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By Igor REZANOV, Dr. Sc. (Geol. & Mineral.), S. I. Vavilov Institute of the History of Natural Science and Technology, Russian Academy of Sciences
UNESCO has proclaimed 2002 a year of mountains. For centuries it has been an intriguing question to us: How did these giants come into being to form ridges thousands of kilometers long? We cannot tell for certain yet.
Articles in this rubric reflect the opinion of the author. - Ed.
Back in the middle of the 19th century two Englishmen, George Airie (corresponding member of the St. Petersburg Academy of Sciences) and Joseph Pratt, proceeding from geodetic measurements in India, formulated the principle of isostasy (equal balance, equilibrium) whereby the earth's crust kind of "floats" on the heavier and denser mantle in compliance with Archimedes' principle. In the 20th century this idea was confirmed by seismic methods: the light earth crust was found to be thicker under high-mountain ridges. In the 1980s Russian geo-physicists built a unique seismic profile across the Pamir and the Hindu Kush Mountains. It showed that under the highland the earth's crust thickness reached 75 km, while it was only 40 km under the Hindustani lowland to the south. The earth's crust is also found to be thicker under the Caucasian Range, under the Alps and other high-mountain massifs. By contrast, the crust is much thinner under the negative relief features, that is seas and oceans. It may be just a mere 5 to 10 kilometers thick under deep-see troughs. Before elucidating the mountain-building mechanism, we should first find out what caused the earth's crust to become thicker under the mountain masses.
We know it from geological history: high-mountain ridges often appear where there had been major down-warpings of the crust, a process accompanied by the accumulation of thick (10 - 15 km) strata of sedimental rock - what we call geosynclines. We should find out why these masses started rising in the Neogene- Quaternary period (in the last 25 million years) to become high-mountain ridges. Now why?
And the last, which is the third and the trickiest question. If the earth's crust "floats" above the denser mantle by Archimedes principle, what caused it to move upwards, i.e. against the gravitation force, in some zones of our planet?
The hypotheses meant to explain to mountain-building process breakdown into three groups.
"COMPRESSION" OF THE EARTH CRUST
The earliest and thus far most widespread theory explains mountain building by the compression of separate, pliant zones of the crust from two sides by "harder" crustal plates. This idea was born in the mid-19th century when the French geologist and full member of the St. Petersburg Academy of Sciences Jean Batiste de Beaumont formulated the now wide-known contraction hypothesis. Starting out from the then dominant concept of the gradual cooling of the earth (whereby the terrestrial radius was to decrease respectively), he pictured the mountain formation process as follows. The cooling liquid core of the earth shrinks; the thin earth shell, hard as it is, cannot keep pace with the contraction of the inner masses, and thus a void appears between it and the core; finally the shell (crust) is rent and contacts the fiery-liquid core again. The crust develops a lateral (side) pressure which piles up rock and gives rise to mountains. This theory gained recognition at the turn of the 19th and 20th centuries, when geologists identified two types of structures in the earth crust: stable platforms (cratons, or force sources) and mobile geosynclinal belts. The platforms, pressing upon the geosynclines filled with sediments, crushed them to give birth to fold mountains.
In the closing decades of the 20th century, as the plate tectonics theory gained currency, orogeny (mountain making) came to be associated with the convergence of crustal plates-an advancing plate pressed upon a geosynclinal belt and caused the crust to bulge up and form a high-mountain ridge. For instance, the building of the Himalaya Mountains is attributed to the northward movement of the Indian plate.*
ROCK VOLUME INCREASE UNDER MOUNTAINS
An essentially new approach to the problem of orogeny (mount building) is offered by a hypothesis postulating decompactification of the crustal or subcrustal matter under mountain ridges: for this or that reason their rock "swells" and expands to a larger volume; as a result, you have an uplift of the earth surface. Geophysics has brought a compelling argument in support of this theory. Using seismic methods*, geologists have found the following: the subcrustal matter under mountains, Tien Shan for instance, is characterized by a lower rate of seismic waves. This means that the underlying mantle (unlike that of lowlands) is decompacted, something that should be accompanied by a volume increase. A study of the gravitational field of mountain countries invites a similar conclusion: a deficit of the mass under highlands can be explained by decompactification only. One of our scientists, M. Artemyev, has calculated that the mass deficit under Tien Shan is but partially caused by the thickening of the lighter crust-by and large, decompactification of the upper mantle is responsible for that. The chief cause of this process is in the mantle's partial melting (the melt takes up a larger volume than crystalline rocks). The zones of enhanced electric conductivity discovered there argue in favor of the latter scenario: the melt shows weaker resistance to the electric current.
