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Structure and Composition of the Earth
The interior of planet Earth is a remote and inaccessible place. Humans have barely scratched the surface, penetrating just 12.8 km into a planet averaging 6771 km in radius. Nevertheless, much has been learned about Earth’s interior, and much will be learned. The purpose of science is to determine the true nature of Earth and Universe, and to convey that understanding truthfully to people everywhere. The following presents a step-by-step logical progression of understanding of important, fundamental discoveries and insights about the interior of the Earth.
The progress of scientific discovery, as noted by J. Marvin Herndon , is much like progressing along a path through the wilderness. As long as one stays on the correct path, new insights and new discoveries will point the way toward further insights and discoveries. But, stray from the path and confusion reigns; further progress becomes impossible. Currently, in the scientific literature and in textbooks, there is much confusion about the interior of Earth. The source of confusion can be traced to a fundamental mistake made more than sixty years ago, which has been exacerbated by the practice of making models based upon assumptions, rather than making discoveries.
Investigations of earthquake waves, augmented by studies of the planet’s spin, provide knowledge of the physical structures within the Earth (Figure 1). The chemical compositions of those structures, though, are deduced from implications derived from meteorites. The Sun, Earth, Moon, meteorites, and presumably all the objects in the Solar System formed from matter of common origin . That primordial composition, except for quite volatile elements, is yet manifest in chondrite meteorites and in the photosphere of the Sun. There is good reason to think that the elemental composition of the Earth as a whole is quite similar to that of a chondrite meteorite. The problem is there are three distinct groups of chondrites, each characterized by a different relative amount of oxygen. The fundamental mistake, made more than sixty years ago, was in assuming that the Earth is like a chondrite from the wrong chondrite group.
Figure 1.Velocities of earthquake compression waves (solid) and shear waves (dashed) as a function of radius within Earth compared to the structures they delineate.
Scientific knowledge of the interior of Earth had its beginnings in the recent past. In 1788, Henry Cavendish measured the average density of the Earth, reporting it to be 5.48 times the density of water , a result quite close to the modern measured value. 5.53. In 1897, Emil Wiechert realized that Earth’s density is too great for it to be composed solely of rock, and suggested that the Earth might have a core made of iron metal, like the iron meteorites he had seen in museums . In 1906, from investigations of earthquake waves, Richard Dixon Oldham discovered the Earth’s core, an object later found to be fluid and to comprise nearly one-third the mass of the planet.
By the mid-1930s, the picture that had emerged was of the fluid core surrounded by a rocky mantle, topped by a thin crust, which had been discovered by Andriaj Mohorovicic in 1909 . It was thus easy to imagine that the Earth is like an ordinary chondrite, one of the most common types of meteorites that fall to Earth, and which appear to consist of a mixture of iron metal, iron sulfide, and silicate rock. At elevated temperatures, the iron metal and iron sulfide would alloy together as a melt.
When earthquake waves pass from one substance into another at an angle, they change speed and direction. Consequently, there is a region, called the “shadow zone”, where earthquake waves should be undetectable, but earthquake waves were in fact detected in the shadow zone. In 1936, the Danish seismologist, Inge Lehmann, proposed the solution to the mystery and in doing so discovered the Earth’s inner core . As shown in Figure 2, Lehmann imagined that if a solid object, now called the inner core, exists within the fluid core, earthquake waves would be reflected into the shadow zone. The inner core is slightly smaller than the Moon and nearly three times as massive. So, what is the composition of Earth’s inner core? This became the problem to solve and ushered in the mistake which would confuse geophysics for more than half a century.
Figure 2.Inge Lehmann’s original diagram of the discovery of Earth's inner core. The added red arrow points to the ray reflected into the shadow zone, which is the region between her numbers 2 and 3.
