Unit 6 metamorphic rocks
Words to be remembered:
primary = original – первоначальный, изначальный
because of – из-за
texture – текстура
mineral composition – минеральный состав
to transform – превращать, изменять, трансформировать
gneiss – гнейс
mica – слюда
mica schist – слюдяной сланец
marble – мрамор
slate = shale – сланец
quartz – кварц
unlike – в отличие от
schistose structure – сланцеватая, слоистая структура
band = layer – слой, пласт
to run parallel – располагаться (лежать) параллельно друг другу
amount – количество
grade – сорт
excess – избыток
deficiency – недостаток
absence – отсутствие
consist of – состоять из
flaky – слоистый, похожий на хлопья
to cause – вызывать, заставлять
to split (split, split) – раскалываться, расщепляться, трескаться
sheet – полоса
foliated – листоватый, тонкослоистый
‘Metamorphic’ means ‘changed from’. It shows that the original rock has been changed from its primary form to a new one. Because of pressures, heat and chemically active fluids under the Earth's surface various rocks in the Earth's crust change in texture, in mineral composition and structure and are transformed into metamorphic rocks. This process is called metamorphism.
Metamorphic rocks have been developed from earlier igneous and sedimentary rocks by the action of heat and pressure. Gneisses, mica schists, phyllites, marbles, slate, quartz, etc. belong to the same group of rocks. They have the same mineral composition as granite, but gneisses consist 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.
The role of water in metamorphism is determined by four 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 are defined by some deficiency of water and the high-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, 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 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.
Metamorphic rock, 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.
Questions on the text:
1) What does the word ‘metamorphic’ mean? 2) How have metamorphic rocks been formed? 3) 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) How does the amount of water affect (влияет на) the grade of metamorphic rocks? 6) What rocks do we call foliated? 7) What can you say about non-foliated metamorphic rocks? 8) Why is the structure of metamorphic rocks of importance? 9) Where do we find metamorphic rock?