Which shifts of the earth crust are of decisive importance: vertical or horizontal? In dealing with this question geologists and geophysicists have splitted into two camps. Some, known as fixists, believe in more or less stable positions of the continents being preserved when parts of them rise or settle. Others - the mobilists - advocate the absolute importance of continental drifts, or dislocations of big crustal plates. Their collisions produce mountain ranges, and in the process of their "movements apart" oceanic depressions are formed. In the heat of their disputes members of both "clans" seem to have forgotten about a concept which supplements, and partially brings together, their "schools of thought". This "bridge" is the idea of turnover, or cycles of the lithosphere. Further progress in this field could be the most promising trend in the geotectonics of the 21st century.
Articles in this rubric reflect the opinion of the author. -Ed.
By Rudolf BALANDIN, geologist
EXCURSUS INTO HISTORY
It were thinkers of the antiquity, who studied marine sediments and seashells found in the mountains and who began to have some "suspicions" about the possibility of the earth crust moving, or shifting in the vertical direction. Their ideas caught the imagination of scholars of the epoch of the Arabic and European Renaissance, such as Ibn-Sina (circa 980 - 1057) and Leonardo da Vinci (1452 - 1519).
The first to voice his ideas about the horizontal shifts of big "segments" of our planet was the French cleric Dr. Placet in a book published in 1668. He linked phenomena of this kind with the Flood as described in the Bible. The possibility of continental shifts, or drifts was also discussed by the British geologist Ch. Layelle (1830 - 1833). And the prevailing opinion, even among experts, was that the first "mobilist" was really the German meteorologist, geophysicist and explorer of Arctic Regions, Prof. A. Wegener (1880 - 1930).
According to Prof. Layelle, significant changes of climate on our planet were precipitated by "concentrations" of continents and islands in the equatorial zone (warm epoch) or their movements into polar regions (coolings). A century later this concept was developed and substantiated by Soviet geologist and geomorphologist Prof. Boris Lychkov (1888 - 1966) in his book "Continental Shifts and Climates of the Earth's Past"
Rotations of the lithosphere according to V. Bonchkovsky (1), L King (2) and D. Panov (3).
Diagram of geochemical rotations.
1-sedimentary rock, 2-weakly metamorphosed deposits, 3-granitoids, 4-basaltoids, 5-ultrabasic rock.
(Dvizhenye materikov i klimaty proshlogo Zemli).
In 1858 French scientist Prof. Henri Snider-Pellegrini noted the similarity of contours, or outlines, of the eastern coast of North and South America and the western coasts of Europe and Africa. He expressed the assumption that the Atlantic Ocean was formed by a split of what used to be a single "supercontinent". In the late 19th century the same idea was pursued by the Italian geologist Prof. F. Sacco. But it was only Prof. A. Wfegener who managed to translate it into a more or less comprehensive theory in his book on the origin of continents and oceans (1915). As different from his predecessors, he brought together facts from different fields of science: geomorphology, paleontology, geophysics and paleography on the basis of the principle of isostasy (equal balance)*.
In the beginning, the Wegener theory received a rather favorable response. But after 1923 it ran into a mounting opposition on the part of British geologists and geophysicists. They were joined by prominent American and later Soviet specialists although there appeared in many other countries convinced proponents of mobilism.
Studies of the bottom of the World Ocean and of paleomagnetism of rocks provided new data in support of the Wegener theory. But there remained a significant snag: it was necessary to explain which forces can really move continents. In 1960 American geologist Prof. H. Hess put forward the idea of abyssal "cycles", or rotation of the earth mantle. He pointed out that continents do not "rift up" the oceanic crust, being set into motion by unknown forces, but are carried up passively on the matter of the mantle, which gets to the surface in the form of mid-oceanic ridges and drifts in the horizontal direction.
