HOW THE EARTH MAY HAVE ORIGINATED
The problem of the origin of the earth is essentially astronomical
rather than geological, because geological history is considered to
have begun when common earth processes, such as erosion, deposition,
and transportation of sediments, etc., were brought into play. It
is quite certain, however, that the earth in its pregeologic state
gradually merged into its geological condition. For this reason the
geologist is interested in the more important doctrines or hypotheses
which have been put forth to account for the origin of the earth.
In fact, one of the few hypotheses which must be taken seriously is
largely the work of a geologist. The most acceptable hypothesis not
only best satisfies the facts regarding the earth's astronomical
relationships, but also best harmonizes with our knowledge of the
oldest known rocks and their history.
Since the problem of the origin of the earth is an essential part of
the problem of the origin of the solar system, the following well-known
facts should be clearly in the mind of the reader. Eight planets,
including the earth, revolve in nearly circular paths around the
central sun, whose diameter is 866,000 miles. The radius of the solar
system is at least 2,800,000,000 miles, this being the distance of
the outermost known planet (Neptune) from the Sun. Neptune requires
164 years for a trip around the sun, while the earth, which averages
about 93,000,000 miles from the sun, makes its circuit once a year. The
planets all revolve around the sun in the same direction, and in nearly
the same plane. The sun and all eight planets rotate on their axes
in the same direction, the earth's rotation being accomplished every
twenty-four hours. Most of the planets have one or more smaller bodies
called satellites revolving about them, such as Earth, with its one
satellite (the moon), and Saturn, with its eight satellites, etc. It is
well known that this solar system is only a very small part of the vast
universe, as shown by the facts that no star is nearer the earth than
several trillion miles, and that some stars are so far away that light
traveling at the rate of 186,000 miles per second requires a thousand
years to reach the earth!
Toward the end of the eighteenth century the famous nebular or ring
hypothesis was set forth by the astronomer named Laplace. This assumes
an original very hot incandescent mass of gas spheroidal in shape and
greater in diameter than the present solar system. This mass rotated
in the direction of rotation of our sun and its planets. Loss of heat
by radiation caused the mass to shrink, and this in turn not only made
it rotate faster, but also caused the centrifugal force (i.e., the
force whose direction was from the center) in its equatorial portion
to gradually become stronger. Finally a time came when the force of
gravity (i.e., the force whose direction was toward the center) and the
centrifugal force became equal and a ring was left (not thrown) off,
while the rest of the mass of gas continued to shrink. After a time the
material of the ring collected to form the outermost planet. The other
planets were similarly formed from other rings which were left off as
contraction of the great mass of gas went on. The sun represents the
remainder of the great mass of rotating gas.
What is the bearing of this nebular hypothesis upon the early
geological history of the earth? According to the hypothesis the earth
must once have been much more highly heated and larger than now. It
condensed to a liquid and then it cooled enough to permit the formation
of a solid crust over a liquid interior. It then had a hot dense
atmosphere containing all the water of the earth in the form of vapor,
and this atmosphere steadily became thinner due to absorption by the
earth. When the pressure and temperature conditions became favorable,
much of the water vapor condensed to form the ocean and the atmosphere
gradually changed to its present condition. According to this view the
oldest rocks of the earth must have been igneous because they resulted
from the solidification of the outer part of the molten globe.
Within recent years certain serious objections to the nebular
hypothesis have been raised, and Chamberlin and Moulton have formulated
the planetesimal or spiral hypothesis as an attempt at a more rational
explanation of the origin of the solar system. Some of the objections
to the older doctrine are that among the many thousands of known
nebulæ in the universe very few only are of the Laplacian or ring
type, while spiral forms are abundant. Spectroscopic study shows
that the nebulæ are not gaseous, but made up of either liquid or
solid particles, and that the leaving off of rings would necessitate
the assumption of an intermittent process which could scarcely have
operated under the conditions of the hypothesis.
[Illustration: Plate 9.--(_a_) Molten Lava Flowing Over a Cliff Into
Water in the Hawaiian Islands. (_After Diller, U. S. Geological
Survey._)]
[Illustration: Plate 9.--(_b_) Dikes of Granite (Light Gray) Cutting
an Old Dark Rock. While the granite on the right was being forced in
molten condition upward into the earth's crust, tongues of it (dikes)
were sent off into the adjacent rock. (_Photo by Howe, U. S. Geological
Survey._)]
[Illustration: Plate 10.--(_a_) Lassen Peak, Northern California, in
Eruption August 22, 1914. The great cloud of steam and volcanic ash
rose several miles. This is the only active volcano in the United
States proper, and it is now included in Lassen Volcano National Park.
(_By permission of R. E. Stinson, Red Bluff, Cal._)]
[Illustration: Plate 10.--(_b_) Devil's Tower, Wyoming. This great mass
of rock was forced in molten condition through strata which, because
of their weakness, have been eroded away all around the hard igneous
rock. This is probably the core or neck of a former volcano. (_Photo by
Darton, U. S. Geological Survey._)]
[Illustration: Fig. 31.--Diagram showing the origin and character of a
spiral nebula according to the planetesimal hypothesis of the origin of
the solar system. (Modified after Moulton.)]
Anything like a full understanding of the planetesimal hypothesis
would be difficult to obtain, and, in the brief space at our disposal,
we shall attempt to make clear only a few of the salient points.
According to this hypothesis the solar system was, during a previous
stage of its evolution, a great, flat, spiral nebula, made up of finely
divided solid or possibly liquid particles called planetesimals,
among which were scattered some larger "knots" or masses. Each tiny
particle and larger mass or knot is considered to have traveled in
its own particular orbit or path about a central very large mass--the
future sun. It is even suggested that the spiral nebula originated by
disruption of one star by a swift-moving passing star. Each disrupted
particle and large mass at first started straight for the large passing
star, but because of change of position of the latter the particles and
larger masses were gradually pulled around so that their paths curved
into spirals. Because of crossing of paths, the larger masses or knots
gradually increased in size by accretion of the small particles or
planetesimals. Meteors (so-called "shooting stars") which now strike
the earth are thought to be disrupted materials still being gathered
in, though very slowly at present. After the passing star got well out
of range, the spiral paths of the disrupted masses gradually changed to
nearly circular, due to a wrapping-up process around the central body
(sun) which then controlled the movements of the both larger masses
(future planets) and small masses (planetesimals).
Let us now inquire briefly into the bearing of this planetesimal
hypothesis upon the early geological history of the earth. According
to this doctrine the earth was never in the form of a highly heated
gas, nor was it ever necessarily hotter than now. Instead of beginning
as a much larger body which has gradually diminished in size, the
earth steadily grew, up to a certain stage, by ingathering of
planetesimals. Increase in size caused the force of gravity to increase
and this caused not only steady contraction of the earth's matter,
but also a development of greater internal heat. The earth has been
getting smaller ever since the force of compression has predominated
over the building-up process, because of the diminishing supply of
planetesimals. Due to steadily increasing internal pressure and heat
the various gases, including water vapor, have been driven out of the
earth to form an atmosphere which has gradually become larger and
denser. After sufficient accumulation of water vapor, condensation
and rainfall took place; the waters of the earth began to gather to
form the oceans; and the ordinary geologic processes of erosion and
deposition of strata were initiated. According to this view stratified
rocks could have been formed very early in the history of the earth,
and in this connection it is interesting to note that the oldest known
rocks are actually of sedimentary origin.