Text 5 the great dying

About 225 million years ago, at the end of the Permian period, fully half the families of marine organisms died out during the short span of a few million years — a prodigious amount of time by most standards, but merely minutes to a geologist.

This late Permian extinction was the greatest of several mass dyings that have punctuated the evolution of life during the past 600 million years. No problem in paleontology has attracted more attention or led to more frustration than the search for the causes of these extinctions. Since the Permian extinction dwarfs all the others, it has long been the major focus of inquiry. If we could explain this greatest of all mass dyings, we might hold the key to understanding mass extinctions in general.

During the past decade, important advances in both geology and evolutionary biology have combined to show which one of the many J proposals is correct and even how it happened. This solution has 1 developed so gradually that some paleontologists scarcely realize that their oldest and deepest dilemma has been resolved.

If we reconstruct the history of continental movements, we realize that a unique event occurred in the latest Permian: all the continents coalesced to j form the single supercontinent of Pangaea. Quite simply, the consequenc- 1 es of this coalescence caused the great Permian extinction. But which consequences and why? Such a fusion of fragments would produce a wide array of results, ranging from changes in weather and oceanic circu­lation to the interaction of previously isolated ecosystems. Here we must look to advances in evolutionary biology — to theoretical ecology and our new understanding of the diversity of living forms.

Many studies now indicate that diversity — the numbers of differ­ent species present in a given area — is strongly influenced, if not 148

largely controlled, by the amount of habitable area itself: the larger the area, the greater the number of species.

We must first understand two things about the Permian extinction and the fossil record in general. First, the Permian extinction primarily affected marine organisms. It did not strongly disturb the few terrestri­al plants and vertebrates then living, and diversity of land organisms may have increased at the time. Second, the fossil record is very strongly biased toward the preservation of marine life in shallow water. We have almost no fossils of organisms inhabitating the ocean depths. Thus, if we want to test the theory that reduced area played a major role in the Permian extinction, we must look to the area occupied by shallow seas. We can identify, in a qualitative way, two reasons why a coalescence of continents would drastically reduce the area of shallow seas. The first is basic geometry.

If each separate land mass of pre-Permian times were completely sur­rounded by shallow seas, then their union would eliminate all area at the sutures. The second reason concerns the mechanics of plate tectonics.

That paleontology's outstanding dilemma should be solved with the help of advances in two other disciplines is not surprising. When a problem has proved intricable for more than 100 years, it is not likely to yield to more data collected in the old way and under the old rubric. Theoretical ecology and plate tectonics have provided paleontologists with the right questions to solve their hardest riddle.

Ш Give the Russian for:

a short span of; a prodigious amount of time; by most standards; to lead to frustration; the key to understanding; to coalesce to form the single continent; a wide array of results; to bias toward

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■ Give the English for:

вымирание; привлекать внимание; поиск чего-либо; в общем; важные продвижения; предположение; решение проблемы;! постепенно; едва ли; осознавать; разрешение дилеммы; по­следствия соединения (сращение); разнообразие видов; в дан­ном районе

Ш Focus on structures.

• If we could explain..., we might hold the key to understanding...

• If we reconstruct the history..., we'll realize that...

• The larger the area, the greater the number of species.

• The diversity of organisms might have increased.

• That dilemma should be solved is not surprising.

Explain and expand.

• When a problem has proved intricable for more that 100 years, it is not likely to yield to more data collected in the old way and under the old rubric.

Text 6

CORALS AND

PALAEONTOLOGICAL CLOCKS

The astronomers, geophysicists and other investigators whose con­cern is the origin and evolution of the earth are handicapped by a shortage of evidence. The events of interest to these workers occurred in times so distant that even geological records are seldom available. As a result the theories that have been advanced about such matters as the origin of the continents are largely conjectural. Moreover, as might be expected in the circumstances, the theories differ considerably and therefore are highly controversial.

An example of the kind of information that would help to overcome the handicap is a reliable measurement of the length of the day, that is, the speed of the earth's rotation on its axis. It is clear that the length of the day has increased slowly throughout geologic time; the earth's rotation has been slowed by the friction of the tides and may also have been changed slightly be internal processes. Hence the number of days in the year has decreased. If a "clock" could be found that had record­ed the days of ancient geological periods, it would be possible to arrive at a more precise measurement of a number of days in the year and so to obtain evidence about the earth's rotation and the factors affecting it.

