Table 6 Classification of Common Metamorphic Rocks
UNIT 7 METAMORPHIC ROCKS 1. COMPREHENSION
Vocabulary
1. Pay attention to the pronunciation of the following terms:
gneiss slate granite
mica quartz orthoclase
phyllite schistose
marble chlorite
2. Pay attention to the underlined terms and expressions in the text:
| schistose | сланцеватый |
| constituent | составная часть; компонент |
| band | слой; прослоек |
| to cleave | расщепляться |
| plate | пластина |
| excess | излишек |
| low-grade | низкая ступень (метаморфизма) |
| medium-grade | средняя ступень (метаморфизма) |
| high-grade | высокая ступень (метаморфизма) |
| flaky | чешуйчатый; расслоенный |
| split into | раскалываться на |
| sheet | пластина, лист |
| foliated | листоватый; сланцеватый |
| dislocation | нарушение, дислокация |
| distribute | распределять; раздавать |
| disturb | нарушать; смещать |
3. Read the following word formations and remember their pronunciation:
| cleave | cleavage | ||
| foliate | foliated | foliation | |
| relate | related | relationship | relative |
| differ | different | difference |
Read the text and fulfill the after reading exercises. (R.P – 7.2, 7.3)
METAMORPHIC ROCKS
Metamorphic rocks 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 are transformed into metamorphic rocks. This process is called metamorphism. As is known, metamorphic rocks developed from earlier igneous, sedimentary or metamorphic rocks by the action of heat and pressure.
Gneiss, mica schist, phyllite, marble, slate, quartzite belong to the same group of rocks. Having the same mineral composition as granite, gneiss consists chiefly of quartz, orthoclase and mica. However unlike granite, they have a schistose structure. It means that their constituents are distributed in bands or layers and run parallel to each other in one direction. If disturbed the rock cleaves easily into separate plates.
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Fig. 38 Gneiss Fig. 39 Marble
The role of water in metamorphism is determined by at least four variable geologically related parameters: rock pressure, temperature, water pressure and the amount of water present.
During a normal progressive metamorphism rock pressure and temperature are interdependent and the amount of water and the pressure of water are related to the sediments and to the degree of metamorphism in such a way that the excess of water characterizes the low-grade metamorphic rocks. The medium-grade metamorphic rocks are characterized by the absence of water.
Many of the metamorphic rocks consist of flaky materials such as mica and chlorite. These minerals cause the rock to split into thin sheets and rocks become foliated. Slate, 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 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 analyzing the causes of its metamorphism. 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 took place.
(М.Я. Баракова, Р. И. Журавлева, Английский язык для горных инженеров, М. 2001)
| TEXT URE | DIAGNOSTIC FEATURES | ROCK NAME |
| Granulitic texture, medium to coarse, even grained. Foliation present in light-colored quartzo-feldspathic rocks, but virtually lacking in pyroxene-plagioclase-bearing rocks. | GRANULITE | |
| Gneissic texture. Coarse grained. Foliation present as macroscopic grains arranged in alternating light and dark bands. Abundant quartz and feldspar in light-colored bands. Dark bands may contain hornblende, augite, garnet, or biotite. | GNEISS | |
| Schistose texture. Medium to fine grained. Common minerals are chlorite, biotite, muscovite, garnet, and dark elongate silicate minerals. Feldspars commonly absent. Recognizable minerals used as part of rock name. Porphyroblasts common. | SCHIST | |
| Phyllitic texture. Fine grained to dense. Micaceous minerals are dominant. Has a sparkling appearance. | PHYLLITE | |
| Slaty texture (slaty cleavage apparent). Dense, microscopic grains. Color variable; black, and dark gray common. Also occurs in green, dark red, and dark purple colors. | SLATE | |
| Crystalline. Hard (scratches glass). Breaks across grains as easily as around them. Color variable; white, pink, buff, brown, red, purple. | QUARTZITE | |
| Dense, dark-colored; various shades of gray, gray-green, to nearly black. | HORNFELS | |
| Crystalline. Composed of calcite or dolomite. Color variable; white, pink, gray, among others. Fossils in some varieties. | MARBLE | |
| Black, shiny luster. Conchoidal fracture. | ANTHRACITE COAL |
Table 6 Classification of Common Metamorphic Rocks
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EXERCISES
2.1 Define the following terms with their similar meaning in Russian metamorphic rocks; some changes in texture; in mineral composition; the description of metamorphism; schistose structure; the role of water; four variable geologically related parameters; flaky materials; the mechanism of metamorphic deformation; crystalline schists; the great dislocation of the Earth’s crust; during normal progressive metamorphism.
2.2 Match the English equivalents to the Russian terms .
1. as a result of the chemical and a. составляющие пород physical changes
2. constituents of rocks b. расщепляться на отдельные слои
3. to be subjected to constant c. в результате химических и development физических изменений
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4. to undergo changes d. избыток воды
5. excess of water e. находиться в постоянном
развитии
6. to cleave into separate layers f. следы первоначального строения
7. traces of original structure g. изменяться
2.3 Match the Russian equivalents to the English terms .
1. сланцеватая текстура a. unlike granite
2. в отличие от гранита b. schistose structure
3. недостаток воды c. pre-existing rocks
4. существовавшие ранее породы d. to define (determine) rock texture
5. расслоенные породы e. deficiency of water
6. мрамор и сланец f. flaky rocks
7. гнейс g. marble and slate
8. определить структуру h. gneiss
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