Unit_4_ТЕКСТ А Sedimentary Rocks

Date: 2015-10-07; view: 557.

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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 rods have origi­nated by sedimentation. They are layered or stratified. Thus, statification 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.

Fig. 1 shows relative abundance 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.