Figure 4 Pangaea
Reprinted from "The Breakup of Pangaea," by R. S. Dietz, By permission, copyright 1970 Scientific American, Inc.
The idea of a large scale movement and displacement of continents has a long history. About 1800 the German naturalist Alexander von Humboldt, because of the apparent fit of the bulge of eastern South America into the bight of Africa, theorized that the lands bordering the Atlantic Ocean had once been joined. Some 50 years later Antonio Snider-Pellegrini, a French scientist, argued that the presence of identical fossils plants in both North American and European coal deposits could indicate the two continents were formerly connected.
The idea of a large ancient continent, composed of several of the present-day smaller ones, had been put forth in the late 19th century by the Austrian geologist Edward Suess. 38 Suess, however, was not thinking of continental drift. He assumed those portions of a single enormous southern continent, designated Gondwana or Gondwanaland, foundered to become the Atlantic and Indian oceans. Sinking continents and vanishing land bridges were frequently invoked in the late 1800s to explain sediment sources apparently present in the ocean and to account for similar floral and faunal connections between continents. This idea remained popular until the 1950s and stimulated people to believe in ancient Atlantis. The idea even made its way into literary works.
In 1908 Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's mountain ranges. The first truly detailed and comprehensive theory of continental drift was proposed in 1912 by Alfred Wegener, 39 a German meteorologist, and was published as "Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans)." Wegener introduced his continental drift proposal by pointing out that the concept of isostasy rendered large sunken continental blocks geophysically impossible. He concluded that if the continents had been once joined together, drifting of their fragments rather than their foundering would have been more probable. The assumption of a former single continent could be tested geologically, and Wegener displayed a large array of data to convince the scientific community. Even today his evidence, ranging from the continuity of fold belts across oceans and similarities of sequences of strata on their opposite sides to paleobiogeographic and paleoclimatological arguments, would be judged worthy of serious consideration. He argued that, if continents could move up and down in the mantle as a result of buoyancy changes produced by erosion or deposition, they should be able to move horizontally as well. The driving forces he considered, however, were unconvincing: both pole fleeing and the westward tidal force appeared to most to be entirely inadequate.
Wegener's proposition was attentively
received by many European geologists, and in England Arthur Holmes
pointed out that the lack of a driving force was hardly sufficient
grounds to scuttle the entire concept. As early as 1929, Holmes
proposed an alternative mechanism, convection of the mantle, which
remains today a serious candidate for the force driving the plates.
Wegener's ideas also were appreciated by geologists in the Southern
Hemisphere. The South African, Alexander Du Toit, remained a lifelong
believer. After Wegener's death, Du Toit continued to amass further
evidence in support of continental drift. Like certain other scientists
before him, Wegener became impressed with the physical match in
the coastlines of eastern South America and western Africa and
he speculated that those lands had once been joined together.
Pangaea
Figure
4 Pangaea
Reprinted from "The
Breakup of Pangaea," by R. S. Dietz, By permission, copyright
1970 Scientific American, Inc.
In about 1910 he proposed that in
the Late Paleozoic era (about 240 million years ago) all the present-day
continents were connected, and the continents formed a single
large mass, or super continent, which had subsequently broken
apart. Wegener postulated that throughout most of geologic time
there was only one continent which he called Pangaea (from Greek
pangaia, "all earth"), Figure 4. According
to this theory, Pangaea was composed of continental sial (granitic
rock), which was balanced isostatically in a layer of denser material
(basalt), called sima, constituting the uppermost portion of the
Earth's mantle. The protocontinent supposedly covered about half
the Earth and was completely surrounded by a world ocean called
Panthalassa.
Laurasia- Gondwana
Figure
5 Laurasia - Gondwana
Reprinted, by permission
from "The Breakup of Pangaea," by R. S. Dietz. Copyright
1970 Scientific American, Inc.
Alexander L. Du Toit, 40 a South African geologist, modified Wegener's hypothesis by suggesting two primordial continents: Laurasia in the north and Gondwana in the south. Gondwana, also called Gondwanaland, and the hypothetical former super continent in the Southern Hemisphere, included South America, Africa, peninsular India, Australia, and Antarctica, Figure 5. The name was coined by the Austrian geologist Edward Suess 41 in reference to the Upper Paleozoic and Mesozoic formations of the Gondwana region of central India, which display typical developments of some of the shared geologic features. Late in the Triassic Period, which lasted from 245 to 208 million years ago, Pangaea fragmented and the parts began to move away from one another.
