić analyzed the seismograms of a Balkan earthquake. He found a marked change in the velocities of seismic waves when they penetrated more than 10 miles (16 kilometers) below the surface. The P and S waves traveled at slower and more-variable speeds before they reached this level than they did below it. Later it was discovered that this boundary extends around the planet.

Moving deeper in the mantle, the next boundary encountered is the transition zone. The transition zone separates the upper mantle from the lower mantle. It is found between 255 and 410 miles (410 and 660 kilometers) below Earth's surface. The transition zone indicates a shift to a denser crystal structure in the lower mantle due to increased pressure. The signature of water has also been found in magma at the base of the transition zone. The water is not in liquid form—the ingredients are bound up in the magma. But the discovery suggests that the movement of plate tectonics may be cycling water between Earth's surface and interior reservoirs. Geologists suspect that a sea of water is dissolved in the transition zone.

The deepest boundary is the Gutenberg discontinuity—the boundary between the lower mantle and Earth's molten core. This separation line—also referred to as the core-mantle boundary—is located about 1,700 miles (2,700 kilometers) beneath Earth's surface. It was named for German-American geophysicist Beno Gutenberg. Gutenberg calculated this boundary in 1914 from an analysis of earthquake-wave velocities. The narrow, uneven Gutenberg discontinuity does not remain constant. It stretches and shrinks as the molten core beneath it swells and shrinks.

Seismic Waves and Earth's Interior

Both P waves and S waves travel throughout the mantle. Since S waves do not pass through fluid matter, the mantle must therefore be solid, except for pockets of volcanic matter. The asthenosphere is softer and more easily deformed than the lithosphere above it. The mesosphere, beneath the asthenosphere, is mainly rigid.

Beneath the Gutenberg discontinuity, the mantle gives way to the outer core. The fact that S waves do not enter the outer core indicates that it is in a liquid state. P waves do pass into the core of the planet. As they do so, their velocity drops suddenly, and their direction is refracted sharply. This refraction causes a great reduction in the intensity of waves penetrating to the surface in an area called the shadow zone. It was the presence of the shadow zone that first provided geophysicists with convincing evidence of the core's existence.

In 1987, scientists at the California Institute of Technology (Caltech) in Pasadena used a technique called seismic tomography to produce computer-generated maps of Earth's outer core. The technique is similar to a medical CAT scan. The results revealed that the core is not a smooth, round surface. It instead contains huge peaks and valleys extending thousands of miles horizontally and varying by several miles vertically.

When P waves reach the inner core, they display a marked increase in velocity. The finding suggests that this part of the core is solid. The inner core has a radius of about 800 miles (1,300 kilometers). The thickness of the outer core is greater than 1,000 miles (1,600 kilometers). The entire core has a radius of approximately 2,200 miles (3,500 kilometers).

Densities and Pressures

By calculating Earth's mass and its volume, scientists have determined that Earth as a whole has a density about 5.5 times that of water. The density of a substance compared with that of water is called its specific gravity. The figure 5.5, therefore, represents the average specific gravity of Earth.

Most rocks at Earth's surface have specific gravities of less than 3. Since the specific gravity for Earth as a whole is more than 5, Earth's interior must contain denser material than the surface rock. Studying seismic patterns, scientists can calculate the densities in the different parts of the mantle and core, since the wave velocities in a region depend on the density of the materials through which they pass.

Other kinds of evidence are helpful in estimating densities at various points below Earth's surface. One source of information is based on Earth's spin. Roughly speaking, Earth rotates about an axis—an imaginary line passing through the center of Earth and connecting the North and South poles. This rotation produces a slight bulge around the planet's equator. The gravitational effect of the Moon on the bulge at the equator brings about certain small changes in the direction of Earth's axis. All these changes provide clues to the geophysicist as to the density of the planet as a whole and that of its interior. Geophysicists then test their conclusions against various physical theories of elasticity and gravitational attraction. Laboratory experiments on actual rock help to further confirm or refute the facts.

