For the most part, this book is concerned with geologic, geophysical, and geochemical processes that take place on or near Earth's surface. Even the essay Plate Tectonics, which takes up one of the central ideas in modern earth sciences, discusses only the lithosphere and crust but not the depths of the mantle or the core. Yet there are several good reasons to study Earth's interior, even if it is not immediately apparent why this should be the case. At first glance it would seem that activities in Earth's interior could hardly be removed further from day-to-day experience. By contrast, even the Moon seems more related to daily life. At least it is something we can see and a place to which humans have traveled; on the other hand, no human has ever seen the interior of our planet, nor is anyone likely to do so. What could Earth's interior possibly have to do with everyday life? The answer may be a bit surprising. As it turns out, many factors that sustain life itself are the result of phenomena that take place far below our feet.
HOW IT WORKS
The Core: Gravity and Density
In the essay on Planetary Science, there is a discussion of an age-old question: "Why is the earth round?" or rather "Why is Earth a sphere?" The answer, explained in more detail within the context of that essay, is that gravitational force dictates a spherical shape. As far as we know, there is no such thing as a planet or sun in the shape of a cube, because for every large object, the gravitational pull from the interior forces it to assume a more or less uniform shape. Since there is no shape more perfectly uniform than that of a sphere, this is the typical form of bodies that possess large mass.
In fact, the greater the mass, the greater the tendency toward roundness. Although they are less dense than Earth, Jupiter and Saturn are certainly more massive, and therefore they are more perfectly round. Mars and the Moon, on the other hand, are less so. Earth is not perfectly round, owing to the fact that its mass bulges at the equator because it is moving; if it were still, it would be quite round indeed, a result of the great mass at its core.
A MASSIVE CORE.
As we shall see, nearly a third of Earth's mass is at its core, even though the core accounts for only about a fifth of its total volume. In other words, Earth's core is exceptionally dense, and this has several implications. First of all, the planet has a powerful gravitational pull, which not only serves to keep people and other objects rooted on the surface of the solid earth but also holds our atmosphere in place.
The gravitational attraction between any two objects is related directly to mass and inversely to the distance between them. Everything in the universe exerts some degree of gravitational pull on everything else, but unless at least one of the objects is of significant mass, the total gravitational force is negligible. The reason for this—as determined by the English mathematician and physicist Sir Isaac Newton (1642-1727)—is that gravitational force between two objects is the product of their mass divided by the distance between them and multiplied by an extremely small quantity known as the gravitational constant.
In the case of Earth, there is an extremely large amount of mass at the interior. Moreover, that mass is at a relatively short distance from objects on the planet's surface—or, to put it another way, Earth has a relatively small radius. Hence its powerful gravitational pull—one of many ways that the interior of Earth affects the overall conditions of the planet.
DENSITY OF TERRESTRIAL AND JOVIAN PLANETS.
Saying that a large amount of mass is concentrated in a small area on Earth is another way of saying that the planet's interior is extremely dense. As it turns out, Earth is, in fact, the densest planet in the solar system; indeed the only other planets that come close are Mercury and Venus.
Mercury, Venus, Earth, and Mars together are designated as the terrestrial planets: bodies that are small, rocky, and dense; have relatively small amounts of gaseous elements; and are composed primarily of metals and silicates. (See the essay Minerals for more on metals as well as the extremely abundant silicates.) By contrast, the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are large, low in density, and composed primarily of gases. (Scientists know little about Pluto, which was discovered in the early twentieth century. It has a density higher than any Jovian planet, but there is little basis for classifying it as a terrestrial planet.)
Saturn, which is the least dense among the planets, has a mass only about 100 times as great as that of Earth, while its volume is almost 800 times greater. Thus its density is only about 12% of Earth's. And whereas Jovian planets, such as Saturn, are mostly gaseous and solid only in their small, dense cores, Earth is extremely solid. For a Jovian planet, there is little distinction between the "atmosphere" and the surface of the planet itself, whereas anyone who has ever jumped from a great height on Earth can attest to the sharp difference between thin air and solid ground.
Beneath that solid ground is a planetary interior composed of iron, nickel, and traces of other elements. The vast mass of the interior not only gives Earth a strong gravitational pull but also, in combination with the comparatively high speed of the planet's rotation, causes Earth to have a powerful magnetic field. Furthermore, Earth is distinguished even from most terrestrial planets (among which the Moon sometimes is counted) owing to the high degree of tectonic activity beneath its surface.
Plate Tectonics and the Interior
Of all the terrestrial planets, Earth is the only one on which the processes of plate tectonics take place. Tectonism is the deformation of the lithosphere, the brittle area of Earth's interior that includes the crust and upper mantle. (We take a closer look at these regions later in this essay.) The lithosphere is characterized by large, movable segments called plates, and plate tectonics is the name both of a theory and of a specialization of tectonics, or the study of tectonism.
As a realm of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement. This theory, discussed in detail within the Plate Tectonics essay, brings together aspects of seismic (earthquake) and volcanic activity, the structures of Earth's crust, and other phenomena to provide a unifying model of Earth's evolution. It is one of the dominant concepts in the modern earth sciences.