The westward drift of the Americas opened the Atlantic Ocean, Figure 6.
Figure
6 Laurasia Breakup - India Moved
Reprinted by permission
from "The Breakup of Pangaea," by R. S. Dietz. Copyright
1970 Scientific American, Inc.
India supposedly drifted to the North
(as Shown in Figure 6), crossed the equator and later collided
with Asia. Its segments, Laurasia (composed of all the present-day
northern continents) and Gondwana (all of the present southern
continents) gradually receded, resulting in the formation of the
Atlantic Ocean. By contrast, Wegener proposed that Pangaea's constituent
portions had slowly moved thousands of miles apart over long periods
of geologic time. His term for this movement was Die Verschiebung
der Kontinente ("continental displacement"), which gave
rise to the term continental drift. Wegener found data in the
scientific literature for both geological and paleontological
evidence that supported his theory. These were closely related
fossil organisms and similar rock strata that occurred on widely
separated continents, particularly in South America and in Africa.
Wegener's theory of continental drift did receive some support,
but his speculations on the driving forces behind the continents'
movement were not generally accepted. By 1930 his theory had been
rejected by most geologists, and it sank into obscurity for the
next few decades, only to be resurrected as part of the theory
of plate tectonics during the 1960s.
The breakup of Pangaea is now explained in terms of plate tectonics.
This theory states that the earth's outer shell, or lithosphere,
consists of large rigid plates, which move relative to each other
and interact at their margins, where they diverge, converge, or
slip past one another (See Chapter Three, Plate Tectonics).
Pangaea split apart at one of the divergent plate boundaries,
and a rift developed beneath the continent. As the two segments
of the continent pulled farther apart, molten rock material from
the asthenosphere, the layer underlying the lithosphere, flowed
upward to fill the void, creating the floor of the new Atlantic
Ocean basin. Other scientists had proposed such a continent, but
they had explained the separation of the modern world's continents
as having resulted from the subsidence, or sinking, of large portions
of the super continent to form the Atlantic and Indian oceans.
The geologic evidence for a former land connection between the currently separated continents and other areas includes the occurrence of tillites (glacial deposits) of Permo-Carboniferous age (the time boundary between the Carboniferous and Permian periods is 286 million years ago) and similar floras and faunas that are not found in the Northern Hemisphere. The widely distributed seed fern Glossopferis is particularly cited in this regard. The rock strata that contain this evidence are called the Karroo (Karoo) System in South Africa, the Gondwana System in India, and the Santa Catharina System in South America. The sequence of layered rocks on the land masses that constituted Gondwana is strikingly similar for those time periods when the land masses are believed to have been together. In these areas, glacial deposits are overlain by coal-bearing shales and sandstones containing fossils of Glossopteris and Mesosaurus, which are in turn overlain by thick sequences of mafic (Basaltic) volcanic rocks.
The idea of Gondwana surfaced again
in the scientific community in the 1960s, when evidence of sea-floor
spreading from the loci of oceanic ridges proved that the ocean
basins are not permanent global features, and these data vindicated
Wegener's hypothesis of continental drift. Although the term Gondwanaland
or Gondwana does not appear in the modern literature with great
frequency, the concept of continental drift and former continental
connections is widely accepted.
Modern Interests in Continental Drift Theory
Aside from the congruency of continental shelf margins across the Atlantic, modern proponents of continental drift have amassed impressive geologic evidence to support their views. 42 Indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. These data have led to the assumption that these continents were adjacent at some point during this time frame. If these continents were once united around the south polar region, this glaciation would become explicable as a unified sequence of events in time and space. Also, fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks and geologic structures. Modern methods of fitting the coastlines consist of computer generated "best fit" maps of the continents at lines of various ocean depths from the actual coast line. One computer generated map proposed by the British geophysicist, E. C. Bullard 43 is shown in Figure 7.