Such a fossil clock may be at hand in certain corals. These organ­isms have long been known to have distinct bands that represent annu­al growth. The bands are themselves made up of narrower bands that seem to represent monthly growth and are probably related to the tides and monthly cycle of the moon. The intriguing possibility now under discussion is that the still finer ridges or bands found in some of the

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corals represent daily growth. If this is the case, a coral that could be accurately assigned to a particular geological period (by radioactive dating or the evidence of stratigraphy) would provide a measurement of the number of days in the year at that time.

It was once taken for granted that the earth originated as a molten object and has gradually cooled. Mountains were thought to have formed through the consequent contraction of its interior, so the length of the day will shorten as the earth contracts and its mass moves toward its axis.

Since the discovery that rocks likely to be those making up the earth's interior contain radioactive elements, it has been proposed to assume that the earth's heat has been generated by radioactive decay and that the earth was originally cold. This theory fits in with modern ideas on the origin of the solar system, which is thought to have start­ed as a defuse cloud of gas and dust, in which the planets grew by accretion. Such an evolution would cause the day to shorten gradually, but by a much larger amount than on the basis that the earth began hot and has cooled.

There are some other hypotheses according to which the earth con­tracts and the length of the day shortens.

In contrast to these theories postulating a gradually shortening day, certain other theories assume that the earth has expanded and that the day has grown longer. The earth that has heated up will have expand­ed. The long ridges that run down the middle of several of the oceans offer some evidence that the earth has expanded and not contracted.

These various theories involve considerable disagreement over the length of the day in distant geological periods.

It seems clear that more observations on corals of different geolog­ical age will make it possible to determine the length of the day and

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month throughout the geologic past. Those data in turn will yield im­portant information on the earthly history of the earth-moon system and may provide an important clue to their origin.

Corals may not be alone in this field. If other marine organisms have recorded time in the same say as the corals, we shall indeed have factual information on the early history of the earth.

Ш Give the Russian for:

to be handicapped by; to be available; to be conjectural; as might be expected in the circumstances; to be controversial, to overcome the handicap; to arrive at a more precise measurements; to be at hand; to be under discussion; to be assigned to; to be taken for granted; in contrast to these theories...; to provide an important clue to...

■ Give the English for:

недостаток доказательств; события, представляющие интерес для...; выдвигать теорию; значительно различаться; надежное измерение; на протяжении геологического времени; получать доказательства; в этом случае; если это имеет место; предлагать принять; соответствовать современным представлениям о ...; значительное расхождение; относительно...; в свою очередь; получать важную информацию; таким же образом

И Focus on structures.

• These organisms have long been known to have distinguished bands...

153

• The bands themselves are made up of narrower bands that seem to represent monthly growth.

• Mountains were thought to have formed through the contraction of the earth's interior.

• This theory fits in with modern theory on the origin of the solar system, which is thought to have started as a defuse cloud of gas.

• Such evolution would cause the day to shorten.

Explain and expand.

• The investigators are handicapped by a shortage of evidence.

• If a "clock" could be found, it would be possible to obtain evidence about the earth's rotation and the factors affecting it.

• Corals may not be alone in this field.

Text 7 THE FUTURE OF THE EARTH

What does modern science have to say about the future develop­ment and fate of the earth? Any prediction, of course, can be no more than a speculation. Astrophysicists who have been studying the devel­opment and the life cycles of typical stars have accumulated enough information to make trustworthy predictions about the future of the sun.

At some time in the future the hydrogen supply of the sun will begin to run low. It might be concluded that this will cause a slow but steady decrease of the intensity of solar radiation. However, this will not be the case. Paradoxically, a star increases its emission of light when its hydrogen supply is being reduced. The sun in the far distant future will not become hotter, but will substantially expand.

It is not difficult to imagine what will happen when the increasing solar radiation gradually affects the earth.

First, the oceans will evaporate; the water vapour, together with the atmosphere, will escape into space. Then, the temperature of the sur­face will rise until the surface materials again assume the liquid state. Life on earth will have terminated by that time.

The next step in the development of the sun will occur suddenly, within a few days or weeks, the sun will change into a completely different type of star, a white dwarf. It will be a sphere no bigger than the earth, with almost the same mass. The sun will still be able to hold all its planets in their orbits. In fact, the planets, including the earth, will survive this cosmic catastrophe without any significant change in their orbital parameters. As the sun is transformed from a red giant into a white dwarf, its total luminosity will again decrease

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substantially. It is quite possible that the sun will then continue to shine for some further billions of years, with a brightness much like the present.

