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 Plate Tectonics are
a theory of global tectonics (geological structural deformations) that
has served as a master key, in modern geology, for understanding the
structure, history, and dynamics of the earth's crust. The theory is
based on the observation that the earth's solid crust is broken up into
about a dozen semi-rigid plates. The boundaries of these plates are
zones of tectonic activity, where earthquakes and volcanic eruptions
tend to occur.
Background. Although
the plate tectonics revolution in geological thought occurred only
recently (in the 1960s and 1970s), the roots of the theory were
established by earlier observation and deduction. In one such discovery,
James Hall, a New York geologist, observed that sediments accumulated in
mountain belts are at least ten times thicker than those in the
continental interiors of the earth.
This planted the seeds for the later geosynclinal theory that continental crust grows by progressive
additions that originate as ancient and folded geosynclines, hardened
and consolidated into plates. This theory was well established by the
20th century. Another 19th-century discovery was that there was a
mid-ocean ridge in the Atlantic Ocean; by the 1920s scientists had
concluded that this ridge was continuous almost all the way round the
world.
In
the period 1908-1912, the German geologist Alfred Lothar Wegener
proposed theories of continental drift and others, who recognized that
continental plates rupture, drift apart, and eventually collide with
each other. Such collisions crumple geosynclinal sediments, thus
creating future mountain belts. Geophysical work on the earth's density
and observations by petrologists had previously shown that the earth's
crust consists of two quite different materials: sima, a
silicon-magnesium rock, typically basalt, which is characteristic of
oceanic crust; and sial, a silicon-aluminium rock, typically granite and
characteristic of continental crust. Wegener thought that the sial
continental plates sail across the sima ocean crust like icebergs in the
ocean. This reasoning was fallacious, because the melting point of sima
is higher than that of sial. Geologists subsequently discovered the
so-called asthenosphere, a layer of relatively low strength in the
earth's mantle that underlies the crust at depths of 50 to 150 km (30 to
80 mi). First deduced hypothetically, it was later seismically
demonstrated to be a low-velocity, plastic material capable of flow.
One
of Wegener's strongest arguments for continental drift was the geometric
matching of continental margins, which he postulated had rifted apart.
To support his theory, he pointed out that rock formations on opposite
sides of the Atlantic Ocean—in Brazil and West Africa—match in age,
type, and structure. Furthermore, they often contain fossils of
terrestrial creatures that could not have swum from one continent to the
other. These palaeontological arguments were among the most persuasive
to many specialists, but they did not impress others (mostly
geophysicists).
Wegener's
best examples of rifted continental borders, as mentioned, were along
the two sides of the Atlantic Ocean. Sir Edward Crisp Bullard, in fact,
tested their precise fitting by computer-based analyses and presented
his results to the Royal Society of London: the fit was perfect. Along
many other ocean margins, however, no such match is found—for example,
along the entire circum-Pacific belt or along the Burma-Indonesian
sector of the Indian Ocean. This discrepancy points out a characteristic
of continental margins that had been noted by a famous Viennese
geologist, Eduard Suess, in the 1880s. He recognized an “Atlantic
type” of margin, identified by abrupt truncation of former mountain
belts and rifting structures, and a “Pacific type”, marked by
parallel cordillera-type mountains, lines of volcanoes, and frequent
earthquakes. To many geologists, the Pacific-type coasts appeared to be
located where geosynclines are in the process of becoming crumpled and
uplifted to create mountains.
Seafloor
Spreading. In
the 1920s, the study of seafloors was advanced when sonar, the
echo-sounding device, was modified to measure ocean depths. With sonar,
submarine topography could be surveyed and the seafloor mapped. Next,
geophysicists adapted the airborne magnetometer so that it would record
variations in geomagnetic intensity and orientation. Shipborne
magnetometric traverses across the mid-ocean ridges showed that the
rocks on one side of the ridge produced a mirror-image geomagnetic
pattern of the rocks on the other. Age dating of the basaltic crustal
rocks of the seafloor showed that those nearest the ridge were
distinctly younger (relatively recent, in fact) than those further away.
In addition, no blanket of marine sediment was found at the ridge crest,
but it appeared on each side and grew older and thicker with increasing
distance from the ridge.
These observations, added to those of the high
heat flow, led to the conviction that the ridge is where new ocean crust
is being created; it is carried up by convection currents as hot lava,
but is rapidly cooled and consolidated on contact with the cold
deep-ocean water. To make room for this continual addition of new crust,
the plates on each side of the ridge must slowly but constantly move
apart. In the North Atlantic, the rate of movement is only about 1 cm
(0.4 in) per year, whereas in the Pacific Ocean it amounts to more than
4 cm (2 in) annually. It is these relatively slow rates of movement,
driven by thermal convection currents originating deep in the earth's
mantle, that have, over the course of millions of years, been generating
the phenomenon of continental drift.
