Geologist Richard Oldham's 1906 Paper on Seismic Wave Transmission Establishes the Existence of Earth's Core and Demonstrates the Value of Seismology for Studying the Structure of Earth's Deep Interior

Overview

In 1906 Richard Dixon Oldham (1858-1936) established the existence of Earth's core and the importance of seismic data for studying the structure of the planet's deep interior. By observing ways in which seismic waves were reflected and refracted, different boundary layers were later identified. Differences in wave speeds also provided information about density and rigidity as a function of radius. Seismic analyses allowed Andrija Mohorovicic (1857-1936) to identify the core-mantle boundary (1910), Beno Gutenberg (1889-1960) to deduce the core boundary (1912), Harold Jeffreys (1891-1989) to demonstrate that the core is liquid (1926), and Inge Lehmann (1888-1993) to discover the solid inner-core (1936).

Background

Magnetism provided one of the first clues to the structure of Earth's interior. Using a spherical piece of magnetized iron to simulate Earth, William Gilbert (1544-1603) found that the pattern of magnetic lines around the ore matched compass-needle patterns observed over Earth's surface (1600). This suggested that Earth contains substantial amounts of magnetized iron. However, as compass data accumulated it appeared Earth's magnetic field was slowly drifting westward—an effect that seemed impossible if Earth's interior were solid iron.

In 1692 Edmond Halley (1656-1742) proposed a core-fluid-crust model to explain this effect. According to Halley, Earth's magnetic field is produced by a solid iron core. Earth's outer shell or crust is separated from the core by a fluid region. The westward drift of the magnetic field was the result of the core rotating eastward slightly slower than the crust.

Evidence increasingly mounted indicating that Earth had a completely molten interior. Volcanoes were observed to spew forth hot material; many surface rocks appeared to have formed by crystallization from extremely hot material; and measurements indicated temperature increases with depth that extrapolated to temperatures well beyond the melting point of all known rocks at a depth of about 50 miles (80.5 km). The idea that Earth had a relatively thin crust and molten interior thus gained ascendancy after the late seventeenth century.

This view came under heavy attack beginning in the 1830s. André-Marie Ampère (1775-1836) argued that the tidal forces exerted on such an enormous volume of liquid by the Moon would render the surface unstable (1833). William Hopkins (1793-1866) echoed Ampère's critique and also noted that since the melting temperature of most substances increases with pressure, the central core might well be solid. Hopkins also adduced astronomical evidence that indicated the crust must be at least 621 miles (1,000 km) thick (1842). Lord Kelvin (1824-1907) produced similar arguments and suggested that experiments be conducted to determine the vertical motions of Earth's surface with respect to lunar and solar positions. These measurements seemed to support Kelvin's claim that Earth was "as rigid as steel," and by the end of the nineteenth century most geologists accepted that Earth was completely solid.

A new source for analyzing the structure of Earth's interior emerged at the end of the nineteenth century when British seismologist John Milne (1850-1913) suggested that with sufficiently sensitive instruments one could detect earthquakes anywhere in the world (1883). This was realized in 1889 when Ernst von Rebeur-Paschwitz (1861-1895) demonstrated that seismometers in Germany had detected vibrations that had passed through Earth's interior from an earthquake that had occurred in Tokyo. These seismic waves could be analyzed using the mathematical theory of wave motion to extract information about the physical constitution of Earth's interior.

Wave theory as applied to seismic analysis predicts three types of waves: primary (Pwaves), secondary (S-waves), and surface waves. P-waves can be transmitted though solids, fluids, and gases. S-waves, however, can be transmitted only through solids, not fluids.

In his landmark report on the 1897 Assam, India, earthquake, Richard Oldham provided the first clear evidence for P-waves, S-waves, and surface waves (1899). Because seismometers located on the other side of Earth from the earthquake detected P-waves later than expected, Oldham concluded Earth must have a central core through which P-waves travel with a substantially lower velocity than the surrounding material. Also, since Earth was believed to be completely solid, he concluded the slower velocity could only be due to a much denser core, which he suggested was probably iron.

The details of Oldham's initial arguments were unconvincing. After collecting further data and refining his ideas he reasserted his initial conclusion in "The Constitution of the Interior of the Earth as Revealed by Earthquakes" (1906). In this work he conclusively established the existence of Earth's core. He did not however, as is widely believed, discover the core to be liquid. He maintained his original position—later shown to be incorrect—that the core is extremely dense and probably iron.

