Introduction
Plate tectonics, a theory
of the dynamics of the Earth's outer shell, the lithosphere, rests
on geologic and geophysical data, and dominates current thinking
in the Earth sciences. The Plate tectonic theory suggests that
the lithosphere consists of about a dozen large plates and several
small ones, each moving in a predetermined manner. These moving
plates interact at their boundaries, where they diverge, converge,
or slip relatively harmlessly past one another, Figure
13. These interactions are responsible for most of the
seismic and volcanic activity of the earth, although earthquakes
and volcanoes do occur in plate interiors. The moving plates cause
mountains to rise where they push together, and continents to
fracture and oceans to form where they pull apart. The continents
sitting passively on the backs of plates drift with them and thereby
bring about continual changes in the earth's geography.
Figure
13 Moving Plates
Reprinted by permission
from "Trenches of the Pacific," by Robert L. Fisher.
Copyright 1972, Scientific American, Inc.
The theory of plate tectonics formulated
during the late 1960s is now almost universally accepted and has
had a major impact on the development of the earth sciences. Its
adoption represents a true scientific revolution, analogous in
its consequences to the Bohr atomic models in physics or the discovery
of the genetic code in biology. Incorporating the much older idea
of continental drift, the theory of plate tectonics has made the
study of the Earth more difficult by doing away with the notion
of fixed continents, but it has at the same time provided the
means of reconstructing the past geography of continents and oceans.
Historical Overview
Any major new idea in science appears to lead instantly to a search of the past for those who might once have proposed similar concepts. In the case of plate tectonics, the primary candidate is obvious: Alfred Wegener of Germany who explicitly presented the concept of continental drift for the first time at the outset of the 20th century. 69 Though plate tectonics is by no means synonymous with continental drift, it encompasses this idea and derives much of its impact from it. The theory of plate tectonics has revolutionized much of the thinking by Earth Scientists since the late 1960s and early 1970s. It has served as a unifying model for explaining geologic phenomena that were formerly considered in unrelated fashion. Plate tectonics describes seismic activity, volcanism, mountain building, and various other earth processes in terms of the structure and mechanical behavior of a small number of enormous rigid plates thought to constitute the outer part of the planet, the lithosphere. This all-encompassing theory grew out of observations and ideas about continental drift and seafloor spreading. Most of the considerations in Chapter 2 on the Continental Drift can be applied to the theory of Plate Tectonics.
In the early 1960s studies of the ocean floor provided insight to explain the continental drift and plate tectonic theories. First, the American geophysicists Harry H. Hess 70 and Robert S. Dietz 71 suggested that new ocean crust was formed along mid-oceanic ridges between separating continents. Second, Drummond H. Matthews 72 and Frederick J. Vine 73 of Britain proposed that the new oceanic crust acted like a magnetic tape recorder insofar as magnetic anomaly strips parallel to the age had been magnetized alternately in normal and reversed order, reflecting the changes in polarity of the earth's magnetic field. This theory of seafloor spreading was tested by the major advances in deep-water drilling technology. The Mohole Project was initiated in the late 1950s to test the feasibility of tapping the thermal energy of the earth's interior. The project was designed to drill through 18,000 feet of crust below 14,000 feet of water off the coast of South America, Figure 14.
Figure 14 Mohole Project
According to the literature, the project was canceled for "money and political reasons." 74 However, the main reason Mohole was terminated was because the project was proved not feasible in this location. As the South American plate moved toward the west, the ocean floor was being subducted, Figure 14. As the drilling proceeded past point A, it encountered a movement to the west in thesubsurface, but as it passed point B the movement of the subsurface was toward the east. This resulted in continual binding of the drill stem and required redrilling and finally abandonment of the project. The Joint Oceanographic Institutions Deep Earth Sampling (JOIDES) project began in 1969, continued with the Deep Sea Drilling Project (DSDP) and since 1976 with the International Phase of Ocean Drilling (IPOD) project. 75 These projects have produced more than 500 boreholes in the floor of the world's oceans, and the results have been as outstanding as the plate-tectonic theory itself. They confirm that everywhere the oceanic crust is younger than about 200,000,000 years and that the stratigraphic age determined by micropaleontology of the overlying oceanic sediments is close to the age of the oceanic crust calculated from the magnetic anomalies.
