Gutenberg Discovers Earth’s Mantle-Outer Core Boundary

Beno Gutenberg expanded the use of seismographs to the global scale when he discovered the boundary between the earth’s outer core and the lower mantle.

Summary of Event

Very little is known about the interior of the earth because of its inaccessibility, even to modern technology. Based on evidence gained through indirect and secondary research and experiments, scientists have been able to determine that the earth consists of layers. The uppermost layer, on or in which most life exists, is the crust. Scientists have determined the structure of the remainder of the earth’s interior and the size and composition of the deeper layers through various methods. The original source of information that founded modern understanding of the earth’s structure was seismology, Seismology the study of earthquakes and vibrations of the earth. Earth’s structure[Earths structure]
Mantle-outer core boundary[Mantle outer core]
Geology;Earth’s structure[Earths structure]
[kw]Gutenberg Discovers Earth’s Mantle-Outer Core Boundary (1913)
[kw]Earth’s Mantle-Outer Core Boundary, Gutenberg Discovers (1913)[Earths Mantle Outer Core Boundary, Gutenberg Discovers (1913)]
[kw]Mantle-Outer Core Boundary, Gutenberg Discovers Earth’s (1913)[Mantle Outer Core Boundary, Gutenberg Discovers Earths (1913)]
[kw]Core Boundary, Gutenberg Discovers Earth’s Mantle-Outer (1913)
Earth’s structure[Earths structure]
Mantle-outer core boundary[Mantle outer core]
Geology;Earth’s structure[Earths structure]
[g]Germany;1913: Gutenberg Discovers Earth’s Mantle-Outer Core Boundary[03270]
[c]Science and technology;1913: Gutenberg Discovers Earth’s Mantle-Outer Core Boundary[03270]
[c]Earth science;1913: Gutenberg Discovers Earth’s Mantle-Outer Core Boundary[03270]
[c]Geology;1913: Gutenberg Discovers Earth’s Mantle-Outer Core Boundary[03270]
Gutenberg, Beno
Milne, John
Mohorovičić, Andrija

The pioneers in the field of seismology used needle-and-paper seismographs Seismographs —instruments that measure and record vibrations—to study wave patterns. In 1893, one of these pioneers, John Milne, perfected a clockwork-powered seismograph that produced a record of vibrations on light-sensitive film. The Milne seismograph was a self-recording instrument capable of preserving a record of ground movement. Milne discovered that the vibrations from a distant earthquake arrived in a series of separate vibrations traveling at different speeds. The greater the separation between the waves of different types, the farther away the earthquake. The method was identical to that used to calculate the distance of a storm from the time between the arrival of the lightning and the sound of the thunder.

The Milne seismograph allowed for the identification of three different kinds of waves. The fastest waves are those of compression, or P waves, which move through the air in a manner similar to sound waves. Another type are shear waves, or S waves, which move by the sideways motion of the particle. The third type, surface waves, move along boundaries, such as the boundary between the rock and the air or the water. Surface waves are a varied group that behave in ways similar to waves in the sea.

Milne’s early seismograph allowed for the base recording of earthquake waves. Later improvements on his model were computerized, with the waves recorded in binary code rather than on paper. Although the later models were more precise, recording new waves that aroused new investigations, Milne’s early seismograph allowed researchers to make great discoveries. Later research in the field of seismology used the records from the seismograph—called seismograms—to learn the composition of the earth by comparing density curves to wave curves. Their research revealed that the innermost section of the earth is composed possibly of pure iron. Surrounding the inner core is the molten outer core, with a radius of approximately 2,175 miles (3,500 kilometers) and composed of an iron alloy with a metal lighter than nickel. The mantle of the earth is the area between the outer core and the crust. Probably the most active area in the layers of the earth, the mantle contains large amounts of the mineral olivine. The mantle is approximately 1,864 miles (3,000 kilometers) thick and consists of two parts, with the lower mantle forming the boundary between the upper mantle and the core. This boundary between the mantle and the outer core was discovered by Beno Gutenberg.