Swelling is possible in the crust as well. It may be caused by various processes depending on the mineral composition of the rock: by hydration of ultrabasic rocks (serpentinization); by silica, carbonates, and so forth brought in solutions. According to this hypothesis, no lateral pressure is required here-the mountains grow by themselves, just like the raised dough.
"SQUEEZING OUT" OF MATTER FROM UNDER TROUGHS
As said above, the uplifting of mountain massifs kind of contradicts the principle of isostasy-crustal rock rising against the force of gravity. Even more surprising is the formation of large depressions (troughs) on our planet-
* See: D. Rundkvist et al., "Geodynamics of the 21st Century", Science in Russia, No. 6,1998. - Ed.
* See: B. Dyakonov, A. Troyanov, "Voices from the Earth's Interior", Science in Russia, No. 3, 1998. -Ed.
Possible in-depth processes of orogeny. I - convergence of crustal plates causing compression of sediments, folding and the bulging up of mountains; II - deconsolidation in the upper mantle (or in the crust) causing the uplifting of the crust and the growth of mountains; III - crustal material squeezed out upwards from a depression being formed nearby.
when lighter crustal rocks sink into the denser and heavier mantle!
Origination of such sags, or troughs, is one of the most formidable problems in geology which, so far, has been tackled depending on the "taste" or preferences of a particular researcher. For instance, the proponents of the theory of major horizontal displacements of the crust believe that troughs appear as a result of its fragmentation, with the fragments moving apart and giving rise to an oceanic-type crust lying on the mantle. Since this is a thin crust, it occurs deeper. However, the configuration of most of the troughs and their interconnection with the surrounding uplifts cannot always be explained by the crustal plates moving apart. This can well be seen in the example of the Mediterranean folded zone, where ridges and troughs form an intricate pattern. The same holds for the deep oval trough off the Caspian.
The geological history of troughs in the ocean and on continents alike shows that their origin is related to the subsidence of the earth's crust. Two facts point to this. First, continental-type rocks found on the bottom of oceanic troughs (for instance, granite- gneiss rock was detected in well No. 547 in the Atlantic Ocean as well as on the Pacific Ocean floor between the faults Clarion and Clipperton). Second, the presence of shallow-water deposits on the floor of the sedimentary section of sea troughs, as discovered in the process of drilling.
Now why did the lighter crust found itself within the dense and heavy mantle? There can be but one explanation only: the bottom strata of the crust beneath troughs consolidated to the state of the mantle. According to one hypothesis, this phenomenon was caused by ultrabasic magma substituting for acid (persilicic) crustal rocks.
The author of these lines suggests taking a different look at this process. Implicated in it are median massifs and tectonic platforms, i.e. geological structures that used to be somewhat uplifted before. A larger (lower) part of the underlying crust, or the seismic "basalt" stratum, is built of serpentinous ultraba-
sic rocks. Say, a slight temperature rise here in consequence of volcanic activity causes dehydration of serpentinites which regain the density proper to the mantle. The thus the consolidated, compact crust sinks.
There may be other mechanisms implicated in the compactification (consolidation) of the lower crust under platforms and median massifs, for we have no reliable data on the mineral composition of the seismic "basalt" stratum. Yet the very fact of such compactification is obvious. Acted upon by buoyancy forces, the heavier platforms and median massifs subside. Thus a significant volume of material from the former earth crust comes to be drawn into the mantle. But the subsidence of matter in one place should lead to its upward "squeezing out" in another, especially in slackened crustal zones. Above all this is true of geosynclinal troughs (sags) arising along in- depth faults. This process can be seen in the example of the Mediterranean. At the start of the Neogene (ca. 25 million years ago) the once uplifted median massifs happened to be involved in subsidence. They sank to make room for the Adriatic and Tyrrhenian Seas and the Algeria-Provence basin. The geosynclinal troughs nearby turned out to be uplifted-today these are the Pyrenees, the Alps, the Apennines, the Carpathians, the Balkans, and the Dinaric Alps; and south of the Mediterranean depression there rise the Atlas Mountains.
A similar situation is obtained around oceanic depressions (troughs) as well. The subsidence of the Pacific Ocean floor was accompanied by the uplifting of the coast. This is conspicuous in the example of the Cordilleras and the Andes mountains that stretch along the ocean coast for thousands of kilometers. Many other zones came to be uplifted, such as the coastal areas in the western Pacific, the shores bordering on the Indian and Atlantic oceans; such rises are in stark contrast with adjacent oceanic troughs. In Australia, the positive morphostructures are situated along
its western and eastern coasts, with a depression in its heartland. The African continent, which is surrounded by the ocean on all sides, is raised by about 1,000 m. On the other side of the Atlantic, in Brazil, mountains come closer to the ocean, while the central part of South America is sunken. Mexico offers a graphic example, with its mountains situated along the coasts of the Pacific and Atlantic. A similar picture holds for the northern Atlantic where the mountains of Scandinavia as well as Greenland and Baffin Island rise along the shoreline.
Compensatory rises appeared as the framing of local depressions: the Red Sea graben (fault trough) is surrounded on both sides by shores "upraised" by a thousand meters. Looking at the hypsometric map of the earth, we can give more and more examples. They go to show that the subsidence of the lithosphere under the oceans was partially redressed by the uplifts on adjacent continents.
High-mountain massifs may also be found at some distance from oceanic troughs. Like Tibet, for instance. In a way, this may be explained by the following: situated south of Tibet are submontane troughs. They were subject to major subsidences in the Cenozoic (in the past 70 mln years), i.e. at the same time as Tibet was rising.
The overall volume of oceanic and other troughs appearing on our planet in the last 60 to 80 million years makes up no less that 1,200 mln km 3 , with the volume of compensatory rises being about 120 mln km 3 . Which means that the newly arising mountain ridges have compensated for no more than 10 percent of the erstwhile mantle sunken into the mantle. The other 90 percent has been added to the earth's mantle. Some traces of its extension in the crust (like grabens) may be due to this particular process.
Now let us try to evaluate the probability of the above mechanisms of mountain making. First, we should consider the history of mountain relief formation as evidenced by summit plains of leveling. Fragments of the oldest ones lie in the central (axial) zones of ridges. Their birth dates back to the Paleogene or the Miocene (25 to 2 million years ago). Preserved on the flanks of the ridges are late Miocenic levels (15 - 10 million years old), and there are even younger surfaces of planation (leveling) in the foothills. This is what has happened: first the axial part of a ridge bulged up, but then this process slackened or discontinued for a while; and on the periphery of the uplifted mountain ridge the next and younger surface of planation was formed. Afterwards the uplifting process went on, taking up a wider zone. Thus the mountain ridge grew both upwards and in width.
This very fact is a trenchant counterargument against the "compression" hypothesis. Had the mountains originated in consequence of the horizontal pressure of "cratons" on an "orogen" (birthplace of mountains), the marginal parts of the ridge would have been deformed and raised first, with the axial part corning but next.
The "swelling" hypothesis relative to the crust and the upper mantle seems more acceptable from the standpoint of geomorphology, but it does not explain the causes of plutonic deconsolidation. Now why do mountains "grow up" in some particular places, while the earth's surface remains plane elsewhere?
The "squeezing out" hypothesis is rather cogent in explaining the formation of ridges framing in the oceans, especially where the Pacific is concerned. But it does not hold for the world's largest mountainous region, Tibet. The adjacent troughs are not large enough to "squeeze out" such a giant rise.
Thus the problem of orogeny, or mountain building, is yet to be solved. Each of the above hypotheses has its pros and cons. Our data on the nature of physical processes occurring at a depth of 20 to 50 km, and even 100 km deep, are far from exhaustive, they are contradictory in many ways. It will take us years and years to learn how giant peaks have come to be on the level expanses of our planet.
Illustrations supplied by the author and Yu. Suprunenko
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