in Christian Perspective
RECENT CONCEPTS OF THE ORIGIN AND EVOLUTION OF THE EARTH
Heinrich D. Holland
From: JASA 3
(December 1951): 23-28.
members of the American Scientific Affiliation, and guests. As
Dr. Eckert has announced this paper is concerned with recent concepts
and evolution of the earth. To do justice to this topic would require a number of
one-hour lectures. It will therefore be my aim to present a summary of results together with but a minimum of methodology. The method will be as follows: I will
first present some facts concerning the earth,, and then deductions from such data
based on assumptions which appear reasonable today. I would like to consider under
this heading evidence as to the chemical constitution and the physical
especially the rigidity end temperature of the Earth. The same approach will then
be used for the solar system as a whole, and an attempt will be made to point to the
more probable origin and mode
development of the solar system.
The first slide presents some of the data basic to a study of the origin,, development.. and present status of the earth. The mass of the earth has been deduced from
experiments with the torsion balance: the radius of the planet is determined from
geodetic data., and calculations using these two figures yields the mean density.
The oblateness or ellipticity of the earth is defined by the equation
f = a - b/a
where a and b are respectively the equatorial and the polar semi-diameters of the
earth. The semi-diameters can be determined by geodetic measurement of distances on
the earth's surface or by measuring the difference in the gravitational force of attraction at the equator and at the poles.
The moment of inertia, I, of a rotating body is defined by the expression
E = I w2/2
where E is the kinetic energy and v the angular velocity. It can be shown that for a
sphere of uniform density the Inertia
is 2/5 Mr2where M is the mass of the sphere
and r its radius. It follows that I/Mr2 for such a body is 0.4. The moments of inertia of the earth can be calculated with a knowledge of the precessional constant
and the ratio of the centrifugal force to the gravity at the equator.2 0.334, the
value thus obtained, indicates that the earth cannot be a sphere of uniform density
Paper presented at the American Scientific Affiliation
New York August 26-31, 1951.
a substance is the friction which opposes plastic deformation.
From the deformation produced by the moon
the earth's surface the viscosity of the
earth is estimated to be about 1018 to 1020 poises This is to be compared with a
viscosity of 0.02 poises for water and of about 10166 poises for lead at 200C.
Turning now from the physics of the earth to the chemistry of the igneous rocks
available for inspection at the surface.4 we note that over
of the weight and
almost 100% of the volume of these rocks is contributed by 8 elements, all of
atomic weight. It follows that the only common compounds are the oxides and silicates of the metals listed,
and that, from the volumetric point of view, the crust
can be considered as consisting largely of oxygen ions and interstitial metallic ions.
From geologic studies to date we know that the continental masses are primarily
blocks of aluminum-silicates of potassium, sodium, and calcium floating on a substratum
consisting predominantly of magnesium and iron silicates. In the ocean basin
nental material seems to be essentially absent.
This conclusion is supported both by samples dredged from the ocean floor5 and
by evidence from the study of the velocity of seismic waves within the earth.6 It is
the data from this branch of geophysics which gives us the best insight into the internal structure of the earth. Consider the earth as shown
the next slide, and
specifically consider an earthquake occurring at the indicated spot. Both compressional and shear waves will usually be generated by the shock., and these will radiate
out in all directions from the focus of the quake. The time of arrival of the waves
at the surface can be recorded by seismographs. Clearly., if the earth were completely
uniform., the arrival times would be equal at equal distances from the focus of the
quake, and there would be a uniform change in the intensity of the compressional and
shear waves with distance from the focus. That this is not the case was recognized
the history of seismology. Specifically it was found that at angular distances of IMO to 1430 from the focus the amplitude of the compressional
waves in very
small. This can be very satisfactorily explained by assuming the presence of an earth
core extending from a depth of 2900 km to the center of the planet. In this core the
velocity of the compressional waves would be smaller than at the base of the overlying
stratum, so that the core would act as a convergent lens. Such a view is confirmed
by the disappearance of transmitted shear waves at the same depth as the boundary of
the core. This disappearance, indicative of a very much smaller shear strength in
the core than in the mantle, is strong evidence for the liquid nature of the core.