The plate-tectonic
theory which embraces both the continental drift and seafloor
spreading was formulated in the mid-1960s by the Canadian geologist
J. Tuzo Wilson, 76 who described the
network of mid-oceanic ridges, transform faults, and subduction
zones as boundaries separating an evolving mosaic of enormous
plates. He also proposed the idea of the opening and closing of
oceans and eventual production of an orogenic belt by the collision
of two continents.
Up to this point no one had considered in any detail the implications
of the plate-tectonic theory for the evolution of continental
orogenic belts; most thought had been devoted to the oceans. In
1969 John Dewey 77 of the University
of Cambridge outlined an analysis of the Caledonian-Appalachian
orogenic belts in terms of a complete plate-tectonic cycle of
events, and this provided a model for the interpretation of other
pre-Mesozoic (Paleozoic and Precambrian) belts.
In 1968, W. Jason
Morgan 78 introduced the concept of
plate tectonics in which the earth's crust is considered to be
divided into a series of rigid plates bounded by mid- oceanic
ridges, oceanic trenches, great faults, and active fold belts.
According to this theory the movements of the continents and the
sea-floor spreading are part of large-scale movements of plates
and are not the "cause-effect" as proposed by other
scientists.
Gestation and Birth of the Plate Tectonics
Theory
When Wegener
developed his idea relatively little was known regarding the nature
of the ocean floor. After World War II, however, rapid advances
were made in the study of the relief, geology, and geophysics
of the ocean basins. Due in large part to the efforts of Bruce
C. Heezen and Henry W. Menard 79 of
the United States, these features which constitute more than two-thirds
of the earth's surface became well enough known to permit serious
geologic analysis. Several major topographic and tectonic features
distinguish the ocean basins from the continents. The first of
these is the mid-ocean ridge system. Mid-ocean ridges are broad,
elongated elevations of the ocean floor rising to about 1.5 or
2 miles below sea level with widths ranging from a few hundred
to more than 500 miles.
Figure
15 Trenches in the Pacific
Reproduced from
World Physical/Ocean Floor Map, by permission from National Geographic
Society.
Their crests tend to be rugged and are often endowed with a longitudinal rift valley where fresh lava flows, high heat flows, and shallow earthquakes of the extensional type are found. Mid-ocean ridges nearly girdle the globe. Trenches constitute another type of seafloor feature. In contrast to mid-ocean ridges, they are long, narrow depressions containing the greatest depths of the ocean basins. Trenches virtually ring the Pacific; a few also occur in the northeastern part of India, and some small ones are found in the Atlantic, Figure 15, but elsewhere they are absent. Trenches have low heat flow, are often filled with thick sediments, and lie at the upper edge of the Benioffzone of compressive earthquakes. Trenches border continents, as in the case of western Central and South America, but they also may occur in mid-ocean, as, for example, in the southwestern Pacific.
Mid-ocean ridges and, more rarely, trenches are offset by fracture zones, Figure 16. These are transverse features consisting of linear ridges and troughs approximately perpendicular to and offset by a few to several hundred miles from the ridge crest. Fracture zones often extend over long distances in the ocean basins but generally end abruptly against continental margins. They are not volcanic, and their seismic activity is restricted to the area between offset ridge crests where earthquakes indicating horizontal slips are common.
Figure
16 Formation of Trenches from Subduction
Reprinted, by permission
from Encyclopaedia Britannica, Inc.
Plate Motion.
The movement of a plate across the surface of the Earth can be
described as a rotation around a pole, and it may be rigorously
described with the theorem of spherical geometry formulated by
the Swiss mathematician Leonhard Euler during the 18th century.
Similarly, the motions of two plates with respect to each other
may be described as rotations around a common pole, provided that
the plates retain their shape, Figure 17.
Figure
17 Rotation of Plates Around a Pole
Reprinted, by permission
from Encyclopaedia Britannica, Inc.
