Download Continents Adrift and Sea-Floors Spreading: The Revolution of Plate

Continents Adrift and Sea-Floors Spreading:
The Revolution of Plate Tectonics
Stephen Brusatte
1
As the mercury inched below -54 °C, the sense of panic and fear suddenly became
palpable. Faced with the uncertainty of a long polar march, a group of 12 native
Greenland guides decided they had had enough. If the two European scientists wanted to
continue their trek to a remote weather station in the center of Greenland’s barren ice cap,
they would have to do it alone. The leader of the team, a German-born earth scientist by
the name of Alfred Lothar Wegener, was unfazed by the desertion of his guides. His
friend and former student, Johannes Georgi, lay stranded at the station without supplies,
and the long Arctic winter was quickly morphing into an agent of death. Wegener
elected to proceed, and set out with two companions during the waning days of
September 1930.
If anybody could survive such an arduous trip it was Wegener. A Ph.D. who
spent years scrutinizing ancient astronomical measurements, a world-record setting hot
air balloonist, and a veteran of both World War I and several previous trips to Greenland,
Wegener was noted for his determination and rock-solid mentality. As a scientist he was
accustomed to the blinding cold of academia and the frostbite of scientific controversy,
and as an intrepid explorer he was familiar with the dangers of the Arctic wasteland. A
little over a month after their launch, Wegener’s team reached Georgi and delivered the
necessary supplies. Knowing that the rations could only feed two men, Wegener found it
necessary to return to the coast, and set out soon after. He never made it back alive.
Today Alfred Wegener is entombed in the Greenland ice cap, a fitting resting
place for a man whose life was gradually enveloped by the freezing gusts of scientific
controversy. During his short 50-year lifespan, Wegener was the consummate polymath,
a dabbler in meteorology, climatology, geology, paleontology, and physics whose refusal
2
to specialize long hampered his search for an academic position. At the time of his death
Wegener embodied an ultimate paradox. He was both a climate specialist of international
fame and an outsider geologist of international infamy. Fifteen years before the relentless
Arctic winter snuffed out his young life, Wegener had outlined a provocative and
controversial theory: continental drift. At the time of his death, Wegener was ridiculed
and his theory was scorned. Today, it is renowned as an essential element of the unifying
theory of earth science: plate tectonics.
Long before Wegener pieced together the notion of continental drift, philosophers
and geographers had recognized that the coasts of Africa and South American snugly fit
together like a child’s jigsaw puzzle. As early as 1596 the geographer Abraham Ortelius
even suggested that the Americas were once conjoined with Europe and Asia, but later
“torn away” by earthquakes and other catastrophes. In recent years, historians of science
have revealed nascent hints of continental drift in the writings of Francis Bacon, Scottish
philosopher Thomas Dick, and the famous French scientist Comte de Buffon. It was the
genius of Wegener, however, that assembled widely divergent lines of evidence into the
first coherent theory of continental motion.
Like the early geographers before him, Wegener was intrigued by the congruence
of the South American and African coastlines as a young boy. It was only during the
final days of 1910, after completing his Ph.D., finishing his first expedition to Greenland,
and receiving his first lectureship, that Wegener seriously began to give currency to
continental drift. In the autumn of 1911 his suspicions would be strengthened, when he
came “quite by accident” upon a paper describing similar Paleozoic fossils from South
America and Africa. This paper argued for a former land bridge connecting the two
3
continents, but Wegener frowned on such a suggestion. The issue ate at his mind so
forcibly that the 31-year-old scientist soon after launched a massive literature search in
the hopes of finding additional data to support continental drift. What he found surprised
him, and gave birth to a revolution in the earth sciences.
The data Wegener uncovered were varied and wide-ranging. Not only did the
coastlines of South America and Africa match, but so too did the coasts of Newfoundland,
England, parts of Greenland, and Scandinavia. Interestingly, the Paleozoic fossils shared
by South America and Africa were only the tip of a paleontological iceberg of supporting
observations. As it turned out, South America, Africa, Australia, Antarctica, and India
shared a mélange of Paleozoic and Mesozoic fossils, including a unique tropical plant
flora characterized by the fern Glossopteris and a reptile fauna that included the tusked,
pig-like Lystrosaurus. Modern animals do not range across all continents, because it is
often impossible to disperse across oceans and other barriers. However, the modern
lands supporting these unique fossils are widely divergent today. This suggested to
Wegener that they must have been connected in the past. Additionally, the presence of
tropical fern fossils in Antarctica makes no sense if the continent has always occupied a
polar position. Other evidence revealed by Wegener included closely matching rock
units shared by Africa and South America; the distributions of former equatorial climate
belts (as shown by coals and fossil reefs shared by the five forementioned lands); and the
locations of past Paleozoic glaciations. Taken together, these facts suggested to Wegener
that all of the continents had once been joined together into a supercontinent named
Pangaea, which later split into two large fragments. The first included North America,
Europe, and Asia, while the second consisted of Africa, South America, India,
4
Madagascar, Australia, and Antarctica, all of which separated later. There was no
conceivable way for land bridges to explain these varied observations. The continents
must have moved.