A second discontinuity at a depth of about 30 km was discovered in 1909 by
Mohorovicic and was assigned to the base of the continental blocks mentioned above.
Further breaks have been observed at a depth of 400 km. and within the core at a distance of about 1300 km from the center of the earth.
As mentioned before, the geologic evidence points strongly to a magnesium-iron
silicate layer, presumably of the mineral olivine, below the continental crust. The
velocity of propagation of seismic waves through this portion of the earth checks
well ,with that to be expected for material of this composition. The problem now remains
to assign definite compositions to the various layers within the earth. Bullen, on
the assumption that the compressibility of subcrustal material is adiabatic, has calculated the probable
pressure and density variations down to the earth's center, using
the observed arrival times of seismic waves, the moment of inertia of the earth as a
whole and the average density shown In the first slide. As shown in the next slide.
the pressure varies rather steadily, showing only a small inflection at the core boundary, and flattening near the center due to the decrease of the gravitational acceleration. According to this interpretation the pressure at the center of the earth is
about 3.64 x
1012 dynes/ cm2 or about 3 1/2 million atmospheres.
The calculated density variations are more complex. There is a general rise
from the surface to the boundary of the core., with irregularities at 30 km and At
400 km and a considerable curvature between 4M and 1200 km. At the core boundary
there is a large density increase followed by a continual rise until a distance
1300 km from the center of the earth is reached. There appears to be further rapid
increase at this point. However, the present evidence for the value 17.2g/cc proposed by Pullen for the inner core is not too strong.
Such a density distribution is in good accord with traditional earth models
based on the study of meteorites. The variations between the depths of 30 km and
2900 km can be explained in terms of changes in the density of olivine due to increasing pressure, although there may well be some compositional
escape definition by the proposed density curve. The estimated values for the density of
core check well with calculated values for the density of iron at pressures
above 10 2 dynes/cm2
whereas the composition of the inner core is still somewhat
Within the last ten years two rival interpretations of earth structure have
been proposed. Kuhn
arguing from the genetic difficulties or accounting for the proposed distribution of silicates and metallics within the earth, suggest that our planet consists of a thin crust of silicates and metallic iron underlain by more or less unaltered solar material. This removes the necessity of separating a large
amount of silicate material from the native iron, but fails to account
quantitatively for the density distribution within the Earth and for the very rapid
changes in the physical constants at a depth or 2900 km.10
A second Interpretation due to Ramsey11 is to the effect that the changes at
the boundary of the core are not chemical but purely physical.
It is probable that
hydrogen collapses into a metallic state at a pressure of 0.7xlO 12 dynes. Above this
pressure electrons no longer "belong" to specific nuclei but form an electron gas
similar to that postulated to account for the properties of true metals. During this
transition the density of the gas increases from about 0.35 g/cc to about 0.7
Reasoning by analogy Ramsey suggests that a collapse of the structure of olivine to
a metallic state might account for the observed density jump at the core boundary. A
logical extension would be to suggest a further collapse involving the
lose of identity of electrons in
the inner shells of magnesium, iron, silicon, and oxygen to account for the density changes at the boundary of the inner core. That such collapsed
states exist is most probable since it has been observed that the radius of white
dwarf stars decreases with increasing mass., but whether the pressure in the earth's
interior is sufficient to produce collapse in the case of olivine has not been demonstrated.
Furthermore, recent work suggests that the density to be expected of such
collapsed material would probably be less than the observed value at the core boundery7
as deduced from seismologic evidence. It is therefore to be concluded that the Pullen
earth model is probably essentially correct.
The present and past temperatures of the earth at the surface and at depth have
been a topic of great interest ever since Lord Kelvin's celebrated work on the subject. Yet even today, after considerable
the earlier data, it is difficult to make definitive temperature estimates for the first few hundred kilometers
of crust and mantle, and it is almost impossible to assign values which are
than educated guesses for temperatures near and within the core. The main difficulties rest in the absence of data
the initial temperature distribution within the
earth and in the uncertainty of the mode of heat transfer within the earth's" mantle.
