Discontinuities

Discontinuities are boundaries within the earth that divide the crust from the mantle, the mantle from the core, and the outer core from the inner core. The term is also used to describe the less dramatic boundaries within layers.

Earth's Interior

Discontinuities are underground boundaries between layers of the earth. The closest discontinuity to the earth's surface is the Mohorovičić, which divides the earth's crust from the mantle underneath. Other discontinuities divide the mantle from the outer core and the outer core from the inner core. Minor discontinuities are found within these layers.

The interior of the earth has been the object of speculation and interest for t3housands of years. Because direct observation of the earth's interior is usually impossible, however, inferences about its structure and characteristics must be made from phenomena seen and felt at or near the earth's surface. Several phenomena do give indications of the subsurface earth: caves that are often cool and damp, cool water emanating from springs and artesian wells, hot water spewing upward from geysers, and volcanoes from which extremely hot lava erupts. These phenomena give a mixed and incomplete picture of the earth beneath the surface.

Seismic Waves

The structure and composition of the interior of the earth can, however, be inferred from the study of earthquake waves. Seismographs can detect three types of vibrations: surface waves (the ones that can cause damage when there is an earthquake), P (primary) waves, and S (secondary) waves, which are also generated by every quake. P waves are compressional (pushing) waves, in which earth or rock particles move forward in the direction of wave movement; S waves are shear waves, in which the particle motion is sideways or perpendicular to the direction of wave movement. The more efficient P waves travel twice as fast as S waves and thus are always detected first by a seismograph. Seismographs record these waves on charts, called “seismograms,” attached to moving drums. By noting the arrival times of the various waves, seismologists can determine the distance to an earthquake and can see the effects on these waves caused by the type of rock through which the waves have moved.

Seismic waves travel through rock layers at specific speeds, which are different for each type of mineral or rock. For example, waves travel through basalt at 5 miles per second and through peridotite at 8 miles per second. Seismogram study has shown that the earth's interior is not homogeneous, but rather is composed of several major layers and many sublayers.

Discontinuity

In 1906, British geologist Richard Dixon Oldham discovered that S waves are never detected on the opposite side of the earth from any earthquake. As he already knew that S waves cannot travel through liquid substances, Oldham postulated that the center of the earth must be composed of a molten core and that the materials above this core are not molten. The depth of the boundary between this core and the material above it was discovered eight years later by Beno Gutenberg. Now called the Gutenberg discontinuity, it is located about 1,800 miles beneath the earth's surface.

Mohorovičić Discontinuity

When Oldham made his discovery of a central core, Andrija Mohorovičić was the director of the Royal Regional Center for Meteorology and subduction at Zagreb, one of the leading seismological observatories in Europe. In 1909, his meticulous study of a Croatian earthquake showed that some of the P waves from that quake had traveled faster than others. He already knew that other waves speed up or slow down when they move from one medium into another (as when light moves from air into water) and that this change in speed can result both in reflection, a bouncing back of waves, and in refraction, or a change in wave direction through the new medium. He deduced that the faster-moving P waves had traversed down through the earth, through a discontinuity to a material of a different density, and then had come back up to the surface. Deep in the earth was a material that allowed for faster transmission of P waves. Above this discontinuity, seismic waves travel at about 4.2 miles per second; below the boundary, they travel at about 4.9 miles per second.

When Mohorovičić's results were replicated by other seismologists, it was concluded that the discontinuity was a global phenomenon. Data from these studies showed that there were two very distinct layers of the earth: an upper, less-dense layer now called the “crust,” and a denser layer below called the “mantle.” Thus Andrija Mohorovičić had discovered what is now called the Mohorovičić discontinuity, the boundary between the earth's crust and mantle (it is often called the “Moho”).

The crust of the earth is made up of continents and ocean basins that are very different from one another. Continental crust is made primarily of granite. Covering this granite over much of the earth's continents may be found layers of younger sedimentary rock such as sandstone, limestone, and shale. Ocean basins, in contrast, are composed of the dark, heavy rock basalt.

Mohorovičić believed the discontinuity between the crust and the mantle to be about 30 to 35 miles below the surface of the earth. Subsequent studies have shown that it is usually at a depth of about 21 miles. However, the Moho has an irregular shape that is roughly a mirror image of the surface of the earth. Under the continents, the Moho is much deeper; under the oceans, the crust is very thin, and the Moho is as close as from 3 to 5 miles from the surface. The greatest depth of the Moho is probably beneath the Tibetan Plateau, where it reaches a depth of 42 miles.

