Text b Metamorphic Rocks

The problem discussed concerns metamorphic rocks which compose the third large family of rocks. "Metamorphic" means "changed from". It shows that the original rock has been changed from its primary form to a new one. Being subjected to pressure, heat and chemically active fluids beneath the Earth's surface, vari­ous rocks in the Earth's crust undergo changes in texture, in mineral composition and structure and are transformed into metamorphic rocks. The process described is called metamorphism.

As is known, metamorphic rocks have been developed from ear­lier igneous and sedimentary rocks by the action of heat and pressure. Gneisses, mica schists, phyllites, marries, slate, quartz, etc. belong to the same group of rocks, having the same mineral composition as granite, gneisses consist chiefly of quartz, orthoclase and mica. However unlike granite, they have a schistose structure.

It means that their constituents are distinguished in bands or layers and are parallel to each other in one direction. If disturbed the rock cleaves easily into separate plates.

The role of water in metamorphism is determined by at least four variable geologically refuted parameters: rock pressure, temperature, water pressure, and the amount of water present.

The low-grade metamorphic rocks are characterized by the excess of water. The medium-grade rocks defined by some deficiency of water and the high-grade metamorphic rocks are characterized by the absence of water.

Many of the metamorphic rocks mentioned above consist of flaky materials such as mica and chlorite. These minerals cause the rock to split into thin sheets, and rocks become foliated.

Slate, phyllite, schist and gneiss belong to the group of foliated metamorphic rocks. Marble and quartzite are non-foliated metamorphic rocks,

The structure of metamorphic rocks is of importance because it shows the nature of their pre-existing rocks and the mechanism of metamorphic deformation. Every trace of original structure is of great importance to geologists. It gives an opportunity of analysing the causes of its metamorphism.

Being often called crystalline schists, metamorphic rocks such as gneisses and mica have a schistose structure. Metamorphic rocks represent the oldest portion of the Earth's crust. They are mostly found in the regions of mountain belts where great dislocations on the Earth once took place.

Ex. 15. Which sentences are true according to the text?

1. Generally speaking, metamorphic rocks have been developed from ores.

2. Marble, slate and phyllite belong to the group of metamor­phic rocks.

3. As is known, unlike granite metamorphic rocks have a schistose structure.

4. It is quite obvious that the role of water in metamorphism is great.

5. As a rule, low-grade metamorphic rocks are characterized by the absence of water.

6. Flaky materials cause the rock to split into thin sheets.

7. It should be noted that marble and quartzite are foliated metamorphic rocks.

8. The structure of metamorphic rocks shows the nature of older pre­-existing rocks and the mechanism of metamorphic deformation as well.

9. All metamorphic rocks are non-foliated.

Ex. 16. Answer these questions.

1. Do you know how metamorphic rocks have been formed?

2. Which rocks belong to the group of metamorphic?

3. Does gneiss have the same structure as granite?

4. Is the role of water great in metamorphism?

5. What rocks do we call foliated? What can you say about non-foliated metamorphic rocks?

6. How can geologists trace the original structure of metamorphic rocks?

7. Why are metamorphic rocks often called crystalline schists?

Ex. 17. Find in the right column Russian equivalents of these words.

as a result of the chemical and physical changes constituents of rocks to be subjected to constant development to undergo changes excess of water low-grade ores coal band to cleave into separate layers traces of original structure generally speaking

а) полоса (или прослоек) угля б) составляющие пород в) расщепляться на отдельные слои г) вообще говоря д) в результате химических и физических изменений е) избыток воды ж) изменяться з) находиться в постоянном развитии и) низкосортные руды к) следы первоначальной структуры

Ex. 18. Find in the right column English equivalents of these words.

иметь значение упомянутые выше сланцеватая структура в отличие от гранита недостаток воды существовавшие ранее породы слоистые породы мрамор и сланец гнейс давать возможность определять структуру

unlike granite to be of importance mentioned above schistose structure to give an opportunity (of doing smth) to define (determine) rock texture deficiency of water flaky rocks marble and slate gneiss pre-existing rocks

Ex. 19. Find in columns A and B words close in meaning.

