TEXT 1 - 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 any type of rocks (igneous, sedimentary or metamorphic).

Sedimentary rocks represent one of the three major groups of rocks that make up the Earth’s crust. 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, seas and lakes), glaciers, wind and organisms.

It should be noted that 95% of the Earth’s crust is made up of igneous rocks and that only 5% 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 action of water, wind, frost and organic decay. Such sediments as gravel, sand and clay can by transformed into conglomerates, sandstones and claystones 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 every similar to the loose sediments whose particles are firmly cemented to one another by some substance. The usual cementing substances are sand, clay, calcium carbonates 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, and coal may serve as an example of organic rocks.

The most principal kinds of sedimentary rocks are conglomerate, sandstone, siltstone, shale, limestone and dolomite. Many other kinds with practical value include common salt, gypsum, phosphorite, 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.

(М.Я. Баракова, Р. И. Журавлева, Английский язык для горных инженеров, М. 2001)