Rocks


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Rocks

 

natural aggregates of minerals of more or less constant composition which form the independent geological bodies that comprise the earth’s crust. The Russian term for rocks in its modern sense was first used by the Russian mineralogist and chemist V. M. Severgin (1798).

Rocks are mechanical combinations of minerals, including liquid minerals, that differ in composition. The percentage of minerals in a rock determines its mineral composition. The shape, dimensions, mutual arrangements, and orientation of mineral grains or particles of the rock determine its structure and texture.

Rocks are divided into three groups on the basis of origin: magmatic (igneous), sedimentary, and metamorphic. Magmatic and metamorphic rocks constitute about 90 percent of the volume of the earth’s crust, with the other 10 percent falling to sedimentary rocks, although sedimentary rocks occupy 75 percent of the area of the earth’s surface.

Igneous rocks are formed as a result of the solidification of magma. In deep-lying parts of the earth’s crust magma cools slowly and crystallizes well, and crystalline granular rocks form from it. They are called intrusive rocks and include granites, syenites, and diorites. These rocks occur in the earth’s crust in the form of batholiths, stocks, laccoliths, and other bodies. Magma, which is poured out onto the earth’s surface in the form of volcanic lava, cools rapidly (part of it, instead of crystallizing, may harden in the form of volcanic glass), forming effusive rocks (such as basalts, andesites, and rhyolites) and also volcanic tuffs, which are cemented solid products of volcanic eruption (for example, ash, lapilli, and volcanic bombs). Effusive rocks often occur in the form of lava flows and sheets. The main rock-forming minerals of igneous rocks are the aluminosilicates and silicates (feldspars, quartz, and mica).

Sedimentary rocks are formed on the earth’s surface and near it as a result of the transformation of marine and continental sediments under conditions of relatively low temperature and pressure. By method of formation sedimentary rocks are subdivided into three genetic groups: detrital rocks (breccia, conglomerates, sands, and silts), which are coarse products of primarily mechanical breaking down of parent rocks and usually inherit the more stable mineral associations of the parent rocks; clay rocks, which are the dispersion products of the deep-seated chemical transformation of silicate and aluminosilicate minerals of parent rocks that have changed into new mineral types; and chemogenic, biochemogenic, and organogenic rocks, which are the products of direct precipitation from solutions (for example, salt) with the participation of organisms (for example, siliceous rocks), products of the accumulation of organic matter (for example, coal), and products of the activity of organisms (for example, organogenic limestone). The group of effusive-sedimentary rocks occupies an intermediate position between sedimentary and volcanic rocks. Mutual transitions are observed among the basic groups of sedimentary rocks; they arise as the result of displacement of material of different genesis. A typical characteristic of sedimentary rocks, related to the conditions under which they were formed, is their layered quality and occurrence in the form of more or less regular strata.

Metamorphic rocks are formed deep in the earth’s crust as a result of the alteration (metamorphism) of sedimentary or igneous rocks. Factors that may cause these alterations include the proximity of a cooling magmatic body and the related heating of the rock undergoing metamorphism; the action of active chemical compounds leaving this body, above all, various water solutions (contact metamorphism); or burying of the rock deep in the earth’s crust where factors of regional metamorphism—high temperatures and pressures—act on it. Regionally metamorphosed rocks are characterized by schistosity, the presence of a number of specific minerals (for example, cordierite, andalusite, and kyanite), and also structures that sometimes preserve traces of the structures of the original rocks (so-called relict structures). Typical metamorphic rocks are crystalline schists of various composition, contact hornfels, skarns, gneisses, amphibolites, and migmatites. The difference in origin and, as a result of this, in the mineral composition of rocks is sharply reflected in their chemical composition and physical properties.

The chemical composition of igneous rocks consisting primarily of silicate minerals is characterized by a large amount of silicic acid. Igneous rocks are divided on the basis of SiO2 content into acid (more than 65 percent), average (55–65 percent), and basic (less than 55 percent). In addition there are certain rarer ultra-acid rocks that are very rich in SiO2 (certain aplites) and ultrabasic rocks containing less than 45 percent SiO2. and a great deal of magnesium oxide. Rocks that are rich in alkali metals form a separate group known as the alkalis. Rocks that differ in content of the main elements also differ in content of admixture elements. Thus, the acid rocks have increased concentrations of such elements as Be, W, Sn, Pb, Zn, Cu. and Au. while the basic rocks have more Ni, Cr, and Pt. Large concentrations of phosphorus are frequently contained in alkaline rocks. In addition to the general distribution of various elements, there are specific relations between particular elements and/or deposits and rocks of a certain region (the so-called metal-logenic specifics of intrusive rocks). The chemical composition of sedimentary rocks differs from that of igneous rocks by a much greater differentiation, a broad range of fluctuation in the content of rock-forming components (for example, SiO2 varies from 0 percent [salt] to 100 percent [pure quartz sand] and CaO varies from fractions of a percentage point [pure kaolin clays] to 56 percent [limestones]), the increased content of water, carbonic acid, organic carbon, and “excess volatiles” (such as S, CI. and B), and also by high ratios of ferric iron to ferrous iron. In composition metamorphic rocks are close to parent sedimentary or igneous rocks, although during the process of recrystallization or metasomatism many ore elements may become concentrated in them, creating ore deposits.

