EVALUATION.
- Continental Drift.
- Rigid Earth Theory.
The traditional rigid earth theory holds that the earth, once hot,
now cooling, became rigid at a very early date and that the contraction
accompanying the cooling process creates compressive forces that, at intervals,
squeeze up the earth along the weak margins of the continent or in deep basins
filled with sediments. This view, first suggested by Isaac Newton, was
quantitatively established during the nineteenth century to fit the ideas
then prevailing. It was claimed that the initially hot, molten earth had
cooled to its present temperature in about 100 million years and that, in so
doing, the circumference of the earth contracted by at least tens or hundreds
miles. It was thought that the granitic blocks of the continents were
differenttiated from the rest of crustal rock and were frozen in place at the
close of the first, fluid chapter of earth's early history. Since then the
continents were modified in situ, without migrating.
This hypothesis, in its essentials, still has adherents and defenders among
the geologists, except those who work around the margins of the southern
continents. Some physicists also defend the validity of the underlying theory.
But a number of formidable objection have been raised by those who have studied
radioactivity, ancient climates, terrestial magnetism, and most recently
submarine geology. The radioactivity is still generating heat and must have
slowed, although it did not stop, the cooling of the earth. And the rigid earth
is not so rigid. The great continental ice sheets had depressed the earth's
crust, just as the loads of ice that today cover Greenland and Antarctica
depress the crust of those regions. Many biologist had a problem with tracing
the evolution and distribution of early life forms on the existing pattern
of continents. The distribution of the earlier forms of life required either
land bridges across the oceans, the origin and disappearance of which are
difficult to explain, or a different arrangement of the continents.
- Early Theories.
- As early as 1620 the English philosopher, Francis Bacon, discussed
the possibility that the Western Hemisphere had once been joined to Europe and
Africa.
- In 1668 P. Placet wrote a work titled La corruption du grand
et dù petit monde, ou il est montré que devant le déluge,
l'Amérique n'&ecute;tait point séparée des autres
parties du monde ("The Corruption of the Great and Little World,
where it is shown that before the deluge, America was not separated from
the other parts of the world")
- Some 200 years later, Antonio Snider was struck by the similarities
between American and European fossil plants of the Carboniferous period
(about 200 million years ago) and proposed that all the continents were
once part of a single land mass. His work of 1858 was called La
Création et Ses Mystères Dévoilés
("The Creation and Its Mysteries Revealed")
- By the end of the nineteenth century the geologists became involved
in the discussion of the problem. At that time the Austrian geologist
Eduard Suess had noted such a close correspondence of geological
formations in the lands of the Southern Hemisphere that he fitted them into
a single continent that he called Gondwanaland. The name comes from
the name of a key geological province in east central India, Gondwana.
[1]
- Modern Theories.
- In 1908 F. B. Taylor in the U.S. and in 1910 the meteorologist
Alfred L. Wegener of Germany independently suggested mechanisms
that could account for the large lateral displacements of the earth's crust
and thus show how the continents could be driven apart. Wegener advanced a
number of correlations, drawn from geology and paleontology, indicating a
common historical record on both sides of the Atlantic Ocean. He proposed
that all the continents were once joined into a single vast land mass before
the start of the Mesozoic Era (about 200 million years ago). Wegener called
this supercontinent Pangea or Pangaea. Wegener in 1912 argued
that if the earth could flow vertically in responce to vertical forces, it
could also flow laterally. Under the action of forces associated with the
rotation of the earth, the continents had broken apart, opening up the
Atlantic and Indian oceans. Between 1920 and 1930 Wegener's hypothesis
excited great controversy. Physcists found the mechanism that he proposed
inadequate and expressed doubts that the continents could move laterally in
any case. Geologists showed that some of Wegener's suggestions for
reassembling the continents into a single supercontinent were certainly wrong
and that the drift was not necessary to explain the similarities of geology in
many areas. They could not dispute the validity of most of the transatlantic
connections. Indeed, more such connections were being steadily added.
- Many geologist of the Southern Hemisphere, led by Alex. L. Du Toit
of South Africa, welcomed Wegener's views. They sought to explain the mounting
evidence that an ice age of 200 million years ago had spread a glacier across
over the now scattered continents of the Southern Hemisphere. At the same
time, according to the geological record, the great coal deposits of the
Northern Hemisphere was being formed in tropical forest as far north as
Spitzbergen. To resolve the climate paradox Du Toit proposed a different
reconstruction of the continents. He brought the southern continents together
at the South Pole and the northern coal forests toward the Equator. Later, he
thought, the southern continent broke up and its component subcontinents had
drifted northward.
