Plate Tectonics and the Expanding Earth
© Copyright, 1995 by R.A.Kanen, All Rights Reserved
Plate Tectonics is one of the most important geophysical/structural geology subjects today. To determine the cause of the movement of the plates is the most studied problem. The first evidence for plate movement was, of course, found by Wegener in 1925. This was a result of a comparison of the continental edges of South America and South Africa. It was not until the 1950s, however, that Carey (1954) found the remarkably good fit between the continents using a modeled globe. Wegener's evidence was primarily geological and paleo-climatological.
The model of the Earth developed by the seismologists, at this time, was a liquid iron core surrounded by a solid mantle with no convection movements. When Elsasser and Bullard (1965) developed their geomagnetic field theory, postulating that there are convective motions in the fluid iron core, there was no real objection by the seismologists since the core did not transmit s-waves, indicating it is a classical fluid. It was not until the development of paleomagnetism that there was new evidence for continental drift; then later on, geophysical measurements of the ocean floor swept away most of the doubts geophysicists had about continental drift. This now constitutes part of the subject called plate tectonics.
Many theories on the mechanism for plate movement have been developed. The most popular and widely held view is that convection currents below the lithospheric plates, in the mantle, are responsible for their movement. This involves hot spots and subduction zones. The most radical view was that that developed by Carey (1954), Heezen (1959) and others, that the Earth is expanding causing the continents to break up and form plates.
The Plume Hypothesis
Morgan (1971, 1972) advocates the idea of mantle plumes to explain continental drift. Briefly, he advocates deep mantle convection in which narrow plumes of deep material rise and spreads out laterally in the asthenosphere. This convective movement causes stresses on the bottoms of the lithospheric plates, causing them to move. He suggests that "hot spots", areas of upwelling visible in the Earth's surface, provide the motive force for continental drift. It is based on three facts: (1) Most of the hot spots are near a ridge and a hot spot is near each of the triple ridge junctions; (2) the gravity and regional high topography suggests that more than just surface volcanism is involved at each hot spot; and (3) neither rises nor trenches appear capable of driving the plates, implying that asthenospheric currents acting on the plate bottoms must exist. He bases his theory on data and observations made worldwide. This explanation is convincing as his observations are simple and sound.
Runcorn (1980) discounts Morgan's reasoning because it is based on the analogy with plumes in the asthenosphere; that a plume maintains a small horizontal width as it rises to a very great height. The reason this occurs in the atmosphere, according to Runcorn, is because the inertia term in the Navier-Stokes equation is much greater than the viscous force. In the mantle, the reverse is true.
Before the evidence for convection became known, geophysicists tried to explain plate movement as do to their own inherent properties i.e. gravity force. Hales (1969) suggested that the plates were moving away from the mid-oceanic ridges. Isac's and Molnar(1969), after the discovery that the plates were sinking into the asthenosphere along the trenches, suggested that since the overhanging part of the plate was colder than the surrounding mantle, it would also be denser, thus, a downward gravity force might cause the horizontal movement of the plate. Runcorn (1974) showed that by using magnitude calculations, it is possible a sufficient force would be produced. Therefore, this theory cannot be rejected on the grounds of magnitude, but should be rejected because it is impossible to explain how the process began and it does not enable any understanding of plate movements. Morgan (1972) rejected this theory, but on the grounds that small trench bounded plates, such as the Cocos, do not move faster than the larger Pacific Plate as would be expected.
In contrast to the Plume theory of convection (Morgan, 1972), Runcorn (1980) promotes the theory of large-scale convection. He believes the only way the continents and plates could move in the regular way they have for the past hundred million years is by convection ion a large scale cell structure. This convection pattern changed with time from a one cell, to a two cell, to a three cell and then to a four cell pattern; thereby, explaining the breakup of Gondwanaland and Laurasia (Pangea) in only the last 150 million years. Evidence for this is sparse, however, one reason for large scale convection is that the Earth formed by accretion with the heavy elements, such as iron, sinking to the center forming a core (Urey, 1951); thus, the core may be continually growing. Runcorn (1980) says it is expected there would be a greater geothermal gradient in the lithosphere above the rising convection currents, thus, it would be possible to find the size and location of the currents. However, the time constant for the lithosphere 100m thick is about 100 million years in which time a plate would have moved a considerable distance relative to the origin of a heat source.
Runcorn (1980) uses the shape of the geoid to support this theory. He says, "if a planet departs from hydrostatic equilibrium on a large scale then there are only two possible explanations. Either, density anomalies were acquired by the planet in its early history or the distortion of the planet is being produced dynamically". In other words, "…the geoid is the primary evidence for convection patterns in the mantle". Jeffrey's (1975) has discovered the Earth's gravitational field departs from the accepted hydrostatic equilibrium model. However, the shape of the geoid and its relationship remain open to interpretation.