Wegener first presented his hypothesis of continental drift in a series of lectures
and journal articles in 1912. Three years later, after a successful expedition to Greenland
and a stint as a solider in World War I, he outlined his ideas in a short, 94-page book, Die
Entstehung der Kontinente und Ozeane. Today regarded as one of the rarest and most
treasured books in the history of science, this short compendium is catalogued in only
two libraries in the United States, the New York Public Library and the John Crerar
Library. Fortunately, Wegener’s book would be revised three more times before his
death, and issued in English and French in 1924. As would be expected, this book caused
a considerable stir in the geological community. University of Chicago geologist R.T.
Chamberlin ridiculed the theory as being “of the foot-loose type,” while Edward Berry
bluntly labeled Wegener’s method as “unscientific.” While a good number of geologists
accepted Wegener’s data, many geophysicists scoffed at Wegener’s failure to provide a
plausible mechanism for continental motion. In his book, Wegener suggested that the
centrifugal force resulting from the Earth’s rotation, or possibly the pull of gravity from
the moon, drove the lighter, granite-rich continents through the denser, basalt-rich
oceanic crust like a ship plowing through water. However, eminent Cambridge geologist
Harold Jeffreys, one of the most respected scientists of his time, did the calculations and
found these forces insufficient to move something as large as a continent.
At the time of his death, Wegener was the subject of ridicule and contempt from
the geological community. However, as a new age of science dawned in the shadow of
5
World War II, an avalanche of observations would drag continental drift back into the
light. Although Wegener himself could never appreciate it, the new data strongly
vindicated him. Over the course of the 1960s, a handful of young earth scientists from
across the globe would mold the new observations into the theory of plate tectonics,
which today explains everything from animal dispersal and mountain building to
volcanism and earthquakes.
One key observation involved the Earth’s magnetic field. As lava cools into solid
rock, tiny crystals of a magnetic mineral called magnetite are “locked” into position,
thereby recording the strength and direction of the Earth’s magnetic field at the time of
the rock’s formation. Using relatively simple trigonometric equations, geologists can
take this data and determine the latitude at which a certain rock formed. This procedure
was applied to igneous rocks from across the globe, and it was discovered that the
latitudes at which the rocks formed were different than the latitudes they occupy today.
At first it was thought that the Earth’s magnetic pole simply wandered over time, but
further research showed that the pole was essentially fixed. Instead, it was the continents
that moved.
Paleomagnetic data strongly supported continental drift, but current geologists
view Wegener’s hypothesis as only half of the unifying theory of plate tectonics. The
other contribution is sea-floor spreading, a mechanism that was long hinted at but only
rigorously demonstrated in the 1960s. Leading the revolution was a Princeton geologist
by the name of Harry Hess, a mild-mannered scientist who discreetly took measurements
with a Fathometer while captaining a Navy ship during World War II. Hess’
measurements detailed the topography of the sea-floor, and revealed that a series of long
6
mountain ranges, deep trenches, and extinct volcanoes littered the deep abyss. Later
evidence suggested that a large percentage of the world’s earthquakes were occurring in
these submerged mountains, hinting that the ocean bottom was a dynamic place. Over
time Hess became convinced that sea-floors were actually spreading in both directions
away from the mid-ocean mountain ridges. He presented his data in a landmark 1962
paper, and was soon supported by a slew of additional observations. For example,
Cambridge geologists Drummond Matthews and Fred Vine measured the magnetic fields
in the rocks of the ocean floor, and found that the strength and direction of the fields
followed striped patterns that were the same on both sides of the ridges. Later studies
directly dated the rock of the sea-floor, and found that the absolute dates were also
symmetrical around the ridges, with rocks becoming older with distance from the
hypothesized spreading center. An influx of additional data only confirmed what Hess,
Matthews, and Vine suspected: that new sea-floor was created in mid-ocean ridges and
gradually spread outward in both directions.