A B

1. band a) allow (let)

2. cleave b) size

3. constituent c) fissure

4. foliated d) take place

5. be like e) speed

6. permit f) layer

7. crack g) split

8. occur h) component

9. rate i) flaky

10. dimension j) be similar to

Ex. 20. Finish the sentences using the words from text B.

1. As for the origin of metamorphic rocks they ....

2. When we say that a mineral has a schistose structure it means ... .

3. It should be noted that the role of water in the formation of metamorphic rocks is great. It is known that high-grade metamorphic rocks are characterized by the absence of water while medium-grade rocks... .

4. Geologists should know every trace of the original structure of metamorphic rocks because ....

Ex. 21. Summarize the contents of text B using these words:

The process of metamorphism. (to mean, to show, to undergo changes, to be transformed, to call)

The role of water in the formation of metamorphic rocks, (to be interdependent, to be related to, in such a way, generally speaking, to be characterized by)

The difference between foliated and non-foliated metamor­phic rocks, (flaky, to cause, to split into, to cleave)

The structure of metamorphic rocks, (to be of importance, trace, pre-existing rocks, to give an opportunity of)

Ex. 22. Read teaxt C without a dictionary.

Text C

Minerals that make up rocks, are defined as inorganic sub­stances which occur naturally and have a definite chemical composi­tion and physical properties which vary within known limits.

The major properties are colour, crystal form, hardness, cleavage and others. Cleavage is one of the most diagnostically useful mineralogical properties which can be found throughout the mineral.

Minerals of use to man can be grouped into two broad catego­ries: 1) metals, such as aluminium, copper, gold, silver, iron, tin, platinum, chromium, nickel, lead and zinc, and 2) non-me­tallic minerals, such as diamonds, salt, limestone, cement, sul­phur, and asbestos. When minerals occur so that they can be worked at a profit they are called ore deposits. Mineral deposits are seldom equally rich throughout.

Economic minerals are those which are of economic impor­tance and include both metallic and non-metallic minerals.

Most minerals consist of several elements. Such elements are oxygen, silicon, titanium, aluminium, iron, magnesium, calcium, sodium, potassium and hydrogen. They make up more than 99 per cent by weight of all the rock-forming minerals. Of these, alu­minium, iron and magnesium are industrial metals. The other metals are present in small quantities, mostly in igneous rocks.

For example, iron is one of the most abundant metals in the Earth's crust. There are three important classes of iron deposits: deposits associated with igneous rocks; residual deposits and sedi­mentary deposits. Iron deposits associated with igneous rocks are usually small but very rich bodies either of haematite or magnetite. Large concentrations have been successfully mined in Pennsylvania (the USA) and in the Russian Federation.

Residual deposits of iron minerals are formed wherever weather­ing occurs. Iron deposits formed this way are very widespread. It should be stressed that the residual deposits were among the first to be exploited by man.

Sedimentary iron deposits make up most of the world's current production.

As the essential component of every variety of steel, iron is ob­viously the most important of all industrial metals. It has played a large part in the development of our modern civilization. Iron ores are mainly used for producing cast iron, steels and ferro-alloys. From a scientific point of view, iron's most important property is that it becomes magnetized.

The magnetic iron ore is the main wealth of the Kursk Magnetic Anomaly (KMA). It is necessary to say that only in the last century was the secret of the unusual magnetism of enormous iron ore masses discovered underground.

Iron fields are worked by surface mining which is more eco­nomical. But the KMA is rich not only in iron ores. Its deposits contain bauxite, phosphorite, cement, sand and clays.

Ex. 23. Divide text C into logical parts, find in each part a sentence expressing its main idea, give each part a title.

Ex. 24. Find in the text answers to these questions:

Why is cleavage the most important property of minerals?

How can ore deposits be defined?

Ex. 25. Find in the text information on these points:

• The main groups of minerals.

• The composition of minerals.

• The important classes of iron ore deposits.

• Industrial importance of iron ores.

• The characteristic feature of the KMA.

Ex. 26. Which of these titles best suits the tex?

Mineral Deposits.

The Composition of Minerals.

Iron Ores of Importance to Man.

Ex. 27. Say what information from the text was new to you.

Ex. 28. Read the text without a dictionary.