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

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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

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

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.

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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).

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.

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■ Give the Russian for:

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

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.

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?