These thought have stimulated a great deal of speculation. It is not impossible that at that time a second act in the history of life on earth will start anew. When the solar radiation on earth has gradually been reduced, the Earth's crust will resolidify, and volcanic activity will per­sist sufficiently to give our planet a new atmosphere — and even a new ocean. The new atmosphere and the new ocean may have a lifetime of several billion years. So, there will be a chance for life to develop. Thus the fantastic story of life on Earth might repeat itself.

All these events, if they occur at all, are mostly of academic interest only. They will not directly influence the fate of mankind. Mere is, however, the chance that, in the less distant future, other events and changes of earth will threaten mankind — for instance, natural catas­trophes, such as floods or earthquakes. They occurred in the past, and they will surely occur in the future.

Earthquakes and volcanic activity are closely related. Volcanic erup­tions have been of great significance during the geological evolution of the earth. There would otherwise be no oceans. Earthquakes, there­fore, should be considered as a necessary evil, as they are among the geophysical processes that the earth will never be without.

Still other phenomena on the earth's surface cause catastrophes, though of less immediate danger because they arise from gradual pro­cesses. It is known, for example, that the level of the sea has been rising slowly ever since the end of the last ice age. The reason for this rise is that much of the ice that covered large areas on the earth a hundred thousand years ago has melted, and the melt water has caused the oceans to spill over. This will go on for the next fifty thousand

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years; as a result, the level of the oceans will rise by another hundred, two hundred, or even three hundred feet.

A new ice age would be another catastrophe of the slow kind. This could occur during the next fifty thousand to one hundred thousand years. It would prevent the further rise of the sea level but it would cause a sharp worsening of the climate all over the earth.

In fact, an extensive glaciation of the planet would cause consider­able lowering of the sea level, which in turn would uncover millions of square miles of new land in nonglaciated areas. These drastic changes would not have catastrophic consequences because mankind would have thousands of years to adapt to them.

Ш Give the Russian for:

Trustworthy predictions; to run low; this is not the case; it is not difficult to imagine that...; to assume the liquid state; within a few days; a great deal of speculation; to be of academic interest; to be of great significance

■ Give the English for:

предсказания; накапливать информацию; делать предположе­ния; когда-нибудь в будущем; можно ли сделать вывод, что ...; уменьшать; увеличивать; сокращать; расширять(ся); под­ниматься; высвобождаться; в самом деле; вполне возможно, что ...; начаться заново; угрожать человечеству; вызывать катастрофы; в результате; ухудшение климата; понижение уров­ня моря; однако; следовательно; в противном случае

157

■ Focus on structures.

• The level of the sea has been rising ever since the end of the last ice 1 age. i

• The meet water has caused the oceans to spill over.

• What does sciences have to say about..:?

• Mankind will have thousands of years to adapt to drastic changes, j

• A new ice age would be another catastrophe.

• There would otherwise be no oceans.

• As the sun is transformed..., its total luminosity will again decrease.

Explain and expand.

• Any prediction can be no more than a speculation.

• It is not impossible that at that time a second act in the history of] life on earth will start anew.

• All these events are mostly of academic interest only. There ard other events that will threaten mankind in the less distant future.

Text 8 KIMBERLITE PIPES

I

These remarkable fossil volcanoes rise from a great depth. They are the ultimate source of diamonds and also of rocks that may be speci­mens of materials from the earth's mantle.

Living on the surface of the earth, geologists have little direct knowl­edge of the planet's interior. Of the three broad layers that make up the earth's structure — the crust, the, mantle and the core — only the crust is accessible, and even in its thickest regions the crust represents only about 1 per cent of the earth's radius. Certain physical characteristics of the deeper layers, such as their average density and the speed with which they transmit earthquake waves, can be deduced from the sur­face. For studies of chemical composition, however, there is no ade­quate substitute for a specimen of mantle material.

An extraordinary source of such specimens is the rare rock type called kimberlite. Kimberlite formations generally take the form of small vertical shafts, called pipes, which are demonstrably of volcanic ori­gin. The pipes have been studied extensively, in large part because they are of economic importance: they are the ultimate source of natu­ral diamonds. For the geologist, however, kimberlite pipes supply gems of a different kind: rocks brought up from a great depth. Some of these rocks may be samples of material characteristic of that found in the upper portions of the earth's mantle.

Until about 100 years ago the only known deposits of diamonds were in river gravels. In 1870, however, allivial diamond deposits in southern Africa were traced to their source, the kimberlite pipes near a

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town that is now the South African city of Kimberley. Several other pipes have since been discovered at Kimberley, and isolated pipes and small groups of pipes are scattered in other parts of southern Africa. Elsewhere in the world the only comparable concentration of kimber-lite deposits is in the Yakutsk Republic in Siberia.

