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By Valery TRUBITSYN, RAS Corresponding Member, Otto Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences
Russian scientists have pioneered a new global geodynamics concept: the tectonics of floating continents. It relies on the data of planetary seismic "tomography", the continental drift theory propounded by the German geophysicist Alfred Wegener (1912), and the theory of earth mantle matter convective circulations first suggested by the English geophysicist Arthur Holmes (1945). The plate tectonics theory, which has become so popular in the last few decades, has been seriously challenged.
Articles in this rubric reflect the opinion of the author .-Ed.
Distribution of mantle flow temperature and velocity. Earth section along 15E on the basis of the convection model. Two hot ascending streams can be observed under Africa and the Pacific Ocean (red) and fragments of oceanic plates drawn into the mantle (green).
Our correspondent Rudolf Timofeev interviews Valery Trubitsyn, the author of the concept of floating continents tectonics.
- Valery Petrovich, the constructions of the plate tectonics theory reduce continents to a passive role, despite the common fact that it is the continental lithosphere that is most active, as manifested by its structure, composition and evolution. What about your model?
- As distinct from plate tectonics, the theory of floating continents takes a fresh look at mantle matter dynamics. In fact, the continental lithosphere substantially affects convection processes in the mantle. This elucidates the laws of continental movement and the causes of global stages of geologic history.
- What do you base your proof upon?
- Primarily, on mathematical modeling. A system of differential equations describing the energy, mass and pulse transfer has received a numerical solution for the interaction of viscous mantle and solid continents...
- Could you spell it out?
- Seismic tomography shows the distribution of seismic wave velocities. From these data we have found the pattern of density and temperature within our planet. On the basis of energy, mass and pulse equations we have calculated convective flows in the unevenly heated viscous mantle. The obtained system of equations was transformed into a computer program by Cand. Sc. (Phys. and Math.) Vitaly Rykov. The computer films (cinetomography) produced on its basis demonstrate the global processes going on in the bowels of the earth, the movement of continents and the change of the ocean floor relief features. The results obtained were just fantastic! The computer showed the distribution of heat flows escaping from the mantle, fitting in with the readings registered in 100,000 different earth locations.
The Atlantic Ocean has begun to expand, and the continents to move exactly as satellite measurements show: Eurasia eastwards rotating clockwise, South America-to the north, North America-westwards
Computed heat stream escaping the mantle and the vectors of continents' and ocean floor's displacement velocities. A high mantle heat flow travels along ocean ridges (brown and red).
revolving counter-clockwise. As for Africa, it is moving northeast. Moreover, the monitor displays the deep "roots" of continents the nature of which had heretofore remained obscure. Now to obtain the true picture of the earth's surface and womb and to get to the core of the global processes so profoundly affecting the planet's geological history and relief features we should consider the continental lithosphere displacement. This enables forecasts for the continental/oceanic pattern in the remote future.
- Why not begin with the past then ? After all, we know more of it than of the earth's geological future. Besides, it can serve as an acid test of theoretical constructions, while events eons ahead cannot be put to trial. So, how do reconstructions based on your model relate to the established facts?
- Let me start with the fundamentals of the model. I would like to stress again: this is a mathematic model, it relies on geophysical data, describes the most generic geodynamic mechanisms and does not purport to address the details, at least as yet. Clearness is among its advantages. Suffice if we look at the monitor.
The screen shows a picture reminiscent of an atmospheric circulation pattern: flows of warm air are rising, moving horizontally and going down when cooled. Then they close the loop by returning to the points of ascent.
Don't be confused by the similarity with the global atmospheric circulation pattern, Dr. Trubitsyn went on to say. This is the same physical process of convective movement of matter heated from underneath and cooled from above. That produces nuclei of circulation. Unlike atmospheric streams, the mantle matter circulates all too slowly from our perspective-some 2 - 3 cm per year. However, measured against the millions of years of geological history, the results are impressive. Although the mantle's viscosity is twenty orders of magnitude as high as that of ordinary liquids, it is thermally inhomogeneous. So, it generates turbulent flows which permeate it as hot ascending and cold descending streams. There are also separate vortices and local upward flows termed "plumes".