Figure
7 Computer Continent Match
Reprinted, by permission
from Encyclopaedia Britannica, Inc.
A belt of ancient rocks along the
Brazilian coast, for example, matches one in West Africa. Moreover,
along the Atlantic coastlines of either South America or Africa,
the earliest marine deposits are Jurassic in age (208 to 144 million
years old), suggesting that the ocean did not exist before that
time. The problem with these computer generated matches is they
ignore the information regarding Central America, the Southeast
Asian countries and islands.
Paleomagnetism
The fact that some rocks are strongly magnetized has been known
for centuries, 44 and geologists recognized
more than 100 years ago that many rocks preserve the imprint of
the earth's magnetic field as it was at the time of their formation.
Volcanic rocks such as basalt are especially good recorders of
paleomagnetism, but some sediments also align their magnetic particles
with the earth's field at the time of deposition. Investigators
have at their disposal fossil compasses that indicate the direction
to the magnetic pole and that yield the latitude of their origin.
Interest in continental drift increased in the 1950s
45 as knowledge of the earth's magnetic field during the
geologic past was developed by the studies of the British geophysicists
Stanley K. Runcorn, 46 P. M. S. Blacket
47 and others.
Figure
8 Polar Wanderings - Pangaea
Reprinted, by permission
from Encyclopaedia Britannica, Inc.
Ferromagnetic minerals, such as magnetite, acquire a permanent magnetization when they crystallize as constituents of igneous rock. The direction of their magnetization is the same as the direction of the earth's magnetic field at the time and place of crystallization. Particles of magnetized minerals released from their parent igneous rocks by weathering may later realign themselves with the existing magnetic field at the time these particles are incorporated into sedimentary deposits. This must be taken into consideration if these data are used to determine the position of the magnetic poles. Studies of the remanent magnetism in suitable rocks of different ages from all over the world indicate that the magnetic poles were in different places at different times. 48 The polar wandering curves are different for the various continents, but in some instances such differences are reconciled on the assumption that continents now separated were formerly joined, Figure 8. The curves for Europe and North America 49 are reconciled by the assumption that the latter have drifted about 30 degrees westward relative to Europe since the Triassic Period (245 to 208 million years ago). 50 These paleomagnetic studies showed that in the late Paleozoic the north magnetic pole, as seen from Europe, seems to have wandered from a Precambrian position near Hawaii to its present location by way of Japan. This might mean that the magnetic pole itself had migrated or that Europe had moved relative to a fixed pole. Therefore, either continental drift or polar wandering would be a reasonable explanation. Paleomagnetic data from other continents yield apparent polar wandering paths different from the European one. Separate wanderings of many magnetic poles are not acceptable, but the paths could be closely aligned by connecting the continents as suggested by Wegener.
Runcorn was one of the first of a
new generation of geologists and geophysicists to accept the theory
of continental drift. However, most geologists found sufficient
reason to doubt the paleomagnetic results due to the conflicting
data and to the primitive nature of the early techniques. More
sophisticated modern methods are capable of removing the overprint
of later magnetization and have made paleomagnetic data strong
supporting evidence for continental drift and a major tool for
reconstructing the geography of the past.
Seafloor Spreading
Investigations of oceanic magnetic anomalies corroborate the seafloor
spreading hypothesis. These investigations show that the strength
of the geomagnetic field is alternately anomalously high and low
with increasing distance away from the axis of the midocean ridge
system.
Figure
9 Sea-Floor Spreading
Reproduced from
World Physical/Ocean Floor Map, by permission from the National
Geographic Society.