The Expanding Earth
Numerous authors, such as Egyed (1957), Cox and Doell (1961), Ward (1963), Creer (1965), Heezen (1960) and especially Carey (1954, 1970) have supported the theory that continents have moved apart because of an expanding Earth. Carey based his theory on geologic and tectonic observations while most other authors have used paleomagnetic data to supplement his initial theory. Carey (1970) proposes the Earth is made up of eight first order polygons, analogues to the lithospheric plates, with accretion occurring on all sides of each polygon. He says sea floor spreading supports his argument that new crust must be forming between continents for expansion to occur. Thus, each of them has increased greatly in area, irrespective of how much or how little swelling of crust along trenches occurred. Thus, this means the Earth has increased in total surface area by a large amount. Based on the area of oceanic crust on each polygon, Carey has calculated the amount of expansion, which has taken place, is 76%. This equivalent to a 33% increase in radius.
As further evidence, Carey suggests the following: if you stand on any polygon, it has moved away from every other polygon and if you face about, the distance to each polygon has also increased. This is assuming the Earth consisted almost entirely of continental crust with oceanic crust only being produced in the Mesozoic-Phanerozoic. There is evidence that oceanic crust must have been present before this time, for instance, along Lower Paleozoic accreted continental margins and in Precambrian greenstones.
That oceanic area has increased is consistent with Egyed's observations that each polygon shows progressively less marine transgression through geologic time. Since oceanic crust can have twice as much water as continental crust Carey (1970), the theory that the Earth's surface has increased with time by progressive increase in the area of the ocean basins is supported.
Island arcs appear to be supportive of a subduction type environment in which case convective movement on a non-expanding Earth would be the obvious mechanism for plate movement. However, Carey (1970) denies that island arcs are the result of volcanism along a zone where oceanic crust is being subducted under continental plate. Instead, because all island arcs are bowed eastward, he suggests they are tensional features. To illustrate this, Carey used an analogy with a glacier. On the western side is a dilation zone of new oceanic crust with high heat flux and repeated horsts and grabens, in contrast with the other side which is passive and quiet with little disturbed sediments. Dilation rifts occur in similar fashion at the head of a glacier and the graben arc. Thus, he says, “trenches are dilation rifts". This theory is in stark contrast to calculations made by various authors, such as Morgan (1972) and Runcorn (1980), who state the plates are actually colliding, moving in opposite directions. This data is widely accepted as the norm today.
Carey (1970) makes a bold suggestion when he says his data indicate expansion must be occurring at the rapid rate of 8mm a year and almost all of it occurring since the Late Paleozoic-Mesozoic. Almost universally, other authors (Dooley, 1973; Creer, 1965; Egyed, 1960) have through their calculations based on paleomagnetic data suggested this is impossible. They have in turn, however, stated that the Earth could expand at a slower rate, up to 0.5mm a year, for long periods.
These authors, Creer (1965) in particular, suggest the Earth may have been expanding all its life at a slow rate. Therefore, at the time Pangea began to breakup, the Earth's radius would have been similar to its present radius. Numerous models have been constructed (Dooley, Creer) to illustrate how well the continents fit together to form Pangea. All agree that the best is obtained at present. This destroys Carey's model, which assumes to have a radius 76% of the present radius at the time of Pangea breakup. This does not destroy the suggestion that the Earth is expanding at a slower rate.
Creer (1965) says,” I think expansion should be regarded as something which may be gently, but persistently, occurring in the background. There may be little obvious geological evidence of expansion; most of this could easily have been obscured by more vicious and rapid processes, such as continental drift and orogeny." He goes on to say that to obtain a satisfactory explanation of expansion we may well have to wait until the origin of the universe has been successfully deciphered.
Criticism can be fired at all the theories expounded to explain the mechanism of plate tectonics. Therefore, it is best to choose the theory, which contains only minor holes and explains the mechanism in a simple, clear and distinct way.
Convective plume theory, developed by Le Pichon (1968), Morgan (1968), Runcorn (1980) and others has three major flaws: (1) plate boundaries are not distinct; (2) the condition that each plate having its own accretion and consumption boundary, as for the case for the African Plate, is violated; and (3) if the plates are rigid, as assumed, deformation should have occurred in bottle necks where part of a plate margin was subducted and the rest was not. Of course, the presence of island arcs, subduction zones, hot spots and basalt relationships support the convective-plume theory.
The expansion theory of Cary has major flaws in it, among others, these are: (1) that the Earth was assumed to consist entirely of continental sialic crust; and (2) that a rapid expansion at a rate of 8mm/year had to occur in the last 200my; and (3) that the Earth had radius 76% of its present radius when Pangea broke up.
The slow-expanding Earth theory of Creer (1965) and others is more plausible but lacks evidence. It does not suggest why the Earth would expand, why continental drift began so late in the Earth's history or where the energy source for expansion is derived from.
The conclusion is that the convective-plume theory is the most plausible, based on evidence available.
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