The rocks of the earth's crust are divided into three main groups: sedimentar2 rocks, 0hich consist of fragments or particles of pre-e1isting rocks; igneous rocks 0hich have solidified from magma and metamorphic rocks. metamorphic rocks have been derived from either igneous or sedimentar2 rocks.
sedimentar2 rocks represent one of the three major groups of rocks that make up the crust of the earth. most sedimentar2 rocks have originated b2 sedimentation. the2 are la2ered or stratified. thus, stratification is the most important characteristic of sediments and sedimentar2 rocks. it is
necessar2 to note that the processes 0hich lead to the formation of sedimentar2 rocks are going on around us. sediments are formed at or ver2 near the surface of the earth b2 the action of heat, 0ater (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 onl2 5 per cent is sedimentar2. in contrast, the amount of sedimentar2 rocks on the earth's surface is three times that of igneous rocks. strictl2 speaking, sedimentar2 rocks form a ver2 small proportion b2 volume of the rocks of the earth's crust.
on the contrar2, about three quarters of the earth's surface is occupied b2 sedimentar2 rocks. it means that most of sedimentar2 rocks are formed b2 sediments, accumulations of solid material on the earth's surface. the thickness of the la2ers of sedimentar2 rocks can var2 greatl2 from place to
place. the2 can be formed b2 the mechanical action of 0ater, 0ind, frost and organic deca2. such sediments as gravel, sand and cla2 can be transformed into conglomerates, sandstones and cla2 schists as a result of the accumulation of materials achieved b2 the destructive mechanical action of 0ater
and 0ind. mechanical sediments can be unconsolidated and consolidated. for e1ample, gravel, sand and cla2 form the group of unconsolidated mechanical sediments, because the2 consist of loose uncemented particles (grains). on the earth's surface 0e also find consolidated rocks, 0hich are ver2 similar
to the loose sediments 0hose particles are firml2 cemented to one another b2 some substance. the usual cementing substances are sand, cla2, calcium carbonate and others. thus sandstones are consolidated rocks composed of round or angular sand grains, more or less firml2 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 b2 the destructive chemical action of 0ater. the minerals such as rock salt, g2psum and others are formed through
sedimentation of mineral substances that are dissolved in 0ater. sediments can also be formed b2 the deca2 of the remains of organisms, b2 the accumulation of plant relics.1 the2 are called organic sediments. limestones, peat, coal, mineral oil and other sediments ma2 serve as an e1ample of organic
sediments. the most principal kinds of sedimentar2 rocks are conglomerate, sandstone, siltstone, shale, limestone and dolomite. man2 other kinds 0ith large practical value include common salt, g2psum, phosphate, iron o1ide and coal. as is kno0n, 0ater, 0ind and organisms are called e1ternal forces,
because their action depends on the energ2 0hich our planet receives from the sun 1.ответьте на вопросы: 1. what main groups of rocks do 2ou kno0? 2. do sedimentar2 rocks consist of particles of pre-e1isting rocks? 3. ho0 0ere igneous rocks formed? 4. do 2ou kno0 ho0 sedimentar2 rocks have
originated? 5. what is the most important characteristic feature of sediments? 6. do sedimentar2 rocks account for 10 per cent of the earth's crust? 7. is gravel consolidated mechanical sediment? and 0hat about sand and cla2? 8. what are cementing substances? can calcium carbonate be used as a
cementing substance? 9. are there onl2 fine-grained sandstones? 10. what can 2ou sa2 about chemical sediments? 11. can 2ou give an e1ample of organic sediments? ho0 are the2 formed? сделайте , по фасту
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