Gutenberg’s method of research was first used by Andrija Mohorovičić, a Yugoslavian meteorologist who was one of the pioneers in the science of seismology. The key event that spurred Mohorovičić’s inspiration was a minor earthquake in 1909 in Zagreb, Yugoslavia (now in Croatia). When Mohorovičić examined the seismogram of the earthquake, he found two primary (P) and two secondary (S) waves recorded for each tremor. The separate groups of P and S waves appeared to be traveling at different velocities. Because the type of material as well as its density changes the rate, and sometimes the direction, of the wave’s speed, the seismogram indicated the presence of a layer of material under the earth’s outer crust that was dense enough to alter the velocity of the second group of P and S waves. Because the second wave group reached the recording stations before the first wave group, Mohorovičić deduced that the boundary—now called the Mohorovičić Discontinuity Mohorovičić Discontinuity[Mohorovicic Discontinuity] —was denser than the crust. Along with the discovery of the boundary between the earth’s outer crust and the upper mantle, Mohorovičić contributed a new application of seismology in exploring the interior structure of the earth.

Gutenberg, a graduate student in Germany, was encouraged to begin his own studies in seismology two years after the discovery of the Mohorovičić Discontinuity. Gutenberg’s area of concentration was the mysterious “shadow zone” of seismology. For some unknown reason, P waves disappeared when passing through an area 2,734 miles (4,400 kilometers) wide on the side of the earth opposite to the focus, or epicenter, of the earthquake. Gutenberg began a mathematical investigation to explain the temporary disappearance of P waves. The most likely answer was suggested by the work of Richard Dixon Oldham, Oldham, Richard Dixon a geologist, and Emil Wiechert, Wiechert, Emil a geophysicist. Their independently formed theories proposed that the center of the earth is composed of a large, dense, and perhaps partially molten core.

Assuming that Oldham and Wiechert were correct, Gutenberg made mathematical models of the effects a dense core would have on P waves. He positioned his hypothetical core at various depths in the earth and then calculated the course and behavior of P waves in each. After comparing his models to actual seismograph readings, he discovered one that confirmed his work. The model that eventually matched the real graphs was based on a core 1,802 miles (2,900 kilometers) below the surface of the earth. The shadow zone was the boundary between the lower mantle and the outer core. Later research that continued with Gutenberg’s original outline altered the accuracy of his placement only slightly and revealed that the outer edge of the core must be molten. Gutenberg’s discovery of the boundary between the mantle and the outer core is called the Gutenberg Discontinuity. Gutenberg Discontinuity


Gutenberg’s confirmation of the presence of the earth’s core and his discovery of the boundary between the outer core and the lower mantle in 1913 solved the old mystery of the shadow zone and raised new questions regarding the composition of these areas. In addition to the significance of the discovery of the boundary between the earth’s lower mantle and outer core, the research techniques that Gutenberg applied opened new avenues for tackling old problems and also provided a built-in continuation of the original research.

As a sideline to Gutenberg’s major discovery, later research found that the Gutenberg Discontinuity also marks the lowest edges of the continental plates. Through the use of seismographs, scientists can apply the speed of earthquake waves to learn the composition of a material through which a random wave will pass. Because he found that the waves traveled more slowly in the lower part of the mantle, Gutenberg postulated that the lower mantle is softer than the upper mantle. This theory shed new light on the study of plate tectonics, for if the lower mantle is the layer in which the plates extend the deepest and it is partially molten, the movement of plates is more easily explained.