- But similar arguments are applied to mantle circulations by the plate tectonics theory too. What is so special about your model?
- Now look. The screen shows convective nuclei with a floating continental plate. The frame sequence shows a subsequent course of events. The oceanic lithosphere is fundamentally different from the continental one in that it is implicated in mantle circulations. The mantle matter heated up at the core rises to the surface. Here the flow changes to horizontal and gradually cools down as it travels from the Mid-Atlantic Ridge. Getting heavier, the oceanic lithosphere submerges near the subduction zone as it approaches the continent. Involved in the circulation, it never stays afloat for more than 200 mln years. The computer first showed that continental lithosphere "freezes" to the continent from beneath to form an integral whole. Therefore, it has been in existence for no less than 3 bin years.
- Have you been able to resolve all problems related to oceanic lithosphere dynamics?
- No. The theory quantifies magma upheaval in oceanic ridges, its cooling down, expansion of the ocean floor, bulging of the oceanic lithosphere with time and its submerging in subduction zones. But we are still unable to compute the process of lithosphere fracturing
Pangea's formation dynamic model. Mantle matter flowing towards the downward stream causes the continents to converge to form a supercontinent (time scale-million of years, unit of distance-mantle thickness of 3 thous. km).
into plates with the formation of deep transformation rifts.
- Does it mean your role is reduced to some mathematical brush-up and visualization of the model?
- Not quite. Note one very important circumstance: on the current tectonic maps none of the lithospheric plates is purely continental, that is, their contours do not match perfectly. The continents are assumed to be passive formations imbedded in lithospheric oceanic plates just like isolated iceberg boulders are frozen into polar ice fields. It is true that continents, due to their floatability, cannot be immersed in the mantle. They drift together with mantle streams to which they adhere. Unlike the oceanic lithosphere whose thick-
Dynamic model ofPangea disintegration. Overheating of mantle generates a hot ascending stream tearing the supercontinent apart. Within the rift, an Atlantic- Ocean-type structure appears, outside-a Pacific-type structure.
ness never exceeds 100 km, continental lithospheric "roots" may go much deeper. Classic plate tectonics cannot explain any of these global laws. But the theory of floating continents can.
- What else is explained by your model?
- In the recent 3.5 bin years continents have come together now and then to form supercontinents like Pangea and then fall apart. If it were just two continents, that could be ascribed to mantle convection: the continents are passively pulled to the location of the largest descending stream. But why did all continents come together? And why did Pangea disintegrate so soon? Moreover, the classic plate tectonics fails to explain why it is continents rather than lithospheric plates that would converge and diverge. While the floating continents theory helps understand
the mechanism without flimsy arguments like chance or randomness.
We proceed from the assumption that it is not just the mantle that acts upon the continents, but the latter, in turn, may restructure mantle flows. My model helps visualize the process by simulating two continental plates converging to the downstream mantle flow. Our calculations make it plain that after they have closed up, the effect of a heat shield starts to develop. A large continental plate itself works as such a shield. As shown by a computer model, roughly 200 mln years later a huge ascending heat stream develops instead of a descending one under the supercontinent. It sucks in heat from the bottom of the mantle. A new powerful flow of mantle matter starts to form. According to our calculations, in another 250 mln years the supercontinent breaks up, and the two parts drift from the central ascending mantle stream. As they move away with the speed of roughly 1 cm per year, the mantle stream builds up.
As a result, 900 mln years after the beginning of convergence the continents would be more than 10 thous. km apart (that phase of earth evolution corresponds to the appearance of the Atlantic Ocean). Computations of a three-dimensional model determine the full cycle between the formations of continents at 500 mln-1 bin years.