The requirement that plates are not internally deformed has become one of the postulates of plate tectonics. It is not totally supported by evidence, but it appears to be a reasonable approximation of what actually happens in most cases and is needed to permit the mathematical reconstruction of past plate configurations. The joint pole of rotation of two plates can be determined from their transform boundaries and from their divergent plate boundaries usually by means of magnetic anomalies. Because all plates form a closed system, all movements can be defined by dealing with them two at a time. It is conceivable that the entire lithosphere might slide around over the asthenosphere like a loose skin, altering the positions of all plates with respect to the spin axis of the earth and the equator. To determine the true geographic positions of the plates in the past which is so important in paleoclimatology and paleoceanography, investigators have to define their relative motions not to each other but rather to this independent frame of reference. The hot spot island chains serve this purpose; their trends provide the direction of motion of a plate. The speed of the plate can be inferred from the increase in age of the volcanoes along the chain. 80 It is assumed that the hot spots themselves remain fixed with respect to the earth, an assumption that appears to be reasonably accurate for at least some hot spots.
Another method of determining absolute plate movements relies on the fact that the equatorial waters of the ocean are and always have been very fertile. The high biological productivity yields an enormous quantity of calcareolls microfossils, which like a gigantic natural chalk line, marks a narrow equatorial zone. The displacement of the equatorial deposits over time, traced by means of deep-sea drill cores, enables investigators to determine the direction and rate of plate movement. The development of the satellite and the Global Positioning Systems have enabled scientists to measure with some degree of accuracy these plate movements, and these data are also used to determine by extrapolation the past positions of the plates. The GPS measurements are most cost effective and yield greater quantities of reliable data. These GPS data were not available until this decade.
Because the plates all interlock,
any change in motion anywhere must reverberate throughout the
entire system. If two continents collide, their edges will crumple
and shorten, but eventually all motion must stop at this boundary
and adjustments will be required in other parts of the system.
Earth scientists are thus able to reconstruct the positions and
movements of plates in the past so long as they have the ancient
oceanic crust to provide them with plate speeds and directions,
and these data can be verified by satellite measurements. Since
old oceanic crust is continuously consumed to make room for new
crust, this kind of evidence is eventually exhausted. The oceanic
crust is younger than the Jurassic, the geologic age that began
approximately 208 million years ago, and this method fails to
define the history of drifting continents during earlier geologic
periods.
Early Plate Activity.
Whatever the forces may be that drive the plates, they consume
energy. It is postulated that by far the largest part of this
energy is derived from the decay of radioactive isotopes within
the earth, and the energy flow has therefore declined through
the 4.5 billion years of the earth's history -- rapidly at first
and then at a slowly diminishing though not a negligible rate.
Accordingly, it is quite likely that the behavior of the lithospheric
plates on the early, more energetic earth was different from what
it is today, and what prevails at present will certainly differ
from what will prevail in the future. A thickening lithosphere,
a decreasing heat flow, a temperature gradient that decreases
with depth within the earth, and enlarging convection cells in
the mantle have all been postulated as unidirectional changes
that affected the behavior of the lithosphere. Any of these events
or some combination may be the driving force of the plate movement.
But they could also be the results of such movement and not the
driving force. It is possible that the initial plates were too
small, too hot, and hence too light to be subducted. In this case,
the first subduction would mark the coming of age of classical
plate tectonics, and, indeed, clear evidence is lacking for subduction
until rather late in the Precambrian period. The evidence that
bears on the existence, nature, and movements of the plates during
the first several billion years of earth history are very limited.
The continental nuclei of the early and middle Precambrian seem
to have been small plates on a more vigorously convecting mantle,
though admittedly other explanations are equally possible. These
nuclei are thought to have been embedded in strongly deformed
complexes of sediments and basic igneous rocks. However, in most
cases, paleomagnetic data do not leave room for the existence
of sizable oceanic areas between such nuclei. Investigators are
thus forced to contemplate the possibility that in the Precambrian
Period intensive deformation took place within plates, because
of higher flow of heat toward the surface. On the other hand,
current knowledge of this long but obscure portion of earth history
is so deficient that some geologists have emphatically denied
that there might have been a remote past to which classical plate
tectonics could not be applied.
Plate Boundaries.