As a physical body a rock is characterized by a group of basic properties that include density, elasticity, and strength, as well as thermal, electrical, and magnetic properties. The most probable limits of change in the basic properties of rocks are given in Table 1.

Table 1. Limits of change In the basic properties of rocks
Porosityup to 60 percent
Density800–8.000 kg/m3
Young’s modulus10–200 giganewtons/m3
Poisson’s ratio0.07–0.38
Compression strengthup to 500 meganewtons/m3
Tensile strengthup to 20 meganewtons/m3
Heat conductivity0.1–10 watts/(nv°K)
Coefficient of linear expansion1 .100-9.1081/° C
Specific electric resistance103-1O14 ohms.m
Relative permittivity2–30
Relative magnetic permeability0.9998–4

The properties of rocks are determined by their mineral composition and structure and also by external conditions. The important parameters that determine the properties of the rock are its porosity and jointing. The pores may be partially filled with liquid, and therefore the properties of the rock depend simultaneously on the properties of the solid, gas, and liquid phases and their interrelations. Porosity and jointing are particularly important in evaluating rocks as oil and water reservoirs, assessing the speed of the oil’s or water’s flow to a well or borehole, and the like. Porosity and jointing also determine the moisture and gas capacity of rocks and their water and gas permeability. In igneous rocks the gas cavities may reach 60–80 percent (pumice and pumice tuffs). In sedimentary rocks pores are created at the moment of sediment formation (intergranular pores), and they may close or be preserved when cementation occurs. A large number of pores arises when porous grains (shells of radiolar-ia and diatoms) accumulate. Metamorphic rocks usually have very few pores: they have only the cracks caused by the cooling of the rock.

The density of a rock is closely connected with porosity and mineral composition. In rocks that have no porosity it is determined by the component minerals. Ore minerals have a high density (up to 5.000 kg/m3 for pyrite and 7.570 kg/m3for galena): lower density is typical of the minerals of sedimentary rocks (for example, rock salt has a density of 2.100 kg/m3). Because of porosity the density of rocks may differ sharply from the density of their component minerals. Thus Armenian pumice tuffs have a density of about 800–900 kg/m3. while the density of granites, marbles, compact limestones, and sandstones is about 2,600 kg/m3. The density of rocks is easy to calculate on the basis of mineral composition and porosity: reverse calculations are possible and very useful.

Rock properties such as thermal capacity and coefficient of volumetric thermal expansion are determined primarily by the mineral composition, while the strength and elastic properties of rocks and their heat and electric conductivity depend mainly on the structure of the rocks and especially on the strength of the bonds between grains. Thus the presence of a predominant orientation of grains leads to anisotropy of properties. Oriented jointing may also be a factor in creating anisotrophy of properties.

Rock properties determined along or across layers or veins usually differ. In this case Young’s modulus, maximum tensile strength, thermal conductivity, electric conductivity, permittivity, and magnetic permeability are greater along the layers, while maximum compression strength is greater across the layers. Strength properties are higher in small-grained rocks than in large-grained ones. Small-grained rocks with fibrous structure have especially high values for maximum compression strength (for example, nephrite has a value up to 500 meganewtons/m2). Many sedimentary rocks (including rock salt and gypsum) have a low maximum compression strength. The elastic properties of rocks determine their acoustic properties (speed of propagation and the index of refraction, reflection coefficient, and coefficient of absorption of elastic waves) and electromagnetic properties (correspondingly, speeds of propagation and the absorption coefficient, reflection coefficient, and refraction index of electromagnetic waves). Rocks are usually poor conductors of heat, and with an increase in porosity their heat conductivity worsens. Rocks that contain semiconductors, such as graphite and iron and complex ores, have greater heat conductivity. In terms of electrical conductivity most rocks are nonconductors or semiconductors. The magnetic properties of rocks are determined primarily by the ferromagnetic minerals present in them (magnetite, titanomagnetite, hematite, and magnetic pyrite).