THE IMPORTANCE OF TECTONIC ACTIVITY.
As discussed in Plate Tectonics, there is a difference in thickness between continental and oceanic plates on Earth. By contrast, the other terrestrial planets have crusts of fairly uniform thickness, suggesting that they have experienced little in the way of tectonic activity. Several other factors indicate that Earth is by far the most prone to tectonic activity.
Earth's core is enormous, larger than the entire planet Mercury. This means that there is a large area of high pressure and high heat driving tectonic processes, as we discuss later in this essay. In addition, Earth has a relatively thin lithosphere, meaning that the effects of heat below the lithosphere are manifested dramatically above it in the form of shifting plates and the results of such shifts—for instance, mountain building.
"Digging to China"
As children, many people growing up in the West heard something along these lines: "If you could dig a hole straight through the earth, you would end up in China." This might be more or less literally true, since eastern China is on the opposite side of the planet from eastern North America. (Southeast Asia, however, is farther away, because it is more exactly opposite the eastern seaboard.) Even with the most sophisticated equipment imaginable, however, it is unlikely that anyone will put a hole straight through Earth.
The idea of "digging to China" may have raised a new question in many a child's mind. Suppose a person were to dig a hole through the Earth and jump down into it. What would happen? Gravity would carry the person to the center of the earth, but after that, would he or she just go on flying past the gravitational center of the planet? It is a good question, and the likelihood is that the powerful gravitational force at the center of the earth would hold the person there. Again, however, the likelihood of ever conducting such an experiment—for instance, with a steel ball that emitted a radio signal—is slim.
The reason for this slim likelihood can be illustrated by visiting some of the world's deepest mines. There is, for instance, the Homestake Gold Mine in South Dakota, one of the deepest mines in the United States, which extends to about 8,500 ft. (2,591 m) below the surface. This is about 1.6 mi. (2.6 km), almost six times as deep as the height of the world's tallest building, the Petronas Towers in Malaysia.
Impressive as the Homestake is, it is almost insignificant when compared with the Western Deeps Gold Mine, near Carletonville, South Africa, which reaches down about 13,000 ft. (3,962 m)! In a mine such as the Western Deeps, or even the Homestake, temperatures can reach 140°F (60°C), which makes working in such an environment extremely hazardous. Mines are air-conditioned to make them bearable, but even so, there are other dangers associated with the great depth. For instance, the pressure caused by the rocks lying above the mine may become so great that rocks in the wall shatter spontaneously.
It is no wonder, then, that workers in extremely deep gold mines and diamond mines are well paid or that their insurance premiums are very expensive. Yet even the Western Deeps is not the deepest spot where humans have drilled holes on Earth. Scientists in Sweden and Russia have overseen the drilling of deep holes purely for research purposes, while in Louisiana and Oklahoma, a few such holes have been drilled in the process of exploring for petroleum. The deepest of these holes are at Andarko Basin, Oklahoma, and the Kola Peninsula, Russia, where artificial holes extend to a staggering 7.5 mi. (12 km). This is more than three times as far down as the Western Deeps, and it is hard to imagine how any human could survive at such depths.
HOW DO WE KNOW?
Even these deepest excavations represent only 0.2% of the distance from Earth's surface to its core, which is about 3,950 mi. (6,370 km) below our feet. Given that fact, one might wonder exactly how it is that earth scientists—particularly geophysicists—claim to know so much about what lies beneath the crust. In fact, they have a number of fascinating tools and methods at their disposal.
Among these tools are such rocks as kimberlite and ophiolite, which originate deep in the crust and mantle but move upward to the surface. In addition, meteorites that have landed on Earth are believed to be similar to the rocks at the mantle and core, since the planet was originally a cloud of gas around which solid materials began to form as a result of bombardment from outer space (see the essays Sun, Moon, and Earth and Planetary Science). Most important of all are seismic, or earthquake, waves, whose speed, motion, and direction tell us a great deal about the materials through which they have passed and the distances over which they have traveled.
An Imaginary Journey
Having established just how far humans would have to go to penetrate even just below Earth's crust with existing technology, let us now pretend that such obstacles have been overcome. In this imaginary situation, through a miracle of science let us say that there really is a hole straight through the earth and an elevator that passes through it.
This, of course, raises still more complications, aside from the gravitational problem mentioned earlier. Among other things, our elevator would have to be made of a heat-resistant material, given the temperatures we are likely to encounter in our descent. It may have sounded hot in the gold mines of South Africa and South Dakota, but that will seem cool by the time we reach Earth's core, which is as hot as the Sun's surface.
For now, however, we will throw all those logistical problems out the window and begin our journey to the center of the earth. In so doing, we will pass through three major regions—crust, mantle, and core—as well as several subsidiary realms within. By far the smallest of these is the crust, which is also the only part about which we know anything from direct experience.
Very quickly we find ourselves passing through the A, B, and C horizons of soil discussed in the Soil essay, and soon we are passing through bedrock into the main part of the crust. Bedrock might be only 5-10 ft. deep (1.5-3 m), or it might be half a mile deep (0.8 km) or perhaps even deeper. Although this is a long way for a person to dig, we still have barely scratched the surface.