The continents are higher because they are composed of granite, which is a lower-density rock than basalt or the materials of the mantle. Even though the mantle is composed of solid rock, under long-term stresses, the rock moves slowly like a liquid. Thus, just as ice floats in water, the continents actually are floating upon the heavier mantle rock. The Moho is the boundary between continental granitic rocks and the denser peridotite rock of the mantle.

The mantle extends from the bottom of the crust to a depth of 1,800 miles. It appears to be made of the rocks somewhat similar chemically to those in the earth's crust but more “basic”—that is, having more of the heavy iron and magnesium minerals such as olivine, and less lightweight aluminum. The mantle also appears to be composed of layers with discontinuities about 220 and 400 kilometers beneath the earth's surface. Although mantle rock is solid, it can, under certain conditions, behave somewhat like a liquid. Under long-term pressures, the molecules of this solid rock can move like liquids, but under sharp, short-term stresses, mantle rock fractures like a brittle solid.

In 1957, a project was conceived to drill a hole through the thin oceanic crust down past the Mohorovičić discontinuity to bring up rock from the mantle. Although the “Mohole” project was approved and funded by the National Science Foundation, funds for it were cut off by the U.S. Congress in 1966.

Mantle Convection Currents

Heat within the earth is created through the decay of radioactive isotopes. Although this generated heat is very small compared to the heat received from the sun, it is well insulated and is enough to create volcanoes and the convection currents of the mantle. Mantle rock is extremely hot; because of the pressure on it from the crust above, however, it cannot melt, except where there is a decrease in this pressure.

Studies at the surface of the earth have revealed areas where great heat flow comes from the mantle. Near the center of the Atlantic Ocean, the basaltic ocean bottom has split; the two sides are being pulled away from each other as Europe and Africa move away from the American continents. At this split, a decrease in pressure allows the hot mantle rock to melt and well upward, filling the gap between the dividing ocean bottoms. Thus the new ocean basin is made of material directly from the mantle. Within the mantle are large, slow-moving convection currents where hot mantle rock moves upward, cools off, and slowly sinks. These currents are believed to be the driving forces of continental drift.

Inner Earth Discontinuities

At the bottom of the mantle, beneath the Gutenberg discontinuity, is the earth's core. Seismic studies have shown that the outer core, which extends from roughly 1,800 to 3,100 miles beneath the earth's surface, is not a perfect sphere. The core rises in areas where hot mantle rock is moving upward and is depressed where cooler mantle rock is moving downward. The density of the core is much greater than that of the mantle. It is believed that this core is made of molten nickel and iron and that its motion generates the earth's magnetic field, which produces the aurora borealis.

In 1936, Danish seismologist Inge Lehmann discovered evidence for a solid core within the molten center of the earth by detecting seismic waves that had been deflected back to the surface from within the core. When she realized that these waves, though very weak, travel faster through this most-central part of the earth than through the rest of the molten core, she was able to infer that this inner core was composed of solid material completely surrounded by the molten outer core. This most central layer of the earth extends from 3,100 miles beneath the earth's surface to the center of the earth, 4,000 miles down.

Seismographs

The seismoscope is an ancient instrument that shows earth movements. A Chinese seismoscope of the second century C.E. had eight dragon figures each with a ball in its mouth. When the earth trembled, a ball would fall from the dragon's mouth into the mouth of a frog figure underneath it. European seismoscopes often used bowls of water that would spill when agitated. In 1853, Italian physicist Luigi Palmieri designed a seismometer that used mercury-filled tubes that would close an electric circuit and prompt a recording device to start moving when the earth vibrated.

In 1880, British seismologist John Milne invented the first modern seismograph, which employed a heavy mass suspended from a horizontal bar. When the earth would quake, the bar would move, and that movement would be recorded on light-sensitive paper beneath. Most seismographs employ a pendulum, which, because of inertia, remains still as the earth moves underneath it.

When seismographs measure shock waves from nearby earthquakes, they first receive the P waves, which vibrate in the direction in which the waves are moving. S waves, which vibrate perpendicular to the direction in which the waves are moving, then arrive, followed by surface waves. When seismographs record more distant earthquakes, the results are complicated by the reflection and refraction of seismic waves resulting from the various discontinuities underground. As the complications were deciphered, seismologists realized that the recordings described the rock layers below and between the quake and the seismograph.

