Geological Classification of Rocks

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热心网友 时间：2022-04-11 11:03

It is convenient to divide the rocks in the earth's crust into three different types based on their origin，namely igneous，sedimentary and metamorphic rocks. Igneous rocks are those formed by the solidification of molten magma，either at depth in the earth's crust or by extrusion，hence， their classification as plutonic，hypabyssal，or volcanic，depending on the depth and rate of their cooling and its effect on their texture or crystal size. Igneous rocks are also subdivided by their composition into acid，intermediate and basic rocks，depending on the amount of silica in their composition. A full classification is given in Table 1. 3.

Table 1. 3 Geological classification of igneous rocks

Sedimentary rocks，the second major type，are those rocks formed by the deposition( usually under water) of procts largely formed by the destruction of pre-existing igneous rocks. They tend to be weaker than igneous rocks，largely because of the hydration of feldspars to form kaolinite and the introction of organic minerals such as calcite. Sedimentary rocks can be subdivided into three main groups according to their method of formation，namely those mechanically formed， those formed from organic remains and those chemically deposited ( Table 1. 4) . From an engineering point of view，the most important are arenaceous ( sand) rocks，argillaceous ( clay ) rocks and calcareous ( limestone) rocks. Typical arenaceous rocks consist of discrete fragments of mineral—usually quartz，held together by a matrix of calcite. Thus when a sandstone is broken，the fracture follows the weaker calcareous cement rather than cutting across the stronger grains. An argillaceous rock such as a clay or shale consists of minute particles held together weakly and comprising largely kaolinite. The calcareous rocks consist of organic remains or precipitates，mainly in the form of calcite and limestone.

Table 1. 4 Geological classification of sedimentary rocks

Metamorphic rocks may be either igneous or sedimentary rocks which have been altered physically and sometimes chemically by the application of intense heat or pressure at some time in their geological history. They are classified by their physical structure，i. e. massive or foliated structure ( Table 1. 5) .

Table 1. 5 Geological classification of metamorphic rocks

It has been estimated that the earth's crust is made up of about 95% igneous rocks，about 5% sedimentary rocks and an insignificant proportion of metamorphic rocks. This does not， however，give a completely true picture of the rocks likely to be encountered by engineers working in rock. The earth's crust may be assumed to be from 30 to 50 km in thickness and virtually all major works take place in the top a fewkilometers which contain the major part of the sedimentary rocks. In addition，a high percentage of these sedimentary rocks will be argillaceous，and the majority of the remainder being arenaceous or calcareous.

This is in some ways unfortunate，igneous rocks are with fewexceptions competent， massive and strong，while sedimentary rocks are weak and strongly foliated and jointed. Of the sedimentary rocks，the arenaceous and calcareous，under favorable conditions，approach nears to the ideal of the igneous rocks. The argillaceous rocks depart furthest from them.

Argillaceous rocks comprise mainly shale，normally closely bedded or laminated. The former are reasonably strong in a dry state，but weak when wet; the latter tend to have intermediate strength under most conditions，but are easily deformed under load. The problems encountered in mining，tunneling or foundation work in such rocks are immediately apparent.

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关于岩石的分类词汇扩展

( 1) 地球 ( Earth)

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( 2) 地貌 ( Geomorphology)

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( 3) 地质结构 ( Geologic structure)

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( 4) 不利地质现象( Adverse geologic phenomena)

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( 5) 地质年代 ( Geologic chronicle)

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( 6) 矿物的物理性质( Physical properties of minerals)

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( 7) 矿物的十级硬度( Mineral hardness)

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( 8) 岩体( Rock mass)

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( 9) 岩石的性质指标 ( Index of rock properties)

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( 10) 岩石类别( Rock classification)

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