Compared with the commoner remnants of volcanic activity on the earth's surface, kimberlite pipes are quite small features. The largest have diameters at the surface of less than two kilometers, and many pipes of economic importance are only a few hundred meters in diam­eter. The pipes generally have the form of a cylinder or a narrow cone that tapers slightly with increasing depth. In the vicinity of the pipes kimberlite can also be found in associated formations called dikes, which are vertical slabs fanned by the intrusion of molten material into fissures in the surrounding rocks.

The pipes probably erupted at the surface when they were formed and were then marked by an open crater and a small cone of ejected material. In almost all cases, however, subsequent erosion has removed the surface features and the uppermost strata of both the kimberlite and the surrounding rocks. The pipes now available for study are ex­posed at deeper erosion levels.

Diamonds are released from kimberlite in stream beds. Subsequent geological changes may bury and consolidate these alluvial deposits, but the diamonds, being extremely durable, remain unaltered. Most of the known kimberlite pipes were emplaced in the Cretaceous period, some 70 million to 130 million years ago. Diamonds are found in allu­vial deposits of several geological ages, however, indicating that there were also pipes in earlier periods.

Kimberlite is a highly variable rock type. Most kimberlite exposed at the surface, called "yellow ground" by miners and prospectors, is severely weathered. At deeper levels there is a material that is better

160

preserved called "blue ground", but only in recent years have samples of the native kimberlite become readily available. Fresh kimberlite is a hard, dark gray or blue rock whose structure gives unmistakable evi­dence of an igneous origin. The kimberlite was extruded into its present position as a molten liquid; it was then cooled by contact with the volcanic conduit and finally solidified.

П

The major constituents of kimberlite are silicates, that is, compounds of silicon and oxygen with metal ions, m general, minerals cannot be defined as simple chemical compounds because their composition is not deter­mined by fixed ratio of atoms. Often two or more compounds are present and are said to be in solid solution with one another. As in a liquid solution, the component substances can be mixed in any ratio over a wide range. One important constituent of kimberlite is the mineral called olivine, which is a solid solution of magnesium silicate and iron silicate. Another silicate is phlogopite, a kind of mica rich in potassium and magnesium, and there are also various silicate minerals that are classified as serpentines. The serpen­tines are formed by the hydration of olivine, or in other words by chemi­cally adding water to it. Kimberlite also contains the mineral calcite, which is not a silicate but consists of more or less pure calcium carbonate.

Of the materials found in kimberlite pipes kimberlite itself may be less interesting than some of the foreign bodies that appear as inclusions within the kimberlite matrix. Among these inclusions, of course, are dia­monds, and it is to their presence that we owe much of our knowledge of these remarkable volcanoes. Another type of inclusions in kimberlite, and one that is far commoner than diamond, consists of rocks torn loose from the walls of the volcanic pipe during the eruption. These inclusions are called xenoliths (from the Greek for foreign rocks).

161

Perhaps the greatest scientific interest in kimberlites derives from a third kind of intrusion: the rocks called ultramafic nodules. Like dia­monds, they are thought to come up from a great depth, perhaps as j much as 250 kilometers below the surface. They have a characteristic j rounded form, like beach stones, caused by abrasion in the pipe.

The interpretation of kimberlites is complicated by the eventful his- 1 tory of the upper mantle. Even several hundred kilometers under the surface the composition and crystal structure of rocks are altered re­peatedly by a variety of chemical and physical processes. For exam­ple, fluids containing dissolved salts can penetrate the grain bound- I aries and microfractures of solid rock. Chemical reactions with the dissolved ions can completely change the character of the host rock.

Melting followed by slow cooling and recrystallization has also prob­ably altered the structure of many rocks incorporated in kimberlite nodules. Much of the evidence required for recognition of their source is thereby destroyed.

The origin of the kimberlite matrix is perhaps even more obscure than that of the ultramafic nodules. The interpretation of kimberlite and the nodules it contains would surely be more secure if their history were less complicated. Even if the story they tell is for now confusing one, however, they remain among the best available sources of infor­mation about the material of the upper mantle.

I ^I I

■ Give the Russian for:

ultimate source; to deduce from; adequate substitute for; to be de­monstrably of volcanic origin; to be studied extensively; in large

162

part; to be traced to the source; to be scattered; the commoner remnants of volcanic activity; the pipes of economic importance; associated formations; to be extremely durable; to give unmistak­able evidence of

■ Give the English for:

прямые/косвенные знания; быть доступными; средняя плот­ность; иметь экономическое значение; где-либо еще в мире; сравнимые концентрации; по сравнению с; в окрестностях чего-либо; почти во всех случаях; последующие геологичес­кие изменения; чрезвычайно разнообразный тип породы; в последующие годы; то есть

Ш Focus on structures.