As noted earlier, there is a difference between continental crust and oceanic crust. We will ignore the details here, except to say that the continental crust is thicker but the oceanic crust is denser. Thus, the continents are at a higher elevation than the oceans around them. Depending on whether the crust is oceanic or continental, we have between 3 mi. and 40 mi. (5-70 km) to travel before we begin to pass out of the crust and into the mantle.
THE LITHOSPHERE, SEISMOLOGY, AND REMOTE SENSING.
The transition from crust to mantle is an abrupt one, marked by the boundary zone known as the Mohorovicic discontinuity. Sometimes called the M-discontinuity or, more commonly, the Moho, it was the discovery of the Croatian geologist Andrija Mohorovicic (1857-1936). On October 8, 1909, while studying seismic waves from an earthquake in southeastern Europe, Mohorovicic noticed that the speed of the waves increased dramatically at a depth of about 30 mi. (50 km).
Since waves travel faster through denser materials, Mohorovicic reasoned that there must be an abrupt transition from the rocky material in the Earth's crust to denser rocks below. His discovery is an excellent example of remote sensing (see Remote Sensing), whereby earth scientists are able to study places and phenomena that are impossible to observe directly.
After the Moho, which is only about 0.1-1.9 mi. (0.2-3 km) thick, we enter the mantle—or, more specifically, the lithospheric mantle. This subregion may extend to depths between 30 mi. and 60 mi. (50-100 km) and is much more dense than the crust. Like the crust, it is brittle, solid, and relatively cool compared with the regions below; hence, the crust and lithospheric mantle are lumped together as the lithosphere.
The Asthenosphere and Its Impact
At the base of the lithosphere, we pass through another transition zone, known as the Gutenberg low-velocity zone (named after the German-born American seismologist Beno Gutenberg [1889-1960]), where the speed of seismic waves again increases dramatically. After that, we enter a layer of much softer material, known as the asthenosphere. The material in the asthenosphere is soft not because it is weak—on the contrary, it is made of rock—but because it is under extraordinarily high pressure.
What happens in the asthenosphere plays a powerful role in life on the surface. The plates of the lithosphere float, as it were, atop the molten rock of the asthenosphere, which forces these plates against one another as though they were ice cubes floating in a bowl of water in constant motion. This motion is the phenomenon of plate tectonics, which, as we have discussed, quite literally shapes the world we know.
VOLCANISM AND THE ATMOSPHERE.
Plate tectonics is responsible not only for such phenomena as the creation of mountains but also, by influencing the development of volcanoes, indirectly for Earth's atmosphere. In the first few billion years of the planet's existence, the action of volcanoes brought water vapor, carbon dioxide, nitrogen, sulfur, and sulfur compounds from the planet's interior to its surface. This was critical to the formation of the air we breathe today. Additionally, volcanic activity plays a significant role in the carbon cycle, whereby that vital element is circulated through various earth systems (see Biogeochemical Cycles and Carbon Cycle).
Earth and Venus stand alone among terrestrial planets as the only two still prone to volcanic activity. (By contrast, Mercury and the Moon have long been dead volcanically, and volcanism on Mars seems to have ended at some point during the past billion years.) This is significant, because even though all the planets possess more or less the same chemical elements, volcanoes are critical to distributing those elements.
In addition, volcanic activity, as well as the heat from Earth's interior that drives it and other tectonic phenomena, is an important influence on the separation of chemical compounds. When Earth formed some 4.5 billion years ago, heavier compounds—among them, those containing iron—sank toward the planet's core. At the same time, lighter ones began to rise into the atmosphere. Among these compounds was oxygen, which is clearly essential to the life of humans and other animals. This separation of compounds continues on Earth, owing to the large amount of heat that emanates from the interior.
One would hardly guess that our atmosphere—or the circulation of carbon, a key component in all life-forms—could be the indirect product of activity that takes place at least 60 mi.(100 km) below our feet. Nor is this the only illustration of the impact that Earth's interior exerts on our world. The interior of Earth is also responsible for the action that produces geothermal energy, discussed in detail within Energy and Earth.
Geothermal energy provides heating and electricity for several countries and is responsible for the dramatic effect of such phenomena as "Old Faithful" at Yellowstone Park in Wyoming. It is also the source behind the soothing natural springs found in such well-known resorts as Warm Springs, Georgia (a favorite getaway for President Franklin D. Roosevelt, who died there in 1945), and Hot Springs, Arkansas, the home-town of another president, Bill Clinton.
Mesosphere to Inner Core: Geomagnetism and Gravity
After we pass through the base of the asthenosphere, we are still only 155 mi. (250 km) deep. Now we are in the mesosphere, which extends to a depth of 1,800 mi. (2,900 km) and includes several other discontinuities, or thresholds of change. We will not discuss the details of these discontinuities here, except to note that they indicate changes in geochemical composition: for example, at 400 mi. (650 km) there appears to be a marked increase in the ratio of iron to magnesium.