Francis Birch thought that nickel and iron were always alloyed together in meteorites. He also knew that all elements heavier than nickel and iron, even if taken together, were insufficiently abundant to comprise a mass as great as the inner core. So, Birch made the assumption that the inner core is nickel-containing iron metal which has started to freeze from the liquid iron alloy core, although there is no reason from that standpoint for the inner core to be the size observed; it might be any size. Beginning in 1940, the idea of the Earth’s inner core being partially crystallized iron metal became firmly entrenched and much of geophysics was subsequently built around that idea and the idea that the Earth is like an ordinary chondrite meteorite .
In the 1970s, J. Marvin Herndon began investigating the rare, oxygen-poor group of enstatite chondrites that Birch had ignored. Herndon realized that two discoveries made in the 1960s admitted a different idea for the inner core. Elemental silicon was found in the metal of enstatite chondrites  and a compound of nickel and silicon, nickel silicide, was observed . To Herndon, the implication was clear. If silicon exists in the Earth’s core, as would be the case if the interior of Earth is like an enstatite chondrite, then under appropriate circumstances the silicon could cause nickel to precipitate from the fluid core as the compound nickel silicide. The inner core, being fully crystallized nickel silicide, would have precisely the mass observed for the inner core. Herndon published the idea of the inner core being nickel silicide in the Proceedings of the Royal Society of London .
Herndon next showed that in matter, like that of ordinary chondrites, too much iron is combined with oxygen in the silicates so that there is an insufficient proportion of iron alloy to account for the Earth’s massive core [11-13] (Figure 3). The Earth is not like an ordinary chondrite meteorite.
Figure 3.Only the enstatite chondrites have a sufficiently high percentage of iron alloy to be like Earth. The reason has to do with oxygen; the greater the relative proportion of oxygen, the more iron occurs as FeO in silicates, leaving less iron in the alloy.
Herndon has shown that the mass ratio of alloy to silicates of the Abee enstatite chondrite is virtually identical to the ratio of the core to lower mantle of the Earth, leading to a precise prediction of the boundary between the upper and lower mantle, implying that earthquake waves change speed and direction at that boundary because of differences in composition, not simply differences in crystal structure as had been thought.
Fundamental Earth Mass Ratio
Earth Ratio Value
Abee Ratio Value
lower mantle to total core
inner core to total core
0.052 – 0.057 (theoretical)
inner core to (lower mantle+core)
In 1946, Keith Bullen first discussed the possibility of some seismic irregularity at the boundary between the core and the lower mantle . Subsequent investigations confirmed the existence of “islands” of matter at the boundary of the core [15, 16], Those “islands” of matter have no logical basis in Birch’s view of Earth being like an ordinary chondrite [17, 18], but instead are readily understandable, as Herndon has shown, in a logical and causally related way as low-density, high-temperature precipitates from the Earth’s enstatite-chondritic-like core [12, 13, 19, 20] (Figure 4).
Figure 4.Composition of major parts of the Earth as derived by Herndon. The compositions of the upper mantle shells are not known with certainty.
In the highly reduced enstatite chondrites, like the Abee meteorite, and in the Interior of Earth, the availability of oxygen during formation was limited. Thus, some oxygen-loving elements, like silicon, magnesium, and calcium, occur in part in the alloy portion (Figure 5).
Figure 5.Bar graph showing the amounts of the major and minor elements in the Earth’s lower mantle and core, by the identity with the Abee enstatite chondrite.
Uranium to a great extent occurs in the alloy portion of the Abee meteorite and thus is expected to occur in Earth’s core. J. Marvin Herndon has demonstrated the feasibility of a natural occurring nuclear fission reactor at the center of the Earth, called the georeactor, as the energy source and mechanism for generating the geomagnetic field through a dynamo mechanism in the presumably fluid fission product sub-shell surrounding the uranium sub-core [19, 21, 22]. Figure 6 shows schematically the major sections of the interior and the anticipated location of the georeactor.
Figure 6.Schematic representation of the major parts of the Earth and showing the location and structure of Herndon’s georeactor.