- Many suggestions have been made to explain how to create and destroy
land bridges needed to explain the biological evidence without moving the
continents. Some involved isthmuses and some involved whole continents that
had subsided below the surface of the ocean. But the chemistry and density of
the continents and ocean floors are now known to be so different that it seems
more difficult today to raise and lower ocean floors than it is to cause
continents to migrate.
- One of the best leads to a mechanism that would move continents came about
in the 1930 when the sensitive techniques of gravimetry that had established
the rule of hydrostatic equilibrium, or isostasy, ashore, was extended to the
sea floor. The Dutch geophysicist, Felix A. Vening-Meinesz, demonstrated that
a submerged submarine would provide a sufficiently stable platform to allow the
use of gravimeter at sea. Over the abyssal trenches in the sea floor that are
associated with the island arcs of Indonesa and the western side of the
Pacific, Vening-Meinesz found some of the largest deficiencies in gravity ever
recorded. It was clear that isostasy does not hold in the trenches. Some
force at work there pulls the crust down into the depths of the trenches
more strongly than the pull of gravity.
Arthus Holmes of the University of Edinburg and D. T. Griggs at the University
of California at Los Angeles, were stimulated by these observations to
re-examine and restate in modern terms an old idea of geophysics: that the
interior state of the earth is that of extremely sluggish thermal convection,
turning over the magma like boiling water does in a pan. They showed that
convection currents were necessary to account in full for the transfer of heat
flowing from the center of the earth through the poorly conductive material
of the mantle; the region lies between the core and the crust of the earth.
The trenches, they said, mark the place where currents in the mantle descend
again into the interior of the earth, pulling down the ocean floor.
Convection currents in the mantle now play the leading role in every discussion
of large-scale and long-term processes that go on in the earth. It is true
that the evidence for their existence is indirect; they flow too deep in the
earth and too slowly, a few centimeters a year, to be directly observed.
Never the less, their presence is supported by an ever increasing body of
independently established evidence. Perhaps the strongest evidence has come
from the discovery of the regions where the currents ascend towards the
earth's surface. This was the major discovery on the floor of the oceans
of a continuous system of ridges. Across the floor of all oceans, for a
distance of 40,000 miles, there are these ridges. In the mid-Atlantic for long
streches these ridges are faulted and rifted by under the tension of forces
acting at right angles to the axis of the ridge. Measurements undertaken by
Sir Edward Bullard of the University of Cambridge showed that the flow of heat
is usually great along these ridges, exceeding by two to eight times the
average flow of a millionth of a calorie per square centimeter per second
observed on the continents and elsewhere on the ocean floor. Most of
oceanographers are now agreed that the ridges form where convections currents
rise in the earth's mantle and that the trenches are pulled down by the descent
of these currents into the mantle. Here then is a mechanism, in harmony with
physical theory and much geological observations, that provide a means for
disrupting and moving continents. This theory, in contrast to earlier theories
of continental drift where the continents drive through the crust like ships
through a frozen sea, this mechanism conveys contiments passively, carrying
them along by the lateral movement of the crust from the source of the
convection current to its sink. The continents having been built up by the
accumulation of lighter and more siliceous materials brought up from below,
are not dragged down at the trenches where the currents descend but pile up
there as mountains. The ocean floor, being essentally altered mantle, can be
carried downward; such sediments as have accumulated in the trenches descend
also and, by a complicated processes, may add new mountains to the continents.
Since the materials near the surface are chilled and brittle, it fractures,
causing earthquakes, until it is heated by its descent.
[2]
ENDNOTES
[1] Patrick M. Hurley, "The Confirmation of Continental Drift", April, 1968, in
Continents Adrift,
Readings from Scientific American
(San Francisco, Calif.:W. H. Freeman and Company,
1952, 1955, 1962, 1963, 1968, 1969, 1970, 1971, 1972), pp. 57-58.
[2] J. Tuzo Wilson, "Continental Drift", April, 1963, in
Continents Adrift,
Readings from Scientific American
(San Francisco, Calif.:W. H. Freeman and Company,
1952, 1955, 1962, 1963, 1968, 1969, 1970, 1971, 1972), pp. 41-55.