Geology Reference
In-Depth Information
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Figure 2.1
Fossil
Glossopteris
Leaves Plant fossils, such as these
Glossopteris
leaves from the Upper Permian Dunedoo Formation in
Australia, are found on all fi ve of the Gondwana continents. Their
presence on continents with widely varying climates today is evidence
that the continents were at one time connected. The distribution of the
plants at that time was in the same climatic latitudinal belt.
In his book,
The Face of the Earth
, published in 1885,
Suess proposed the name
Gondwanaland
(or
Gondwana
as we will use here) for a supercontinent composed of the
aforementioned southern continents. Abundant fossils of
the
Glossopteris
fl ora are found in coal beds in Gondwana, a
province in India. Suess thought these southern continents
were at one time connected by land bridges over which
plants and animals migrated. Thus, in his view, the similari-
ties of fossils on these continents were due to the appearance
and disappearance of the connecting land bridges.
The American geologist Frank Taylor published a pam-
phlet in 1910 presenting his own theory of continental drift.
He explained the formation of mountain ranges as a result of
the lateral movement of continents. He also envisioned the
present-day continents as parts of larger polar continents that
eventually broke apart and migrated toward the equator after
Earth's rotation was supposedly slowed by gigantic tidal forces.
According to Taylor, these tidal forces were generated when
Earth captured the Moon approximately 100 million years ago.
Although we now know that Taylor's mechanism is in-
correct, one of his most significant contributions was his
suggestion that the Mid-Atlantic Ridge, discovered by the
1872-1876 British HMS
Challenger
expeditions, might mark
the site along which an ancient continent broke apart to
form the present-day Atlantic Ocean.
the various continents to their present-day locations. Wegener
amassed a tremendous amount of geologic, paleontologic, and
climatologic evidence in support of continental drift; however,
initial reaction of scientists to his then-heretical ideas can best be
described as mixed.
Nevertheless, the eminent South African geologist
Alexander du Toit further developed Wegener's arguments and
gathered more geologic and paleontologic evidence in support
of continental drift. In 1937, du Toit published
Our Wandering
Continents
, in which he contrasted the glacial deposits of Gond-
wana with coal deposits of the same age found in the continents
of the Northern Hemisphere. To resolve this apparent climato-
logic paradox, du Toit moved the Gondwana continents to the
South Pole and brought the northern continents together such
that the coal deposits were located at the equator. He named this
northern landmass
Laurasia
. It consisted of present-day North
America, Greenland, Europe, and Asia (except for India).
CONTINENTAL DRIFT?
What then was the evidence Wegener, du Toit, and oth-
ers used to support the hypothesis of continental drift? It
includes the fi t of the shorelines of continents, the appear-
ance of the same rock sequences and mountain ranges of the
same age on continents now widely separated, the matching
of glacial deposits and paleoclimatic zones, and the similari-
ties of many extinct plant and animal groups whose fossil
remains are found today on widely separated continents.
Wegener and his supporters argued that this vast amount of
evidence from a variety of sources surely indicated that the
continents must have been close together in the past.
Figure 2.2), is gener-
ally credited with developing the hypothesis of
continental drift
.
In his monumental book,
The Origin of Continents and Oceans
(fi rst published in 1915), Wegener proposed that all landmasses
were originally united in a single supercontinent that he named
Pangaea
, from the Greek meaning “all land.” Wegener por-
trayed his grand concept of continental movement in a series
of maps showing the breakup of Pangaea and the movement of
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Wegener, like some before him, was impressed by the close
resemblance between the coastlines of continents on opposite
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