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UNIT 9. METAMORPHIC ROCKS

II. Прочитайте и переведите текст:

Metamorphism may be defined аs any change in mineral composition, structure or texture of an igneous or a sedimentary rock whereby the original rock character is notably altered. Rocks which have been so modified constitute the metamorphic group. Sometimes a rock may be so thoroughly metamorphosed that it loses its original character. All metamorphic rocks are divided into two major groups those which have been derived from igneous and those which have been derived from sedimentary rocks.

Kinds of Metamorphism. We may divide metamorphism into (1) contact or local metamorphism and (2) general or regional metamorphism.

Contact metamorphism. By contact metamorphism are meant the changes produced by intrusive magmas in contact with other rocks which they invade. The rock invaded may be sedimentary, igneous or metamorphic.

The changes are usually most pronounced in sedimentary rocks such as limestones, clay shales, and slates, and to a less extent in sandstones. Shales and slates are baked to a hard siliceous rock called "hornfels", while limestones are converted into marble, and sandstones are usually changed to quartzites.

Regional Metamorphism. Over many parts of the earth there are extensive regions of rocks that have been altered more or less profoundly and which, because of great areal extent involved and the character in part of changes produced, cannot be considered to be local or contact metamorphism. The principal metamorphic rocks composing such extensive regions are gneisses, crystalline schists and slates. Alteration on such an extensive scale is known as general or regional metamorphism.

Agents of Metamorphism. The principal agents involved in the alteration of igneous and sedimentary rocks are (1) earth movement and pressure, (2) liquids and gases, chiefly water and (3) heat.

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MINERAL COMPOSITION OF METAMORPHIC ROCKS

The mineral composition of metamorphic rocks is subjected to wide variation. Certain silicate minerals are known to be characteristic of the igneous rocks, whereas hydrated oxides, carbonates and the like occur chiefly in sedimentary rocks. Other minerals such as quartz, feldspar and mica are found in both igneous and metamorphic rocks, whereas chlorite, talc, serpentine and common garnet are common minerals in metamorphic rocks.

TEXTURE OF METAMORPHIC ROCKS

Most metamorphic rocks resemble the igneous rocks, by being highly crystalline, but they differ from them in possessing a parallel structure which sometimes closely resembles classification. This parallel structure is characterized by a foliated, laminated, or as it is often termed, schistose structure. Well developed foliation causes rock cleavage, that is a tendency for a rock to split in layers parallel to the foliation. But there are some metamorphic rocks, such as serpentine, marble and quartzite, which do not show any trace of this parallel schistose structure and cleavage.

III. Переведите без словаря:

rock character, modified rocks, contact or local metamorphism, general or regional metamorphism, intrusive magmas, conversion, variation, crystalline parallel structure, tendency, cleavage.

VI. Заполните пропуски соответствующим по смыслу глаголом:

1. Metamorphism may be … as any change of mineral composition, structure

VII. Поставьте вопросы к подчеркнутым членам предложения:

Well developed foliation causes rock cleavage.

VIII. Перескажите текст по плану, используя слова, данные в скобках:

1.Definition of metamorphism (to define, any change, original rock character, notably altered).

2.Definition of contact metamorphism (changes, to produce, to invade).

3. Definition of regional metamorphism (extensive regions, to involve, to alter).

4.Agents of metamorphism (principal agents, to involve, alteration).

5.Mineral composition of metamorphic rocks (to be subjected, wide variation).

6.Texture of metamorphic rocks (to resemble, highly crystalline, to differ, to possess a foliated structure).

IX. Прочитайте и переведите текст:

METAMORPHIC ROCKS

Metamorphic rocks are rocks which, originally sedimentary or igneous, have been changed in mineral composition or in texture, or in both, so that their original characters have been altered or even entirely disappeared.

Rocks for most part are composed of minerals, and minerals are mostly of definite chemical combinations. If minerals are subjected to new conditions which are quite different from those under which they were formed, with new physical factors operating (действовать) upon them, they will change into new chemical combination which will be the most stable under new conditions. For example, feldspar may change into clay and other substances through the action of water and carbon dioxide.

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Factors of Metamorphism

Temperature is one of the most powerful factors of metamorphism. Rise of temperature speeds up chemical reactions, and at higher temperatures there occur such reactions which cannot take place at lower temperatures. Besides temperature is highly effective in increasing the plasticity of minerals, thereby (тем самым) increasing the deformability of rocks (способность пород к деформации) in which they occur.