• Most kimberlites exposed at the surface, called "yellow groud", is severely weathered.

• Melting followed by cooling has altered the structure of many rocks incorporated in kimberlite nodules.

• Some of these rocks may be samples of material characteristic of that found in the upper portions of the earth's mantle.

• Kimberlite pipes were traced near a town that is now the city of Kimberley.

• The major constituents of kimberlites are silicates, that is, compounds of silicon and oxygen with metal ions.

• It is to their presence that we owe much of our knowledge of these volcanoes.

163

Questions to discuss.

• Why can kimberlite pipes be called "fossil" volcanoes?

• What sort of information on the earth's interior can be obtained from the surface?

• What sort of information can be obtained from kimberlite pipes?

В Focus on structures.

• Ultramafic nodules are thought to come up from a great depth.

• The origin of kimberlite matrix is more obscure than that of the ultramafic nodules.

• The interpretation of kimberlite and nodules it contains would be more secure if their history were less complicated.

Explain and expand.

Explain and expand.

• Kimberlite pipes are the ultimate source of diamonds. For the geologists, however, kimberlite pipes supply gems of different kind.

II I

■ Give the Russian for:

to derive from; to be complicated by; eventful history; to be ob­scure; to be secure ; to be confusing; thereby

■ Give the English for:

основные компоненты; химические соединения; другими сло­вами; инородные тела; включения; материнская порода; име­ющиеся в наличии источники информации; в конце концов; очевидно; широко; вероятно, легко; повторно

164

kimberlite —

silicate —

serpentines —

inclusions —

diamond —

solid solution —

hydration —

ultramafic nodules —

Text 9 THE LAVA LAKES OF KILAUEA

The eruptions of the Hawaiian volcano leave pools of molten basalt that can take as long as 25 years to solidify. They provide a natural laboratory for studying the nature of magma from the earth's mantle.

Magma—molten rock—from the interior of the earth is responsible for a host of phenomena at the earth's surface. The flow of magma out of the mid-ocean rifts adds to and pushes apart the rigid plates that make up the earth's surface and carry the continents on their backs. All igne­ous rocks are by definition formed by congealing of magma. If the magma is erupted at the surface as lava, it forms extrusive igneous rocks such as basalt; if it slowly crystallizes below the surface, it forms intru­sive igneous rocks such as granite. In spite of the importance of magma, however, there is much that is not known about it. Most studies of the cooling crystallization and other properties of magma have centered on the laboratory analysis of small samples and on theoretical extrapolation from already solidified lava. A different approach is the studying of molten and solidifying lava in situ by examining three lakes of lava left in the wake of eruptions of the volcano Kilauea on the island of Hawaii.

The three lakes are filled with basaltic lava. Basalt is the commonest rock formed by the solidification of magma extruded to the surface of the Earth, the Moon and perhaps other bodies in the solar system. Basalt is found on all the continents and covers huge expanses of land. Basaltic lavas, erupting from the mid-ocean rifts to create the floor that underlies the sediment of the ocean basins, have poured forth throughout geolog­ic time from the early Precambrian to the present. Although basalt vary significantly in chemical and mineralogical composition, they have all

166

formed at high temperatures. In principle the high temperature of the molten rock makes it attractive as source of energy, although in practice numerous obstacles stand in the way of tapping its heat.

Most of the more than 500 active volcanoes on the earth are entirely or predominantly basaltic, including the active volcanoes that make up the southern two-thirds of the island of Hawaii. The basaltic magma that feeds the eruptions comes from the earth's mantle at depths of at least 50 kilometers below the surface. Geological and geophysical data suggest that magma rising from these depths is stored in an irregularly shaped reservoir. In the formation of a lava lake lava from the reservoir erupts to the surface and flows into a depression. Eventually the natu­ral dikes that channel the lava into the lake collapse, and so the lake is cut from a source of lava and starts to solidify.

Like a freezing lake of water, a lava lake solidifies from the top down. The surface of a lava lake is cooled by air and particularly by rain, which falls copiously on Hawaii. Unlike a water lake, however, a lava lake also solidifies from the bottom up. That happens because the rock under a lava lake is cooler than the molten lava. As a result the molten lava is sandwiched between two layers of solidified crust and takes the shape of a lens.