The departing continents get onto the cooled areas of the mantle; therefore under most of them abnormally low heat streams develop as compared to oceans. This law is hard to explain from the standpoint of classic plate tectonics, as distinct from the theory of floating continents.
- Judging by your theory, continental plates move horizontally undergoing no changes. But in actual fact their crust thickness varies roughly within 30 - 80 km, while the thickness of the lithosphere changes in a broader range, which is commonly rationalized from the positions of isostasy: some blocks of the crust are dipped into a less viscous asthenosphere to different depths depending on the weight and the size. How can you explain that?
- In my model the continental lithosphere is first assumed as a smooth plate. However, in the process of horizontal displacement it builds up, attracting the highly viscous upper mantle matter sticking to it from underneath and forming a heterogeneous continental lithosphere with deep "roots". The point is that under the continent there are stagnant cold zones moving together with it.
- But you have mentioned a heat stream formed under the supercontinent to melt- at least partway-the plate and make it thinner.
- Initially this is really what happens. However, following the split-up, the two parts of the supercontinent move to the cooled areas of the upper mantle. That is as important as the fact that as it moves on, the continental lithosphere may pass through plume areas. Here it melts and becomes thinner. Thus, the dimensions of the continental plate should vary a great deal while tending to gain in thickness.
- It is common knowledge that the variations of continental crust thickness follow a distinct pattern: the crust is thin in stable areas, or plates reaching a maximum in mountain regions. Does your theory take that into account?
- I am considering the continental lithosphere evolution in general outline. But processes it the upper part of the lithosphere, in the crust, are studied by geologists. So you'd better ask them.
- I see, you do not seem to be much involved with geological processes. But the energy potential of the biosphere is enormous due to solar radiation. The biosphere is doing colossal geological and geochemical work.
- No doubt, the flow of energy radiated by our luminary is three orders as powerful as the amount of heat coming from the bowels of the earth. Its surface undergoes deep geochemical transformations affecting the composition and the structure of the planet's crust. That is so. But the theory of floating continents is basically geophysical. It proceeds from the assumption that the mantle heats up on the border of the planet's core producing convective flows. The deep heat of the earth is critical for mantle processes. This is the starting point of our constructions. Next come computations, mathematical and computer models. What is most important, however, is that my postulates largely agree with a number of global laws. For example, there is an enormous hot upward mantle flow under Africa. Why and how has it developed? Why is this continent standing on top of it for a relatively long time without breaking up?
Together with Cand. Sc. (Phys. and Math.) Alexander Bobrov we undertook an additional modeling. A relatively small continental plate was placed over a downward heat stream. Now if it stays put, approximately in 300 mln years it will find itself on top of a hot ascending stream similar to that of the African one. Hitting the platform the heat stream is split and starts to wash it on two sides. In fact, presently in the east, south and west Africa is surrounded by oceanic ridges emitting high heat flows. They should have pushed it north. But Eurasia impedes the movement. As a result, along the contact line of the two continental plates a gigantic mountain belt is developing.
- In the east Africa is latitudinally crossed by a rift zone. According to your model, the split heat stream pulls the continent apart, does it not?
- Apparently, yes.
- Why is nothing of the kind observed under Australia?
- Due to its floatability the continent cannot immerse into the bowels of the earth. And because it is rather small in size and moves fast, no thermal anomaly forms beneath it, at least, not quickly enough.
- Does your model take into account processes going on inside the continental lithosphere, for example, isostasy, vertical displacement of geological blocks, geological and geochemical evolution of the crust?
- All that is outside the scope of the floating continents theory. We have just taken the top off the issue without going into particulars. Next, I am planning to expand the convection model by including processes of global chemical differentiation of matter. We have managed to create a computer program for that purpose. Let me stress it again: the proposed model does not set the high aim of presenting the earth's general theory. We have already discussed its advantages as compared with the widely popular plate tectonics theory. More of the in-depth studies are still ahead.
Illustrations supplied by the author.
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