Fоr every aggregate of minerals there is a minimum temperature that must be reached before any reaction between them begins and then new minerals begin to be formed.

Liquids, particularly water on and in the earth, are often effective agents of alteration of rock material by dissolving (растворение) it, after which it may crystallize into new mineral combinations.

Various gases and vapors (пары), especially those which escape (выделяться) from molten masses into surrounding rocks, often cause important and chemical changes and rearrangement of mineral matter in rocks.

Pressure is also a very important factor in metamorphism. It may be lateral (боковой) and downward (направленный вниз).

X. Определите абзац, в котором можно найти ответ на вопросы:

a.What is a metamorphic rock?

b.What happens with minerals when they are subjected to new condition with new chemical and physical factors operating upon them?

с. How does the rise of temperature help the process of metamorphism?

d.How do liquids help the alteration of rocks?

e.In what way do various gases and vapours take part in rock metamorphism?

XI. Кратко изложите содержание текста на английском языке.

XII. Прочитайте и переведите текст:

METAMORPHIC ROCKS

Metamorphic rocks are formed from igneous and sedimentary rocks by the process of changes by which the original rock may be more or less completely altered in its composition and texture. The change may be so great, that the metamorphic rock does not resemble the original rock from which it was derived and looks like a rock of a new kind. For example, sedimentary rocks after metamorphism become much harder, denser, more crystalline and may loose their stratification.

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The factors of metamorphism are: mechanical movements of the earth's crust, pressure, chemical action of liquids and gases and the effect of heat.

Most metamorphic rocks have a highly crystalline structure and a foliated or banded texture.

Since the metamorphic rocks are derived from sedimentary and igneous rocks there are very many different kinds of metamorphic rocks.

XIII. Ответьте на вопросы:

1.How are metamorphic rocks formed?

2.Does the metamorphic rock resemble the rock from which it was derived?

3.What are the chief factors of metamorphism?

4.What is the structure and the texture of metamorphic rocks?

5.Why are there so many different kinds of metamorphic rooks?

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Crystalline Limestones and Dolomites (Marbles). Crystalline limestones and dolomites are the metamorphic equivalents of ordinary limestones and dolomites and are known geologically as marbles.

Marbles, when pure, are compact crystalline granular rocks composed of calcite or dolomite, or a mixture of the two. They show a wide range of colour, dependent chiefly upon their degree of purity. The pure ones are white, others grey to black, and still others may show shades of red, pink, yellow, green, brown and other colours. The principal impurities which act as a pigment influencing colour are carbonaceous matter and the oxides of iron, as well as finely divided mica. The colour may be entirely uniform in the pure marbles, but more often it is spotted, blotched or streaked.

VII. Заполните пропуски соответствующим по смыслу глаголом:

VIII. Поставьте вопросы к подчеркнутым членам предложения:

1.Quartz is the most frequently occurring constituent in schists.

2.Gneiss is а banded metamorphic rock.

IX. Отвечая на вопросы, составьте сообщения о главных видах метаморфических пород:

1.What kind of a rock is quartzite?

2.In what properties do quartzites differ from sandstones?

D.Marbles

1.What kind of a rock is marble?

2.What is the mineral composition of marble?

3.What is the colour of marble?

X. Прочтите текст без словаря и кратко передайте его содержание по-английски:

Metamorphic rocks are formed in deep-seated (глубинные) layers of the Earth by recrystallization of magmatic and sedimentary rocks under the influence of high temperatures, pressures and other agents.

Metamorphism changes the primary structure and texture of the original rock and its mineralogical and chemical composition.

Metamorphic rocks have a crystalline structure and a foliated or banded texture.

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The most widespread metamorphic rocks are:

Quartzite is a rock composed of quartz grains so firmly cemented that fracture takes place not around the grains but through them.

Originally the rock was a sandstone but has been changed to quartzite:

1)by filling the pore space of the original sandstone through the deposition of quartz from circulating ground water,

2)by contact or dynamic metamorphism.

Quartzites of metamorphic origin are commonly interbedded (переслаиваются) with gneisses and mica schists. They are generally compact, hard, resistant (устойчивые) rocks of light colours.