This picture of plate tectonics, a unification of continental drift and sea-floor
spreading, was largely pieced together during an eight-year period in the 1960s. Today,
geologists view the Earth as composed of two principle layers, the brittle, outer
lithosphere and the denser, warmer asthenosphere. The lithosphere, akin to the fragile
shell of an egg, is broken into some 20 distinct plates, which are rigid but deformable at
their edges. When two plates meet one of three general interactions occur: they can move
away from each other, move toward each other, or slide past each other. When plates
move away from each other, at what are called divergent boundaries, sea-floor spreading
takes place, and new oceanic crust is formed. When plates come together at a convergent
7
boundary, the result is more complicated. If denser oceanic crust meets lighter
continental crust, the oceanic crust is subducted beneath the continent, which gives rise to
volcanism, explaining the Ring of Fire. When two segments of continental crust meet
they will collide and wrinkle, forming mountains. This is precisely what happened when
India and Tibet collided, uplifting the Himalayas. And, when two plates slide past each
other, earthquakes frequently occur, as manifested by the San Andreas Fault of California.
Despite this resolution, the mechanism of continental drift is still somewhat murky.
However, it is thought that convective currents in the wax-like asthenosphere drive the
motion of the overriding lithosphere.
The development of plate tectonics, stretching from Wegener’s initial
observations to the more rigorous experiments of Hess and cohorts, constituted nothing
less than a revolution, a complete transformation of the practice of geology. Unlike most
traditional sciences, geology deals with large-scale patterns and processes that operate
over unthinkable lengths of time. Early in the formation of geology as a scientific
discipline, practitioners found it necessary to describe the geological past of Earth based
on processes that occur in the present. Although intuitively rational, this method of
“uniformitarianist” thinking disabled many geologists from recognizing complex
processes that are difficult to observe. This often led to rampant speculation and frankly
unscientific methods when geologists were faced with explaining complicated issues,
such as earthquakes or mountain building. For a long period it was widely accepted that
mountains formed as the warm surface of the Earth gradually cooled and contracted,
much as the skin of an apple wrinkles as it rots. There was little rational evidence to
support this suggestion, but geology was unable to do much better.
8
Today, geologists have a much easier time explaining how mountains form. The
new answer is not only simpler, but also more scientific. It is based on numerous
observations, a healthy gathering of collected data, and years of testing and refinement.
This new answer is based on plate tectonics, a beautiful and elegant theory that unifies
centuries of divergent observations into a complete, understandable, working theory of
the Earth. Mountains form not via contraction, but when continents collide. Earthquakes
are not a mysterious force of nature, but instead a consequence of plate motion.
Volcanism doesn’t reflect the wrath of the gods, but rather a chaotic and vibrant Earth
whose sea-floors spread and continents drift. This is the beauty and importance of plate
tectonics. It is the grand unifying theory of earth science, the geological equivalent to
Darwinian evolution or Newtonian and Einsteinian physics. Plate tectonics has truly
delivered geology from a historical science based solely on modern observation to a
pulsating discipline rooted in a solid framework.
Although critical for those studying the past evolution of life or the changing
nature of the Earth’s magnetic field, plate tectonics saves its most important contributions
for modern, industrial societies. Finding the Higgs Boson will have few practical
applications, but understanding plate tectonics has already saved innumerable lives and
promises to better our standard of living. It is near impossible to understand the
frequency and potential destruction of earthquakes if we view these tremors as mysteries
of nature or the forces of a god. However, now that we have a general understanding of
why and where earthquakes occur we can better predict their damage, and better survive
their wrath. The study of earthquakes has allowed governments to update building codes,
construct safer freeways and power lines, and enact detailed responses to a tremor. The
9
same is true of volcanism. Knowing where volcanism occurs and how potentially
dangerous an eruption can be has allowed for safe and easy evacuations of potential hot
spots.
Not only does an understanding of plate tectonics help save lives, but it also
fosters a deep respect for the power of nature and the mind-numbing length of geologic
time. Human time, measured on the span of days and years, rarely coincides with
geologic time, which is enumerated by epochs, periods, and eras. People have little
appreciation for the vast history of our planet, and the slow, gradual geologic forces that
can build a mountain, split a continent, or close an entire ocean. We are using our natural
resources at an alarming rate, exhausting within decades supplies of oil, coal, and ores
that took millions of years to form. Similarly, biodiversity is rapidly decreasing, as the
extinction rates of species have risen with the evolution of the human race. Much of this
isn’t directly our fault. We simply find it difficult to understand how we, a single species
existing for hundreds of thousands of years, fit into the larger picture of a planet that has
been around for 4.5 billion years. Plate tectonics puts the enigma of deep time into
perspective. Sea-floors spread at 2-4 centimeters per year, a rate comparable with the
growth of our fingernails. However, as is obvious by a quick glance at the topography of
Earth, these slow forces can create incredibly significant results. Plate tectonics is exhibit
A in the case that the Earth is ancient and ever-changing, and that we are only a small
part of the history of such an amazingly complicated and dynamic planet.