Through such means, Keith Bullen calculated in 1936 that the density of Earth, expressed in terms of its specific gravity, ranges from a little over 3 just below the crust to about 5.5 at the base of the mantle. The increase of density with depth is brought about mainly by the tremendous pressure exerted by the overlying rocks. It may also be due in part to changes in the chemical composition of the mantle. Thus, pressure at the center of Earth is greater than pressure at the outer boundary of the core.

At the boundary between the mantle and the core, the specific gravity jumps suddenly from 5.5 to nearly 10. Inside the outer core, it increases, because of the greater pressure, until it reaches 11.5 at the bottom. These figures are widely accepted. Bullen estimated that the specific gravity of Earth at its center is about 13.

Once the density variation within Earth is known, the pressure distribution can be calculated. Atmospheric pressure is about 14.7 pounds per square inch at sea level. This figure is insignificant when compared with the pressure in the very deep interior of the planet. Bullen has estimated that the pressure at Earth's center is about 660 tons per square inch.

Earth's Elasticity

The rigidity of a body is its resistance to forces that tend to distort shape. Earth is not a completely rigid body. It yields elastically under stress, just as some metals do. For this reason, the attraction of the Sun and the Moon causes tidal movements to arise. The tides occur not only in the oceans but in the solid Earth as well. The elasticity of the planet also causes certain fluctuations in the position of its axis.

In 1863, British physicist Lord Kelvin calculated the extent of these fluctuations. On the basis of the above effects, he found that the average rigidity of Earth is somewhat greater than that of steel. This means that, considered as a whole, the planet is much more rigid than the average surface rock. Additional knowledge of Earth's density variations since Kelvin's time has made it possible to calculate the rigidity and seismic velocities in Earth's interior. The rigidity increases throughout the mantle. Bullen estimates that the rigidity at the bottom of the mantle is about three times that of ordinary steel.

In the 1950s, Japanese scientist H. Takeuchi reviewed data on earthquakes and tidal motion of the solid Earth. He concluded that the greater part of the outer core is far less rigid than the mantle. This finding gives the strongest support to the theory that the outer core is in a molten state. An independent series of calculations by Bullen makes it highly probable that the inner core is solid. This part of the central core has a rigidity comparable with that of ordinary steel.

Temperature and Heat Outflow

Scientists have no direct evidence regarding Earth's internal temperature. But knowing Earth's core temperature is key to understanding many of the planet's processes. Iron, however, is a key ingredient in all layers. Scientists reason that the temperature at the boundary of the mantle and the core should be the melting point of pressurized iron. Moreover, the fact that Earth's mantle is not molten sets an upper limit to the possible temperature at its bottom.

Researchers have used a synchrotron accelerator to simulate Earth's interior conditions. They placed iron in the accelerator, compressed it under pressure, and studied its changes when exposed to laser-beam heat. The results suggest that the temperature at the bottom of the crust is probably about 900° to 1,800° F (500° to 1,000° C), and rises steadily with increasing depth. Temperatures in the mantle range from about 3,600° to 7,200° F (2,000° to 4,000° C). The temperature at the center of Earth is about 10,800° F (6,000° C).

There is a small but steady outflow of heat from Earth. By far the greater part of this heat comes from the decay of naturally radioactive material, especially uranium and thorium, in rocks near the surface. In continental regions, the radioactivity is mainly in granitic rocks in the crust. Such rocks are absent in most oceanic regions. Scientists therefore assumed that the heat flow through the ocean floors would be much less than that from the continents. More-recent measurements, however, seem to indicate that the heat outflow from Earth is fairly uniform over the entire surface, even from the ocean floor, and that fluctuations do not exceed 20 percent.

Composition

It is generally believed that the material immediately below the crust consists of silicate rock. Silicates are rocks containing one or more elements combined with silicon and oxygen. Although scientists are unable to obtain samples by drilling, they believe that olivine (an iron-magnesium silicate) is the predominant mineral in the mantle. The evidence for this comes from matching earthquake-wave velocities for this region against the results of experiments made on rock, conducted in geophysical laboratories.

In the lower part of the mantle, the composition of rock appears to be much the same as higher up. The material in this part of the mantle may consist of distinct silica, magnesia, and iron oxide phases.