With the use of seismic waves, Gutenberg and Mohorovičić revealed the interior structure of the earth. The improved sensitivity of later technology revealed a previously unrecorded set of P waves in the shadow zone of the earth. Based on Gutenberg’s findings, the understanding that seismologists had of the core did not coincide with this energy rebound. Using the later, more sensitive records for her research, Inge Lehmann solved the mystery by proposing the existence of an inner core that could reflect the waves. Through the use of seismic velocities, K. E. Bullen Bullen, K. E. proved that the earth’s inner core is solid. Despite Bullen’s ingenious argument, however, one important wave pattern is missing from the records. If the inner core is solid, there should be an extra wave recorded, but it has not appeared in seismographs. Although the interior of the earth is solid, the outer core is molten. The use of densities, as indicated by wave patterns, is essential in determining the composition of the earth’s layers. The easier method of comparing infrared densities in the earth with known densities is inapplicable in this case because of the temperatures and pressures found in the earth that cannot be duplicated in laboratories. Thus scientists must compare observed seismic wave speeds and estimated densities.

Gutenberg later used the seismograph again while working with his colleague Charles Francis Richter Richter, Charles Francis at the California Institute of Technology. A universal way of judging the absolute size of earthquakes was needed, as well as a scale on which to measure earthquakes. Richter developed a way to define the amplitudes of magnitudes of earthquakes. Richter scale One forms magnitudes by plotting a curve based on the amplitudes of ground motion against the distance of seismograph recording stations from the epicenter, or heart, of the earthquake. Richter used the logarithms of the amplitudes of the earthquakes as they could be determined to the nearest tenth magnitude. Along with Gutenberg, he developed three scales by which to judge earthquakes: one for near earthquakes, one for distant shallow earthquakes, and one for deep-focus earthquakes. Earth’s structure[Earths structure]
Mantle-outer core boundary[Mantle outer core]
Geology;Earth’s structure[Earths structure]

Further Reading

  • Bolt, Bruce A. Inside the Earth: Evidence from Earthquakes. 1982. Reprint. Fairfax, Va.: Techbooks, 1991. Intended as an introductory textbook for students in the fields of earth sciences, physics, and engineering. Nonmathematical discussion can be understood by students in other fields. Gives the reader a clear and basic knowledge of seismology and its uses and discoveries.
  • Clark, Sydney P., Jr. Structure of the Earth. Englewood Cliffs, N.J.: Prentice-Hall, 1971. Introductory volume discusses the structure of the earth and its composition as well as the methods of seismology used to determine these. A good source of information for anyone interested in learning more about the dynamics of the earth.
  • Hodgson, John H. Earthquakes and Earth Structure. Englewood Cliffs, N.J.: Prentice-Hall, 1964. An excellent study of the relationship of earthquakes to revelations of earth structure. Presented in language understandable to the general reader interested in the science and applications of seismology.
  • Jacobs, J. A. The Earth’s Core. New York: Academic Press, 1975. Intended for graduate students and researchers. Discusses the geophysics of the layers of the earth, with a concentration on the core and its special properties.
  • Phillips, O. M. The Heart of the Earth. San Francisco: Freeman, Cooper, 1968. A technical volume on the science of geophysics, recommended for readers with a serious interest in earth science. Describes the workings of the earth as a complex machine.
  • Stein, Seth, and Michael Wysession. An Introduction to Seismology, Earthquakes, and Earth Structure. New York: Blackwell Science, 2002. Introductory textbook intended for advanced undergraduate and first-year graduate courses in seismology. Heavily illustrated. Includes suggestions for further reading and index.
  • Weiner, Jonathan. The Planet Earth. New York: Bantam Books, 1986. Companion volume to a PBS television series provides an introductory-level discussion of earth science in general and also addresses many topics in greater depth.
  • Wood, Robert Muir. Earthquakes and Volcanoes: Causes, Effects, and Predictions. New York: Weidenfeld & Nicolson, 1987. Provides excellent explanations of the science behind earthquakes, the cause of earthquakes, and the history of earthquakes for the average reader. Fully illustrated and includes a glossary and a list of major earthquakes.

Oldham and Mohorovičić Determine the Earth’s Interior Structure

Richter Develops a Scale for Measuring Earthquake Strength

Lehmann Discovers the Earth’s Inner Core