The anomalous features are nearly symmetrically arranged on both sides of the axis and parallel the axis, creating bands of parallel anomalies, as shown in Figure 9. Measurements of the thickness of marine sediments and absolute age determinations of such bottom material have provided additional evidence for seafloor spreading. The oldest sediments so far recovered by a variety of methods, including coring, dredging, and deep-sea drilling, dated only to the Jurassic Period -- that is, they do not exceed 208 million years in age. Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among earth scientists for many years. In the 1920's the study of sea-floors was greatly enhanced when sonar was modified to measure ocean depths. Submarine topography could be surveyed, and the seafloor was mapped as shown for the North Atlantic in Figure 9. The adaption of airborne magnetometers enabled geophysicists to record variations in geomagnetic intensity and orientation. The magnetometric measurements were also conducted by ship borne units, and the midocean ridges showed that the rocks on one side of the ridge produced a mirror geomagnetic image of the other side.
It also revealed that there was not any marinesediment at the ridge crests, but that it did appear on the down slopes of the ridges. These observations 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 it is rapidly cooled and consolidated on contact with the cold, deep-ocean water. This is illustrated in Figure 10. This creation of new ocean floor forces the continents to move away from the ridges, causing the continents to drift. 51 Increased knowledge of the floor configurations and the subsequent formulation of the concepts of seafloor spreading and plate tectonics provided further support for continental drift. During the early 1960s the American geophysicist Harry H. Hess 52 incorporated these data into his proposed model for Seafloor Spreading. He proposed that new oceanic crust is continually generated by igneous activity at the crests of midocean ridges. These ridges are submarine mountains that follow a sinuous course of about 37,000 miles along the bottom of the major ocean basins. Molten rock material from the earth's mantle rises upward to the crests, cools, and is later pushed aside by new intrusions, as shown in Figure 10. As the magma cools, it is pushed away from the ridges.
Figure
10 Plate Slipping
Reprinted by permission
from J.R. Heirtzler, "Sea Floor Spreading," Copyright
Scientific American, Inc.
This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The ocean floor is thus pushed at right angles and in opposite directions away from the crests. This idea played a pivotal role in the development of plate tectonics. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, under thrusting the continents and ultimately reentering and dissolving in the earth's mantle from which it originated.
There is impressive
evidence that supports the seafloor spreading hypothesis. Studies
conducted with thermal probes indicate that the midocean ridges
have a heat flow through bottom sediments of up to four times
greater than that which flows through the continents. 53
These high values reflect the intrusion of molten material near
the crests of the ridges. Research has also revealed that the
ridge crests are characterized by anomalously low seismic-wave
velocities which can be attributed to thermal expansion and micro
fracturing associated with the up-welling magma.
Hess's Seafloor Spreading Model
The existence of these three types of striking, large seafloor
features, which had gradually become evident during the late 1940s
and 1950s, clearly demanded a global rather than local tectonic
explanation. The first comprehensive attempt at such an explanation
was made by Harry H. Hess 54 of the
United States. In this paper Hess, drawing on Holmes's model of
convective flow in the mantle, suggested that the mid-ocean ridges
were the surface expressions of rising and diverging convective
flow while trenches and Benioffzones with their associated island
arcs marked descending limbs, Figure 11.
Figure
11 Seafloor Formation & Spreading
Reprinted by permission
from J. R. Heirtzler, "Seafloor Spreading," Copyright
Scientific American, Inc.
At the ridge crests, new oceanic crust would be generated and then carried away laterally to cool, subside, and finally be destroyed in the nearest trenches. Consequently, the age of the oceanic crust should increase with distance away from the ridge crests, and because recycling was its ultimate fate, very old oceanic crust would not be preserved anywhere. This explained why only rocks younger than Mesozoic had ever been encountered in the oceans, whereas the continents bore ample evidence of the presence of oceans for more than three billion years. Hess' model, later dubbed seafloor spreading by the American oceanographer Robert S.Dietz, 55 appeared to account for most observations and was favorably received by marine geologists. Confirmation of the production of oceanic crust at ridge crests and its subsequent lateral transfer was not long in coming. Fracture zones had thus far been widely regarded as transcurrent faults that gradually displaced one crustal block to the right or left relative to the other, Figure 12.
Figure
12 Continental Drift and Transform Faults
Reprinted by permission,
Encyclopaedia Britannica, Inc.
