/rok/, n.
a male given name.

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In geology, a naturally occurring and coherent aggregate of minerals.

The three major classes of rock
are based on the processes that formed them. These three classes are further subdivided on the basis of various factors, especially chemical, mineralogical, and textural attributes (see e.g., acid and basic rocks; crystalline rock; extrusive rock). See also felsic rock; intrusive rock; mafic rock.
(as used in expressions)
Hudson Rock

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      in geology, naturally occurring and coherent aggregate of one or more minerals. Such aggregates constitute the basic unit of which the solid Earth is comprised and typically form recognizable and mappable volumes. Rocks are commonly divided into three major classes according to the processes that resulted in their formation. These classes are (1) igneous rocks, which have solidified from molten material called magma; (2) sedimentary rocks, those consisting of fragments derived from preexisting rocks or of materials precipitated from solutions; and (3) metamorphic rocks, which have been derived from either igneous or sedimentary rocks under conditions that caused changes in mineralogical composition, texture, and internal structure. These three classes, in turn, are subdivided into numerous groups and types on the basis of various factors, the most important of which are chemical, mineralogical, and textural attributes.

General considerations

Rock types
      Igneous rocks (igneous rock) are those that solidify from magma, a molten mixture of rock-forming minerals and usually volatiles such as gases and steam. Since their constituent minerals are crystallized from molten material, igneous rocks are formed at high temperatures. They originate from processes deep within the Earth—typically at depths of about 50 to 200 kilometres (30 to 120 miles)—in the mid- to lower-crust or in the upper mantle. Igneous rocks are subdivided into two categories: intrusive (emplaced in the crust), and extrusive (extruded onto the surface of the land or ocean bottom), in which case the cooling molten material is called lava.

      Sedimentary rocks (sedimentary rock) are those that are deposited and lithified (compacted and cemented together) at the Earth's surface, with the assistance of running water, wind, ice, or living organisms. Most are deposited from the land surface to the bottoms of lakes, rivers, and oceans. Sedimentary rocks are generally stratified—i.e., they have layering. Layers may be distinguished by differences in colour, particle size, type of cement, or internal arrangement.

      Metamorphic rocks (metamorphic rock) are those formed by changes in preexisting rocks under the influence of high temperature, pressure, and chemically active solutions. The changes can be chemical (compositional) and physical (textural) in character. Metamorphic rocks are often formed by processes deep within the Earth that produce new minerals, textures, and crystal structures. The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water. Metamorphism often produces apparent layering, or banding, because of the segregation of minerals into separate bands. Metamorphic processes (metamorphism) can also occur at the Earth's surface due to meteorite impact events and pyrometamorphism taking place near burning coal seams ignited by lightning strikes.

Rock cycle
 Geologic materials—mineral crystals and their host rock types—are cycled through various forms. The process depends on temperature, pressure, time, and changes in environmental conditions in the Earth's crust and at its surface. The rock cycle illustrated in Figure 1—> reflects the basic relationships among igneous, metamorphic, and sedimentary rocks. erosion includes weathering (the physical and chemical breakdown of minerals) and transportation to a site of deposition. diagenesis is, as previously explained, the process of forming sedimentary rock by compaction and natural cementation of grains, or crystallization from water or solutions, or recrystallization. The conversion of sediment to rock is termed lithification.

Abundance of rock types
  An estimate of the distribution of rock types in large structural units of the terrestrial crust is given in the Table—>. The relative abundance of main rock types and minerals in the crust is shown in the Table—>.

      The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (crystal) (for igneous and metamorphic rocks). Also of importance are the rock's extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy. The latter is the extent to which the bulk structure and composition are the same in all directions in the rock.

      Analysis of texture can yield information about the rock's source material, conditions and environment of deposition (for sedimentary rock) or crystallization and recrystallization (for igneous and metamorphic rock, respectively), and subsequent geologic history and change.

Classification by grain or crystal size
 The common textural terms used for rock types with respect to the size of the grains or crystals, are given in the Table—>. The particle-size categories are derived from the Udden-Wentworth scale developed for sediment. For igneous and metamorphic rocks, the terms are generally used as modifiers—e.g., medium-grained granite. Aphanitic is a descriptive term for small crystals, and phaneritic for larger ones. Very coarse crystals (those larger than 3 centimetres, or 1.2 inches) are termed pegmatitic.

      For sedimentary rocks, the broad categories of sediment size are coarse (greater than 2 millimetres, or 0.08 inch), medium (between 2 and 1/16 millimetres), and fine (under 1/16 millimetre). The latter includes silt and clay, which both have a size indistinguishable by the human eye and are also termed dust. Most shales (shale) (the lithified version of clay) contain some silt. Pyroclastic rocks are those formed from clastic (from the Greek word for broken) material ejected from volcanoes. Blocks are fragments broken from solid rock, while bombs (bomb) are molten when ejected.