The Gutenberg discontinuity, or the core-mantle boundary (CMB), marks our entrance to the core. By now it has become very, very hot. Whereas the lithospheric mantle is about 1,600°F (870°C), the bottom of the lithosphere is about 4,000-6,700°F (2,200-3,700°C). By the time we get to the inner core, we may be confronted with temperatures as high as 13,000°F (7,200°C)—which, if this is true, would make the center of Earth about 50% hotter than the surface of the Sun!
EARTH'S FIERY HEART.
At such temperatures, one would expect the rock of the outer core to be entirely molten, and indeed it is. In fact, it is this difference in phase or state of matter that marks the change from mantle (which is partially solid) to the liquid outer core. The region between the mantle and the outer core is one of undulating boundaries due to convection (see Convection), which may be the driving force behind the plate tectonic activity that occurs at a much higher level. In addition, the eddies and currents of molten iron in the core are ultimately responsible for the planet's magnetic field (see Geomagnetism).
The boundary between the mantle and core is lower today than it once was, a sign that our planet is slowly aging. If there ever comes a point when the heat is entirely dissipated, as may perhaps happen many billions of years from now, that could well be the end of Earth as a "living" planet. If we did not have a mantle and core, with all their heat, pressure, and resulting tectonic activity, Earth would be as dead as the Moon, whose interior is relatively cool.
The distinction between the outer and inner cores, which starts at a depth of about 3,150 mi. (5,100 km), comes from the fact that here, too, there is a phase change—in this case from liquid, molten material back to solid. This has nothing to do with cooling, since, as we have noted, the inner core is almost unimaginably hot; rather, it is a result of the immense pressures apparent at this depth.
GRAVITY ALWAYS WINS.
It is interesting to note that the core constitutes only about 16% of the planet's volume but 32% of its mass. As we discussed near the beginning of this essay, enormous gravitational force exists between two objects when at least one of them has a relatively large amount of mass and the distance between them is great. Thus, Earth's mass, concentrated deep in its interior, helps hold our world—people, animals, plants, buildings, and so forth—in place. It also keeps our atmosphere firmly rooted as well. Without an extensive gravitational field of the kind that Earth possesses, significantly less massive bodies, such as the Moon or Mercury, have no atmosphere. In this and many another way, it turns out that life on Earth's surface depends heavily on what goes on its ultra-hot, extremely pressurized interior.
A Bizarre Postscript
Given the vast amount of power in Earth's interior, it is no wonder that it has long fascinated humans—even before science possessed any sort of intelligent understanding with regard to the contents of that interior. The ancients offered all manner of fascinating speculation regarding the contents of Earth: it was hollow, some said, while others claimed that it contained one substance or another—perhaps even a heart of gold.
Such imaginative musings continued well into the Middle Ages, when the Italian poet Dante Alighieri (1265-1321) described an allegorical journey through Earth's interior in his epochal Divine Comedy. This epic poem depicts the inside of the planet as concentric circles of hell, descending toward the fiery core, where Satan himself resides. Beyond this lies Purgatory and further still—on the other side of Earth—Heaven, the New Jerusalem.
By the time the French writer Jules Verne (1828-1905) wrote Journey to the Center of the Earth almost six centuries later, scientific knowledge regarding Earth's interior had increased dramatically, though many of the significant discoveries we have examined here—for example, the Moho—still lay in the future. In any case, the tone of Verne's work was that of a new literary style, science fiction. Pioneered by Verne and the British writer H. G. Wells, science fiction could not have been less like Dante's poetry, infused as it was with spirituality and mystery.
SCREAMS OF THE DAMNED?
In the early 1990s, an urban legend of sorts brought together the science fiction of Verne, the religious vision of Dante, and a number of other, less pleasant strains—including ignorance and, on the part of its originators, the willingness to deceive. This "urban legend" did not involve crocodiles in sewers, ghostly hitchhikers, or the other usual fodder; instead, it concerned the center of the earth—where, it was claimed, hell had been discovered.
The full account appeared on Ship of Fools (see "Where to Learn More"), a Web site operated by Rich Buhler—himself a Christian minister and a debunker of what he has called "Christian urban legends." As Buhler reported, the story gained so much support that it appeared on Trinity Broadcasting Network (TBN), a major evangelical television outlet. According to the TBN report, Russian geologists had drilled a hole some 8.95 mi. (14.4 km) into Earth's crust and heard screams, which supposedly came from condemned souls in the nether regions.
As the embellished details of the story began to unfold, it turned out that the Russian geologists had found the temperatures to be much higher than expected: 2,000°F (1,093°C). Also, their drilling had unleashed a bat that flew out of hell with the words "I have conquered" inscribed in Russian on its wings. Buhler and his team traced this bizarre tale to Finland and then back to southern California. As to how the story originated, Buhler noted the drilling at the Kola Peninsula, which we mentioned earlier in this essay. The depth cited in the rumor, however, was greater than that which the drilling at Kola reached, and the temperatures claimed were much higher than what one actually would encounter at that depth.
"It is possible that somewhere in the world there has been a spooky experience during deep drilling operations," Buhler concluded. Nonetheless, "characteristic of many urban legends, this story was alleged to have occurred in an obscure part of the world where it would be virtually impossible to track down the facts. And once the story got started, people began quoting one another's newsletters to validate their own. This is the stuff of which tabloid newspapers are made." In the end, the "screams of hell" offered nothing of value in terms of either science or religion, but it proved to be an excellent example of human beings' fascination with, and latent terror of, Earth's interior.