Until quite recently it was almost universally believed that the central core of Earth was made up chiefly of nickel-iron. This idea was based to a large extent on the study of meteorites—material that has fallen to Earth's surface from outer space. It is generally agreed today that meteorites are fragments of much larger bodies. Scientists traditionally divide meteorites into three main classes: "irons," consisting of iron often mixed with nickel; "stones," resembling rocks at Earth's surface; and "stony irons," a mixture of the first two types.

An analysis of the "irons" indicates that the crystals they contain were formed through the slow cooling of solid nickel-iron. This cooling took place at pressures occurring only deep in bodies of considerable size. It has been calculated that the "irons" were formed inside a body perhaps as large as the Moon.

These calculations suggest that at least some meteorites have come from a planet that was once like Earth, but which has been broken up into fragments. The fragments of such a body would be similar to the materials deep in our planet. If, as was once believed, most meteorites are "irons," nickel-iron must predominate in Earth's core.

Today we realize that the proportion of "irons" to "stones" is not nearly as great as it was once thought by scientists to be. A recent survey by J. Öpik put the proportion as low as 2 percent by mass. Other investigators put the "iron" percentage at a level considerably higher than did Öpik. Still, the matter is far from settled. Whether we accept Öpik's figure or a higher figure, it is clear that evidence provided by meteorites for the nickel-iron content of the central core is not as conclusive as it formerly seemed.

Most geophysicists still believe that the inner core of Earth is made up of nickel-iron, with perhaps some slightly denser materials as well. Many hold that the outer core also consists chiefly of nickel-iron—not solid, but in molten form. Others have their doubts. In 1948, British scientist W. H. Ramsey advanced the idea that the outer core has the same chemical composition as the lowest part of the mantle. He suggested that the difference in density between the two regions is due to the increased pressures below the mantle. Recent computer simulations suggest that Earth's solid core is actually a giant iron crystal.

Earth's Magnetic Field

In the year 1600, English physician William Gilbert set forth the theory that Earth has the properties of a huge magnet, one whose magnetic poles nearly coincide with its geographic poles. He also suggested that Earth's magnetic field originates mainly in the planet's deep interior. These ideas have been confirmed by many investigators.

Many explanations have been offered to account for the magnetism of the planet. One theory assumed that permanently magnetized iron was present in the deep interior of Earth. This theory was disproved when it was shown that Earth's core was partly fluid, and therefore could not hold permanent magnetism. It seems likely that permanent magnetism elsewhere in Earth's interior would not provide a sufficiently strong field.

Patrick M. S. Blackett of the University of London in Britain suggested in 1947 that any massive rotating body generates a magnetic field solely as a consequence of the rotation. However, laboratory experiments with large rotating objects did not reveal any such field. A test on Earth itself carried out in deep mines also failed to lend any substantial support to Blackett's theory.

It now seems probable that the magnetic field of Earth is generated by ordinary electric currents circulating through the planet's interior. Sir Harold Lamb pointed out in 1893 that such currents would have to be continuously supplied from some source of energy within Earth. It is natural to suppose that this would take place in the part of Earth where there is the least electrical resistance—that is, in the fluid outer core.

In 1939, German-born American physicist Walter Elsasser suggested that such a current might arise in the core. He posited that currents could be produced when materials of different electric properties and at slightly different temperatures came into contact. This is called the thermoelectric hypothesis. Thermoelectricity is produced by the unequal heating of an electric circuit composed of two dissimilar metals. In 1954, Stanley K. Runcorn suggested that there might be a thermoelectric effect at the boundary between the mantle and the outer core.

A highly developed theory of Earth's magnetism is the dynamo theory of Elsasser and Sir Edward Bullard. It holds that a huge natural dynamo deep within Earth converts mechanical energy into magnetic energy. The mechanical energy would be supplied by a special type of fluid motion, called convection, carrying electric currents inside the outer core. Elsasser calculated that such motion is possible. It is now known that Earth's inner core rotates faster than the rest of the planet, suggesting that the planet's magnetic field may result from the interaction between the solid inner core and liquid outer core.

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