Encyclopedia of Evolution.pdf - Online Reading Center

continental drift
was an animal that spread widely through the world continents
after the Permian extinction (see figure).
There were two major reasons that the proposal of
Wegener and du Toit met resistance and criticism, sometimes
even hostility. First, Wegener had set out to prove, rather than
to test, his proposal; he was preaching it, rather than investigating
it, according to his critics. Second, Wegener could not
explain how continents could drift. What would move them?
And how could they plow through the ocean floor without
leaving huge piles of rubble? Scientists now understand that it
is not the continents themselves that move, but the heavy rock
of the crustal plates of the Earth, upon which the lighter rock
of the continents rests (see plate tectonics). Plate tectonics
provides the explanation for how continental drift occurs.
A major breakthrough that enhanced the credibility of
continental drift was the discovery of the Mid-Atlantic Ridge,
a mountain range that runs the entire length of the bottom
of the Atlantic Ocean. The existence of this ridge had been
known ever since it was encountered while the transatlantic
cable was being laid in the late 19th century. Princeton mineralogist
Harry Hess designed a depth-sounder during World
War II that used sound waves to produce images of underwater
objects. After the war he used this technique to study the
Mid-Atlantic Ridge. By the 1950s, oceanographers had found
ridges through all the world’s oceans, with a total length of
46,600 miles. All of these ridges looked like fractures from
which volcanic material emerged, and which could produce
new ocean floor, the driving force for plate tectonics.
Another advancement in scientific understanding that
paved the way to the acceptance of continental drift was the
discovery of paleomagnetism. Molten lava contains small
pieces of iron that align with the Earth’s magnetic field. When
the lava hardens, the pieces of iron are locked into place. If
the rock subsequently moves, the pieces of iron will no longer
line up with the Earth’s magnetic field. Also, the Earth’s magnetic
field changes polarity every few hundred thousand years.
Volcanic rocks that hardened during an earlier period of Earth
history will have pieces of iron that align with the direction
that the Earth’s magnetic field had at that time, not what it
is today. Geologist Fred Vine, along with Harry Hess, discovered
bands of alternation of paleomagnetism in the rocks of
the Atlantic Ocean floor (see figure on page 87). These bands
were symmetrical on either side of the Mid-Atlantic Ridge.
This evidence strongly indicated that the Mid-Atlantic Ridge
had, for millions of years, been producing new seafloor rock,
both to the east and to the west. This demonstrated the mechanism
of plate tectonics and made continental drift a compelling
theory. The newly developed techniques of radiometric
dating, furthermore, revealed that no part of the ocean floor
was more than 175 million years old, while many continental
rocks were more than three billion years old. This can now be
explained because the continents have rafted along on top of
the continually recycling rock of the ocean floors.
Acceptance of continental drift theory represented a
swift revolution in geological science. Surveys indicated
that by 1950 about half of geologists accepted continental
drift. Almost no scientists had accepted it in 1930, and by
the 1960s nearly all geologists and evolutionary biologists
accepted it. One exception was George Gaylord Simpson,
who resisted it for many years (see Simpson, George Gaylord).
The modern reconstruction of continental drift reveals
a complex and surprising history of the continents. What is
now Kazakhstan was once connected to Norway and to New
England. One corner of what is now Staten Island is European,
as is a corner of what is now Newfoundland. What is
now Massachusetts is part of the North American continent,
but part of its coast is of European origin. The Scottish Highlands
and Scandinavia contain much rock of American origin.
The future movement of continents is no less surprising.
Coastal California will move northwest, pulling Los Angeles
past Oakland and, later, Seattle. Africa will push into Europe,
destroying the Mediterranean and creating a mountain chain
all the way to India. Australia will run into Asia. This will
occur at about the pace that a fingernail grows.
The further back in time one looks, the less evidence is
available for reconstructing the patterns of continental drift.
Most rocks over a billion years old have been destroyed by
crustal movements. Geologist J. J. W. Rogers summarized
available evidence and proposed that four continents merged
together about a billion years ago to form a single continent,
Rodinia. Others have proposed that the effect of this worldcontinent
on oceanic circulation patterns caused the Earth to
freeze, forming Snowball Earth. By 700 million years ago,
when the Snowball was beginning to thaw, Rodinia split into
separate continents, which became Laurasia and Gondwana
during the Paleozoic era.
In the middle of the Paleozoic era, most of the landmass
of the Earth was in the Southern Hemisphere. Toward the
end of the Paleozoic, the continents crashed back together,
forming once again a single world-continent, Pangaea (see
figure on page 88). The loss of shallow oceans, and changes
in world climate, that accompanied the formation of Pangaea
contributed to the Permian extinction.
Near the beginning of the Mesozoic era, Pangaea began to
split again, into continents roughly corresponding to modern
Asia, Europe, and North America; and a cluster that was later
to become South America, Antarctica, Australia, Africa, and
India (see figure on page 88). By about 50 million years ago,
North America and Europe were separated, as well as Africa
and South America; Antarctica separated from South America
and Australia. India separated from the other southern continents
and moved northward, crashing into Asia and forming
the Himalayan mountains. Only about a million years ago, the
isthmus of Panama formed, connecting North and South America
for the first time since Pangaea (see figure on page 89).
Continental drift also helps to explain otherwise puzzling
features about modern distributions of organisms. For example,
the animals of Madagascar resembled those of India, several
thousand miles away, rather than nearby Africa. This can
be explained by the previous unification of these continents in
Gondwanaland. Many types of trees (for example, oaks) are
found in Eurasia and North America but not in the Southern
Hemisphere. Apparently the first oaks evolved at a time when