      The term rock refers to the bulk volume of the material, including the grains or crystals as well as the contained void space. The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material is termed porosity. That is to say, porosity is the ratio of void volume to the bulk volume (grains plus void space). This void space consists of pore space between grains or crystals, in addition to crack space. In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting. Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals.

 Sorting is the tendency of sedimentary rocks to have grains that are similarly sized—i.e., to have a narrow range of sizes (see Figure 2—>). Poorly sorted sediment displays a wide range of grain sizes and hence has decreased porosity. Well-sorted indicates a grain size distribution that is fairly uniform. Depending on the type of close-packing of the grains, porosity can be substantial. It should be noted that in engineering usage—e.g., geotechnical or civil engineering—the terminology is phrased oppositely and is referred to as grading. A well-graded sediment is a (geologically) poorly sorted one, and a poorly graded sediment is a well-sorted one.

      Total porosity encompasses all the void space, including those pores that are interconnected to the surface of the sample as well as those that are sealed off by natural cement or other obstructions. Thus the total porosity (ϕT) is

      where VolG is the volume of grains (and cement, if any) and VolB is the total bulk volume. Alternatively, one can calculate ϕT from the measured densities of the bulk rock and of the (mono)mineralic constituent. Thus,

      where ρB is the density of the bulk rock and ρG is the density of the grains (i.e., the mineral, if the composition is monomineralogic and homogeneous). For example, if a sandstone has a ρB of 2.38 grams per cubic centimetre (g/cm3) and is composed of quartz (SiO2) grains having ρG of 2.65 g/cm3, the total porosity is

      Apparent (effective, or net) porosity is the proportion of void space that excludes the sealed-off pores. It thus measures the pore volume that is effectively interconnected and accessible to the surface of the sample, which is important when considering the storage and movement of subsurface fluids such as petroleum, groundwater, or contaminated fluids.

Physical properties
      Physical properties of rocks are of interest and utility in many fields of work, including geology, petrophysics, geophysics, materials science, geochemistry, and geotechnical engineering. The scale of investigation ranges from the molecular and crystalline up to terrestrial studies of the Earth and other planetary bodies. Geologists (geology) are interested in the radioactive age dating of rocks to reconstruct the origin of mineral deposits; seismologists (seismology) formulate prospective earthquake predictions using premonitory physical or chemical changes; crystallographers study the synthesis of minerals with special optical or physical properties; exploration geophysicists (geophysics) investigate the variation of physical properties of subsurface rocks to make possible detection of natural resources such as oil and gas, geothermal energy, and ores of metals; geotechnical engineers (engineering geology) examine the nature and behaviour of the materials on, in, or of which such structures as buildings, dams, tunnels, bridges, and underground storage vaults are to be constructed; solid-state physicists study the magnetic, electrical, and mechanical properties of materials for electronic devices, computer components, or high-performance ceramics; and petroleum reservoir engineers analyze the response measured on well logs or in the processes of deep drilling at elevated temperature and pressure.

      Since rocks are aggregates of mineral grains or crystals, their properties are determined in large part by the properties of their various constituent minerals. In a rock these general properties are determined by averaging the relative properties and sometimes orientations of the various grains or crystals. As a result, some properties that are anisotropic (i.e., differ with direction) on a submicroscopic or crystalline scale are fairly isotropic for a large bulk volume of the rock. Many properties are also dependent on grain or crystal size, shape, and packing arrangement, the amount and distribution of void space, the presence of natural cements in sedimentary rocks, the temperature and pressure, and the type and amount of contained fluids (e.g., water, petroleum, gases). Because many rocks exhibit a considerable range in these factors, the assignment of representative values for a particular property is often done using a statistical variation.

      Some properties can vary considerably, depending on whether measured in situ (in place in the subsurface) or in the laboratory under simulated conditions. Electrical resistivity (resistance), for example, is highly dependent on the fluid content of the rock in situ and the temperature condition at the particular depth.

      Density varies significantly among different rock types because of differences in mineralogy and porosity. Knowledge of the distribution of underground rock densities can assist in interpreting subsurface geologic structure and rock type.

      In strict usage, density is defined as the mass of a substance per unit volume; however, in common usage, it is taken to be the weight in air of a unit volume of a sample at a specific temperature. Weight is the force that gravitation exerts on a body (and thus varies with location), whereas mass (a measure of the matter in a body) is a fundamental property and is constant regardless of location. In routine density measurements of rocks, the sample weights are considered to be equivalent to their masses, because the discrepancy between weight and mass would result in less error on the computed density than the experimental errors introduced in the measurement of volume. Thus, density is often determined using weight rather than mass. Density should properly be reported in kilograms per cubic metre (kg/m3), but is still often given in grams per cubic centimetre (g/cm3).