Once geologists realized that they could learn about the earth by examining seismograms, some researchers became impatient when they wanted to study a particular area but had to wait for an earthquake to occur. This became particularly difficult in areas where earthquakes did not occur frequently. Milne solved the problem by dropping a one-ton weight from a height of about 25 feet. The impact of this weight on the ground generated seismic waves that were weaker than, but similar to, those generated by earthquakes. To create stronger waves, seismologists explode charges of dynamite. These artificially induced shock waves have enough energy to reach deep into the planet. Since the 1970s, pistons on large trucks have been used to strike the earth and create artificial seismic waves.

When charges are exploded and the vibrations recorded by several nearby seismographs, a detailed description of rock layers can be detected. Since 1923, when a seismograph was first used to locate a large underground pool of petroleum, seismology has played a large part in the oil and gas industries. Earthquake waves artificially produced by explosions are also able to determine the location of underground geologic structures that may contain mineral deposits.

With the advent of the space age, seismographs connected to radio transmitters have been placed on the surfaces of the moon and Mars. There are more than a thousand seismographs in constant operation gathering seismic data around the world. Data from the National Earthquake Information Center are updated daily and are available on the Internet.

Earthquake Study and Prediction

The same technology that has indicated the location of discontinuities deep within the earth has also provided a greater knowledge about the crust. Whereas ancient civilizations feared earthquakes as manifestations of angry gods, quakes are now seen as the results of energy being released when plates of the earth's crust move past one another.

Although earthquakes do occur in many places on the earth's crust, they are most common in certain areas such as the “Ring of Fire” around the Pacific Ocean. Most earthquakes are linked directly to the movement of the earth “plates,” or sections of the crust. The Pacific plate and the North American plate meet along the San Andreas fault, which runs from western Mexico through California to the Pacific Ocean. The two plates are moving past each other along this fault. Each time there is movement along the fault, tremendous amounts of energy are released, and the earth quakes. Quakes along this fault and others have caused untold damage.

One of the primary goals of seismologists is to determine a way to predict exactly when earthquakes will occur. If this information were known in advance, people could prepare for quakes, and far fewer deaths would occur. Many phenomena have been observed before quakes, such as increased strains upon bedrock, changes in the earth's magnetic field, changes in seismic wave velocity, strange movements of animals, changes in groundwater levels, increased concentrations of rare gases in well water, geoelectric phenomena, and changes in ground elevation. Because none of these dependably occurs before every quake, these signs have not become reliable indicators.

Seismologists cannot prevent earthquakes from occurring, nor can they yet predict the exact time of a major quake, but they can predict where earthquakes are likely to occur. It is believed that certain active faults where there has been no earthquake activity for thirty years or so are about ready for an earthquake. With this information, urban and regional planners can provide for quake-resistant roads, bridges, and buildings.

Search for Oil and Gas Reservoirs

Seismology and the search for minor discontinuities play a great part in the search for oil, gas, and mineral resources. Since much petroleum is retrieved from off-shore locations where the crust of the earth is thinner, knowing the location of the Mohorovičić discontinuity sets the lower boundary for exploration.

Seismic studies are used regularly to assist in the search for oil and gas reservoirs. Natural gas and petroleum both can become trapped under some geologic formations. Seismologists routinely create a survey of an area before drilling to find minor discontinuities or boundaries between two different rock types, such as shale and sandstone. These surveys are made by measuring the reflection of seismic waves from the underlying rock layers. Geologic structures that can contain petroleum, natural gas, or mineral deposits can be identified from these surveys. Seismic surveys can show the distance and direction to these structures.

Principal Terms

crust: the top layer of the earth, composed largely of the igneous rock granite; it ranges from 3 to 42 miles in thickness

earthquake: a tremor caused by the release of energy when one section of the earth rapidly slips past another; earthquakes occur along faults or cracks in the earth's crust

earthquake waves: vibrations that emanate from an earthquake; earthquake waves can be measured with a seismograph

inner core: the innermost layer of the earth; the inner core is a solid ball with a radius of about 900 miles

mantle: the largest layer of the earth, about 1,800 miles in thickness; the mantle is within 3 miles of the earth's surface at some locations

outer core: the outer portion of the core, about 1,300 miles in thickness; it is believed to be composed of molten iron

seismograph: a device that measures earthquake waves

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