CONTINENTS ADRIFT
During the 19th and early 20th century, several geologists explored the idea
that the continents may have moved across the Earth's surface. They were all
inspired by the remarkable fit between the Atlantic coasts of Africa and
South America. The hypothesis of continental drift was largely developed by
the German Alfred L. Wegener, a lecturer in astronomy and meteorology, who
suggested that the Earth's continents had at one time been joined in two
supercontinents. In the year 1912, Wegener made the proposal that all the
continents were previously one large continent, but then broke apart, and
had drifted through the ocean floor to where they are now located. Apart from
using the fit of the two continents already mentioned, Wegener also used
fossil distribution and lithological similarity as evidence. Wegener was
born in Berlin on November 1, 1880 the youngest child of an evangelical
preacher. By the time he was in his teens, he had developed a strong
interest in the earth sciences. He studied astronomy at the University of
Berlin, where he received a doctoral degree in 1904. Wegener exhibited a
talent for expounding complex subjects with great ease. Together with the
force of his personality, the clarity of Wegener's vision inspired great
enthusiasm and loyalty among his students.
Of course, the "Drift" theory was not immediately accepted by Wegener's
peers, as it is difficult in the world of science to change accepted or
established doctrines or views. Two other viewpoints prevailed at this time.
Those who believed that the continents and basins were basically unchanged
in their position and relative configuration since they were formed were
called "Permanentists". Others believed that as a result of the gradual
contraction of the solid earth, ocean floor became dry land, and dry land in
turn became ocean floor; these scientists were called "Contractionists".
Wegener studied the distribution of animals and fossil land plants to help
him in his interpretations. Wegener found that the plant
Glossopteris had left behind leaf remains which were relatively
common in the Southern Hemisphere continents. This supported his hypothesis,
as Wegener reasoned that in order for
Lithology
Wegener also studied the distribution of major geological bodies, such as
crystalline basement (rocks and continental crust) complexes and mineral
deposits. He found that the fit predicted by map estimates was confirmed by
the alignment of geological complexes on either side of the Atlantic Ocean.
For instance when he fitted Africa and South America together along their
continental shelves, he found that large blocks of ancient rock called
cratons formed contiguous patterns across the dividing line. The mountains
that run from east to west across South Africa seemed to link with the range
near Buenos Aires in Argentina. The distinctive rock strata of the Karoo
system in South Africa, which consists of layers of sandstone, shale and
clay laced with seams of coal, were identical to those of the Santa Catarina
system in Brazil.
South Africa's contribution
Wegener's most compelling and enthusiastic support came from a South African
geologist, Alexander Du Toit. South African scientists were far more
favorably disposed to the idea of continenal drift for a simple reason: All
around them they could see a plethora of geological phenomena that closely
resembled those of the other continents in the Southern Hemisphere.
Du Toit spent five months in Brazil, Uruguay and Argentina amassing
evidence. He found it difficult to believe that he was on another continent
as not only did he find the same plant and animal fossils he knew at home,
but he found them in the same complex sequence, embedded layer by layer in
the rock. Du Toit was confident he had found conclusive proof that the
continents were once joined. In a book dedicated to Wegener and entitled
Our Wandering Continents, Du Toit proposed a prior configuration for
the continents that was different from Wegener's.
Instead of a simple supercontinent, Du Toit reconstructed the continents at
the South Pole and grouped the northern continents near the Equator. He
called his southern supercontinent Gondwanaland and the northern land mass
Laurasia. He devoted most of his book to Gondwanaland and as evidence for
its existence he produced an impressive mass of data far more detailed than
anything Wegener had attempted.
Wegeners's findings were published in 1915 in his book Die Entstehung der
Kontinente und Ozeane (The origin of the continents and oceans). His ideas
were not widely accepted as critics thought that the evidence were not
strong enough, that the underlying cause of the drift was not explained and
that the drift was impossible. In fact, he was bitterly attacked by many
critics and these attacks eventually took their toll on his career.
Despite his undisputed talents as a teacher and the continuing loyalty of
his close associates, he was unable to obtain a professorship in a German
university and eventually left Germany for the University of Graz in
Austria.
Was this good enough for the academic critics ? No. Du Toit's flamboyant writing style pained the critics. For instance he would write "the dumbfounding spectacle of the present continental masses, firmly anchored to a plastic foundation yet remaining fixed in space; set thousands of kilometers apart, it may be, yet behaving in almost identical fashion from epoch to epoch and stage to stage like soldiers at a drill; widely stretched in some quarters at various times and astoundingly compressed in others, yet retaining their general shapes, positions and orientations; remote from one another throughout history, yet showing in their fossil remains common or allied forms of terrestrial life; possessed during certain epochs of climates that may have ranged from glacial to torrid or pluvial to arid, though contrary to meteorological principles when their existing geographic positions are considered - to mention but a few such paradoxes!" Du Toit's overdramatization succeeded only in decreasing the value of his substantial contributions to the evidence. "This," sniffed an academic critic, "is the colorful language of a pamphleteer."
How the critics were converted
The discovery of palaeomagnetism and the development of oceanography was a necessary step in the development of science which Wegener's and Du Toit's theories awaited.
