Inge Lehmann discovered Earth's inner core in 1936 , Frances Birch in
1940 (wrongly) pronounced its composition to be partially crystallized
nickel-iron metal  believing that the Earth's mantle was of uniform
composition. But, as early as the 1930s, the seismologist, Keith Bullen,
pictured at left, recognized that the Earth's mantle is not uniform as previously thought.
He discovered that there is a major boundary which
separates the mantle into two parts. The lower mantle is featureless, while the
upper mantle appears like layers of veneer, as illustrated at right by earthquake wave
In the illustration at right, the seismic speeds of the shear
waves are indicated by a dashed line, the compression waves by a solid line.
Believing as Birch did that the Earth's mantle is of uniform composition,
geophysicists since have attempted to explained these layers as
"phase changes", different crystal structures of the same chemical composition
produced by pressure caused by the overburden weight of the mantle above.
also discovered a narrow band between the lower mantle and the core where
seismic "irregularities" occur.
When J. Marvin Herndon first conceived
of the inner core being fully crystallized nickel silicide , it was
like parting the curtains a bit and glimpsing an entirely new realm of
Earth science, a realm where discoveries could be made in a step-by-step
logical progression of understanding based upon the properties and
behavior of matter.
After an inspiring conversation with Inge Lehmann, Herndon, pictured at left, progressed through the following logical exercise: If
the inner core is in fact the compound nickel silicide, as he had suggested
(click here for pdf), then the Earth's core must be like the alloy portion of an
enstatite chondrite meteorite. If the Earth's core is in fact like the alloy
portion of an enstatite chondrite, then the Earth's core must be surrounded by a
silicate-rock shell, like the silicate-rock portion of an enstatite chondrite.
But the enstatite chondrite type of silicate-rock is essentially devoid of
oxidized iron (FeO), unlike the silicate-rock of the upper part of the upper
mantle, which contains appreciable FeO. Thus, the Earth's
enstatite-chondrite-like silicate-rock shell, if it exists, should be bounded by
a seismic "discontinuity", the boundary where earthquake waves change speed and
direction because of the different compositions.
Using only the mass of the Earth's
core, and the silicate-rock to alloy ratio of the Abee enstatite
chondrite, Herndon calculated the mass of the Earth's
enstatite-chondrite-like silicate-rock shell and found it virtually
identical to the mass of the Earth's lower mantle. He calculated the
boundary to within about 1.2% of the radius of the seismic discontinuity
that separates the lower mantle from the upper mantle. The table at left
shows the comparison between the fundamental mass ratios of the inner
82% of the Earth, the region below the seismic boundary that separates
the upper and lower mantle. What this means is that the Endo-Earth, the
inner 82% of the Earth, is like the Abee enstatite chondrite and the
chemical compositions of Endo-Earth parts can be estimated from
compositions of corresponding Abee parts [4-10].
In 1980, J. Marvin Herndon first published the discovery of that identity in the
Proceedings of the Royal Society of London in an article entitled "The
Chemical Composition of of the Interior Shells of the Earth"
(click here for pdf).
This also means that the seismic discontinuity which separates the lower mantle
from the upper mantle is a boundary between regions of different chemical compositions,
not simply different crystal structures having the same composition.
ordinary chondrites and enstatite chondrites consist mainly of iron metal, iron
sulfide, and silicate minerals. Imagine heating one of those meteorites to a
high temperature in a gravitational field. What would happen is this: The iron
sulfide minerals would alloy with the iron metal and that mass, being denser,
would settle to the bottom like steel on a steel-hearth. The Earth is like a
The vertical axis of the figure at left shows
the weight percent alloy of each of 157 ordinary chondrites (shown as open
circles) and 10 enstatite chondrites (shown as filled circles). The dashed lines
show the points on the vertical axis which correspond to the Earth's core,
expressed as weight percent of the whole Earth and as weight percent of the
Endo-Earth (lower mantle plus total core). This figure shows that the alloy
weight percent of the Earth's core is consistent with that for enstatite
chondrites, like the Abee meteorite. It also shows that the Earth cannot be like
an ordinary chondrite, as was assumed by Francis Birch and many who followed
Note the horizontal axis, which show's the
meteorites' oxygen content. See, the enstatite chondrites are relative low in
oxygen compared to ordinary chondrites The low oxygen content of enstatite
chondrites and the Earth, established during formation, has important
consequences on Earth-core composition. If the Earth were like an ordinary
chondrite, the only major and minor elements in the core would be iron (Fe),
nickel (Ni), and sulfur (S). But, being like an enstatite chondrite, the Earth's
core contains in addition to those elements, some silicon (Si), some magnesium
(Mg), and some calcium (Ca), as shown in the graph to the right.
When the Earth formed, and when the matter of
certain enstatite chondrites formed, the availability of oxygen was so limited
that some normally lithophile "rock-loving" elements could not find oxygen and
were thus constrained to reside in the alloy, in the core. The trace element
uranium (U) was among these. Generally lithophile elements are incompatible in
an iron alloy and tend to precipitate out. How these elements precipitate out,
as described by J. Marvin Herndon, determines the structures within the Earth's
figure at left is a schematic representation of Herndon's Earth interior. Note
that the composition of the upper mantle is shown as unknown. Why? Because the
upper mantle has several seismic discontinuities, appearing like layers of
veneer. At present, the compositions of those layers are unknown. Indeed, the
upper mantle may consist of two or more components, one being, for example,
ordinary chondrite matter.
The lower mantle consists of mainly of
enstatite (MgSiO3), like the silicate of the
Abee enstatite chondrite, but in a different (perovskite) crystal structure
because of the pressure by the weight above. Like the Abee-silicate, the lower
mantle has essentially no oxidized iron (FeO).
Now consider the Earth's fluid core. Imagine,
for sake of discussion, that at some point in the past the Earth's core was so
hot that all of its elements were dissolved in the liquid iron alloy. As the
Earth's core began to cool, the incompatible elements would begin to precipitate
At a high temperature, calcium (Ca) and
magnesium (Mg) would grab sulfur to precipitate as CaS and MgS and, being less
dense, would float to the top of the core, causing the seismic "irregularities"
observed there. Other highly incompatible trace elements, such as uranium, would
find a thermodynamically feasible way to precipitate.
At a somewhat lower temperature, under
appropriate conditions, silicon (Si) and nickel (Ni) would combine and
precipitate, settling downward, forming the Earth's nickel silicide inner core.
In the Birchian view of the Earth being like
an ordinary chondrite, on the other hand, the parts of the core are inexplicable
without making ad hoc assumptions for which there is no corroborating
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