Some geophysicists were attracted by the idea which provided the basis for what we now call global plate tectonics**. The number of such plates varies from 7 to 100 and more. But in all cases there remains a basic difference from the Wegener concept, which deserves special mention. In the first instance it is believed that shifting or "drifting" are not continents themselves (because they would have "ripped up" oceanic crust), but large fragments of the stone sheaths in which they are enclosed. That means not taking into account one firmly established and basic regularity: sharp differences in the composition, structure and origin of the oceanic and continental crust. Another important consideration is that the solar energy, reaching our planet, is thousands of times greater than the energy of the deep interior of the earth and performs a staggering amount of "geological work".
* Isostasy-equilibrium state of the earth crust and mantle caused by forces of gravitation, in which the earth crust kind of "floats" in a denser and more plastic subcrustal layer. -Ed.
** See: V. Trubitsyn, "Global Plate Tectonics: A New Turn?", Science in Russia, No. 3, 2003. -Ed.
Diagram of thermal (top) and helium flows on a generalized diagram of earth crust. High temperatures are typical for ascending streams of lithospheric rotations, low-for descending ones. The biggest effluxes of helium are in places of "replenishment" from deep interior.
Bearing in mind these factors several Soviet scientists-geophysicist Vladimir Bonchkovsky and geologist Dmitry Panov, and also American geomorphologist Dr. Lester King, put forward almost simultaneously in the middle of the 20th century the hypothesis of cycles of the lithosphere. And that marked the start of bitter debates between the "fixists" and "mobilists". Both took an uncompromising stand: complete predominance in global tectonics of either vertical, or horizontal movements. And their combination produces circular rotation.
We all know about cycles, or rotations, of the air and water shells (or sheaths) of our planet. The system of geochemical cycles connects data of the geosphere with the earth crust, taking into account the activities of living organisms on the basis of the assimilation, utilization of radiant solar energy. Universal recognition was granted a long time ago to the doctrine of the biosphere-an area of life with a most complex organization which is in constant progression. Seen against the background of its activity, the inertness (inaction) of the lithosphere within the framework of global plate tectonics looks really puzzling. Is that a natural paradox, or flaw in theory, which fails to take into account the most important geomorphological and geochemical regularities?
WHAT ACCOUNTS FOR THE STABILITY OF THE BASTIONS OF CONTINENTS?
Terra firma above the sea level is constantly exposed to erosion. According to geographers' calculations, over a period of 10 thous. years the total dry land surface "sinks" to the depth of one meter and the shoreline "retreats" by 10 meters. Putting these figures on the geological timescale, we discover that all continents will be covered with water over a period of 6 - 8 mn years. According to calculations of Acad. Alexander Ronov (1913 - 1996) with coauthors, from the start of the Devonian and until the end of the Jurassic periods (over some 250 mn years) more than 194 mn km3 of sediments was washed off the continents-twice the present volume of our dry land. That means that the entire earth surface should have been on the bottom of the ocean years ago. Why nothing of this kind has really happened?
And one gets no better picture taking into account the volume of sedimentary rock. The total annual wash-down of solid and dissolved matter from the con-
tinents is of the order of 20 bn tons and the zone of accumulation amounts to 1 mn km2 . That means that the average "rate of accumulation" is 2 kg/m2 a year-a layer of 1 mm. And since the process has been going on for an unusual length of time-over a period of 1 mn years-a layer one kilometer thick is formed over one mn years, and this figure reaches 600 km over 600 mn years. These figures are really incredible!
When one compares the generalized data on erosion and sediments, one inevitably comes to the conclusion formulated by French geologist and Foreign Corresponding Member of the USSR Academy of Sciences, Lucien Kayet (1864 - 1944). He said that matter is moving in closed circles: erosion by water on the continents-transfer-accumulation of deposits on ocean bed and along the coast-"transformation" of sediments into the crust-new erosion on the continents. Over the geological history of the earth this process must have "reprocessed" many times over the rocky "flesh" of the continents. This explains their extraordinary stability and paradoxically large amounts of material "washed off' from them and then accumulated, or precipitated.