The crust at the bottom of the lake is always thinner than the crust at the top because the rock under it has a low thermal conductivity and is not rapidly cooled by rainwater. The molten lens decreases in thick­ness as the lake solidifies from the top down and from the bottom up. A lava lake, like a lake of water, is more shallowed at the edges than it is at the center, so that the top and bottom crusts fuse at the edges of the lake, separating the molten lens from the rock enclosing the lake basin. As the top and bottom crusts continue to thicken the lens de­creases in diameter and thickness until it disappears and the lake be­comes a single body of solidified lava.

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II

The ice on a lake of water is quite distinct from the water below it. That is not the case with a lake of basaltic lava, which consists not of one chemical compound but of different minerals that crystallize at different temperatures and rates. The solidified top crust grades slow­ly downward into the fluid lava through a region of partially molten crust several tens of centimeters thick. Within this region the temperate and the ratio of crystal grains to melt increase smoothly with depth. There is an interface, however, across which the physical properties of the partially molten lava change sharply. Above the interface the lava is solid enough to be drilled. Below the interface the lava is a fluid that yields like taffy when a drill probe is pushed into it. At the interface, whose temperature is 1,070°C, crystal grains and melt are equally abun­dant. At 980°C the lava is entirely crystallized except for a small frac­tion in the glassy state: a supercooled liquid in which the silicon oxide molecules have not been organized into crystals. The rate of thicken­ing of both the top and the bottom crust decreases with time because the solidified material is a poor conductor of heat and so acts as an insulator.

As molten lava cools, gas in the melt is driven out of solution. The gas either escapes into the atmosphere or remains in the lava as vesi­cles, or bubbles. The vesicles that were frozen into the crust at shallow depths are chiefly spheres as much as a centimeter in diameter. They were apparently created at high temperatures when the lava became supersaturated with gas on the reduction of the confining pressure above it, just as bubbles appear in a bottle of soda water when the cap is removed. With increasing depth in the lava the confining pressure increases, and so the vesicles become smaller and scarcer. Below six meters in lava lake most of the vesicles are minute angular pores less

168

than a millimeter in diameter. They were apparently created when gas was driven out of solution by crystallization of the cooling lava. The composition of the gases expelled from the lava also changed as the lake cooled: water vapour increases in abundance at the expense of the more rapidly exsolved gases of carbon and sulphur compounds.

Although a solidifying lava might be expected to sink because its crystals are denser that the melt, the lava in some lakes became more buoyant as it solidified because it was filled with gas vesicles. There­fore as the lens of molten lava below the surface at the center of the lake solidified it pushed up the surface above it. At the edges of the lake, where there was no solidifying lens because the top bottom crusts had fused, the surface subsided as the cooling lava thermally contract­ed and became denser; since this lava had already solidified, no bub­bles were being formed in it that would make it more buoyant. The entire surface of other lakes, on the other hand, has generally subsided as the lakes have cooled. Since these lakes are deeper, the higher pres­sures within them have hindered the formation of vesicular lava that would have pushed up the surface.

Another phenomenon that alters the cooling crust of the lava lakes is that large cracks developed in the crust as cooling basalt contracts. Such cracks open a minute or so after incandescent lava appears at the surface in the course of an eruption. As the crust continues to cool and thicken, the cracks propagate downward by further fracturing and new cracks open that divide the surface of the crust into polygons. Within a few hours the polygons become deformed as their centers are elevated by vesicular expansion of the solidifying lava under them. Then still more cracks open, the rate of cracking being greatest at times when the crust is being chilled by heavy rains.

169

The investigations of lava lakes provide a good opportunity to study he physical and chemical properties of basaltic magma and to discov­er more about its nature.

I

■ Give the Russian for:

to provide a natural laboratory for studying; to be responsible for; a host of phenomena; in situ; in principle; in practice; to feed the eruptions; to be stored; eventually; at last; at least; the least impor­tant; on cooling; when cooled

■ Give the English for:

добавлять; отталкивать друг от друга; по определению; на поверхности; под поверхностью; несмотря на; иной подход к; подстилать; значительно изменяться; при высокой темпе­ратуре; стоять на пути (препятствовать); на глубине; несмот­ря на важность...; подобно; в отличие от; при охлаждении; при затвердевании

Ш Focus on structures.

• Basaltic lava, erupting from the mud-oceanic rift to create the floor of the ocean basins, have poured fourth...

• Rising magma is stored in a reservoir.

• On rising magma is stored in a reservoir.

170

■ The text aims at discussing.

a) the origin of magma. b)the composition of basalts.

3. the distribution of basaltic volcanoes.

4. the process of solidification of molten magma

Ф* Х5Г Questions to discuss.