      Another property closely related to density is specific gravity. It is defined, as noted above, as the ratio of the weight or mass in air of a unit volume of material at a stated temperature to the weight or mass in air of a unit volume of distilled water at the same temperature. Specific gravity is dimensionless (i.e., has no units).

      The bulk density of a rock is ρB = WG/VB, where WG is the weight of grains (sedimentary rocks) or crystals (igneous and metamorphic rocks) and natural cements, if any, and VB is the total volume of the grains or crystals plus the void (pore) space. The density can be dry if the pore space is empty, or it can be saturated if the pores are filled with fluid (e.g., water), which is more typical of the subsurface (in situ) situation. If there is pore fluid present,

      where Wfl is the weight of pore fluid. In terms of total porosity, saturated density is

      and thus

      where ρfl is the density of the pore fluid. Density measurements for a given specimen involve the determination of any two of the following quantities: pore volume, bulk volume, or grain volume, along with the weight.

      A useful way to assess the density of rocks is to make a histogram plot of the statistical range of a set of data. The representative value and its variation can be expressed as follows: (1) mean, the average value, (2) mode, the most common value (i.e., the peak of the distribution curve), (3) median, the value of the middle sample of the data set (i.e., the value at which half of the samples are below and half are above), and (4) standard deviation, a statistical measure of the spread of the data (plus and minus one standard deviation from the mean value includes about two-thirds of the data).

   Dry bulk densities for various rock typesA compilation of dry bulk densities for various rock types found in the upper crust of the Earth is listed in the Table (Dry bulk densities for various rock types). A histogram plot of these data, giving the percent of the samples as a function of density is shown in Figure 3—>. The parameters given include (1) sample division, the range of density in one data column—e.g., 0.036 g/cm3 for Figure 3—>, (2) number of samples, and (3) standard deviation. The small inset plot is the percentage of samples (on the vertical axis) that lie within the interval of the “mode - x” to the “mode + x,” where x is the horizontal axis.

 In Figure 3—>, the most common (modal) value of the distribution falls at 2.63 g/cm3, roughly the density of quartz, an abundant rock-forming mineral. Few density values for these upper crustal rocks lie above 3.3 g/cm3. A few fall well below the mode, even occasionally under 1 g/cm3. The reason for this is shown in Figure 4, which illustrates the density distributions for granite, basalt, and sandstone. Granite is an intrusive igneous rock with low porosity and a well-defined chemical (mineral) composition; its range of densities is narrow. Basalt is, in most cases, an extrusive igneous rock that can exhibit a large variation in porosity (because entrained gases leave voids called vesicles), and thus some highly porous samples can have low densities. sandstone is a clastic sedimentary rock that can have a wide range of porosities depending on the degree of sorting, compaction, packing arrangement of grains, and cementation. The bulk density varies accordingly.

       Typical density ranges for some other rock typesOther distribution plots of dry bulk densities are given in Figures 5 and 6, with a sample division of 0.036 g/cm3 for Figures 5 and 6A and of 0.828 percent for Figure 6B. The Table (Typical density ranges for some other rock types) lists typical ranges of dry bulk densities for a variety of other rock types as prepared by the American geologists Gordon R. Johnson and Gary R. Olhoeft.

      The density of clastic sedimentary rocks increases as the rocks are progressively buried. This is because of the increase of overburden pressure, which causes compaction, and the progressive cementation with age. Both compaction and cementation decrease the porosity.

 Representative densities for common rock-forming minerals (i.e., ρG) and rocks (i.e., ρB) are listed in the Table—>. The bulk densities for sedimentary rocks (sedimentary rock), which typically have variable porosity, are given as ranges of both dry ρB and (water-) saturated ρB. The pore-filling fluid is usually briny water, often indicative of the presence of seawater when the rock was being deposited or lithified. It should be noted that the bulk density is less than the grain density of the constituent mineral (or mineral assemblage), depending on the porosity. For example, sandstone (characteristically quartzose) has a typical dry bulk density of 2.0–2.6 g/cm3, with a porosity that can vary from low to more than 30 percent. The density of quartz itself is 2.65 g/cm3. If porosity were zero, the bulk density would equal the grain density.

 Saturated bulk density is higher than dry bulk density, owing to the added presence of pore-filling fluid. The Table—> also lists representative values for density of seawater, oil, and methane gas at a subsurface condition—pressure of 200 bars (one bar = 0.987 atmosphere, or 29.53 inches of mercury) and a temperature of about 80° C (176° F).