The oceanic crust really reminds one of an icefield, broken into plates. It remains passive and is only "broken up" in places by volcanoes. This inertia has a simple explanation: it is not exposed to solar energy. That is why the earth crust under the oceans has a rather simple structure and exists in a relatively stable condition over hundreds of millions of years.
As for continents, exposed to sunshine, water, air and living organisms, they lead what one can call an active and original life of their own. They are split up into a multitude of geoblocks, big and small (sections of the continental earth crust of 0.1 - 5 mn km2 ). Each of these has a very complicated structure, its own chemical peculiarities and unique history*.
What experts describe as mighty "swirls" of lithosphere are located along the "fringes" of the Pacific and also on island arches (Aleutians, Kuriles, Japanese, Philippines, Kampo, the Marianas, etc.). Cycles of lesser intensity are located in the zones of present-day orogenesis. In other places there occur concurrent up-and-down oscillations. And some blocks turn around and shift in the horizontal direction over tens and even hundreds kilometers. Moving most rapidly towards the ocean are island arches, and at a much slower pace-continents.
MECHANISM OF LITHOSPHERIC ROTATIONS
Crustal blocks which become "unloaded", start "floating up" by the law of isostasy (equal balance). Lifting up to this day are the territories of Scandinavia and North America; 12 - 15 thous. years ago they were "overloaded" with giant glaciers.
Having said all that, let us try and imagine a raised geoblock near a continental fringe. It is constantly disintegrating, and the products of erosion are carried off on the shelf and oceanic slope. As a result the crustal segment, being "unloaded" over the millennia, is
Structure of earth crust of active margins of North America. Below on the right-diagram based on the hypothesis of lithosphere rotations.1-astenosphere rocks, 2-basaltoids, 3-granitoids, 4-sedimentary strata.
* See: L. Krasny, M. Sadovsky, "Mosaic Face of the Earth", Science in Russia, No. 1, 2001. -Ed.
moving upwards and the section upon which the sediments precipitate keeps "sinking" under their weight.
What we get as a result is like a transporter conveyor which carries giant masses of matter. The lifting, horizontal movements "down the slope" and the "sinking" phenomena have to be compensated by rock displacements in the opposite directions occurring in deep interior. But, as different from simple machines of this kind, in our particular "model" an important role belongs to geochemical processes. Mere physical crushing boosts the chemical activity of rock exposed on the surface. The "debris" are impacted even more by the air, water, sunshine and living organisms. That leads to the formation of minerals with increased inner energy thanks to their more complicated chemical composition and structure, and greater atomic distances in crystal lattice.
Acad. Nikolai Belov and Dr. Vladimir Lebedev called such changes of matter geochemical "storage batteries".
Sinking down into the bowels of the earth, such minerals, exposed to pressure and high temperatures, recrystallize with considerable participation of water, and then "discharge" the accumulated solar energy.
Acquiring plasticity, the heated masses of sedimentary rock start "flowing" under the continents. Getting into a rising geoblock, with decreased pressure, they undergo degassing, expand in volume and try to escape onto the surface through cracks (like foaming "stream" from a bottle of champagne).
Thus the volcanic belt "framing" the Pacific, is a zone of uplift and deep-water grooves, or furrows, framing the zone of submergence of the lithospheric "whirls". The foci of earthquakes move under the island arches or continents, thus "stressing" the direction of the flows of plastic rock masses.
According to global plate tectonics, oceanic plates move under the continents at the active continental margins. But in that case local volcanoes would be erupting mostly chemical elements typical of that given type of the earth crust. That, however, is not observed and gases and melts almost always match the composition of the continental geoblocks.
In deep-water trenches, sediments remain mostly "undisturbed". Extension forces prevail there, and had they been spreading out, the picture would have been different. An oceanic plate, colliding with a continental geoblock, would produce a zone of compression in which layers are compressed into folds and crumbled. Without any major assumptions of a dubious nature, one can say that the situation observed in active continental border areas can be explained only as lithospheric rotation.