Impact

Oldham's analyses stimulated further seismic research on Earth's interior. While studying 1909 earthquake data, Andrija Mohorovicic noticed changes in seismic wave velocities. Mohorovicic attributed the effect to transmission through two layers of different density. He identified the boundary between these two layers as a sharp discontinuity now known to vary from 6.21 miles (10 km) under the oceans up to 31 miles (50 km) under continents (1910). Thus the Mohorovicic or "Moho" discontinuity between Earth's crust and mantle was established.

Another discontinuity was discovered by Beno Gutenberg in 1912. Gutenberg was then a student of Emil Wiechert (1861-1928) at Göttingen, Germany—a center for seismic research. Wiechert's theories provided a fairly reliable guide to seismic wave paths through Earth's interior, and mathematicians at Göttingen had developed practical computational methods based on his work for calculating variations in wave velocity with depth. Gutenberg applied these methods to determine P- and S-wave velocities in Earth's interior. His results indicated a major discontinuity at a depth of 1,802 miles (2,900 km). This is now called the Gutenberg discontinuity and marks the boundary between the mantle and core.

By 1913 Oldham became increasingly convinced that his data showed no indication of S-wave transmission through the core. He thus began to move toward the idea that Earth's core was fluid. He was anticipated in this by Leonid Leybenzon (1879-1951), whose 1911 Russian publication attracted little attention. In 1924 Wiechert suggested the sudden velocity drop below the Gutenberg discontinuity was indirect evidence that the core was fluid.

Gutenberg, however, remained firmly convinced that Earth's core was solid. He reviewed six methods for calculating core rigidity and concluded that all but one clearly indicated a solid core. The sixth method only required a fluid core if S-waves were not transmitted. Gutenberg dealt with this possibility by explaining why S-wave transmission had not yet been observed.

The fluidity of Earth's core was finally established in 1926 by Sir Harold Jeffreys. Jeffreys was able to show that each of the methods Gutenberg referred to could be suitably interpreted to support the fluid-core theory. His most conclusive demonstration was to show that the average mantle rigidity was much greater than the average rigidity of the entire Earth, thus requiring a compensating region of lower rigidity below the mantle. This obviously had to be within the core. The low core rigidity of Jeffrey's fluid-model precluded S-waves transmission.

In 1936 Inge Lehmann further refined our understanding of Earth's interior. Analyzing data from the 1929 Buller, New Zealand, earthquake, she showed that P-waves were being reflected from a sharp boundary within the core. She calculated the boundary radius to be 870 miles (1,400 km) at what is now called the "Lehmann discontinuity." Lehmann went on to argue for the solidity of the inner core, but this was not conclusively established until the early 1970s.

This model of Earth's interior—of a central iron core surrounded by a fluid then solid crust—is similar to Halley's 1692 model, but there are significant differences. Halley failed to specify the core radius and the depth of the fluid, and he had no idea how Earth's rigidity varied with depth. Halley's account of terrestrial magnetism also differs from the modern explanation.

Iron loses its magnetism at extremely high temperatures such as those believed to exist at the inner core. Consequently, Earth's iron core cannot generate terrestrial magnetism. However, it was discovered in the nineteenth century that electric currents produce magnetic fields. Ampère suggested that Earth's magnetic field and its variations might be the result of currents flowing in the planet's interior.

Walter Elsasser (1904-1991) developed this idea and in 1946 proposed a geomagnetic dynamo theory. According to Elsasser's theory, electric currents are generated within the outer-core by the induction effects of the moving fluid much as a dynamo generates electricity. Edward Ballard developed a similar though more detailed theory in 1948. Research has continued, but only in the last few years of the twentieth century have scientists been able to produce computer models of the geomagnetic dynamo capable of accurately simulating the westward magnetic drift, spontaneous pole reversals, and other secular magnetic variations.

STEPHEN D. NORTON

Further Reading

Books

Brush, Stephen G. Nebulous Earth. Cambridge: Cambridge University Press, 1996.

Jeffreys, Harold. The Earth: Its Origin, History and Physical Constitution. 6th edition. Cambridge: Cambridge University Press, 1976.

Oldroyd, David. Thinking about the Earth: A History of Ideas in Geology. Cambridge, MA: Harvard University Press, 1996.

Periodical Articles

Brush, Stephen G. "19th-Century Debates about the Inside of the Earth: Solid, Liquid, or Gas?" Annals of Science 26 (1979): 225-254.

Brush, Stephen G. "Discovery of the Earth's Core." American Journal of Physics 48 (1980): 705-724.

Jeffreys, Harold. "The Rigidity of Earth's Central Core." M. N. Royal Astronomical Society 77 (1926): 371-383.

Oldham, Richard. D. "The Constitution of the Interior of the Earth as Revealed by Earthquakes." Quarterly Journal of the Geological Society of London 62 (1906): 456-472.