Alfred Wegener (1880-1930)
"Scientists still do not appear to understand sufficiently that all
earth sciences must contribute evidence toward unveiling the state of our
planet in earlier times, and that the truth of the matter can only be reached
by combing all this evidence. . . It is only by combing the information
furnished by all the earth sciences that we can hope to determine 'truth'
here, that is to say, to find the picture that sets out all the known facts
in the best arrangement and that therefore has the highest degree of
probability. Further, we have to be prepared always for the possibility that
each new discovery, no matter what science furnishes it, may modify the
conclusions we draw."
Alfred Wegener. The Origins of Continents and Oceans (4th edition)
Some truly revolutionary scientific theories may take years or decades to
win general acceptance among scientists. This is certainly true of
plate tectonics,
one of the most important and far-ranging geological theories
of all time; when first proposed, it was ridiculed, but steadily
accumulating evidence finally prompted its acceptance, with immense
consequences for geology, geophysics, oceanography, and paleontology. And the
man who first proposed this theory was a brilliant interdisciplinary
scientist, Alfred Wegener.
Born on November 1, 1880, Alfred Lothar Wegener earned a Ph.D in astronomy
from the University of Berlin in 1904. However, he had always been interested
in geophysics, and also became fascinated with the developing fields of
meteorology and climatology. During his life, Wegener made several key
contributions to meteorology: he pioneered the use of balloons to track air
circulation, and wrote a textbook that became standard throughout Germany.
In 1906 Wegener joined an expedition to Greenland to study
polar air circulation. Returning, he accepted a post as tutor at the
University of Marburg, taking time to visit Greenland again in 1912-1913.
(The above photograph of Wegener was taken during this expedition). In
1914 he was drafted into the German army, but was released from combat
duty after being wounded, and served out the war in the Army weather
forecasting service. After the war, Wegener returned to Marburg, but became
frustrated with the obstacles to advancement placed in his way; in 1924 he
accepted a specially created professorship in meteorology and geophysics
at the University of Graz, in Austria. Wegener made what was to be his
last expedition to Greenland in 1930. While returning from a rescue
expedition that brought food to a party of his colleagues camped in the
middle of the Greenland icecap, he died, a day or two after his fiftieth
birthday.
While at Marburg, in the autumn of 1911, Wegener was browsing in the
university library when he came across a scientific paper that listed
fossils of identical plants and animals found on opposite sides of the
Atlantic. Intrigued by this information, Wegener began to look for, and
find, more cases of similar organisms separated by great oceans. Orthodox
science at the time explained such cases by postulating that land bridges,
now sunken, had once connected far-flung continents. But Wegener noticed the
close fit between the coastlines of Africa and South America. Might the
similarities among organisms be due, not to land bridges, but to the
continents having been joined together at one time? As he later wrote:
"A conviction of the fundamental soundness of the idea took root in my
mind."
Such an insight, to be accepted, would require
large amounts of supporting evidence. Wegener
found that large-scale geological features on separated continents often
matched very closely when the continents were brought together. For example,
the Appalachian mountains of eastern North America matched with the Scottish
Highlands, and the distinctive rock strata of the Karroo system of South
Africa were identical to those of the Santa Catarina system in Brazil.
Wegener also found that the fossils found in a certain place often
indicated a climate utterly different from the climate of today: for example,
fossils of tropical plants, such as ferns and cycads, are found today on
the Arctic island of Spitsbergen. All of these facts supported Wegener's
theory of
"continental drift."
In 1915 the first edition of The
Origin of Continents and Oceans, a book outlining Wegener's theory, was
published; expanded editions were published in 1920, 1922, and 1929.
About 300 million years ago, claimed Wegener, the continents had formed
a single mass, called Pangaea (from the Greek for "all the Earth"). Pangaea
had rifted, or split, and its pieces had been moving away from each other ever
since. Wegener was not the first to suggest that the continents had once
been connected, but he was the first to present extensive evidence from
several fields.
Modern reconstruction of Pangaea, ca. 255 million years ago -- click to view
a much larger version of this map!
Reaction to Wegener's theory was almost uniformly hostile, and often
exceptionally harsh and scathing; Dr. Rollin T. Chamberlin of the University
of Chicago said, "Wegener's hypothesis in general is of the footloose
type, in that it takes considerable liberty with our globe, and is less
bound by restrictions or tied down by awkward, ugly facts than most of
its rival theories." Part of the problem was that Wegener had no convincing
mechanism for how the continents might move. Wegener thought that the
continents were moving through the earth's crust, like icebreakers plowing
through ice sheets, and that centrifugal and tidal forces were responsible
for moving the continents. Opponents of continental drift noted that
plowing through oceanic crust would distort continents beyond recognition,
and that centrifugal and tidal forces were far too weak to move continents --
one scientist calculated that a tidal force strong enough to move
continents would cause the Earth to stop rotating in less than one year.
Another problem was that flaws in Wegener's original data caused him to
make some incorrect and outlandish predictions: he suggested that North
America and Europe were moving apart at over 250 cm per year (about ten
times the fastest rates seen today, and about a hundred times faster than
the measured rate for North America and Europe).
There were scientists who supported Wegener: the South African geologist
Alexander Du Toit supported it as an explanation for the close similarity
of strata and fossils between Africa and South America, and the Swiss
geologist Émile Argand saw continental collisions as the best
explanation for the folded and buckled strata that he observed in the Swiss
Alps. Wegener's theory found more scattered support after his death, but
the majority of geologists continued to believe in static continents and
land bridges.
What prompted the revival of
continental drift?
In large part it was
increased exploration of the Earth's crust, notably the ocean floor, beginning
in the 1950s and continuing on to the present day. By the late 1960s,
plate tectonics
was well supported and accepted by almost all geologists.
We now know that Wegener's
theory was wrong in one major point: continents do not plow through the
ocean floor. Instead, both continents and ocean floor form solid plates,
which "float" on the
asthenosphere, the underlying rock that is under
such tremendous heat and pressure that it behaves as an extremely viscous
liquid. (Incidentally, this is why the older term "continental drift" is
not quite accurate -- both continents and oceanic crust move.)
Since Wegener's day, scientists have
mapped and explored the great system of oceanic
ridges, the sites of frequent earthquakes, where molten rock rises from below
the crust and hardens into new crust. We now know that the farther away you
travel from a ridge, the older the crust is, and the older the sediments
on top of the crust are. The clear implication is that the ridges are the
sites where plates are moving apart (click on the picture at the left to see
a map of the age of the ocean crust). Where plates collide, great mountain
ranges may be pushed up, such as the Himalayas; or if one plate sinks below
another, deep oceanic trenches and chains of volcanoes are formed.
Earthquakes are by far most common along plate boundaries and rift
zones: plotting the location of earthquakes allows seismologists to map
plate boundaries and depths (click on the picture at the right to view a
map of quake epicenters).
Paleomagnetic data have allowed us to map past plate movements much more
precisely than before.
It is even possible to measure the speed of continental plates extremely
accurately, using satellite technology.
Nevertheless, Wegener's basic insights remain sound, and the lines of evidence
that he used to support his theory are still actively being researched and
expanded.
The images of the ocean floor on this page were provided by the
World Data Center A of the
National Geophysical Data Center/ NOAA.
There are a number of excellent WWW sites dealing with the modern theory
of plate tectonics. Here is a modest sample:
Finally, it seems appropriate to mention
the Alfred Wegener Institute,
the German national research center for polar and marine research,
carrying on Wegener's tradition of interdisciplinary earth science.
