Lehmann Discovers the Earth’s Inner Core Summary

  • Last updated on November 10, 2022

Inge Lehmann’s hypothesis that the earth has an inner and outer core led to investigations that confirmed her theory.

Summary of Event

Prior to the development of the seismograph, Seismology an instrument that records vibrations of the earth, or earthquakes, Earthquakes very little was known about the composition of the inner parts of the earth. In the late 1800’s, a basic knowledge of vibrations generated by a seismic Seismic waves source evolved. It was found that these vibrations, or waves, travel outward through the earth at measurable speeds. [kw]Lehmann Discovers the Earth’s Inner Core (1936) [kw]Earth’s Inner Core, Lehmann Discovers the (1936)[Earths Inner Core, Lehmann Discovers the (1936)] [kw]Inner Core, Lehmann Discovers the Earth’s (1936) [kw]Core, Lehmann Discovers the Earth’s Inner (1936) Earth’s structure[Earths structure] Geology;Earth’s structure[Earths structure] [g]Denmark;1936: Lehmann Discovers the Earth’s Inner Core[09070] [c]Science and technology;1936: Lehmann Discovers the Earth’s Inner Core[09070] [c]Geology;1936: Lehmann Discovers the Earth’s Inner Core[09070] [c]Earth science;1936: Lehmann Discovers the Earth’s Inner Core[09070] Lehmann, Inge Milne, John

The science of geophysics recognizes two major types of “elastic” waves, which were defined by the British geologist Richard Dixon Oldham. Oldham, Richard Dixon The first type is called a P wave because it causes a “primary” disturbance that deforms the earth by means of alternately lengthening and shortening in the direction of the source of the wave. P waves are also called “compressional waves” because the volume of the earth that is affected is alternately compressed and expanded. The second type of elastic wave is the S wave, which produces a “secondary” disturbance. The S wave is a transverse body wave that travels through the interior of an elastic medium. S waves do not change the volume of the medium, but they do change its shape; for this reason, they are also called “distortional waves” or “shear waves.” Both P waves and S waves pass through the interior of the earth; for this reason, they are called “body waves.”

When an earthquake occurs, its waves travel through the body of the earth and are recorded by seismographs at earthquake observatories. These seismic waves carry to the surface information about the material through which they have passed. In 1883, the English engineer John Milne surmised that every large earthquake at any point of the globe could be recorded if there were an instrument designed for that purpose; however, it was not until 1893 that he perfected the first clockwork-powered seismograph.

In later years, when extremely sensitive seismographs had been developed, it was found that some weak P waves actually were penetrating a shadow zone, an area opposite the projected core of the earth. The shadow zone, discovered in the research of the Croatian geophysicist Andrija Mohorovičić, Mohorovičić, Andrija was left unexplained in earlier research conducted by pioneers in the use of seismographs to map the interior. Inge Lehmann postulated the existence of an inner core that could reflect the rays back into the shadow zone.

At the Copenhagen Seismological Observatory, Lehmann had for a number of years been clearly observing, through the core, seismic waves caused by earthquakes in the Pacific Ocean. Among these were the shocks that occurred at Murchison and Hawke’s Bay in New Zealand in 1928 and 1931, respectively. It was evident from these records that a P-type wave that should have been within the shadow zone was arriving at seismological stations. This phenomenon could be explained only by the existence of an inner core that was about 1,250 kilometers (roughly 777 miles) in radius and was denser than the outer core.

Lehmann believed that core waves could be classified into three separate types of P wave. The standard explanation for the first two of these wave types was that their rays were refracted at the boundary between the mantle and core and focused toward the antipodes, placed opposite each other on the globe. She explained that waves of the third type were reflections from another sharp discontinuity within the core itself. This family of waves is made up of the core refractions. Beyond about 103 degrees, the direct P wave cannot be recorded because of the shadow effect of the core. Beyond this distance, the first wave to appear on long-period instruments is often a PP wave, which does not penetrate so deeply and therefore is able to avoid the obstacle. Short-period instruments show a refracted wave arising from complexities within the core, but it is not quite as prominent as P when it makes its reappearance at 142 degrees. Because it is deflected from its path and disappears altogether for nearly 40 degrees, it is called a PKP wave (K stands for Kern, the German word for “core”).