• What are the advantages of studying lava lakes in situ?

• What conclusions can be drawned from the fact that basalts are found on the moon and other bodies in the solar system?

I ^П

Ш Give the Russian for:

that is not the case with; to grade slowly downloward; within this region; reduction of the pressure; at the expense of; to be buoyant; within a few hors; to provide a good opportunity; in the course of eruption; on the other hand

И Give the English for:

Отличаться от; расплав; плавиться; плоскость; над поверхно­стью; под поверхностью; полностью кристаллизоваться; пло­хой проводник тепла; пузырьки

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■ Focus on structures.

• Within this region the temperature and the ratio of crystal grains to meet increase smoothly with depth.

• As molten lava cools, gas is driven out of solution.

• Although a solidifying lava might be expected to sink..., the lava in some lakes became more buoyant as it solidified...

• Since these lakes are deeper, the higher pressures within them have hindered the formation of vesicular lava that would have pushed up the surface.

• Eventually the natural dikes that channel the lava into the lake collapse.

Questions to discuss.

• What is the process of the formation of a lava lake?

• How can the lens form of lave lakes be accounted for?

• Compare water and lava lakes.

Text 10

THE DEEP-EARTH-GAS

HYPOTHESIS

I

There is much evidence indicating that earthquakes release gases from deep in the earth's mantle. Such gases may indicate methane of nonbiological origin, which could be a vast resource of fuel.

It is widely believed that the earth's supply of hydrocarbon fuels will be largely used up in the foreseeable future, the most desirable ones (oil and natural gas) within a few decades and coal within a few centuries. Diverse evidence leads us to believe that enormous amounts of natural gas lie deep in the earth and that if they can be tapped, there would be source of hydrocarbon fuel that could last for thousands of years. The hypothesis that there is much gas deep in the earth also provides a unified basis for explaining a number of otherwise rather puzzling phenomena that either give warning of earthquakes or accom­pany them.

The exact composition of the gas is not known, since the observa­tional evidence is scattered and not easily interpreted. Volcanic erup­tions bring gas out from the interior of the earth. It is not possible, however, to deduce from such observations the initial composition of the gas while it was still deep in the earth.

Gases released during earthquakes are probably more reliable sam­ples of what resides in the deep crust and the upper mantle. The sam­pling of such gases is just beginning, and the data will not yet support confident conclusions. One can assume that the composition of the deep-earth gases varies from place to place, since the location of min­eral deposits in the crust suggests that the underlying mantle is quite

173

heterogeneous. For a variety of reasons we think methane of nonbio-logical origin is one of the principle deep-earth gases, and it will be the focus of our discussion here, although we do not mean to minimize the possible importance of other deep-earth gases in the phenomena associated with earthquakes.

The notion of non-biological methane runs counter to the prevail­ing view in petroleum geology that virtually all the oil and natural gas in the earth is of biological origin. In that view the carbon in hydro­carbon fuels was originally derived from atmospheric carbon diox­ide, and the energy to dissociate the carbon and the oxygen came from sunlight in the course of photosynthesis by green plants. The bural of some of these organic compounds before they could be­come oxidized would then have provided the source materials for oil and gas. It cannot be doubted that this process contributed to the genesis of much of the petroleum that has been recovered, but there may be more to the story.

П 1

The hypothesis that the earth contains much non-biological hydro­carbon begins with the observation that hydrocarbons are the domi­nant carbon containing molecules in the solar system. The universe is made mostly of hydrogen, and the evidence of cosmochemistry sug­gests that the earth and the rest of the solar system originally con­densed out of a hydrogen-saturated nebula. Most of the carbon in meteorites, which provide the best clues to the origin, composition of the inner planets, is in the form of complex hydrocarbons with some chemical similarity to oil tars.

The picture we favour is of dual origin, with some hydrocarbons derived from buried organic sediments and probably much larger

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amount added to those hydrocarbons by augmentation from a stream of non-biological methane.

Let us now examine some of the evidence for the escape of meth­ane from the interior of the earth. A likely place to look is along the crustal faults and fissures of the tectonic-plate boundaries, which ought to provide the best access to the deep interior. Indeed, hydrocarbons appear to be clearly associated with such plates.

Another line of evidence connecting non-biological hydrocarbons with such features is the striking correlation between the major oil and gas regions and the principal zones of past and present seismic activi­ty. Oil fields often lie along actiye or ancient lines. Most of the known natural seeps of oil and gas are found in seismically active regions. The association suggests to us that the deep faults may provide a conduit for the continuous input of nonbiological methane and other gases streaming up from below. Moreover, the upward migration of methane and other gases in fault zones may contribute to the triggering of earthqnakes.

