Alfred Wegener (1880-1930)
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.
Reproduced with kind permission
of UCMP