ТЕКСТ А Sedimentary Rocks

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_f Most sedimentary rods have origi­nated by sedimentation. They are layered or stratified. Thus, stntification is the most important characteristic of sediments and sedimentary rocks j 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.

Fig. 1 shows relative abundance¹ of sedimentary rocks and igne­ous rocks. It should be noted that 95 per cent of the Earth's crust is made up of igneous rocks (see Fig. 1 left) and that only 5 per cent is sedimentary. 11n contrast, the amount of sedimentary rocks on the Earth's surface is three times that of igneous rocks (see Fig. 1 right)-:

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.

Fig. 1. Relative abundance of sedimentary and igneous rocks

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 unce-mented 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

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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.² 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

1. relative abundance — относительная распространенность (минералов в

земной коре)

2. plant relics (plant remains) — растительные остатки

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