As a student of geology with an interest in fossils, learning the workings of plate
tectonics has given me a new perspective on the history of our planet. Earth isn’t static, it
is dynamic. The present state of affairs is just a temporary moment in time, subject to
10
change with the next earthquake or volcanic eruption. I will never perceive a sea-floor
spreading or a continent drifting during my lifetime. Human lifetimes are simply too
short. But, extended over millions of years, the very same forces in operation today
constantly reshape our planet. Understanding these forces will not only allow me to be a
better scientist, but will also certainly aide society in coping with nature and humanity in
respecting its true place in the cosmos.
BIBLIOGRAPHY
Bacon, F. 1855. Novum Organum. Oxford University Press, London.
Chamberlin, R.T. 1928. Some of the objections to Wegener’s theory. In: W.A.J.M.
van Waterschoot van der Gracht et al. (eds.), Theory of Continental Drift,
American Association of Petroleum Geologists, Tulsa, Oklahoma, 83-87.
Cutler, A. 2003. The Seashell on the Mountaintop: A Story of Science, Sainthood, and
the Humble Genius Who Discovered a New History of the Earth. E.P. Dutton
and Company, New York.
Deffeyes, K.S. 1972. Plume convection with an upper mantle temperature inversion.
Nature 240:539-544.
Dietz, R.S. 1961. Continent and ocean basin evolution by spreading of the sea floor.
Nature 190:854-857.
Georgi, J. 1935. Mid-Ice: The Story of the Wegener Expedition to Greenland.
E.P. Dutton and Company, New York.
Hallam, A. 1973. A Revolution in the Earth Sciences. Oxford University Press,
Oxford.
Hess, H.H. 1962. History of Ocean Basins. In: A.E.J. Engel, H.L. James, and B.F.
Leonard (eds.), Petrologic Studies: A Volume to Honor A.F. Buddington,
Geological Society of America, New York, 599-620.
Isacks, B., J. Oliver, and L.R. Sykes. 1968. Seismology and the new global tectonics.
Journal of Geophysical Research 73:5855-5899.
Jeffreys, H. 1924. The Earth: Its Origin, History, and Physical Constitution. Cambridge
University Press, Cambridge.
Marshak, S. 2001. Earth: Portrait of a Planet. W.W. Norton & Company, London.
Martin, U.B. 1973. Continental Drift: The Evolution of a Concept. Smithsonian
Institution Press, Washington D.C.
McPhee, J. 1981. Basin and Range. Farrar, Straus, and Giroux, New York.
McPhee, J. 1983. In Suspect Terrain. Farrar, Straus, and Giroux, New York.
McPhee, J. 1993. Assembling California. Farrar, Straus, and Giroux, New York.
Morgan, W.J. 1968. Rises, trenches, great faults, and crustal blocks. Journal of
Geophysical Research 73:1959-1982.
Morgan, W.J. 1972. Deep mantle convection plumes and plate motions. Bulletin of the
American Association of Petroleum Geologists 56:202-213.
11
Oreskes, N. 1999. The Rejection of Continental Drift: Theory and Method in American
Earth Science. Oxford University Press, Oxford.
Oreskes, N. 2003. Plate Tectonics: An Insider’s History of the Modern Theory of the
Earth. Westview Press, New York.
Palmer, D. 2003. Prehistoric Past Revealed: The Four Billion Year History of Life on
Earth. University of California Press, Berkeley, California.
Redfern, R. 2001. Origins: The Evolution of Continents, Oceans, and Life. University of
Oklahoma Press, Norman, Oklahoma.
Repcheck, J. 2003. The Man Who Found Time: James Hutton and the Discovery of
Earth’s Antiquity. Perseus Publishing, Cambridge, Massachusetts.
Sullivan, W. 1974. Continents in Motion: The New Earth Debate. McGraw Hill Book
Company, New York.
Vine, F.J. 1966. Spreading of the ocean floor: new evidence. Science, 154:1405-1415.
Vine, F.J., and D.H. Matthews. 1963. Magnetic anomalies over oceanic ridges.
Nature, 199:947-949.
Wegener, A. 1912. Die entstehung der Kontinente und Ozeane. Geologische Rundschau
3:276-292.
Wegener, A. 1915. Die Entstehung der Kontinente und Ozeane. Friedrich Vieweg & Sons,
Braunschweig, Germany.
Wegener, A. 1924. The Origin of Continents and Oceans. E.P. Dutton and Company,
New York.
Wegener, A. 1928. Two notes concerning my theory of continental drift. In: W.A.J.M.
van Waterschoot van der Gracht et al. (eds.), Theory of Continental Drift,
American Association of Petroleum Geologists, Tulsa, Oklahoma, 97-103.
Wilson, J.T. 1965. A new class of faults and their bearing on continental drift. Nature
207:343-347.
Wilson, J.T. 1968. A revolution in earth science. Geotimes 13(10):10-16.
12