Oldham and Mohorovičić Determine the Earth's Interior Structure

Date 1906-1910

Richard Dixon Oldham, Andrija Mohorovi Mohorovičić, and other seismologists, using data from earthquakes, revealed the layered internal structure of the earth.

Also known as Mohorovičić Discontinuity

Locale England; Croatia

Key Figures

• Richard Dixon Oldham (1858-1936), Irish seismologist
• Beno Gutenberg (1889-1960), German American seismologist
• Andrija Mohorovičić (1857-1936), Croatian meteorologist
• Inge Lehmann (1888-1993), Danish seismologist

Summary of Event

Scientists must rely on indirect evidence to create a picture of the deep interior of the earth. This picture has thus changed over time as new evidence has become available. Only in the twentieth century, with the development of new measurement techniques, did a clear picture of the earth’s interior structure begin to emerge.

The preeminent geological text of the later half of the seventeenth century, German Jesuit and scholar Athanasius Kircher’s Mundus Subterraneus (1664), described the earth with a fiery central core from which emerged a web of channels that carried molten material into fire chambers or “glory holes” throughout the “bowels” of the earth. The “contraction” theory, popular through the first half of the nineteenth century, held that the earth had formed from material from the Sun and had since been slowly cooling, creating a solid crust. As the fluid cooled, it would contract, and the crust would collapse around the now smaller core, causing earthquakes and producing wrinkles that formed the surface features of mountains and valleys.

Physicists in the 1860’s, however, pointed out several physical consequences of the contraction model that appeared to be violated. For example, the gravitational force of the Moon, which creates tides in the ocean, would produce tides also in a fluid interior. The resulting effect should either cause the postulated thin crust to crack and produce earthquakes whenever the Moon was overhead or, if the crust were sufficiently elastic, cause tides in the crust as well. Instruments were developed to detect crustal tides, but no such tides were discovered.

By the beginning of the twentieth century, geologists were forced to abandon the contraction theory and conclude that the earth probably was completely solid and “as rigid as steel.” Thus when the German geophysicist and meteorologistAlfred Wegener suggested the idea of continental drift in 1912, it was dismissed on this basis as being physically impossible. Ironically, the very measuring instruments that overturned the simple liquid interior contraction theory would, in turn, disprove the solid-earth view that had challenged and replaced it. Seismographs—such as the inverted pendulum seismograph invented in 1900 by German seismologist Emil Wiechert—would provide a completely new source of information about the internal structure of the earth.

In 1900, Richard Dixon Oldham published a paper that established that earthquakes give rise to three separate forms of wave motion that travel through the earth at different rates and along different paths. When an earthquake occurs, it causes waves throughout the earth that fan out from the earthquake’s point of origin or “focus.” Primary (P) waves emanate like sound waves from the focus, successively compressing and expanding the surrounding material. Whereas P waves can travel through gases, liquids, and solids, secondary (S) waves can travel only through solids. S waves travel at about two-thirds the speed of P waves. Surface waves, the third type of seismic wave, travel only near the earth’s surface.

Seismologists now speak of seismic data as being able to provide an X-ray picture of the earth. In his groundbreaking 1906 article “The Earth’s Interior as Revealed by Earthquakes,” Oldham analyzed worldwide data on fourteen earthquakes. He also observed that within certain interior earth zones, P waves behaved differently from what he expected. In 1912, Beno Gutenberg was able to establish that at a certain depth the velocity at which the seismic waves traveled changed sharply. He estimated this depth to be about 2,900 kilometers (roughly 1,802 miles—almost half of the earth’s radius).

Oldham had also recognized in his own data the suggestion of a thin outer crust, but his data were insufficient to determine its depth. This estimation was to be made by Andrija Mohorovičić, a professor at the University of Zagreb, from an analysis (published in 1910) of an earthquake that had hit Croatia’s Kulpa Valley in late 1909. Mohorovičić had noticed that at any one seismic station, both P and S waves from the earthquake appeared in two sets, but the time between the sets varied according to how far away the station was from the earthquake’s focus.

Based on these different arrival times, Mohorovičić reasoned that waves from a single shock were taking two paths, one of which traveled for a time through a “faster” type of rock. He calculated the depth of this change in material. His estimate fell within the now-accepted figure of 20 to 70 kilometers (12.4 to 43.5 miles)—the crustal thickness varies under the continents and shrinks to 6 to 8 kilometers (about 3.7 to 5 miles) under the ocean. This boundary between crust and mantle is now called the Mohorovičić Discontinuity, or the Moho.

Significance

The use of seismic data to plumb the earth’s interior produced an outpouring of research, both theoretical and experimental. By the mid-1920’s, seismologists had learned to make subtler interpretations of their data and realized that no S waves had been observed to penetrate through the core. Given that S waves cannot pass through liquid, there was reason to think that the core was liquid.

Independent support for this hypothesis came from rigidity studies. Seismic waves could be used to determine the rigidity of the mantle; it was revealed to be much greater than the average rigidity of the earth as a whole (the existence of a low-density fluid region would account for this discrepancy). In 1936, however, Inge Lehmann was able to show that P waves passing close to the earth’s center changed velocity slightly, and she correctly inferred the existence of an inner core to account for this phenomenon. The current picture of the earth now includes a liquid outer core surrounding a solid inner core about 1,200 kilometers (745.6 miles) in radius.

Bibliography

Brush, Stephen G. “Inside the Earth.” Natural History (February, 1984): 26-34. Historical overview provides a good introduction to the topic for the lay reader. Explains clearly the various theories about the nature of the interior of the earth that have been proposed since the seventeenth century. Includes photographs of some of the scientists who made important contributions to this area in the twentieth century.

Bullen, K. E. “The Interior of the Earth.” In Adventures in Earth History, edited by Preston Cloud. San Francisco: W. H. Freeman, 1970. Reprint of a 1955 Scientific American article is a good source for details regarding the layered structure of the earth as revealed by seismic data. Presents information chronologically and notes persons who were most important in the development of the field, from those involved in the development of seismographs to the seismologists who interpreted the data. Features photographs of early seismometers, graphs, and diagrams.

Davison, Charles. “Richard Dixon Oldham: 1858-1936.” Obituary Notices of the Fellows of the Royal Society 2, no. 5 (1936): 110-113. One of very few published sources of biographical data on Oldham. Covers the major events of Oldham’s life, briefly describes his most important research, and includes a photograph.

Jeanloz, Raymond. “The Earth’s Core.” Scientific American 249 (September, 1983): 56-65. Discusses theoretical and experimental work on the nature of the earth’s core up to the early 1980’s. Does an excellent job of highlighting the lines of evidence for and against alternative hypotheses regarding the composition and development of the core. Explains views on how the core generates the earth’s magnetic field and how data on the field can provide information about the core. Clear diagrams highlight important concepts.

Mather, Kirtley F., ed. Source Book in Geology, 1900-1950. Cambridge, Mass.: Harvard University Press, 1967. Collection of historically important original articles on geological subjects from the first half of the twentieth century includes major portions of Oldham’s “The Constitution of the Interior of the Earth as Revealed by Earthquakes” (omitting only the data sections) and a very brief biographical sketch of Oldham. Also includes Gutenberg’s article “Mobility of the Earth’s Interior” (1941).

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.

Tarbuck, Edward J., and Frederick K. Lutgens. The Earth: An Introduction to Physical Geology. 8th ed. Upper Saddle River, N.J.: Prentice Hall, 2004. Introductory-level text offers a very good overview of the earth’s interior. Well written and illustrated with color graphics.