Given this interpretation, the abrupt termination of many fracture zones against continental margins raised intractable problems. J. Tuzo Wilson, 56 the Canadian geologist, solved these problems in 1965 by arguing that the offset between two ridge crest segments is present at the outset. Each segment generates new crust which moves laterally away. Along that part of the fracture zone lying between crests, the crustal slabs move in opposite directions, even though the axes or rift valleys themselves remain stationary. Beyond the crests, adjacent portions of crust move in parallel and are eventually absorbed in a trench.
Wilson called this a transform fault and noted that on such a fault the seismicity should be confined to the part between ridge crests, as is indeed the case, Figure 12. Shortly afterward, Lynn R. Sykes, 57 an American seismologist, showed that the motions deduced from earthquakes on transform faults conform to the directions of motion postulated by Wilson and are opposite to those observed on a transform fault.
The seafloor-spreading model also specifies that the oceanic crust increases in age as a function of distance from the ridge axis. Wilson had already pointed out that volcanic islands in the Atlantic indeed show this pattern. 58 It is in the nature of these piles of lava and ash that the moment of their birth is difficult to ascertain. Additional evidence was needed, and it soon came from magnetic surveys of the oceanic crust.
A magnetic survey of the eastern Pacific floor off the coast of Oregon and California had been published in 1961 by two geophysicists, Arthur D. Raff and Ronald G. Mason. 59 The results were puzzling and gave rise to many farfetched interpretations. On the continents, magnetic anomaly patterns tend to be confused and seemingly random except on a fine scale, but the seafloor possesses a remarkably regular set of magnetic bands along the ocean ridges, Figure 9, and are alternately higher and lower than the average earth field. These positive and negative anomalies are strikingly linear and parallel with the mid-ocean ridge axis and show distinct offsets along fracture zones. The axial anomalies tend to be higher and wider than the adjacent ones, and they approximate a mirror image of that on the other as described earlier.
In his convection, seafloor-spreading
model, Hess had attributed the formation of the oceanic crust
mainly to the hydration of a peridotitic mantle, a process not
judged likely to produce such regular magnetic anomalies. It also
seemed possible that partial melting of the mantle would yield
a basaltic magma, which would be a much better medium for retaining
a strong imprint of the Earth's magnetic field upon solidification. This second hypothesis
has since been confirmed by deep-sea dredging and drilling. It
has been known since early in the century that the polarity of
the earth's magnetic field reverses from time to time. 60 Studies of the remanent magnetism of
stacks of basalt lavas extruded in rapid succession on land had,
since the late 1950s, begun to establish a sequence of reversals
dated by isotopic methods.
The Vine-Matthews Hypothesis.
Assuming that the oceanic crust is indeed made of basalt intruded
in an episodically reversing geomagnetic field, Drummond H. Matthews
of Cambridge University and a research student, Frederick J. Vine,
61 postulated in 1963 that the new
crust would assume a magnetization aligned with the field at the
time of its formation. If the field were normal, the magnetization
of the crust would be added to that of the earth and produce a
positive anomaly. If intrusion had taken place during a period
of reverse magnetic polarity, it would subtract from the present
field and appear as a negative anomaly. Subsequent to intrusion,
each new block would split and the halves in moving aside would
generate the observed bilateral magnetic symmetry. Given a constant
rate of crustal generation, the widths of individual anomalies
should correspond to the intervals between magnetic reversals. Correlation of magnetic
traverses from different mid-ocean ridges demonstrated in 1966
an excellent correspondence with the magnetic polarity-reversal
time scale just then published by the American geologists Allan
Cox, 62 Richard Doell, and Brent Dalrymaple
63 in a series of timely papers. This
reversal time scale went back some three million years, but further
extrapolation based on marine magnetic anomalies (confirmed by
deep-sea drilling) has extended the magnetic anomaly time scale
far into the Cretaceous period, which spanned from about 144 to
66.4 million years ago. These time scales are based on the isotopic
decay of the material. 64
These confirmations persuaded a large number of marine geologists that seafloor spreading was a reality. However, they focused mainly on the explanations that the concept provided for a host of oceanic features, not on the continental drift. Land geologists were disinterested, viewing the affair as primarily an issue for their marine colleagues.
