Unit 4 sedimentary rocks
Words to be remembered:
the Earth's crust – земная кора
sedimentary rocks – осадочные породы
igneous rocks – изверженные породы, вулканические породы
metamorphic rocks – метаморфические породы
either … or – или…или, либо … либо
pre-existing – ранее существовавший
to solidify from – затвердевать (из)
be derived from – происходить (от), вести свое происхождение (от)
to stratify – напластовываться, залегать пластами
stratification – напластование, залегание
layer =bed = formation – пласт
surface – поверхность
organic decay – органическое разложение, распад
mechanical sediments – механические осадки
unconsolidated – неуплотненный
consolidated – уплотненный
to consist of – состоять из
loose = uncemented – рыхлый = несцементированный
particles = grains – частицы = зерна
gravel – гравий
sand – песок
sandstone – песчаник
clay – глина
clay schist – глинистый сланец
fine-grained – мелкозернистый
medium-grained – среднезернистый
coarse-grained – грубозернистый
calcium carbonate – карбонат кальция
destructive chemical action – разрушительное химическое действие
sedimentation – образование осадочных пород
to dissolve – растворять
limestone – известняк
organic sediments – органические осадки
The rocks of the Earth's crust are divided into three main groups: sedimentary rocks, igneous rocks and metamorphic rocks. Sedimentary rocks consist of fragments or particles of pre-existing rocks. Igneous rocks have solidified from magma. Metamorphic rocks have been derived from either igneous or sedimentary rocks.
Sedimentary rocks have originated by sedimentation. They are layered or stratified. 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. The thickness of the layers of sedimentary rocks can vary greatly from place to place. Sedimentary rocks can be formed by the mechanical action of water, wind, frost and organic decay.
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. Such sediments as gravel, sand and clay can be transformed into conglomerates, sandstones and clay schists. Like sand, sandstones can be divided into fine-grained, medium-grained and coarse-grained.
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. They are called organic sediments. Limestones, peat, coal, mineral oil and other sediments are examples of organic sediments.
The most principal kinds of sedimentary rocks are conglomerate, sandstone, shale, limestone and dolomite. Many other kinds with large practical value include common salt, gypsum, phosphate, iron oxide and coal.
Questions on the text:
1) What main groups of rocks do you know? 2) What are sedimentary rocks? What do they consist of? 3) How were igneous rocks formed? 4) What are metamorphic rocks? 5) How have sedimentary rocks originated? 5) What is the most important characteristic feature of sediments? 6) What groups are all sediments divided into? 7) What are the two types of mechanical sediments? 8) Is gravel a consolidated mechanical sediment? 9) And what can you say about sand and clay? 10) What are the usual cementing substances? 11) Can calcium carbonate be used as a cementing substance? 12) Are there only fine-grained sandstones? 13) What can you say about chemical sediments? 14) Can you give an example of organic sediments? How are they formed?