The properties of rocks also depend on the effect of various fields including the mechanical (pressure), the thermal (temperature), electrical, magnetic, radiation (field intensity), and the material (saturation with liquids, gases, and the like). When hard rocks are saturated with water there is an increase in elastic parameters, heat conductivity, heat capacity, electric conductivity, and permittivity; when easily dissolved minerals (halogen compounds) and clayey rocks are saturated with water, their elastic and strength indexes are reduced. Change in the properties or rocks under the influence of pressure is caused by rock consolidation, crushing of the pores, and an increase in the area of grain contact. With an increase in pressure, properties such as electric conductivity, heat conductivity, and strength usually increase. An increase in temperature reduces the elastic and strength properties and increases the plastic characteristics of rocks; in addition, it reduces heat conductivity and increases heat capacity, electric conductivity, and permittivity. The appearance of internal thermal stresses resulting from a different thermal expansion of particular minerals leads to an increase or decrease in elastic and strength properties of the rocks depending on the direction of the resulting stresses. Restructuring of the crystal lattice of minerals because of heat (polymorphic transformations and the like) causes points of anomaly in the chart showing property dependence on temperature. For example, for quartzites the minimum value of Young’s modulus and the maximum value for the coefficient of linear expansion are observed at the point of polymorphic transition of β-quartz into α-quartz (573° C). The action of heat also leads to agglutination, decomposition, fusion, sublimation, and evaporation of particular minerals, which changes the properties of the rocks accordingly. The intensity and frequency of electromagnetic fields exert the greatest influence on the electromagnetic and radio-wave properties of rocks. This is caused by the energy action of fields on rock particles, which leads to their electric and magnetic reorientation (polarization and magnetization) and excitation of electrons and ions. Thus, an increase in intensity leads to a rise in electric conductivity, permittivity, and magnetic permeability.

As objects of mining work rocks are characterized by various technological properties—such as toughness, abrasive-ness, hardness, resistance to drilling, and explosiveness. (Toughness rates the resistance of rocks to mechanical destruction, while abrasiveness describes the capacity of rocks for wearing down the cutting edges of working mechanisms.) Various rock classifications by technological properties are used to select efficient methods and mechanisms for breaking them up. (For example, in mining practice the classification of rocks by toughness that was proposed by professor M. M. Protod’iakonov, Sr. is widely used.)

The study of the material composition and physical and physicochemical properties of rocks is a primary source of information in geophysics, geology (including engineering geology), and mining production.

REFERENCES

Kuznetsov, E. A. Petrografiia magmaticheskikh i metamorfiche-skikh porod. Moscow, 1956.
Baron, L. I., B. M. Loguntsov, and E. Z. Pozin. Opredelenie svoistvgornykh porod. Moscow, 1962.
Rzhevskii, V. V., and G. Ia. Novik. Osnovy fiziki gornykh porod. Moscow, 1967.
Ronov, A. B., and A. A. Iaroshevskii. “Khimicheskoe stroeniezemnoi kory.” Geokhimiia, 1967, no. 11.
Spravochnik fizicheskikh konstant gornykh porod. Moscow, 1969. (Translated from English.)
Mineraly i gornye porody SSSR. Moscow, 1970.
Shvetsov, M. S. Petrografiia osadochnykh porod. Moscow, 1958.
Huang, W. T. Petrology. New York, 1962.

G. IA. NOVIK, V. P. PETROV, V. V. RZHEVSKII, and A. B. RONOV

References in classic literature ?
Oh, yes," said Dorothy, now observing for the first time the crack in the rock.
He placed a finger in his mouth, and drew a long, shrill whistle, which was answered from the rocks that were guarded by the Mohicans.
It was round one of these terminations of the sweep of the rock that Edwards had ascended, and it was toward the same place that he urged Elizabeth to a desperate exertion of speed.
From above it seemed as though rope and cliff were well-nigh touching, but now, when swinging a hundred feet down, the squire found that he could scarce reach the face of the rock with his foot, and that it was as smooth as glass, with no resting-place where a mouse could stand.
After an hour's painful progress, we reached the verge of another fall, still loftier than the preceding and flanked both above and below with the same steep masses of rock, presenting, however, here and there narrow irregular ledges, supporting a shallow soil, on which grew a variety of bushes and trees, whose bright verdure contrasted beautifully with the foamy waters that flowed between them.
for another creature just like the first had suddenly appeared sitting on another rock, its black side toward them.
Just at the moment when they were taking the dainty animal from the spit, they saw Edmond springing with the boldness of a chamois from rock to rock, and they fired the signal agreed upon.
A little after, as I was jumping about after my limpets, I was startled by a guinea-piece, which fell upon a rock in front of me and glanced off into the sea.
As for you, coxswain, these are your orders; attend to them, for the ship is in your hands; turn her head away from these steaming rapids and hug the rock, or she will give you the slip and be over yonder before you know where you are, and you will be the death of us.
The rock is phonolite, and is divided into irregular columns.
At the distance of a mile and a half from the foot of this narrow channel is a rapid, formed by two rocky islands; and two miles beyond is a second great fall, over a ledge of rocks twenty feet high, extending nearly from shore to shore.
It was with difficulty held to the rocks by a boat hook, for the current rushed furiously round the point.