These difficulties have been emphasized by
Slichter12 and Urry13 in their recent
treatments of the subject. At present the major part of the heat transfer from the
interior of the earth is derived from the energy liberated by the decay of radioactive
atom . The distribution and decay schemes of uranium, thorium, and potassium within
the earth's crust and at the top of the mantle are well enough known at present to
enable us to place considerable faith in this statement. Provided that heat transfer
through the mantle has occurred chiefly by conduction. It can be shown that the
cooling of the earth at depths greater than 700 km has beep negligible even during the
Past 3000 million years. Assuming further with Adaml14 that cooling by-convection
predominated before solidification and that an adiabatic temperature gradient existed
at that time,, the temperature at the boundary of the core can be calculated to be
several thousand degrees centigrade. However, Schlichter points out that slow convective cooling despite the rigidity
of the mantle is not impossible
and may well indeed the effect of cooling by
condution. The demonstration of convection cells within the mantle would also be of extreme
interest geologically in connection with the
causes of mountain building.
The following slide presents Urry's results on temperature variations within the
crust during geologic history. It can be seen that.. if these estimates are
temperatures near the malting point of crustal rocks pertained relatively near the
surface during much of the history of the earth.
We can now use the earth model developed up to this point to shed some light on
the origin of the earth. However.. since the earth appears so closely related to the
sun and to the other planets, it may be well to summarize some of the facts known
about the other inhabitants of the solar system. The nine planets Mercury, Venus)
the Earth, Mars, Jupitero Saturn, Uranus, Neptune, and Pluto describe about the sun
paths which are almost circular and almost coplanar. They all rotate about the sun
in the same direction and also in the same direction about their own axes. This
statement Is true of almost all of the satellites as well. The distance of
planets to the sun can be expressed fairly accurately by the Titius-Bode
D = ( 4 + 3X2n ) x 147.6X105 km
is the distance of nearest approach of each planet to the sun and n Is an
12. L. A. Slichter; Cooling of the Earth; Pull. Geol. Soc. Amer. 52,
integer. Thus for Mercury n
0, for Venus n = 1, for the Earth n = 2 and so forth.
Any theory of the origin of the solar system must satisfactorily explain these
facts and the hosts of others which have been collected through the intensive study
of the system. Two theories have alternately been credited and discarded: the
collision theory of Buffon and the dust-cloud theory of Kant and
The Buffon theory modified by Chamberlin,, Moulton, Jeans end Jeffreys2 was widely
accepted until the close of the
1930's. The principal concept was as follows: a star
passed close to the sun at one point In the past. A large tide, raised on the sun IV
the gravitational attraction of the star, ultimately broke into an extended filament
which later condensed into gaseous globules, the proto-planets. On the basis of this
Idea the concept of the coplanar orbits and the similar direction of rotation of the
planets are easily explained. So also is the original heat to permit fractionation
of the earth into its present compositional configuration. However, the almost circular nature of the orbits, the fact that some of the satellites do not revolve In
the same sense as the majority, and the Titius-Bode law of planetary distances am
Recently the criticism of the collision theory has led to a further analysis of
the discarded Kant-Laplace hypothesis. Under the treatment of Weizaecker,15
ter Haar,17 and others, the
seen to have been removed, and the theory is at the moment very popular. According
Weizsaeckerl5 the planetary
originated as a flat cloud of gas with a density in the neighborhood Of 10-9
It can be shown that at the temperature of a few hundred degrees Kelvin to be expected under these conditions,, condensation of the gas particles Into larger chunks
is inevitable. Presumably the cloud vas relatively homogeneous and of approximately
the composition of the Sun. This would
mean that the bulk of
the material consisted
of hydrogen and helium. As the chunks grow by capture, some probably were again
destroyed by head-on collisions. Gradually, however, larger and larger bodies are
supposed to have formed; these were soon able to capture neighboring particles by
virtue of their large gravitational attraction. By carrying through rigorous arguments regarding this process., justification can be found for the
Intuitively it can be seen that the original roughly circular rotation of the gas
cloud would be transferred to the planets, thus endowing them all with the same direction of rotation in nearly circular orbits. The non-conformist behavior of some
satellites can be ascribed to non-hom6geneouo relations at the edge of the cloud and
at a few points within. The main objection, that a gas cloud of sufficient size
to produce planets would have a moment of inertia much greater than that of the planets as a group
today, can be overcome by assuming the
escape of hydrogen and helium such as seems to have been the case in the vicinity of the inner planets.
