Continental drift is the movement of the Earth's continents relative to each other by appearing
to drift across the ocean bed. The speculation that continents might have 'drifted' was first
put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed
by Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism
and others because of prior theoretical commitments. The idea of continental drift has been subsumed
by the theory of plate tectonics, which explains how the continents move.
History
Early history Abraham Ortelius, Theodor Christoph Lilienthal,
Alexander von Humboldt, Antonio Snider-Pellegrini, and others had noted earlier that the shapes
of continents on opposite sides of the Atlantic Ocean seem to fit together. W. J. Kious described
Ortelius' thoughts in this way:
Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn
away from Europe and Africa ... by earthquakes and floods" and went on to say: "The vestiges
of the rupture reveal themselves, if someone brings forward a map of the world and considers
carefully the coasts of the three [continents]."
Writing in 1889, Alfred Russel Wallace remarks "It was formerly a very general belief, even
amongst geologists, that the great features of the earth's surface, no less than the smaller
ones, were subject to continual mutations, and that during the course of known geological
time the continents and great oceans had again and again changed places with each other."
He quotes Charles Lyell as saying "Continents, therefore, although permanent for whole geological
epochs, shift their positions entirely in the course of ages" and claims that the first
to throw doubt on this was James D. Dana in 1849.
In his Manual of Geology, 1863, Dana says "The continents and oceans had their general
outline or form defined in earliest time. This has been proved with respect to North
America from the position and distribution of the first beds of the Silurian - those
of the Potsdam epoch - and this will probably prove to the case in Primordial time with
the other continents also". Dana was enormously influential in America - his Manual of Mineralogy
is still in print in revised form - and the theory became known as Permanence theory.
This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger
expedition, 1872-6, which showed that contrary to expectation, land debris brought down by
rivers to the ocean is deposited comparatively close to the shore in what is now known as
the continental shelf. This suggested that the oceans were a permanent feature of the
earth's surface, and did not change places with the continents.
Wegener and his predecessors
The hypothesis that the continents had once formed a single landmass before breaking apart
and drifting to their present locations was first presented by Alfred Wegener to the German
Geological Society on 6 January 1912. Although Wegener's theory was formed independently
and was more complete than those of his predecessors, Wegener later credited a number of past authors
with similar ideas: Franklin Coxworthy, Roberto Mantovani, William Henry Pickering and Frank
Bursley Taylor. Eduard Suess had proposed a supercontinent Gondwana in 1858 and the
Tethys Ocean in 1893, from a sunken land-bridge/ geosyncline theory point-of-view, though.
John Perry had written an 1895 paper proposing that the earth's interior was fluid, and disagreeing
with Lord Kelvin on the age of the earth. For example: the similarity of southern continent
geological formations had led Roberto Mantovani to conjecture in 1889 and 1909 that all the
continents had once been joined into a supercontinent; Wegener noted the similarity of Mantovani's
and his own maps of the former positions of the southern continents. Through volcanic
activity due to thermal expansion this continent broke and the new continents drifted away
from each other because of further expansion of the rip-zones, where the oceans now lie.
This led Mantovani to propose an Expanding Earth theory which has since been shown to
be incorrect. Some sort of continental drift without expansion
was proposed by Frank Bursley Taylor, who suggested in 1908 that the continents were
dragged towards the equator by increased lunar gravity during the Cretaceous, thus forming
the Himalayas and Alps on the southern faces. Wegener said that of all those theories, Taylor's,
although not fully developed, had the most similarities to his own.
Wegener was the first to use the phrase "continental drift" and formally publish the hypothesis
that the continents had somehow "drifted" apart. Although he presented much evidence
for continental drift, he was unable to provide a convincing explanation for the physical
processes which might have caused this drift. His suggestion that the continents had been
pulled apart by the centrifugal pseudoforce of the Earth's rotation or by a small component
of astronomical precession was rejected as calculations showed that the force was not
sufficient. The Polflucht hypothesis was also studied by Paul Sophus Epstein in 1920 and
found to be implausible. Evidence of continental 'drift'
Evidence for the movement of continents on tectonic plates is now extensive. Similar
plant and animal fossils are found around different continent shores, suggesting that
they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile,
found both in Brazil and South Africa, are one example; another is the discovery of fossils
of the land reptile Lystrosaurus from rocks of the same age from locations in South America,
Africa, and Antarctica. There is also living evidence—the same animals being found on
two continents. Some earthworm families are found in South America and Africa, for instance.