As conceived by the theory of plate tectonics, the lithospheric plates are much thicker than the oceanic or the continental crust; their boundaries do not usually coincide with those between oceans and continents; and their behavior is only partly influenced by whether they carry oceans, continents, or both. For example the Pacific Plate is purely oceanic, but most of the others contain continents, as shown in Figure 18.
Figure
18 World Wide Plate System
Reprinted by permission
from Encyclopaedia Britannica, Inc.
At a divergent plate boundary, magma wells up from below as the release of pressure produces partial melting of the underlying mantle and generates new crust and, because the partial melt is basaltic in composition, the new crust is oceanic. Diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making. Most divergent plate boundaries are found within continents rather than in oceans because a weak layer is sandwiched between two stronger ones and this renders the continental crust more vulnerable to fragmentation than its oceanic counterpart. This may be the reason for the basin type areas in North America between the Appalachian and Rocky mountains and the desert area between the Rockies and the Sierras in the plains states which would enhance the possibility of this idea, Figure 19.
Figure
19 Basins within the Continental U.S.
Reproduced from
World Physical/Ocean Floor Map,
by permission from the National Geographic Society.
The presence of the Great Salt Lake and the Salton Sea would indicate that at one time there may have been an ocean between the Rockies and the Sierra Nevada Mountains. The creation of the new crust is normally restricted to ocean areas and accompanied by much volcanic activity and by many shallow tension earthquakes as the crust repeatedly rifts, heals, and rifts again. . The continuous formation of new crust produces an excess that must be disposed of elsewhere. This is accomplished at convergent plate boundaries where one plate descends -- i.e., is subducted -- beneath the other. At depths between 150 and 400 miles, the subducted plate melts and is recycled into the mantle. The plates form an integrated system that completely covers the surface of the earth, and the total amount of crust generated equals that destroyed. It is not necessary that new crust formed at any given divergent boundary be completely compensated at the nearest subduction zone. It is in subduction zones that the difference between plates carrying oceanic and continental crust can be most clearly seen, Figure 20.
Figure
20 Subducted Plates
Reprinted by permisssion,
Encyclopaedia Britannica, Inc
If both plates have oceanic edges, either one may dive beneath the other; but if one carries a continent, the greater buoyancy prevents this edge from sinking. Thus it is invariably the oceanic plate that is subducted. Continents are permanently preserved in this manner, while the ocean floor continuously renews itself. If both plates possess a continental edge, neither can be subducted and a complex sequence of events from crumpling under and over thrusting raises lofty mountain ranges. Much later, after these ranges have been largely leveled by erosion, their remains continue as a reminder that this is the "suture" where continents were once fused. The subduction process which involves the descent into the mantle of a slab of cold rock about 60 miles thick is marked by numerous earthquakes along a plane inclined 30 to 60 degrees into the mantle -- the Benioffzone. Most earthquakes in this planar dipping zone result from compression, and the seismic activity extends 150-400 miles below the surface. At a depth of 60 miles or more, the subducted oceanic sediments together with part of the upper basaltic crust melt to an andesitic magma which rises to the surface and gives birth to a line of volcanoes a few hundred kilometers behind the subducting boundary, Figure 20. This boundary is usually marked by an oceanic deep, or trench, where the overriding plate scrapes off the upper crust of the lower plate to create a zone of highly deformed, largely sedimentary rock. If both plates are oceanic, the deformed sediments and volcanoes form two island arcs parallel to the trench. If one plate is continental, the sediments are usually accreted against the continental margin and the volcanoes form inland, as they do in Mexico or western South America.
Along the third type of plate boundary, Figure 21, two plates move laterally and pass each other without creating or destroying crust. Large earthquakes are common along such strike-slip, or transform, boundaries. Also known as fracture zones, these plate boundaries are perhaps best exemplified by the San Andreas fault in California and the North Anatolian fault system in Turkey.
Figure
21 Types of Faults
Reprinted by permission
from Encyclopaedia Britannica, Inc.
Most of the seismic and volcanic activity on Earth is therefore concentrated along plate boundaries where mid-ocean ridges, trenches with island arcs, and mountain ranges are generated. Some seismic and volcanic activity also occurs within plates, as shown in Figure 22.
Figure
22 Distribution of Earthquakes along the Plate Boundaries
Reprinted by permission
from Encyclopaedia Britannica, Inc.