Two concerns remained. The spreading seafloor was generally seen as a thin skin, and the boundary between the crust and mantle was considered of such major importance in the early 1960s that plans were undertaken to sample it by deep drilling in the oceans. If only oceanic crust were involved as seemed to be the case in the Pacific Ocean, the thinness of the slab was not disturbing, even though the ever-increasing number of known fracture zones with their close spacing implied oddly narrow, long convection cells. More troubling was the fact that the Atlantic Ocean, though it had a well-developed mid-ocean ridge, lacked trenches adequate to dispose of the excess oceanic crust. There the adjacent continents needed to travel with the spreading seafloor, a process that, given the thin but clearly undeformed slabs, strained credulity.
By the late 1960s several American investigators, among them Jack E. Oliver 65 and Bryan L. Isacks, 66 had integrated this notion of seafloor spreading with that of drifting continents and formulated the basis of plate tectonic theory. The midocean ridges occur along some of the plate margins. The lithospheric plates separate and the up-welling mantle material forms a new ocean floor along the trailing edges. As the plates move away from the flanks of the ridges, they carry the continents with them.
On the basis of all these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago, when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Subsequent plate movements averaging about 2 cm (0.8 inches) per year have brought the continents to their present position. More recent measurements by NASA 67 indicate the movement is much greater in some areas and is slowing down in others. It is believed that this breakup of a single land mass and the drifting of its fragments is the latest in a series of similar occurrences throughout geologic time.
John Tuzo Wilson, a Canadian geologist
and geophysicist, established global patterns of faulting and
the structure of the continents. His studies in plate tectonics
have had an important bearing on the theories of continental drift,
seafloor spreading, and convection currents within the earth.
In the early 1960s Wilson became the world's leading spokesman
for the revived theory of continental drift at a time when prevailing
opinion held that continents were fixed and immovable. His paper, entitled "A
New Class of Faults and Their Bearing on Continental Drift (1965),"68 introduced the concept of the transform
fault. Previous theories of continental drift had conceived of
plates as moving either closer together (convergent plates) or
further apart (divergent), but Wilson asserted that a third kind
of movement existed whereby plates slide past each other. This
theory became one of the bases for plate tectonics which revolutionized
the geophysical sciences in the 1970s.
Conclusions on the Continental Drift Theory
The concept of continental drift has been shown to have merit.
Studies in Paleomagnetism and Sea-floor Spreading have indicated
that the land masses or continents were connected at some point
in the past. However, the theory does not account for the anomalous
movements of some areas, for example, the Caribbean and Italy.
The scientific community has generally accepted the presuppositions
that:
1) There was a single super continent in the
past, prior to the Triassic Period (245 to 208 million years ago),
Pangaea, or a variation of this continental model.
2) That the super continent fragmented during
the Jurassic period (208 to 144 million years ago).
3) The distribution of plants and animals support
this theory.
4) The center of the continental drift was the
southern tip of Africa as shown in Figures 4, 5, 7 and 8.
5) Time frames are based on radioactive decay,
index fossils and existing strata definitions.
There are two questions remaining that have not been answered.
1) No explanation for the cause of the initial
movement of the continents is provided.
2) If the sea floor spreading is causing the
continents to drift, then the actual mechanism needs further study
for definition of the specific interactions. It appears that the
sea floor spreading is the result of the continental drift and
not the cause.
© 1997, 1998, Aaron C Ministries
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Jack
E, Oliver , "The
Big Squeeze: How Plate Tectonics Redistributes Mineral and Organic
Resources," The Sciences 31 (July 1991): 22-8. Return
66
Bryan
L. Isacks, "Andean
Tectonics Related to Geometry of Subducted Nazca Plate,"
Geological Society of America Bulletin 94 (July
1984): 341-61. Return
67
NASA
Measures Continental Drift,
Earth Sciences 38 (1985): 8-9; "Continental Plates
Break Speed Limit," Geotimes 38 (April 1993): 7; Nigel
Henbest, "Continental Drift: The Final Proof," New
Scientist 102 (May 1984): 6. Return
68
J. T.
Wilson, "A
New Class of Faults and Continental Drift," 343. Return