WHERE TO LEARN MORE
Buhler, Rich. "Drilling for Hell," Ship of Fools (Web site). <http://ship-of-fools.com/Myths/03Myth.html#Top>.
Darling, David J. Could You Ever Dig a Hole to China? Minneapolis, MN: Dillon Press, 1990.
De Bremaecker, Jean-Claude. Geophysics: The Earth's Interior. New York: John Wiley and Sons, 1985.
Earth's Interior (Web site). <http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html>.
Earth's Interior and Plate Tectonics (Web site). <http://www.star.le.ac.uk/edu/solar/earthint.html>.
Earth's Interior and Plate Tectonics (Web site). <http://www.solarviews.com/eng/earthint.htm>.
Physical Geology: Interior of the Earth (Web site). <http://www.uh.edu/~jbutler/physical/chapter19.html>.
Rockdoctors Guide: Earth's Interior (Web site). <http://www.cobweb.net/~bug2/rock7.htm>.
Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.
A region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The asthenosphere liesat a depth of about 60 mi. to 215 mi.(about 100-350 km).
In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).
The center of Earth, an area constituting about 16% of the planet's volume and 32% of its mass. Made primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi.(1,220 km) and a liquid outer core about1,750 mi. (2,820 km) thick. For terrestrial planets, in general, core refers to the center, which in most cases is probably molten metal of some kind.
The uppermost division of the solid Earth, representing less than 1% of its volume and varying in depth from 3 mi. to 37 mi. (5-60 km). Below the crust is the mantle.
A branch of the earth sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes.
The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.
A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet's physical processes as well as its gravitational, magnetic, and electric properties and the means by which energy is transmitted through its interior.
The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
The planets between Mars (the last terrestrial planet) and Pluto, all of which are large, low indensity, and composed primarily of gases.
The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.
The thick, dense layer of rock, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core. In reference to the other terrestrial planets, mantle simply means the area of dense rock between the crust and core.
At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.
The name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As atheory, it explains the processes that have shaped Earth in terms of plates and their movement. Plate tectonics theory brings together aspects of continental drift, seafloor spreading, seismic and volcanic activity, and the structures of Earth's crust to provide a unifying model of Earth's evolution. It is one of the dominant concepts in the modern earth sciences.
Large, movable segments of the lithosphere.
A packet of energy resulting from the disturbance that accompanies a strain on rocks in the lithosphere.
The study of seismic waves as well as the movements and vibrations that produce them.
The study of tectonism, including its causes and effects, most notably mountain building.
The deformation of the lithosphere.
The four inner planets of the solar system: Mercury, Venus, Earth, and Mars. These are all small, rocky, and dense; have relatively small amounts of gaseous elements; and are composed primarily of metals and silicates. Compare with Jovian planets.
It is approximately 3,950 mi (6,370 km) from Earth's surface to its center. Geologists understand the structure and composition of the surface by direct observation and by analysis of rock samples raised by drilling projects; however, the depth of drill holes and, therefore, the depth limit of scientists' ability to directly observe Earth's interior is severely limited. Even the deepest drill holes (7.5 mi [12 km]) penetrate less than 0.2% of the distance to Earth's center. Thus, we know more about the layers near Earth's surface than about the depths, and can only investigate conditions deeper in the interior through indirect means.
Geologists collect indirect information about the deep interior from several different sources. Some rocks found at the surface, such as kimberlite, originate deep in Earth's crust and in the mantle. These rocks provide geologists with samples of the composition of Earth's interior; however, their depth limit is still on the order of a few tens of miles. Another source of information, because of its ability to probe Earth to its very core, is more important: seismic waves. When an earthquake occurs anywhere on the planet , seismic waves—mechanical vibrations transmitted by the solid or liquid rock of Earth's interior—travel outward from the earthquake center. The speed, motion , and direction of seismic waves changes dramatically at depth different levels within Earth, and these are known as seismic transition zones. From such data, scientists have concluded that Earth is composed of three basic parts: the crust, the mantle, and the core.
The outermost layer of Earth is the crust, a thin shell of rock that covers the globe. There are two types of crust: (1) the continental crust, which consists mostly of light-colored rock of granitic composition and underlies the continents, and (2) the oceanic crust, which consists mostly of dark-colored rock of basaltic composition and underlies the oceans. The continents have an average elevation of about 2,000 ft (609 m) above sea level , while the average elevation (depth) of the ocean floor is 10,000 ft (3,048 m) below sea level. An important difference between continental and oceanic crust is their difference in density . Continental crust has a lower average density (2.6 g/cm3 ) than does oceanic crust (3.0 g/cm3). This density difference allows the continents to float permanently on the upper mantle, persisting more or less intact for billions of years. Oceanic crust, in contrast, is barely able to float on the mantle (which has a density of about 3.3 g/cm3). As oceanic crust ages, it accumulates a heavy underlayer of cooled mantle rock; the resulting two-layer structure eventually sinks of its own weight into the mantle, where it is melted down and recycled. Because of this recycling process, no oceanic crust older than about 200 million years exists on the surface of the earth. About 16% of the mantle consists of recycled oceanic crust; only about 0.3% consists of recycled continental crust.