Interesting examples
of this interplate activity are linear volcanic chains in ocean
basins such as the Hawaiian Islands and their westward continuation
as a string of reefs and submerged sea mounts. An active volcano
usually exists at one end of an island chain of this type with
progressively older extinct volcanoes occurring along the rest
of the chain. Such topographic features have been explained by
J. Tuzo Wilson 81 of Canada and W.
Jason Morgan 82 of the United States
as the product of "hot spots," magma generating centers
of controversial origin located deep in the mantle far below the
lithosphere. 83 A volcano builds at
the surface of a plate positioned above a hot spot. As the plate
moves, the volcano dies, is eroded, and eventually sinks below
the surface of the sea, and a new volcano forms above the hot
spot. Hot spot volcanism is not restricted to the ocean basins;
other manifestations occur within continents, as in the case of
Yellowstone National Park in western North America.
Plate Tectonics and Mountain Building.
The "accepted"
methodology for raising mountain ranges is subduction and continental
collisions raise mountain ranges. The implications of plate tectonics
for the processes of mountain building have attracted much attention.
One of the earliest to apply the new theory was Cambridge geologist
John Dewey, 84 who analyzed the Appalachian
and Alpine orogenies. Many other researchers have subsequently
undertaken similar work in the Mediterranean system and the American
Cordilleran ranges, as well as in the Appalachians. 85
Toward
a Unifying Theory
Working
independently but along very similar lines, Dan P. McKenzie and
Robert L. Parker 86 of Britain and
W. Jason Morgan 87 of the United States
resolved these issues. McKenzie and Parker showed with a geometric
analysis that if the moving slabs of crust were thick enough to
be regarded as rigid and thus to remain undeformed, their motions
on a sphere would lead precisely to those divergent, convergent,
and transform boundaries that are indeed observed. Morgan demonstrated
that the directions and rates of movement had been faithfully
recorded by magnetic anomaly patterns and transform faults. He
also proposed that the plates extended approximately 60 miles
to the base of a rigid lithosphere which had long been known to
be underlain by a weaker asthenosphere marked by strong attenuation
of earthquake waves. In
1968 the French geophysicist Xavier Le Pichon refined these propositions
with a computer analysis of all plate data and proved that they
did indeed form an integrated system where the sum of all crust
generated at mid-ocean ridges is balanced by the cumulative amount
destroyed in all subduction zones, Figure 20. 88
That same year the American geophysicists Bryan Isacks, 89 Jack Oliver, 90
and Lynn R. Sykes 91 showed that the
theory, which they called the "new global tectonics,"
was capable of accounting for the larger part of the earth's seismic
activity. Almost immediately others began to consider seriously
the ability of the theory to explain mountain building and sea-
level changes. Only a few years later, details of the processes
of plate movement and of boundaries interactions along with much
of the plate history of the Cenozoic era (the past 66.4 million
years) had been worked out. Yet, the driving forces -- not withstanding
a brief flurry of discussion around 1970 -- remained mysterious
and continue as such. The vast accumulation of data bearing on
plate history and plate processes has yielded surprisingly little
information about what happens beneath them. Pull by the subducting
slab, push at the spreading ridge, convection in the asthenosphere,
and even tidal forces have been considered, but in every case
the evidence has been admitted as inconclusive. Many favor convection,
but if this indeed is the driving force the flow pattern at depth
is clearly not reflected in the surface movements of the plates,
constrained as they are by each other.
Evidence Supporting the Hypothesis
The plate movement analyses also indicate that the continents
were joined together in the Paleozoic era, and supporting evidence
is continuing to accumulate. The opposing Atlantic shores match
well, especially at the 3,300-foot depth contour, which is a better
approximation of the edge of the continental block than the present
shoreline, as Sir Edward Bullard demonstrated in 1964 with the
aid of computer analysis, Figure 23.
Figure
23 Computer Matched Coastlines
Reprinted by permission
from Encyclopaedia Britannica, Inc.