Paleomagnetism
Paleomagnetism is based on the principle that in molten igneous rocks, or
unlithified sediments, magnetic particles will align themselves with the
Earths's magnetic field. This magnetic record is stored within the rocks
when they cool and within the sediments when they become lithified. The
deviations in the alignment of these paleomagnetic particles from the
current direction of the Earth's magnetic filed shows that the continents
have moved. A British physicist Patrick Blackett, who had won the Nobel
Prize in 1948 for his work in nuclear physics and cosmic radioation,
developed a sensitive device called the astatic magnetometer. Using this
equipment, it was possible for the first time to detect the orientation of
extremely weak magnetic fields. This enabled researchers to conduct
paleomagnetic studies of types of rocks whose magnetism could not be
discerned by earlier equipment.
Oceanography
During the 1960's two Cambridge scientists, Drummond Matthews and Fred Vine
discovered that on either side of the Mid-Atlantic Ridge there were a series
of linear magnetic anomalies. Strips of ocean crust had alternating magnetic
orientations. These observations were explained in terms of a
sea floor spreading model by which new oceanic crust forms along mid-ocean
ridges as the two halves of an ocean move apart. From these simple
observations the theory of PLATE TECTONICS developed.
According to the plate tectonic model, the surface of the Earth consists of
a series of relatively thin, but rigid, plates which are in constant motion.
The surface layer of each plate is composed of oceanic crust, continental
crust or a combination of both. The lower part consists of the rigid upper
layer of the Earth's mantle. The rigid plates pass gradually downwards into
the plastic (soft) layer of the mantle, the astenosphere. The plates may be
up to 70 km thick if composed of oceanic crust or 150 km incorporating
continental crust. Plates move at different velocities, The African plate
moves about 25 mm per year, whereas the Australian plate moves about 60 mm
per year.
Most of the Earth's tectonic, seismic and volcanic activity occurs at the
boundaries of neighbouring plates. There are three type of plate boundaries:
divergent, convergent and transform boundaries.
Divergent plate margins
At this type of boundary new oceanic crust is formed in the gap between two
diverging plates. Plate area is increased as the plates move apart.
Plate movement takes place laterally away from the plate boundary, which is
normall marked by a rise or a ridge. The ridge or rise may be offset by a
transform fault. Presently, most divergent margins occur along the central
zone of the world's major ocean basins. The process by which the plates move
apart is referred to as sea floor spreading.
The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this
type of plate margin.
Convergent plate boundaries
At a convergent boundary two plates are in relative motion towards each
other. One of the two plates slides down below the other at an angle of
around 45 degrees and is incorporated into the Earth's mantle along a
subduction zone. The path of this descending plate can be
found from analysis of deep earthquakes and the initial point of descent is
marked on the surface by a deep ocean trench . Plate area
is reduced along the subduction zone. When two plates of oceanic crust
collide a volcanic island arc may form. As one of the
plates is subducted beneath the other it begins to melt at a depth of
between 90 and 150 km and the resulting magma rises to the surface above the
subduction zone to form a chain or arc of volcanoes. The edge of the plate
which is not descending is therefore marked by a chain of volcanic islands.
Conservative or transform margins
The San Andreas fault system is the most famous example of this type of
boundary. Here two plates move laterally past each other and oceanic crust is
neither created nor destroyed.
What causes plates to move ?
This question has yet to be fully resolved. Four main hypotheses have been
put forward to explain this.
Convection currents
This hypothesis suggests that flow in the mantle is induced by
convection currents which drag and move the lithospheric
plates above the astenosphere. Convection currents rise and spread below
divergent plate boundaries and converge and descend along convergent. Three
sources of heat produce the convection currents:
(1) cooling of the Earth's core
(2) radioactivity within the mantle and crust
(3) cooling of the mantle
The convection hypothesis has been proposed in several different forms
throughout the last 60 years. Convective models of plate evolution clearly
show how important convective heat transport is to the modern Earth, over
length scales as small as 100 km and times of 60 million years. Earth is a
spendthrift, living on its inherited capital of primaeval heat, not on its
radiogenic modern income.
Magma injection
This hypothesis invokes the injection of magma at a spreading centre
pushing plates apart and thereby causing plate movement.
Gravity
Oceanic lithosphere thickens as it moves away from a spreading centre and
cools, a configurationwhich might tend to induce plates to slide under the
force of gravity, from a divergent margin towards a convergent margin.
Descending plates
This hypothesis suggests that a cold dense plate descending into the mantle
at a subduction zone may pull the rest of the plate with it and thus cause
plate motion.
To summarize, the plate tectonic model provides a mechanism by which:
(1) continents can move across the surface of the globe
(2) patterns of volcanism can change and shift across the globe as plates
and their boundaries evolve and move
(3) new oceans may grow and different sedimentary basins evolve
(4) oceans and sedimentary basins close and are deformed to produce
mountains
Do measurements using VLBI, SLR and GPS support the findings from
paleomagnetism ?
Yes, it does.
Geodetic data from VLBI, SLR and GPS indicate that plate velocities as
measured over the last 15 years nearly equal those averaged over the past 3
million years.
For more information on plate tectonics, read
http://www.hartrao.ac.za/geodesy/tectonics.html
Produced by Ludwig Combrinck 19/02/99,
e-mail: ludwig@ludwig.hartrao.ac.za
since 19 February 1999.