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The rocks of the Earth's crust are divided into three main groups: sedimentary rocks, which consist of fragments or particles of pre-existing rocks; igneous rocks which have solidified from magma and metamorphic rocks. Metamorphic rocks have been derived from either igneous or sedimentary rocks.
Sedimentary rocks represent one of the three major groups of rocks that make up the crust of the Earth. Most sedimentary rocks have originated by sedimentation. They are layered or stratified. Thus, stratification is the most important characteristic of sediments and sedimentary rocks. It is necessary to note that the processes which lead to the formation of sedimentary rocks are going on around us.
Sediments are formed at or very near the surface of the Earth by the action of heat, water (rivers, glaciers, seas and lakes) and organisms.
It should be noted that 95 per cent of the Earth's crust is made up of igneous rocks and that only 5 per cent is sedimentary. In contrast, the amount of sedimentary rocks on the Earth's surface is three times that of igneous rocks. Strictly speaking, sedimentary rocks form a very small proportion by volume of the rocks of the Earth's crust. On the contrary, about three quarters of the Earth's surface is occupied by sedimentary rocks. It means that most of sedimentary rocks are formed by sediments, accumulations of solid material on the Earth's surface.
The thickness of the layers of sedimentary rocks can vary greatly from place to place. They can be formed by the mechanical action of water, wind, frost and organic decay. Such sediments as gravel, sand and clay can be transformed into conglomerates, sandstones and clay schists as a result of the accumulation of materials achieved by the destructive mechanical action of water and wind.
Mechanical sediments can be unconsolidated and consolidated. For example, gravel, sand and clay form the group of unconsolidated mechanical sediments, because they consist of loose uncemented particles (grains).
On the Earth's surface we also find consolidated rocks, which are very similar to the loose sediments whose particles are firmly cemented to one another by some substance. The usual cementing substances are sand, clay, calcium carbonate and others. Thus sandstones are consolidated rocks composed of round or angular sand grains, more or less firmly consolidated. Like sand, sandstones can be divided into fine-grained, medium-grained and coarse-grained.
On the other hand, chemical sediments are the result of deposits or accumulations of substances achieved by the destructive chemical action of water. The minerals such as rock salt, gypsum and others are formed through sedimentation of mineral substances that are dissolved in water.
Sediments can also be formed by the decay of the remains of organisms, by the accumulation of plant relics. They are called organic sediments. Limestones, peat, coal, mineral oil and other sediments may serve as an example of organic sediments.
The most principal kinds of sedimentary rocks are conglomerate, sandstone, siltstone, shale, limestone and dolomite. Many other kinds with large practical value include common salt, gypsum, phosphate, iron oxide and coal.
As is known, water, wind and organisms are called external forces, because their action depends on the energy which our planet receives from the Sun.
Пояснения к тексту:
· clay – n глина; глинозем
· consolidate – v твердеть, затвердевать, уплотнять(ся); укреплять
· grained - а зернистый
· gravel – n гравий, крупный песок
· decay - v гнить, разлагаться
· extrusive – a эффузивный, излившийся (о горной породе)
· grain – n зерно; angular grains - угловатые зерна (минералов); fine-grained (medium-grained, coarse-grained) ~ мелкозернистый (среднезернистый, грубозернистый)
· layer – n пласт
· lime – n известь; limestone – n известняк
· loose - а несвязанный, свободный; рыхлый
· particle – n частица; включение
· peat – n торф; торфяник
· siltstone – n алеврит
· rock – n горная порода; igneous ~ изверженная порода; sedimentary ~ осадочная порода
· stratification – n напластование, залегание
· sandstone – n песчаник
· sediment – n отложение; осадочная порода
· schist – n (кристаллический) сланец; schistose – а сланцеватый
· shale – n сланец, сланцевая глина, глинистый сланец
2. Укажите, какие предложения соответствуют содержанию текста.Подтвердите свои ответы фактами из текста.
1) The rocks of the Earth's crust are divided into two main groups.
2) Igneous rocks are composed of particles of pre-existing rocks.
3) Sedimentary rocks are stratified.
4) Sediments are formed by the action of glaciers.
5) Igneous rocks make up 75 per cent of exposed rocks.
6) Conglomerates are formed as a result of the accumulation of materials caused by the destructive mechanical action of water.
7) Sandstones are consolidated rocks.
8) Clays are unconsolidated mechanical sediments.
9) Chemical sediments are formed by the destructive chemical action of water.
10) Peat and coal are the organic sediments which are of great practical value.
11) Clay schist was formed at the beginning of the sedimentation period and clay was formed later.
3. Письменно ответьте на следующие вопросы по тексту:
1. What main groups of rocks do you know?
2. Do sedimentary rocks consist of particles of pre-existing rocks?
3. How were igneous rocks formed?
4. Do you know how sedimentary rocks have originated?
5. What is the most important characteristic feature of sediments?
6. Do sedimentary rocks account for 10 per cent of the Earth's crust?
7. Is gravel consolidated mechanical sediment? And what about sand and clay?
8. What are cementing substances? Can calcium carbonate be used as a cementing substance?
9. Are there only fine-grained sandstones?
10. What can you say about chemical sediments?
11. Can you give an example of organic sediments? How are they formed?
4. Найдите в правой колонке русские эквиваленты следующих слов и словосочетаний и выпишите их:
5. Заполните пропуски в предложениях, используя приведенные слова: