Дата публикации: 15 ноября 2022
Автор(ы): S. Prudnikov →
Публикатор: Научная библиотека Порталус
Рубрика: ПРИКЛЮЧЕНЧЕСКАЯ ЛИТЕРАТУРА →
Источник: (c) "Science in Russia" Date:11-01-2000 →
Номер публикации: №1668460439
S. Prudnikov, (c)
Sergei PRUDNIKOV, Tuva Institute of Comprehensive Development of Natural Resources, RAS Siberian Branch (Kyzyl)
Now, what might there be in common between Iceland in the North Atlantic and Tuva, a land of mountains and taiga forests, lying in Asia's heartland? Volcanoes! Yes, what we call shield volcanoes, which are cylindrical mountains with vertical walls and flat caps. These volcanoes are built up of products of eruptions that occurred under water long ago. But while Icelandic volcanoes are famous far and wide, the Tuvinian volcanoes-without peer in their picturesque beauty-are known only to a narrow group of geologists.
SO SCENIC AND SO LITTLE STUDIED...
The East Tuvinian volcanic field has witnessed many geological events over the last two million years. This is the westernmost part of the volcanic belt of the Baikal rift system. The Tuvinian field is a 2,000 km 2 - high mountain basalt plateau situated at the foot of the Eastern Sayan mountain range. I t is there, from mineral springs on the plateau's slopes (the springs appeared as a result of post-eruptive activity) that the Yenisei, the largest river of Eastern Siberia, takes its rise.
The variety of local volcanic structures is amazing indeed- structures illustrative of a multistage pattern of the field's formation. Six solitary shield volcanoes rise on the plateau (formed next to fissure eruptions of basaltic lavas) and on an old planation to the east: Derbi-Taiga (absolute height, 2,652 m), Kok-Kemsky (2,703 m), Priozerny (2,415 m), among others. Some of them have common bases and form large complex volcanic clusters like Sorug-Chushku-Uzu (2,546 m) and Shivit (2,769 m). Another five volcanoes are in the form of conical mountains, e.g. Sagan (2,109 m) and Ulug-Arga (2,252 m).
The burnished summits of shield volcanoes are here and there built of cones of loose material ejected by eruptions: ashes, volcanic bombs and scoria (slags). The Shivit massif is remarkable for numerous maars - vents formed by small portions of gases cum loose material and drops of lava escaping to the surface. Local volcanic structures have produced basalt sheets which, stretching for dozens and hundreds of kilometers, once buried what used to be river valleys. The lava sheet thickness varies in a wide range- from several dozen to a thousand meters, while the gross volume of
volcanic products is estimated at no less than 600 km 3 .
But even today these fantastic and scenic spots are hard of access. From the north and east they are bounded by rocky ridges of the eastern Sayans, while from the west there are river rapids, impassable taiga wilderness and bogs of the Tojinsk basin. The closest population center-the community of Toora- Ichem - lies 150 km to the west. This district can be reached by helicopter only; that is why we know too little about it: for one, we lack reliable data on the chronology and occurrence of recent eruptions-the data we need for assessing the scope of relevant processes in Central Asia in prehistoric ages and for predicting the likelihood of new eruptions.
It was Sergei Obruchev (subsequently elected Corresponding Member of the USSR Academy of Sciences) who first mapped Tuvinian volcanoes way back in 1948. Afterwards they were studied by geologists and prospectors during surveying works; the data thus obtained became the basis for the present-day conception of Tuvinian volcanism. The local field was thought to comprise three layers, or beds: the lower lava layer forming a basaltic plateau; the middle volcanoclastic, or tuff layer that goes to build shield volcanoes; and last, the upper lava bed above. At first the so-called table, or flat-topped mountains (mesas) predominant here were regarded as denudation outliers formed with the washout of horizontal layers of hardened effluvia. But then Dr. Sc. (Geol. and Mineral.) Mikhail Grosswald (RAS Institute of Geography) related them to shield volcanoes as well: shield volcanoes, he believed, were originally bun-shaped mounts with gentle slopes but later, under the action of intensive destructive processes, turned into flat-topped outliers with abrupt walls.
A SCIENTIFIC BREAKTHROUGH
In 1998 and 1999 our research team led by the director of our
Institute Vladimir Lebedev, Dr. Sc. (Geol. and Mineral.), undertook a study of recent volcanism on the southwestern flank of the Baikal rift system. Our findings caused geologists to rethink the orthodox notions about the structure of the East Tuvinian volcanic belt, the composition and age of its rocks. Using the potassium/argon method of dating we obtained the first ever evidence on the chronology of recent volcanic events in Central Asia. That enabled us to identify seven stages of volcanicity on the East Tuvinian plateau over the last two million years. Relatively large volcanic eruptions occurred every 500 to 700 thousand years, with smaller and more frequent events in between (the latest valley basalt discharges took place already within recorded history).
And here's another important result: Dr. Vladimir Yarmolyuk (RAS Institute of Geology of Ore Deposits, Mineralogy, Petrology and Geochemistry) was the first to relate the volcanoclastic stratum of shield volcanoes to hyaloclastites - formerly it was thought to be of tuff. These hyoline (vitreous) fragmental rocks- as to their formative conditions and composition-are essentially different from tuffs formed from the solid products of volcanic eruptions-ash, sand, bombs and fragments of mountain rock of non-volcanic origin, subsequently consolidated and cemented. In hyaloclastites small fragments are mostly represented by black basalt glass, and larger ones have a glassy surface zone and often take the shape of lava balls and pillows, while the cementing mass is made up of small and fine fragments of volcanic glass.
Hyaloclastites are formed in low-depth bodies of water as a result of underwater eruptions-in that case the low pressure of water cannot hinder discharges of volcanic products, and the hot lava turns water to steam. In consequence of various reactions the incandescent fragments of lava are reduced to thin vitreous ash. But once a shield volcano
rises above the water surface, the outpourings continue on the ground, and a horizontal lava blanketing (cap) is formed.
But how could these two different types of volcanicity produce structures similar in form and built to hyaloclastites which are typical of underwater discharges? On the one hand, we have the Icelandic type of volcanism, which is of genuine rift origin but proceeding above the ground, and not under water. And, on the other hand, Tuvinian volcanism, which belongs to the continental rift variety. How come? In my opinion, the reason for such similitude consists in the following: both in Iceland and in Tuva volcanic eruptions occurred under giant glacial shields which, thawing, gave rise to lakes in the center. That is to say, the conditions were similar to those under water. And thus there appeared vast congregations of glassy ashes to build cylindrical mountains with vertical walls and flat tops, i.e. the shield volcanoes.
DATUM MARKS OF ANCIENT GLACIATIONS
So, we have identified seven stages in the volcanicity of the East Tuvinian region. And in three of them we have traced a connection between volcanism and major glacial epochs. Proceeding from this premise we, much to our surprise, could solve an important problem. It turns out that Tuva's shield volcanoes, whose age has been determined by means of potassium-argon dating, can also serve as reliable datum marks of ancient glaciations that have spread to the entire Sayano-Tuvinian plateau in the past 800 thousand years.
The question of the number and age of these glaciations is one of the most controversial problems of the quaternary geology of the region. Their age is still being determined from morphological characters and from random paleontological data; hence the moot points in conclusions. Most geologists still agree on the existence of two grandiose ice sheets in Tuva - of the Upper Pleistocene
(local name, Azasskoye) and of the Middle Pleistocene (Kaakhemskoye). However, the oldest glaciation - that of the Lower Pleistocene-is bypassed now and then. Our investigations have made it possible to solve this problem too and determine the basic datum marks (reference points) of glacial epochs.
It has not been difficult to determine the age of the Upper Pleistocene glaciation with good reliability. It developed via two principal phases: the initial (formation of mountain-and-valley glaciers) and the main (cover shield formation). Deposits of the mountain-and-valley glaciers cover in places lava sheets of the volcano Ulug-Arga; but here and there they have remained just side by side. And this means that the beginning of glaciation concurred with the formation of this volcano which, according to our data, is about 50 thousand years old. Deposits of the next phase-cover shield formation-overlay moraines of mountain-and-valley glaciers, lava sheets and the summit of Ulug-Arga's slag cone. These moraines are represented by many large and very large blocks and boulders of both basalt and bedrock from the nearby mountains rising above the shield surface here and there. And so we can trace the glacier's route.
