Continental drift
Antonio Snider-Pellegrini's Illustration of the closed and opened Atlantic Ocean (1858).
Continental drift is the 1____ of the Earth's continents relative to each other by
appearing to drift across the ocean bed.[1] The speculation that continents 2____ have
'drifted' was first put forward by Abraham Ortelius in 1596. The concept was
independently and more 3____ developed by Alfred Wegener in 1912. The theory of
continental drift was 4_____ by the theory of plate tectonics, which explains how the
continents move.
History
Early history
Abraham Ortelius (Ortelius 1596),[2] Theodor Christoph Lilienthal (1756),[3] Alexander
von Humboldt (1801 and 1845),[3] Antonio Snider-Pellegrini (Snider-Pellegrini 1858),
and others had noted 5_____ that the shapes of continents on opposite sides of the
Atlantic Ocean (most notably, Africa and South America) seem to 6____ together.
Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas
were "torn away from Europe and Africa ... by 7_____and 8_____" 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 9____of the three [continents]."
Writing in 1889, Alfred Russel Wallace 10_____"It was formerly a very general
11_____, even amongst geologists, that the great 12_____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."[6] He 13_______ Charles Lyell as saying "Continents,
therefore, although permanent for whole geological epochs, 14___ their positions
entirely in the course of 15___ ”
Wegener and his predecessors
1
Alfred Wegener
The 16_____ that the continents had once formed a single 17______ before breaking
apart and drifting to their present locations was first presented by Alfred Wegener to the
German Geological Society on 6 January 1912.[11] 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:[12][13]
For example: the similarity of 18_____ continent geological formations had led Roberto
Mantovani to conjecture in 1889 and 1909 that all the continents had once been 19___
into a supercontinent (now known as Pangaea); Wegener noted the similarity of
Mantovani's and his own maps of the former positions of the southern continents.
Wegener was the first to use the phrase "continental drift" (1912, 1915)[11][12] (in
German "die Verschiebung der Kontinente" –20______ into English in 1922) and
formally publish the hypothesis that the continents had somehow "drifted" apart.
Although he presented much evidence for continental drift, he was 21 _____ to provide
a convincing explanation for the physical processes which might have caused this drift.
His 22_____ that the continents had been pulled apart by the centrifugal pseudoforce
(Polflucht) of the Earth's rotation or by a small component of astronomical precession
was rejected as calculations showed that the force was not sufficient.[20] The Polflucht
hypothesis was also studied by Paul Sophus Epstein in 1920 and found to be
23_______.
Evidence of continental'drift'
Fossil 24 ________ across continents (Gondwanaland).
2
Mesosaurus skeleton, MacGregor, 1908.
Evidence for the movement of continents on tectonic plates is now extensive. Similar
plant and animal fossils are found around different continent 25_____, suggesting that
they were once joined. The fossils of Mesosaurus, a 26______ 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 27___ from
locations in South America, Africa, and Antarctica.[21] There is also living evidence—
the same animals being found on two continents. Some earthworm families (e.g.:
Ocnerodrilidae) are found in South America and Africa, for instance.
The complementary 28 _____ of the facing sides of South America and Africa is
obvious, but is a temporary coincidence.
29 _____ 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 30 __ 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.[12]
Rejection of Wegener's theory, and subsequent
vindication
The theory of continental drift was not accepted for many years. One problem was that a
plausible driving force was missing.[1] 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 31 ___ questions is the one
Wegener failed to resolve: what is the nature of the forces propelling the plates?[1]
The British geologist Arthur Holmes championed the theory of continental drift at a
time when it was deeply 32 ___. He proposed that the Earth's mantle contained
convection cells that dissipated radioactive 33 ___ and moved the 34 ___ at the surface.
His Principles of Physical Geology, ending with a chapter on continental drift, was
published in 1944.[22]
David Attenborough, who attended university in the second half of the 1940s, recounted
an incident illustrating its lack of acceptance then: "I 35 ____ asked one of my 36 __
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."[23]
3
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.
Alfred Wegener, around 1925
The determined age of the Earth as 2 billion years opened doors for theories of
continental movement during this vast amount of time.[25] In 1912 Alfred Wegener
proposed the theory of Continental Drift.[26] This theory suggests that the continents
were joined together at a certain time in the past and formed a single landmass known as
Pangaea; thereafter they drifted like rafts over the ocean floor, finally reaching their
present position. The shapes of continents and matching coastline geology between
some continents indicated they were once attached together as Pangea. Additionally, the
theory of continental drift offered a possible explanation as to the formation of
mountains. From this, different theories developed as to how mountains were built.
Unfortunately, Wegener’s ideas were not accepted during his lifetime and his theory of
Continental Drift was not accepted until the 1960s.[27]
In the 1960s new found evidence supported the theory of Continental Drift. The term
Continental Drift was no longer used but was replaced by the concept of Plate Tectonics
that was well supported and accepted by almost all geologists by the end of the decade.
Geophysical evidence suggested lateral motion of continents and that oceanic crust is
younger than continental crust. This geophysical evidence also spurred the hypotheses
of seafloor spreading and paleomagnetism. The hypothesis of seafloor spreading,
proposed by Robert S. Dietz and Harry H. Hess, holds that the oceanic crust forms as
the seafloor spreads apart along mid-ocean ridges. Paleomagnetism is the record of the
orientation of the Earth’s magnetic field recorded in magnetic minerals. British
geophysicist S. K. Runcorn suggested the concept of paleomagnetism from his finding
that the continents had moved relative to the Earth’s magnetic poles.
4
5