Isostasy, as indicated by the term itself, presupposes static nature of phenomena. Indeed, without some permanent external impacts, geoplates and geoblocks would have reached equilibrium comparatively quickly. But practically everywhere on this planet there are gravitational anomalies, indicating either excesses or deficits of lithospheric masses-something which points to either subsidence or elevation of a territory.
And even if and when deep "bowels" of the earth are passive and there are no "cycles" of mantle matter there, the land surface does not remain unchanged because of the exogenic factors. Products of erosion are carried from elevations into depressions where under the pressure of sedimentary masses a geoblock sinks into the asthenosphere*. It is stratified and responds to vertical forces like a spring, shifting rather easily in the horizontal direction.
One can observe an effect of this kind when folding a pile of paper. When the pile is high enough it begins to "slip off" under its own weight. This is nearly what happens with thick diffluent glaciers which can even "cope" with hills. In the case of asthenosphere the picture must be as apparent. Being under great pressure, it is kind of waiting for some slightest pretext for moving into a horizontal shift. That brings us to a picture of "mobile" terra firma, full of complicated interactions between vertical and horizontal forces, surface and abyssal processes. These are the most intensive in zones with the greatest gravitational anomalies and lithospheric rotations. An ideal state, isostasy is being constantly upset, passing into real isodynamics.
It is not surprising that some geo-blocks and their complexes possess clearly expressed "individual traits". They move up and down, shift in the horizontal direction, are partially remelted in subterranean heat, are tipped and even overturned. All of these things take place on the scale of our geological history whose events are being reconstructed by specialists, though not without some appreciable gaps.
In global plate tectonics, the earth crust is schematically represented as an inert cover which is impacted only be hypothetical fluxes of the super-dense mantle of the planet. Isodynamics of geoblocks presupposes active "life" of the rock shell, or sheath, which constantly interacts with the biosphere and solar energy fluxes. And that does not exclude processes occurring in the core and the mantle at depths of more than 500 km. But the central role is attributed to the surface factors.
As has been found out during the past half of a century: global human activities (technogenesis) can upset the relative stability of some geoblocks during the construction of water reservoirs, development of major oil deposits etc. This can cause "man-made" or technogenic earthquakes. Which confirms the concept of isodynamics.
Within the context of global plate tectonics, continents are given the role of icebergs frozen into icefields. And the isodynamics of geoblocks and cycles of the lithosphere make continents and island arcs look like amoebas, creeping up upon the oceanic crust and assimilating it in part. And saying that is not saying too much: on the surface of the earth and in its rocky bowels there occur most complicated geochemical processes at different levels, beginning from atomic-something which is quite comparable with vital activities of living organisms.
Active continental margins, or borderland, like tracks of a caterpillar tractor, crush oceanic plates, producing deep-water trenches. At the same time horizontal shifts also take place. There seem to be all reasons to believe that some (if not all?) oceanic arches, or arcs, are like rotations which split off the continental massifs and drift into the open ocean.
Here we come back to the theory of Prof. A. Wegener, but with some additions and clarifications which help us understand the causes of the movement of big chunks of the earth crust, sliding over the asthenosphere.
As different from the fixists' concept, we probably have to admit the existence of significant horizontal drifts of continents. But the priority should nevertheless be given to vertical (up-and-down) oscillations of geoblocks which produce, under certain conditions, lithospheric rotations.
The concept of izodynamics makes it possible to generalize a wealth of facts accumulated in geomorphology, tectonics, geophysics, geochemistry and the doctrine of the biosphere. And there is no denying the fact that much data still remain unexplained and open for more in-depth and detailed studies. The hypothesis of rotations of the lithosphere was doomed to oblivion for a long time. And is it not high time to put it back into the focus of serious attention? And by doing that open new vistas for our Earth sciences which we are only guessing at so far.
* Asthenosphere-layer of lower density, strength and viscosity in the upper part of the earth mantle at depths of some 100 km under continents and some 50 km under ocean bed. -Ed.
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