Another difference between the oceanic crust and continental crust is their difference in thickness. The oceanic crust is 3–6 mi (5–10 km) thick, while the continental crust averages about 20 mi (35 km) in thickness and can reach 40 mi (70 km) in certain sections, particularly those found under recently elevated mountain ranges such as the Himalayas.
The bottom of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone termed the Mohorovičić discontinuity. The Mohorovičić discontinuity, commonly referred to as "the Moho" or the "M-discontinuity," is the transition or boundary between the bottom of the crust and the solid, uppermost layer of the mantle (the lithospheric mantle). As the thickness of the crust varies, the depth to the Moho varies, from 3–6 mi (5–10 km) under the oceans to 20–40 mi (35–70 km) under the continents.
The Moho was first discovered by the Croatian geophysicist Andrija Mohorovičić (1857–1936) in 1908. On October 8, 1908, Andrija Mohorovičić observed seismic waves from an earthquake in Croatia. He noticed that both the compressional (or primary [P]) waves and the shear (or secondary [S]) waves, at one point in their journey, picked up speed as they traveled farther from the earthquake. This suggested that the waves had been deflected. He noted that this increase in speed seemed to occur at a depth of about 30 mi (50 km). Since seismic waves travel faster through denser material, he reasoned that there must be an abrupt transition at that depth from the material of the crust to denser rocks below. This transition zone was later named for its discoverer. The Moho is a relatively narrow transition zone, estimated to be 0.1–1.9 mi (0.2–3 km) thick. It is defined by the level within the earth where P wave velocity increases abruptly from an average speed of 4.3 mi/sec (6.9 km/sec) to about 5.0 mi/sec (8.1 km/sec).
Underlying the crust is the mantle, which comprises about 82% of Earth's volume and 65% of its mass . The uppermost section of the mantle, which is solid, is called the lithospheric mantle. This section extends from the Moho down to an average depth of 40 mi (70 km), fluctuating between 30 and 60 mi (50–100 km). The density of this layer is greater than that of the crust, averaging 3.3 g/cm3. Like the crust, this section is solid, and is cool relative to the material below. The lithospheric mantle, combined with the overlying solid crust, is termed the lithosphere , a word derived from the Greek lithos, meaning rock. At the base of the lithosphere is another seismic transition, the Gutenberg low velocity zone. At this level, the velocity of S waves decreases dramatically, and seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a "weak" or "soft" zone of partially melted material (1–10% molten material). This zone is termed the asthenosphere, from the Greek asthenes, meaning "weak." This transition between the lithosphere and the asthenosphere is named after German geologist Beno Gutenberg (1889–1960), who made several important contributions to our understanding of Earth's interior. It is at this level that some important Earth dynamics occur, affecting those of us here at the earth's surface. At the Gutenberg low velocity zone, the lithosphere is carried on top of the weaker, less-rigid asthenosphere, which seems to be in continual circulation. This circulatory motion creates stress in the rigid rock layers above it, and the slabs or plates of the lithosphere are forced to jostle against each other like ice cubes floating in a bowl of swirling water . This motion of the lithospheric plates is known as plate tectonics (from the Greek tektonikos, meaning construction), and is responsible for many surface phenomena, including earthquakes, volcanism, mountain-building, and continental drift .
The asthenosphere extends to a depth of about 155 mi (250 km). Below that depth, seismic wave velocity increases, suggesting an underlying denser, solid phase.
The rest of the mantle, from the base of the asthenosphere at 155 mi (250 km) to the core at 1,800 mi (2,900 km), is called the mesosphere ("middle sphere"). Mineralogical and compositional changes are suggested by sharp velocity changes in the mesosphere. Notably, there is a seismic discontinuity at about 250 mi (410 km) of depth, attributed to a possible mineralogical change (presumably from an abundance of the mineral olivine to the mineral spinel), and another at about 400 mi (660 km), attributed to a possible increase in the ratio of iron to magnesium in mantle rocks. Except for these variations, down to 560 mi (900 km) the mesosphere seems to consist of predominantly solid material that displays a relatively consistent pattern of gradually increasing density and seismic wave velocity with increasing depth andpressure . Below the 560 mi (900 km) depth, the P and S wave velocities continue to increase, but the rate of increase declines with depth.
Although much of the mantle is "solid," the entire mantle actually convects or circulates like a pot of boiling water. Images produced by analysis of seismic waves show that dense slabs of oceanic crust plunge all the way through the mantle to the outer surface of the core, which indicates that the entire mantle is in motion, mixing thoroughly with itself over geological time .