Further support for the theory comes from a study by Brovnl8 of the comparison
of the abundance of slightly volatile elements in meteorites and in the sun
atmosphere. The relationship is shown in the next slide. It can be seen that there is
fair correlation between the ratios
the elements in the sun and in the meteorites.
This is to be expected if the gas cloud was of the same composition as the sun's atmosphere today
meteorites are either' uncaptured chunks or parts of fragmented
However, if the earth did grow in an initially cold condition, the present
layered distribution of metallic iron and silicates is hard to explain. It my be
possible to show that the gravitational energy released during accretion and energy
from the disintegration of radioactive elements distributed more evenly throughout
the earth than today were sufficient to cause malting and fractionation of the planet.
R. C. Ureyl9 recently made the statement that "taken with the astronomical and
physical data, the chemical facts are so detailed that it is possible to construct an
acceptable picture of the earth's formation." I doubt that all investigators in
these fields are as optimistic as Dr. Urey. Nevertheless, the impetus which the
recent developments in astrophysics) geophysics, and geochemistry have given to the
other planets, both past and and present, which today may well enable us to discard some of the if's
decorate our statements on these subjects.
1. H. N. Russell, R. S Duganp and J. Q Stewart; Astronmy) Iy The Solar
System; Ginn add Co . 1945
2. R. Jeffreys; The Earth; Canbridge) 1929
3. W. Kuhn-. Stoffliche Homogenitaet des Erdinnern; Naturviss.
4. K. Rankama and Th. G. Sahama; Geochemistry; Univ. of Chicago Press., 1350
5. M. Ewing, J. L. Worzel,, J. B. Hersey., P. Phess, G. R. Hamilton., Short Note.,
PU B - S - A.- _60.j._1303 -1304, 1949
6. K. E. Pullen; An Introduction to the Theory of Seismology; Cambridge, 1947
7. W. M. Elaasser; Quantum-theoretical Densities of Solids at extreme compression;
Science 113, 105-107, 1951
8. W. Kuhn and A. Rittmann; Ueber den Zustand des Erdinnern und seine E ntatehung
aus einem homogenem Urzustand; Geol. Rundsch. L2., 215-255, 1941
9. W. Kuhn; Zur Diskussion ueber dis Homogenitaet des Erdinnern; Experientia 2, 391, 1946
10. A. Eucken; Ueber den Zustand des Erdinnern; Naturviss. 32, 112-121, 1944
11. W. H. Ramsey; On the constitution of the terrestrial planets; Mon. Not. Boy.
Astr. Soc. 108, 406-413, 1948
D. Urry; Significance of radioactivity in geophysics-thermal history of
the earth; Trans. Am. Geophys. Union 30, 171-180, 1949
L. X Adams; Temperatures at moderate depths within the earth; J. Wash. Acad.
sci. 459-472, 1924
15. C. F v. Weizaaecker; Ueber die Entatehung des Planet en systems; Z. f.
Astrophys 22, 319-355, 1943
16. G. Kuiper; On the origin of the solar system. In Astrophysics: A topical
symposium; McGraw Hill, 1951
17. D. ter Haar; Further studies on the origin of the solar system; Astrophys.
Jour. lll'.' 179-190) 1950
18. H. Brown; On_tbe composition and structure of the planets; Astrophys. J. 111,
19. X. C. Urey; On the origin of the earth; Baskerville Chemical Jour. 2, 3-5, 1951