Similarly, the structures and stratigraphic sequences of Paleozoic mountain ranges in eastern North America and northwestern Europe can be matched in detail. This fact was already known to Wegener and has been strengthened substantially in subsequent years. Often cited as evidence have been the strikingly similar Paleozoic sequences on all southern continents and also in India. This Gondwana sequences, so called after one of Suess's large continents, consists of glacial tillites, followed by sandstones and finally coal measures. Placed on a reconstruction of Gondwana, the tillites mark two ice ages that occurred during the long march of this continent across the South Pole, from its initial position north of Libya about 500 million years ago until its final departure from southern Australia 250 million years later, Figure 24.
Figure
24 Gondwana Glacial Areas
Reprinted by permission
from Encyclopaedia Britannica,Inc.
The first of these ice ages left its
glacial deposits in the southern Sahara during the Silurian period
which extended from about 438 to 408 million years ago, and the
second ice age did the same in southern South America, South Africa,
India, and Australia from 380 to 250 million years ago. At each
location the tillites were subsequently covered by desert sands
of the subtropics, and these in turn by coal measures, indicating
that the region had arrived near the equator.
During the 1950s
and 1960s, patient work in isotopic dating showed that the massifs
of Precambrian time found on opposite sides of the South Atlantic
did indeed closely correspond in age and composition, and they
probably originated as a single continent, Pangaea. 92
Plate Tectonics and Life.
Inevitably the continuous rearrangements over time of the size
and shape of ocean basins and continents followed by changes in
ocean circulation and climate, have had a major impact on the
development of life on earth. Active
interest in these aspects of the earth science revolution has
lagged behind that in other areas, even though as early as 1970
the American geologists James W. Valentine 93
and Eldridge M. Moores 94 attempted
to show that the diversity of life increased as continents fragmented
and dispersed and diminished when they were joined together.
The study of plate activity as a force in the evolution of life
is based on the land bridges and continent collisions. 95 Toward the end of the Paleozoic, during
the Permian period about 286 to 245 million years ago, there was
a drastic drop in the variety of animal forms inhabiting the shallow
seas around Pangaea. 96 Well over
half of the total number of known families became extinct. This
drop is attributed to the decrease in biogeographic variety that
marks a world consisting of a single continent rather than one
comprising many widely dispersed land masses. Other factors, such
as a sharp decrease in the area of shallow-water habitats or a
change in ocean fertility due to upwelling, have also been invoked.
Moreover, the extinction had a complex history. The latitudes
were affected first as a result of the ending of the Permian ice
age when the South Pole slipped beyond the southern edge of Pangaea.
The equatorial and subtropical zones appear to have been affected
somewhat later by global cooling; the extinctions were not felt
so strongly on the continent itself. Instead the vast semiarid
and arid lands that emerged on so large a continent, the shortening
of its moist coasts, and the many mountain ranges remaining from
the collisions that led to the formation of the super continent
provided strong incentives for evolutionary adaption to dry or
high-altitude environments.
The impact of plate movements and
interactions on life is perhaps most clearly demonstrated by what
happens when continents diverge or collide. During the middle
Mesozoic period, when the Atlantic Ocean began to open, the similarity
between the faunas of opposite shores gradually decreased in almost
linear fashion -- the greater the distance, the smaller the number
of families in common. The difference increased more rapidly in
the South Atlantic than in the North Atlantic, where a land connection
between Europe and North America persisted until well after the
middle Cenozoic. The inverse, the effect of a collision between
two hitherto separate land masses, is illustrated by the consequences
of the Pliocene emergence of the Isthmus of Panama. In South America
a highly specialized fauna had evolved, rich in marsupials but
with few predators. After the emergence of the isthmus had made
it possible for land animals to cross, numerous herbivores migrated
from north to south. They adapted well to the new environment
and were more successful than the local fauna in competing for
food. The invasion of highly adaptable carnivores from the north
contributed to the extinction of no fewer than four orders of
South American land mammals. Only a few species, notably the armadillo
and the opossum, managed to migrate in the opposite direction.
Ironically, many of the invading northerners, such as the llama
and tapir, subsequently became extinct in their country of origin
and found their last refuge in the south.
DISSENTING
OPINIONS AND UNANSWERED QUESTIONS
The Dissenters.