At a depth of 1,800 mi (2,900 km) there is another abrupt change in the seismic wave patterns, the Gutenberg discontinuity or core-mantle boundary (CMB). The density change at the CMB is greater than that at the interface of air and rock on the Earth's outer surface. At the CMB, P waves decrease while S waves disappear completely. Because S waves cannot be transmitted through liquids, it is thought that the CMB denotes a phase change from the solid mantle above to a liquid outer core below. This phase change is believed to be accompanied by an abrupt temperature increase of 1,300°F (704°C). This hot, liquid outer core material is much denser than the cooler, solid mantle, probably due to a greater percentage of iron. It is believed that the outer core consists of a liquid of 80–92% iron, alloyed with lighter element. The composition of the remaining 8–20% is not well understood, but it must be a compressible element that can mix with liquid iron at these immense pressures. Various candidates proposed for this element include silicon, sulfur , or oxygen .
The actual boundary between the mantle and the outer core is a narrow, uneven zone that contains undulations on the order of 3–6 mi (5–8 km) high. These undulations are affected by heat-driven convection activity within the overlying mantle, which may be the driving force for plate tectonics . The interaction between the solid mantle and the liquid outer core is also important to Earth dynamics for another reason; eddies and currents in the iron-rich, fluid outer core are ultimately responsible for the Earth's magnetic field .
There is one final, deeper transition, evident from seismic wave data: Within Earth's core, at a depth of about 3,150 mi (5,100 km), P waves encounter yet another seismic transition zone. This indicates that the material in the inner core is solid. The immense pressures present at this depth probable cause a phase change, from liquid to solid. Density estimates are consist with the hypothesis that the solid, inner core is nearly pure iron.
The heat that keeps the whole interior of the Earth at high temperatures is derived from two sources: heat of formation and radioactive metals. As the Earth accreted from the original solar nebula, impacts of new material delivered sufficient energy to melt most or all of the forming planet's bulk. As most of the new Earth's iron sank its center through its bulkier, lighter elements (silicon, oxygen, etc.), further energy was released, sufficient to raise the temperature of the core by several thousand degrees Centigrade. Radioactive elements such as uranium and thorium, mostly resident in the mantle, have continued to supply the Earth's interior with heat in the billions of years since its formation; however, the Earth's interior continues to cool, steadily losing its primordial heat to space through the crust. As the core cools, its inner, solid portion grows at the expense of its outer, liquid portion. The current rate of thickening of the inner core is about 0.04 inch (1 mm) per year.
See also Magma.
Magill, Frank N., ed. Magill's Survey of Science: Earth Science. Hackensack, NJ: Salem Press, Inc., 1990.
Tarbuck, Edward. D., Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.
Winchester, Simon. The Map That Changed the World: WilliamSmith and the Birth of Modern Geology. New York: Harper Collins, 2001.
Buffett, Bruce A. "Earth's Core and the Geodynamo." Science. (June 16, 2000): 2007–2012.
Hellfrich, George, and Bernard Wood "The Earth's Mantle." Nature. (August 2, 2001): 501–507.
Mary D. Albanese
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- Continental crust
—Layer of crust (about 35 km thick) that underlies the earth's continents; comprised of light-colored, relatively lightweight granitic rock.
—The part of Earth below 1,800 mi (2,900 km). Comprised of a liquid outer core and a solid inner core.
- Gutenberg discontinuity
—The seismic transition zone that occurs at 1,800 mi (2,900 km) and separates the lower mantle (solid) and the underlying outer core (liquid). Also known as the core-mantle boundary (CMB).
- Gutenberg low velocity zone
—The transition zone that occurs at 30–60 mi (50–100 km), between the rigid lithosphere and the underlying "soft" or partially melted asthenosphere.
- Lithospheric mantle
—The rigid uppermost section of the mantle, less than 60 mi (100 km) thick. This section, combined with the crust, constitutes the lithosphere, or the solid and rocky outer layer of Earth.
—The thick middle layer of the Earth that extends from the core to the crust, a thickness of almost 1,800 mi (2,900 km). The mantle is predominantly solid, although it includes the partially melted asthenosphere.
—The solid section of the mantle directly beneath the asthenosphere. Extends from 150 mi (250 km) down to 1,800 mi (2,900 km).
- Mohorovičić discontinuity
—The seismic transition zone indicated by an increase in primary seismic wave velocity that marks the transition from the crust to the uppermost section of the mantle.
- Oceanic crust
—Thin (3–6-mi [5–10-km] thick) crust that floors the ocean basins and is composed of basaltic rock: denser than continental crust.
- P waves
—Primary or compression waves that travel through Earth, generated by seismic activity such as earthquakes; can travel through solids or liquids.
- S waves
—Secondary or shear waves that travel through Earth, generated by seismic activity such as earthquakes; cannot travel through liquids (e.g., outer core).
- Seismic transition zone
—A layer in the Earth's interior where seismic waves undergo a change in speed and partial reflection; caused by change in composition, density, or both.
- Seismic wave
—A disturbance produced by compression or distortion on or within the earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g., earthquakes) or artificial (e.g., explosions) means.
The distance from Earth's surface to its center is about 3,975 miles (6,395 kilometers). Scientists have divided the interior of Earth into various layers, based on their composition. The crust, or outer portion, varies in depth from 5 to 25 miles (8 to 40 kilometers). Below the crust is the mantle, which extends to a depth of about 1,800 miles (2,900 kilometers). Below that is the core, composed of a liquid outer core about 1,380 miles (2,200 kilometers) in depth, and a solid inner core about 780 miles (1,300 kilometers) deep.