For further information, see http://UnderstandEarth.com
1.Herndon, J.M., Maverick's Earth and Universe. 2008, Vancouver: Trafford Publishing. ISBN 978-1-4251-4132-5.
2.Herndon, J.M., Reevaporation of condensed matter during the formation of the solar system. Proceedings of the Royal Society of London, 1978. A363: p. 283-288.
3.Cavendish, H., Experiments to determine the density of Earth. Philosophical Transactions of the Royal Society of London, 1798. 88: p. 469-479.
4.Wiechert, E., Ueber die Massenverteilung im Inneren der Erde. Nachr. K. Ges. Wiss. Goettingen, Math.-Kl., 1897: p. 221-243.
5.Mohorovicic, A., Jb. Met. Obs. Zagreb, 1909. 9: p. 1-63.
6.Lehmann, I., P'. Publ. Int. Geod. Geophys. Union, Assoc. Seismol., Ser. A, Trav. Sci., 1936. 14: p. 87-115.
7.Birch, F., The transformation of iron at high pressures, and the problem of the earth's magnetism. American Journal of Science, 1940. 238: p. 192-211.
8.Ringwood, A.E., Silicon in the metal of enstatite chondrites and some geochemical implications. Geochimica et Cosmochimica Acta, 1961. 25: p. 1-13.
9.Ramdohr, P., Einiges ueber Opakerze im Achondriten und Enstatitachondriten. Abh. D. Akad. Wiss. Ber., Kl. Chem., Geol., Biol., 1964. 5: p. 1-20.
10.Herndon, J.M., The nickel silicide inner core of the Earth. Proceedings of the Royal Society of London, 1979. A368: p. 495-500.
11.Herndon, J.M., The chemical composition of the interior shells of the Earth. Proceedings of the Royal Society of London, 1980. A372: p. 149-154.
12.Herndon, J.M., Composition of the deep interior of the earth: divergent geophysical development with fundamentally different geophysical implications. Physics of Earth and Planetary Interiors, 1998. 105: p. 1-4.
13.Herndon, J.M., Scientific basis of knowledge on Earth's composition. Current Science, 2005. 88(7): p. 1034-1037.
14.Bullen, K.E., A hypothesis on compressibility at pressures on the order of a million atmospheres. Nature, 1946. 157: p. 405.
15.Lay, T. and D.V. Helmberger, The shear wave velocity gradient at the base of the mantle. Journal of Geophysical Research, 1983. 88: p. 8160-8170.
16.Vidale, J.E. and H.M. Benz, Seismological mapping of the fine structure near the base of the Earth's mantle. Nature, 1993. 361: p. 529-532.
17.Bina, C.R., Mantle Discontinuities. Reviews of Geophysics, 1991. Supplement: p. 783-793.
18.Okamoto, T., et al., Deformation of a partially molten D'' layer by small-scale convection and the resulting seismic anisotropy and ultralow velocity zone. Physics of Earth and Planetary Interiors, 2005. 153(1-3): p. 32-48.
19.Herndon, J.M., Feasibility of a nuclear fission reactor at the center of the Earth as the energy source for the geomagnetic field. Journal of Geomagnetism and Geoelectricity, 1993. 45: p. 423-437.
20.Herndon, J.M., Sub-structure of the inner core of the earth. Proceedings of the National Academy of Sciences USA, 1996. 93: p. 646-648.
21.Herndon, J.M., Nuclear georeactor origin of oceanic basalt 3He/4He, evidence, and implications. Proceedings of the National Academy of Sciences USA, 2003. 100(6): p. 3047-3050.
22.Herndon, J.M., Nuclear georeactor generation of the earth's geomagnetic field. Current Science, 2007. 93(11): p. 1485-1487.
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About the Author
J. Marvin Herndon
President, Transdyne Corporation, Ph.D.-nuclear chemistry, post-doctoral-geochemistry and cosmochemistry, noted for: nickel silicide inner c
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