The complementary arrangement of the facing sides of South America and Africa is obvious,
but is a temporary coincidence. In millions of years, slab pull and ridge-push, and other
forces of tectonophysics will further separate and rotate those two continents. It was this
temporary feature which inspired Wegener to study what he defined as continental drift,
although he did not live to see his hypothesis become generally accepted.
Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa,
Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence
for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial
striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana,
which became a central element of the concept of continental drift. Striations indicated
glacial flow away from the equator and toward the poles, in modern coordinates, and supported
the idea that the southern continents had previously been in dramatically different
locations, as well as contiguous with each other.
Rejection of Wegener's theory The theory of continental drift was not accepted
for many years. One problem was that a plausible driving force was missing. And it did not
help that Wegener was not a geologist. Other geologists also believed that the evidence
that Wegener had provided was not sufficient. It is now accepted that the plates carrying
the continents do move across the Earth's surface; ironically one of the chief outstanding
questions is the one Wegener failed to resolve: what is the nature of the forces propelling
the plates? The British geologist Arthur Holmes championed
the theory of continental drift at a time when it was deeply unfashionable. He proposed
in 1931 that the Earth's mantle contained convection cells that dissipated radioactive
heat and moved the crust at the surface. His Principles of Physical Geology, ending with
a chapter on continental drift, was published in 1944.
David Attenborough, who attended university in the second half of the 1940s, recounted
an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why
he was not talking to us about continental drift and I was told, sneeringly, that if
I could I prove there was a force that could move continents, then he might think about
it. The idea was moonshine, I was informed." Geological maps of the time showed huge land
bridges spanning the Atlantic and Indian oceans to account for the similarities of fauna and
flora and the divisions of the Asian continent in the Permian era but failing to account
for glaciation in India, Australia and South Africa.
As late as 1953 – just five years before Carey introduced the theory of plate tectonics
– the theory of continental drift was rejected by the physicist Scheiddiger on the following
grounds. First, it had been shown that floating masses
on a rotating geoid would collect at the equator, and stay there. This would explain one, but
only one, mountain building episode between any pair of continents; it failed to account
for earlier orogenic episodes. Second, masses floating freely in a fluid
substratum, like icebergs in the ocean, should be in isostatic equilibrium. Gravitational
measurements were showing that many areas are not in isostatic equilibrium.
Third, there was the problem of why some parts of the Earth's surface should have solidified
while other parts were still fluid. Various attempts to explain this foundered on other
difficulties. Geophysicist Jack Oliver is credited with
providing seismologic evidence supporting plate tectonics which encompassed and superseded
continental drift with the article "Seismology and the New Global Tectonics", published in
1968, using data collected from seismologic stations, including those he set up in the
South Pacific. It is now known that there are two kinds of
crust, continental crust and oceanic crust. Continental crust is inherently lighter and
of a different composition to oceanic crust, but both kinds reside above a much deeper
"plastic" mantle. Oceanic crust is created at spreading centers, and this, along with
subduction, drives the system of plates in a chaotic manner, resulting in continuous
orogeny and areas of isostatic imbalance. The theory of plate tectonics explains all
this, including the movement of the continents, better than Wegener's theory.
Bibliography Notes
References Le Grand, Homer Eugene, Drifting Continents
and Shifting Theories, Cambridge University, ISBN 0-521-31105-5 .
Oreskes, Naomi, The Rejection of Continental Drift, Oxford University Press, ISBN 0-19-511732-8 .
Oreskes, Naomi, "Continental Drift", in Munn, Ted; MacCracken, Michael C.; Perry, John S.,
Encyclopedia of Global Environmental Change, Vol. 1, Chichester, West Sussex: John Wiley
& Sons, pp. 321–325, ISBN 0-471-97796-9, OCLC 633880622
Ortelius, Abraham [1570], Thesaurus Geographicus, Antwerp: Plantin, OCLC 214324616
Snider-Pellegrini, Antonio, La Création et ses mystères dévoilés, Paris: Frank and
Dentu . External links
A brief introduction to Plate Tectonics, based on the work of Alfred Wegener.
Maps of continental drift, from the Precambrian to the future
Observe an animation of the breakup of Pangaea - Animation of continental drift for the last
150 million years Four main evidences of the Continental Drift
theory Wegener and his proofs