From direct observation, core samples, and drilling projects, scientists have been able to study the rock layers near the planet's surface. However, this knowledge is limited. The deepest drill hole, just over 9 miles (15 kilometers) in depth, penetrates only about 0.2 percent of the distance to Earth's center.
Geologists collect information about Earth's remote interior from several different sources. Some rocks found at Earth's surface originate deep in Earth's crust and mantle. Meteorites that fall to the planet are also believed to be representative of the rocks of Earth's mantle and core. Meteor fragments presumably came from the interior of shattered extraterrestrial bodies within our solar system. It is likely that the composition of the core of our own planet is very similar to the composition of these extraterrestrial travelers.
Another source of information, while more indirect, is perhaps more important. That source is seismic, or earthquake, waves. When an earthquake occurs anywhere on Earth, seismic waves travel outward from the earthquake's center. The speed, motion, and direction of seismic waves changes dramatically as they travel though different mediums (areas called transition zones). Scientists make various assumptions about the composition of Earth's layers through careful analysis of seismic data, a method called subsurface detection.
The crust, the thin shell of rock that covers Earth, contains all the mountains, valleys, oceans, and plains that make up the surface of the planet. There are two types of crust: the continental crust (which underlies Earth's continents) and the oceanic crust (which underlies Earth's oceans). The lighter-colored continental crust is thicker—yet lighter in weight—than the darker-colored oceanic crust. The crust is composed largely of minerals containing the elements calcium, aluminum, magnesium, iron, silicon, sodium, potassium, and oxygen.
Words to Know
Asthenosphere: Portion of the mantle beneath the lithosphere composed of partially melted material.
Core-mantle boundary (CMB): Also referred to as the Gutenberg discontinuity, the seismic transition zone separating the mantle from the underlying outer core.
Gutenberg low velocity zone: Seismic transition zone between the lithosphere and the underlying asthenosphere.
Lithosphere: Rigid uppermost section of Earth's mantle combined with the crust.
Mohorovičič discontinuity: Seismic transition zone that marks the transition from the crust to the uppermost section of the mantle.
Seismic transition zone: Interval within Earth's interior where seismic waves, or earthquake waves, display a change in speed and shape.
Seismic waves: Vibrations in Earth's interior caused by earthquakes.
The base of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone called the Mohorovičič discontinuity, commonly referred to as the Moho. First discovered in 1909 by the Croatian geophysicist Andrija Mohorovičič (1857–1936), this boundary marks the point where seismic waves pick up speed as they travel through Earth's interior. Since seismic waves travel faster through denser material, Mohorovičič reasoned that there was an abrupt transition from the rocky material in Earth's crust to denser rocks below. The Moho is a relatively narrow transition zone, estimated to be somewhere between 0.1 to 1.9 miles (0.2 to 3 kilometers) thick.
Underlying the crust is the mantle, which is composed mainly of minerals containing magnesium, iron, silicon, and oxygen. The uppermost section of the mantle is a rigid layer. Combined with the overlying solid crust, this section is called the lithosphere, which is derived from the Greek word lithos, meaning "stone."
At the base of the lithosphere, a depth of about 40 miles (65 kilometers), there is another distinct seismic transition called the Gutenberg low velocity zone. At this level, all seismic waves appear to be absorbed more strongly than elsewhere within Earth. Scientists interpret this to mean that the layer below the lithosphere is a zone of partially melted material. This "soft" zone is called the asthenosphere, from the Greek word asthenes, meaning "weak." It extends to a depth of about 155 miles (250 kilometers).
This transition zone between the lithosphere and the asthenosphere is named after American geologist Beno Gutenberg (1889–1960). At the
Gutenberg low velocity zone, the lithosphere is carried "piggyback" on top of the weaker, less rigid asthenosphere, which seems to be in continual motion. This motion creates stress in the rigid rock layers above it, and the slabs or plates of the lithosphere are forced to jostle against each other, much like ice cubes floating in a bowl of swirling water. This motion of the lithospheric plates is known as plate tectonics, and it is responsible for earthquakes, certain types of volcanic activity, and continental drift.
At a depth of 1,800 miles (2,900 kilometers) there is another abrupt change in the seismic wave patterns. This level is known as the coremantle boundary (CMB) or the Gutenberg discontinuity. At this level, certain seismic waves disappear completely, an indication that the material below is liquid. Accompanying this change is an abrupt temperature increase of 1,300°F (700°C). This hot, liquid outer core is thought to consist mainly of iron. Electric currents in the outer core's iron-rich fluids are believed to be responsible for Earth's magnetic field.
Within Earth's core, at about a depth of 3,200 miles (5,150 kilometers), the remaining seismic waves that passed through the outer liquid core speed up. This indicates that the material in the inner core is solid. The change from liquid to solid in the core is probably due to the immense pressures present at this depth. Based on the composition of meteorite fragments that fell to Earth, scientists believe the inner core to be composed of iron plus a small amount of nickel.
[See also Earthquake; Plate tectonics ]