Marble. Marbles are the metamorphic equivalents of sedimentary limestones and dolomites. During metamorphism the minute grains, of which limestones and dolomites are composed, grow in size and become crystalline.

Most marbles are massive showing no foliation even if they are subjected to greet pressure.

Pure marble is white, but it may be of red, grey and black colour due to impurities.

XI. Переведите текст со словарем:

DEFINITION OF METAMORPHISM

The Earth in many places is made up of metamorphic rocks which comprise one of the three major groups into which all the rocks are divided. Metamorphic rocks have been formed by the transformation of sedimentary, igneous and preexistent metamorphic rocks. This transformation has developed new structures, new textures and new minerals.

In some places metamorphic rocks pass gradually into rocks of evidently sedimentary origin, while in other places they pass into rocks of igneous origin. An illustration of the first case we can see in the alteration of sedimentary rocks caused by pressure. When sediments have accumulated to a depth of several kilometres, the lower layers may be affected by the weight of those above, by the internal heat of the Earth and by other agents, so that chemical changes occur. The material begins to recrystallize, new minerals grow from the debris of those in the sediments and finally the rock becomes quite different in appearance.

Sometimes we find such a rock with the marks of its sedimentary origin still visible. Other rocks may be so perfectly recrystallized that there is no direct evidence in their structure that they have ever been sediments. Such rocks are metamorphic.

The word "metamorphic" means changed in form, and "metamorphism" is used as a general term for all those changes by which the original characters of rocks are more or less completely altered in their component minerals or textures, or both. The change may be so great that the metamorphic product does not resemble the rock from which it was derived, but it looks like a rock of a new kind. For example, igneous rocks may lose the particular features which distinguish them and may assume a banded appearance and cleavage which resemble those of sedimentary rocks.

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METAMORPHIC ROCKS

These are rocks which have been altered to their present form by one or more of the agents, such as: heat, pressure and water. These agents may so alter a rock in composition and appearance that its original character can hardly be recognized. The rock surrounding and covering batholiths always changed more or less by the intense heat and the solutions given off.

Sandstones may have the particles so welded together by pressure or so cemented together by silica as to form a hard, compact rock called quartzite, which is very common in gravels. Shale under pressure is metamorphosed to slate. New minerals are formed during this change, and they arrange themselves more or less parallel to each other, thus accounting for the ease with which slate can be split into thin sheets.

Limestone under heat and pressure changes to marble. In this metamorphosis the rock recrystallizes. Anthracite coal may be considered a metamorphic rock.

Schist is formed from igneous and sedimentary rocks under extremely great pressure. The original minerals in the rock may be changed to others, and these latter minerals arrange themselves in thin layers or bands, along which the rock splits fairly easily. Gneiss is formed in the same way as schist, but the banding is not so highly developed. Therefore the rock does not split so easily along the bands. It also exhibits a rougher surface. Both schist and gneiss are found in regions of intense pressure, as in mountainous regions or where mountains once existed.

METAMORPHISM

The outer shell of the Earth in many places is made up of metamorphic rocks which comprise one of the three major groups into which all the rocks are divided. Metamorphic rocks have been made by the transformation of older, pre-existing rocks. This transformation has developed new structures, new textures and new minerals.

FACTORS OF METAMORPHISM

Temperature. Temperature is one of the most powerful factors in causing metamorphism. Rise of temperature speeds up chemical reactions. For every aggregate of minerals a critical temperature must be reached before chemical reactions can begin to take place between the minerals, and only then new minerals begin to form.

Temperature is also highly effective in increasing the plasticity of minerals, thereby increasing the deformability of the rocks which they occur.

Minerals of metamorphic rocks. The chemical compounds which form the minerals, found in the rocks, vary greatly in their ability to withstand the changes of conditions to which different geological processes subject them. Under action of new chemical and physical factors acting upon them, they will tend to change into new minerals or into such chemical combinations which will be the most stable under new conditions. Thus, feldspar is known to alter into clay and other substances through the action of water and carbon dioxide. The igneous rocks are characterized by one set of minerals, chiefly silicates, while carbonates and hydrated oxides and silicates are mostly found in the sedimentary rocks. Some minerals, like quartz, have a wide range of stability and are found in all three classes of rocks, but many minerals when they are subjected to metamorphic processes are converted into other minerals.

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