In 1936, after ten years of interpreting seismograms (records made by a seismograph) and using a well-established scientific method, Lehmann was prepared to discover the inner core. Her first step was to calculate a direct problem. She assumed an earth model that was particularly simple. It had constant velocities in the mantle (10 kilometers, or 6.2 miles, per second) and in the core (8 kilometers, or 5 miles, per second). These were reasonable average values for both regions. She then introduced a small central core, which again had a constant velocity. These simplifications enabled her to view the seismic rays as straight lines; therefore, their travel times could be calculated by using elementary trigonometry. She then showed by making successive adjustments that a reasonable velocity and radius of the inner core could be found that predicted a travel-time curve close to the observations of the third type of P wave. In effect, she proved an existence theorem: A plausible three-shell earth structure could be defined that explained the features of the observed waves.





Lehmann’s discovery of the inner core was very complicated, but it convinced Beno Gutenberg Gutenberg, Beno in the United States and Harold Jeffreys Jeffreys, Harold in England that her hypothesis was a viable one. Within two years, they had independently carried out more detailed calculations involving many observed travel times of P waves and calculated by means of an inverse method both the radius of the inner core and the P-velocity distribution in it.


After the discovery of the earth’s inner core, the measured travel times could be transformed, using inverse theory, into plausible P velocities in the mantle and the outer core. In late 1938 and 1939, Gutenberg and Jeffreys computed, independently, the average velocity based on thousands of observed travel times of P and S waves. Their agreement was extremely close; in fact, their calculations were so well developed that they have not been seriously altered since.

As a result of the development of sensitive seismographs, an increase in the number of seismographic stations around the world, and the availability of large-capacity computers, a better understanding of the earth has become possible. The core has a role in many geophysical studies, and the way it is affected during great earthquakes is being probed actively. If the physical properties inside the earth were better known, the frequencies and amplitude patterns of the resonant vibrations could be calculated, thereby making it possible to prevent loss of life. Earth’s structure[Earths structure] Geology;Earth’s structure[Earths structure]

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Bolt, Bruce A. Inside the Earth: Evidence from Earthquakes. 1982. Reprint. Fairfax, Va.: Techbooks, 1991. An easy-to-read, copiously illustrated introductory textbook covering the evolution of knowledge of the middle earth, types and measurements of earthquake waves, main shells of the earth, structural vibrations densities, elastic properties, and temperatures. Includes a guide to further reading.
  • citation-type="booksimple"

    xlink:type="simple">Clark, Sydney P., Jr. Structure of the Earth. Englewood Cliffs, N.J.: Prentice-Hall, 1971. Brief and very readable introduction to the earth’s structure. Areas covered include geologic structures, the earth’s magnetic field, plate tectonics, seismology, and heat flow and the earth’s temperature. Includes many illustrations and references as well as suggestions for further reading.
  • citation-type="booksimple"

    xlink:type="simple">Jacobs, I. A. The Earth’s Core. New York: Academic Press, 1975. Geared for graduate students and research workers in geophysics. Focuses on seismology and geomagnetism; contains many graphs and mathematical computations. Areas covered include the general properties of the earth, the origin of the earth’s core, and the thermal regime of the earth’s core. Includes both author and subject indexes.
  • citation-type="booksimple"

    xlink:type="simple">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.
  • citation-type="booksimple"

    xlink:type="simple">Strahler, Arthur N., and Alan H. Strahler. Environmental Geoscience: Interaction Between Natural Systems and Man. New York: John Wiley & Sons, 1973. Introductory college textbook weaves the basic principles of geoscience with information on environmental and resource problems. Of particular interest is the discussion of the magnetosphere and the core as the source for the generation of the earth’s magnetism. Abundantly illustrated.
  • citation-type="booksimple"

    xlink:type="simple">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.

Oldham and Mohorovičić Determine the Earth’s Interior Structure

Gutenberg Discovers Earth’s Mantle-Outer Core Boundary

Richter Develops a Scale for Measuring Earthquake Strength

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