Scientific revolutions as far-reaching in their consequences as
the plate tectonics revolution cannot be expected to be accepted
easily. Nevertheless, once the theory had fully emerged acceptance
was quick and widespread, and by the late 1960s its influence
in the West was pervasive. Such was not the case in the Soviet
Union, a country located largely in the continental interior far
from present-day plate boundaries. As a central issue to global
tectonics, Soviet scientists viewed the vertical movements of
continental interiors, phenomena not satisfactorily considered
by the plate tectonics theory. A leading spokesman for the Soviet
position, the academician Vladimir Vladimirovich Belousov, strongly
defended a model of the earth that postulated stationary continents
affected almost exclusively by vertical motions. The model, however,
only vaguely defined the forces supposedly responsible for the
motions. In recent years, a younger generation of Soviet geologists
has very gradually come to regard plate tectonics as an attractive
theory and a viable alternative to the concepts of Belousov and
his followers.
Opposition to plate
tectonics was by no means limited to the Soviet Union. Critics
were heard elsewhere as well. Sir Harold Jeffreys 97
continued his lifelong rejection of continental drift on grounds
that his estimates of the properties of the mantle indicated the
impossibility of plate movements. He did not take note of the
mounting geophysical and geologic arguments that were in favor
of a mobile outer shell of the earth. Others proffered different
explanations of the accumulated evidence, like the suggestion
that new crust was formed at trenches and destroyed on mid-ocean
ridges.
The American geologists A. A. Meyerhoff and Howard A. Meyerhoff,
98 attempted to assemble data that
contradicted the theory and thereby show that the supporting evidence
was wrong, insufficient, or simply misconstrued. Demonstrating
a remarkable command of often quite obscure literature, they issued
a series of negative commentaries in the early 1970s, but they
failed to convince the majority of their colleagues, partly because
they did not offer alternative explanations for the evidence.
The only serious
alternative had been proposed in l958 by the Australian geologist
S. Warren Carey 99 in the form of
a new version of an old idea of the expanding earth model. 100 Carey accepted the existence and early
Mesozoic breakup of Pangaea and the subsequent dispersal of its
fragments and formation of new ocean basins, but he attributed
it all to the expansion of the earth, the planet presumably having
had a much smaller diameter in the late Paleozoic. In his view,
the continents represented the pre-expansion crust, and the enlarged
surface was to be accommodated entirely within the oceans. This
model accounted for a spreading ocean floor and for the young
age of the oceanic crust; however, it failed to deal adequately
with the evidence for subduction and compression. Carey's model
also did not explain why the process should not have started until
some four billion years after the earth was formed, and it lacked
a reasonable mechanism for so large an expansion. Finally, it
disregarded the evidence for continental drift before the existence
of Pangaea.
Unanswered Questions.
As the
philosopher Thomas J. Kuhn 101 has
pointed out, science does not always advance in the gradual and
stately fashion commonly attributed to it. Major breakthroughs
often come from a leap forward that is at least in part intuitive
and may fly in the face of conventional wisdom and widely accepted
evidence while strict requirements for verification and proof
are temporarily relaxed. Revolutions thus often become widely
accepted before the verdict from rigorous analysis of evidence
is complete. Such was certainly the case with the geologic revolution
which also confirms Kuhn's view that a new paradigm is unlikely
to supersede an existing one until there is little choice but
to acknowledge that the conventional theory has failed. Thus,
while Wegener did not manage to persuade the world, his theory
was readily embraced 40 years later, even though it remained open
to much of the same criticism that had caused the downfall of
continental drift.
In 1974, almost alone among the doubters who tried to discredit the new theory with contrary evidence, the American geologist John C. Maxwell, in a closely reasoned paper enumerated all the points on which he believed plate tectonics had failed to offer an explanation. Many of these points have since been resolved, but more than a few remain to suggest that the theory, though in essence valid, may be incomplete.
The greatest successes of plate tectonics
have been achieved in the ocean basins where additional decades
of effort have confirmed its postulates and enabled investigators
to construct a credible history of past plate movements.