The Shivit-Taiga volcano (age, 110-130 thousand years) is a datum mark of the Middle Pleistocene glaciation. This volcano has the lower part formed by sheets of hyaloclastite and globular lavas, and the upper part composed of regular lava. It took body and form during glaciation when the ice cover was 400 m thick. The upper lava mass forms a horizontal plateau composed of high-porosity olivine basalts up to 300 m thick; this plateau is pockmarked by calicular (cup-like) pits which Vladimir Yarmolyuk attributes to the caving of holes that originated with the thawing of the buried ice.
Determining the age of the oldest-Lower Pleistocene-deposits
was a formidable problem for lack of reliable geochronological materials. The geomorphological method, usually applied for dissection of younger glacial formations, is no good here either. For instance, Mikhail Grosswald has inferred the age of the Lower Pleistocene (or "Shivit") glaciation of the Sayano-Tuvinian plateau on the basis of tuff-boulder beds detected in the out- croppings of the northern slope of the Shivit-Taiga volcano. Yet, as shown by potassium/argon dating, its volcanoclastic mass was formed at the beginning of the Late Pleistocene (that is, 110-130 thousand years ago), which means that Mikhail Grosswald erred in his conclusions: the "Shivit" tuff-moraine is much younger - it was formed in the period of the Middle Pleistocene (Kaakhemskoye) glaciation.
But we have determined the age of the Lower Pleistocene glaciation. By our estimates, the Derbi-Taiga volcano (age, 600 to 700 thousand years) can serve as the reference point. This volcano is built of hyaloclastites, globular lavas and lahars - mudstream deposits and ejectamenta erosion products. The volcanoclastic complex, accounting for the bulk of volcanic rock, is more than 500 m thick. Above it is covered with a series of lava sheets of highly porous basalts, 50 to 70 m thick, "iron-cladding" the top of Mount Derbi-Taiga. Judging by the size of the volcanoclastic complex, the Lower Pleistocene glacier was more that 500 m thick, while the basaltic cap shows that lavas flowed out above the ice cap. We called this ice sheet Derbitaiginsky, from the name of the volcano, Derbi- Taiga. The same name can be given to all of the Lower Pleistocene glacial epoch of the Sayano-Tuvinian plateau.
All that goes to show that the impact of volcanism on the course of paleographic processes is enormous and is not confined to lava discharges alone. In the first place
we should point to a close temporal and spatial link between the stages of enhanced eruptive activity and major glaciation periods.
A RETROSPECTIVE LOOK
To conclude, I would like to cite the description of a disastrous eruption of a shield volcano that occurred in southern Iceland in the summer of 1783 (from the book of the German explorer Horst Rast- Volcanoes and Volcanism - off the press in 1982). It began with a sequence of powerful explosions whereby huge masses of ash were ejected to bury, fully or in part, all the pasturelands. Thereupon the grazing animals fell gravely ill because, together with the grass, they ate up sharp particles of natural glass that injured their internal organs. That summer a "blue haze" hung over Iceland for a long time; it must have contained a large concentration of sulfurous gases that inhibited the growth of grass. That was the worst natural calamity in Iceland's history.
Subglacial eruptions are just as dangerous. Thus in southern Iceland two volcanoes, Katia and Grimsvatn, exploded (in 1934 and 1938 respectively) under giant ice shields; thawing, the ice gave rise to lakes that ultimately broke through to the surface. And then violent torrents of water mixed with loose volcanic debris, sand, silt and blocks of ice wrought havoc in the locality wiping out everything on their way.
While studying Tuva's volcanoes, we recalled a unique find we made in 1996 when exploring a section of loose deposits in the Tapsa river valley to the southwest. On a patch of land next to gold placers we hit upon a real graveyard of large mammalians, among them the remains of young and adult mammoths and of a bison. What natural calamity could be behind the death of such large animals? We wondered. But now we know: the animals were killed by powerful volcanic eruptions and their aftermath...
Our data on the distinct cyclic pattern of eruptive activity prompt this conclusion: the Tuvinian volcanic area is quite active. This is also evidenced by post-eruptive manifestations, such as medicinal mineral springs and enhanced seismicity.
Our research studies have been carried out with the financial support of the Russian Foundation of Basic Research (projects 98-05-65242 and 99-05- 65645).
Опубликовано на Порталусе 15 ноября 2022 года
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