Detailed
mapping of the ocean floor was collated in the 1960s and incorporated in
physiographic maps in which the submarine landforms were artistically
rendered by scientists at Columbia University's Lamont Geological
Observatory. They noticed that the crest of a mid-ocean ridge is in the
form of a rift, or cleft, a few kilometres across, that coincides with
the ridge centre. They also found that in the Red Sea the rift enters
the African continent to become an integral part of the famous Great
Rift Valley, which runs from the Jordan Valley and Dead Sea through the
Red Sea to Ethiopia and East Africa. Evidently, the rift marks a split
in the continental crust, as well as that of the ocean.
The
new physiographic maps of the ocean floor also revealed, for the first
time, that the crest of the mid-ocean ridge is extensively offset by
deep cracks, which have been called fracture zones. These cracks mark
the course of transform (“strike-slip”) faults that have developed
to accommodate strain generated by uneven rates of seafloor spreading.
Although most of these faults are hidden below the ocean, one of them,
the earthquake-prone San Andreas Fault, emerges from the Pacific Ocean
near San Francisco and crosses hundreds of kilometres of land.
Volcanic
Arcs and Subduction. Dynamic
problems unique to Pacific-type coasts were recognized as early as the
1930s by American seismologists, who showed that earthquakes associated
with these belts are at shallow depths near the outer (ocean) side of
volcanic island arcs, but that the depth of seismic shocks increases
until it reaches a maximum of about 700 km (430 mi) at a distance of 700
km landward from the front of the arc. By close analysis of a single
instance, the American seismologist Hugo Benioff concluded that this
geometry represented a fault plane extending through the crust into the
upper mantle and inclined downwards, towards land, at an angle of about
45°. A similar underthrusting, of the Southern Alps beneath the
Northern Alps, had been proposed in 1906, and in the 1950s the process
was named subduction.
The
existence of similar subduction planes has now been demonstrated along
almost all Pacific-type coasts. (Those where the zone is absent possess
geological evidence to show that a zone of this type formerly existed,
but that it is simply inactive today.) Most of these belts disclose a
major fault system that runs parallel to the general mountain system. At
long time intervals, the movement on the fault changes from gradual to
abrupt, and a shift of about 1 to 5 m (3 to 15 ft) may be produced by
just a single earthquake. Such faults are found in Chile, Alaska, Japan,
Taiwan, the Philippines, New Zealand, and Sumatra.
During
subduction, ocean crust is constantly being drawn down into the mantle
and melted. Because it is continually recycled, no part of the modern
ocean crust is more than 200 million years old. Indeed, crustal blocks
are constantly moving and jostling as they are carried by the various
plates.
An
important effect of the melting of subducted ocean crust is the
production of new magma. When subducted ocean crust melts, the magma
that forms rises upwards from the plane of subduction, deep within the
mantle, to erupt on the earth's surface. Eruption of magma melted by
subduction has created long, arc-shaped chains of volcanic islands, such
as Japan, the Philippines, and the Aleutians. Where an oceanic tectonic
plate is subducted beneath continental crust, the magma produced by
subductive melting erupts from volcanoes situated among long, linear
mountain chains, such as the Cordillera, up to 100 km inland from the
zone of subduction. (The zone itself is located along a submarine trench
offshore of the continent.) In addition to creating and feeding
continental volcanoes, melting of subducted ocean crust is responsible
for the formation of certain kinds of ore deposits of valuable metallic
minerals.
Integrated
Plate-Tectonics Theory. With
this knowledge of seafloor spreading and subduction zones, all that
remained was for the ideas to be combined into an integrated system of
geodynamics. In the 1950s, the Canadian geophysicist J. Tuzo Wilson
demonstrated the global continuity of the subduction zones, rather like
the stitching on a football. The American geologist Harry Hammond Hess
argued that if the ocean floor were rifted apart in one part of the
globe, the expansion that would result there had to be accommodated by
subduction in another part; otherwise the earth would grow larger and
larger. Xavier LePichon, a French student of seismology at Lamont,
worked out the geometry of the plates from seismic evidence, and the
American geophysicist Robert Sinclair Dietz took Wegener's evidence of
continental drift and reconstructed the positions of the continents and
oceanic plates in successive stages back in time to about 200 million
years ago. Since then, the theory of plate tectonics has been debated,
tested, and expanded and has become both a paradigm and a centre of
controversy for the geological sciences.
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