Inevitably in less rigorous form the reconstruction of early,
Mesozoic and Paleozoic continental configurations have provided
a powerful tool with which to resolve many important questions. On the other hand, the
new paradigm has proved less useful in deciphering mountain-building
processes or in offering explanations for the complex history
of sea-level fluctuations. The American geologist L.L. Sloss 102 has devoted a great deal of effort to
demonstrating that continents do indeed rise and fall in unison,
but the possible mechanisms for such a process remain elusive.
Where plate boundaries adjoin continents, matters often become
very complex and have demanded an ever denser thicket of ad hoc
modifications and amendments to the theory and practice of plate
tectonics in the form of microplates, obscure plate boundaries,
and exotic terrains. A good example is the Mediterranean where
the collisions between Africa and a swarm of microcontinents have
produced a tectonic nightmare that is far from resolved. More
disturbingly, some of the present plate boundaries especially
in the eastern Mediterranean appear to be so diffuse and so anomalous
that they cannot be compared to the three types of plate boundaries
of the basic theory.
There is further evidence held by the American geophysicist Thomas H. Jordan 103 that the base of the plates extends far deeper into the asthenosphere below the continents than below the oceans. How much of an impediment this might be for the free movement of plates and how it might affect their boundary interactions remain open questions. Others have postulated that the lower layer of the lithosphere peels off and sinks late in any collision sequence producing high heat flow, volcanism, and an upper lithospheric zone vulnerable to contraction by thrusting.
It is understandable that any simple
global tectonic model would work better in new oceans, which being
young retain a record of only a brief and relatively uneventful
history. On the continent, almost four billion years of growth
and deformation, erosion, sedimentation, and igneous intrusion
have produced a complex imprint that, with its intricate zones
of varying strength must directly affect the application of plate
forces. Seismic reflection studies of the deep structure of the
continents have demonstrated just how complex the events that
form the continents and their margins may have been, and their
findings sometimes are difficult to reconcile with the accretionary
structures one would expect to see as a result of subduction and
collision. Notwithstanding these cautions and the continuing lack
of an agreed-upon driving mechanism for the plates, one cannot
help but conclude that the plate tectonics revolution has been
fruitful and has immensely advanced scientific understanding of
the earth. Like all paradigms in science, it will probably be
replaced by a better one; yet there can be little doubt that whatever
the new theory may state, continental drift will be part of it.
Conclusions on Plate Tectonic Theory
General Conclusions
The Plate Tectonic theory does describe the observed occurrences
in the movement of the continents, but the theory is in need of
constant revision to account for the many variances. These variances
require the addition of microplates to the base theory and the
trend is more toward the Continental Drift theory than a clean
separation. The Plate Tectonic theory does not develop or explain
an original plate or continent position. The adherents to the
Plate theory have generally accepted the idea of Pangaea and Laurasia
- Gondwana without proposing any additional detailed information.
The acceptance without question of the age of the earth in terms
of billions of years is a basic presupposition of the theory.
The presupposition that the center of movement of the continents
from the Pangaea was the southern tip of Africa is neither questioned
nor supported by the Plate Theory. The assumption is based on
the evidence of similar glacial deposits in the south pole and
adjacent continents, see Figure 19. However, similar data
in Europe and North America are not considered. If it were taken
into account and given the same weight as the other information
the center of Movement conclusion would not have been acceptable
as a plausible explanation.
Advantages
1) The theory does adequately describe the events
at the plate boundaries as observed in modern time, particularly
localized movements and events.
2) It provides an acceptable explanation of
the occurrences of volcanos and earthquakes.
3) The modern scientific investigations in the
area of tectonics are valid and descriptive of current activities.
4) The theory provides a basis for analyses of
current events and the projection of future events along plate
boundaries.
Disadvantages
1) It does not provide an acceptable explanation
of the start of the continental movement or the initial plate
boundaries.
2) The theory assumes the age of the earth to
be billions of years.
3) The time frame for the continent movements
is dependent on the definitions of the various periods in the
history of the earth. The definitions of these time periods are
based on fossil evidence and strata definitions which are a circular
definition.
4) The promoters of the Plate theory do not
utilize all the available data.
5) The explanation of mountain building by the
initial formation of Pangaea is not feasible nor does it reflect
or explain current observations.
The explanation of the connection of
North and South America is highly improbable.
© 1997, 1998, Aaron C Ministries
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