Seismologists have long recognized a difficulty in accounting for deep earthquakes. Yet earthquakes have been recorded from depth of as much as 700 kilometers and if the fracture is strong enough to fracture the ground up to the surface, the gas escaping may generate some of the peculiar phenomena that have been reported to accompa­ny many major earthquakes. The phenomena include flames that shoot from the ground, "earthquake lights", fiece bubbling in bodies of wa­ter, sulphureous air and visible waves rolling slowly along alluvial ground. Tsunamis (large, earthquake-caused waves at the sea that are often highly destructive) may be an analogous phenomenon. It is usually assumed that they are generated by a sudden displacement of an enor­mous area of the sea floor over a vertical distance comparable to the height of the wave.

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There is as yet no proof that any of the effects we have mentioned are caused by eruption of gas during earthquakes, but at least for the flame and bubbling water phenomena it is difficult to imagine a likely alternative.

Many of the precursory phenomena are detected only by instru­ments. Included in this category are changes in the velocity of seismic waves through the ground, in the electrical conductivity of the ground, in the tilt and elevation of the surface, in the chemical composition of gases in the soil and the ground water. The time between the onset of a precursor and the earthquake ranges from minutes to years.

Not all precursors of earthquakes can be detected only by instru­ments. Some are so obvious to the senses that they have been recog­nized since ancient times. We believe these effects too are caused by an increased flow of gas through the ground. Among these "micro­scopic" precursors are dull explosive noises of unknown origin, the strange behaviour of animals, local increases of temperature, bubbling of water in wells and flames from the ground.

Many other lines of investigation can elucidate the degassing pro­cesses of the earth. Variations of the methane content of the atmo­sphere may be observable. Changes of fluid pressure in the ground can be monitored. No one has any firm evidence on the diverse gas regimes more than a few kilometers below the surface or on the quan­tity or frequency of the various gases emerge.

Our present attempt to formulate a relatively simple hypothesis to account for numerous previously unrelated facts will doubtless turn out to be in places oversimplified or overstated. We hope, however, that it will stimulate further research in this fundamental field of geo­physics and geochemistry, leading perhaps to the discovery of large new sources of fuel and in any case to an improvement in the under­standing of the earth and its resources.

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\ I

Ш Give the Russian for:

a vast resource of fuel; within a few decades; there is much evi­dence; to be used up; to provide a unified basis for..; to deduce from the observations; confident conclusions; to run counter to...; it cannot be doubted that...

■ Give the English for:

> полагают, что...; обозримое будущее; разнообразные доказа­тельства; предупреждать о...; первоначальный состав; надеж­ные образцы; по ряду причин; преобладающая точка зрения; по ходу фотосинтеза

Ш Focus on structures.

• The exact composition of the gas is not known, since the evidence

is scattered.

• The earth's supply of hydrogen fuel is believed to be largely used up.

• The hypothesis ... provides a unified basis for explaining a number of otherwise puzzling phenomena.

^w J> Questions to discuss.

• Why is the problem of hydrocarbon fuel supply considered to be urgent?

• Is it possible to deduce the exact composition of the gas released during volcanic eruptions?

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п

Ш Give the Russian for:

Diverse evidence leads us to believe that... .

It is widely believed that...

One can assume that...

For a variety of reasons...

It cannot be doubted that...

In that view...

To account for previously unrelated facts...

In any case...

Another line of evidence...

Many other lines of investigation...

Ш Give the English for:

Широко известно, что...

Разнообразные факты заставляют нас поверить в то, что ...

Гипотеза дает универсальную основу для понимания приро­ды явлений иначе трудно объяснимых.

Данные наблюдений отрывочны и их объяснение затруднено.

Из подобных наблюдений невозможно сделать вывод о...

По ряду причин данные пока не дают достаточных основа­ний для того, чтобы...

Не пытаться приуменьшить важность других исследований... Противоречить общепринятым взглядам... Не может вызывать сомнения тот факт, что...

178

Данные дают основания полагать, что...

Другая линия доказательств заключается в том, что...

Попытка предложить относительно простое объяснение слож­ным, не связанным между собой фактам, может привести к не­сколько упрощенному пониманию...

Ш Explain and expand.

The exact composition of the deep-earth gases is not known be­cause:

• it varies from place to place,

• gases are not accessible for direct observation,

• there are difficulties in interpretation,

• the gas is contaminated while rising to the surface,

• the underlying mantle is heterogeneous.

■ Challenge Translation