Mechanical properties
stress and strain
      When a stress σ (force per unit area) is applied to a material such as rock, the material experiences a change in dimension, volume, or shape. This change, or deformation (deformation and flow), is called strain (ε). Stresses can be axial—e.g., directional tension or simple compression—or shear (shear stress) (tangential), or all-sided (e.g., hydrostatic compression). The terms stress and pressure are sometimes used interchangeably, but often stress refers to directional stress or shear stress and pressure (P) refers to hydrostatic compression. For small stresses, the strain is elastic (recoverable when the stress is removed and linearly proportional to the applied stress). For larger stresses and other conditions, the strain can be inelastic, or permanent.

Elastic constants
      In elastic deformation, there are various constants that relate the magnitude of the strain response to the applied stress. These elastic constants include the following:

      (1) Young's modulus (E) is the ratio of the applied stress to the fractional extension (or shortening) of the sample length parallel to the tension (or compression). The strain is the linear change in dimension divided by the original length.

      (2) shear modulus (μ) is the ratio of the applied stress to the distortion (rotation) of a plane originally perpendicular to the applied shear stress; it is also termed the modulus of rigidity.

      (3) bulk modulus (k) is the ratio of the confining pressure to the fractional reduction of volume in response to the applied hydrostatic pressure. The volume strain is the change in volume of the sample divided by the original volume. Bulk modulus is also termed the modulus of incompressibility.

      (4) Poisson's ratio (σp) is the ratio of lateral strain (perpendicular to an applied stress) to the longitudinal strain (parallel to applied stress).

      For elastic and isotropic materials, the elastic constants are interrelated. For example,


      The following are the common units of stress:

      Thus 10 kilobars = 1 gigapascal (i.e., 109 Pa).

Rock mechanics
      The study of deformation resulting from the strain of rocks in response to stresses is called rock mechanics. When the scale of the deformation is extended to large geologic structures in the crust of the Earth, the field of study is known as geotectonics.

      The mechanisms and character of the deformation of rocks and Earth materials can be investigated through laboratory experiments, development of theoretical models based on the properties of materials, and study of deformed rocks and structures in the field. In the laboratory, one can simulate—either directly or by appropriate scaling of experimental parameters—several conditions. Two types of pressure may be simulated: confining (hydrostatic), due to burial under rock overburden, and internal (pore), due to pressure exerted by pore fluids contained in void space in the rock. Directed applied stress, such as compression, tension, and shear, is studied, as are the effects of increased temperature introduced with depth in the Earth's crust. The effects of the duration of time and the rate of applying stress (i.e., loading) as a function of time are examined. Also, the role of fluids, particularly if they are chemically active, is investigated.

      Some simple apparatuses for deforming rocks are designed for biaxial stress application: a directed (uniaxial) compression is applied while a confining pressure is exerted (by pressurized fluid) around the cylindrical specimen. This simulates (simulation) deformation at depth within the Earth. An independent internal pore-fluid pressure also can be exerted. The rock specimen can be jacketed with a thin, impermeable sleeve (e.g., rubber or copper) to separate the external pressure medium from the internal pore fluids (if any). The specimen is typically a few centimetres in dimension.

      Another apparatus for exerting high pressure on a sample was designed in 1968 by Akira Sawaoka, Naoto Kawai, and Robert Carmichael to give hydrostatic confining pressures up to 12 kilobars (1.2 gigapascal), additional directed stress, and temperatures up to a few hundred degrees Celsius. The specimen is positioned on the baseplate; the pressure is applied by driving in pistons with a hydraulic press. The end caps can be locked down to hold the pressure for time experiments and to make the device portable.

      Apparatuses have been developed, typically using multianvil designs, which extend the range of static experimental conditions—at least for small specimens and limited times—to pressures as high as about 1,700 kilobars and temperatures of about 2,000° C. Such work has been pioneered by researchers such as Peter M. Bell and Ho-Kwang Mao, who conducted studies at the Geophysical Laboratory of the Carnegie Institution in Washington, D.C. Using dynamic techniques (i.e., shock from explosive impact generated by gun-type designs), even higher pressures up to 7,000 kilobars (700 gigapascal)—which is nearly twice the pressure at the centre of the Earth and seven million times greater than the atmospheric pressure at the Earth's surface—can be produced for very short times. A leading figure in such ultrapressure work is A. Sawaoka at the Tokyo Institute of Technology.

      In the upper crust of the Earth, hydrostatic pressure increases at the rate of about 320 bars per kilometre, and temperature increases at a typical rate of 20°–40° C per kilometre, depending on recent crustal geologic history. Additional directed stress, as can be generated by large-scale crustal deformation (tectonism), can range up to 1 to 2 kilobars. This is approximately equal to the ultimate strength (before fracture) of solid crystalline rock at surface temperature and pressure (see below). The stress released in a single major earthquake—a shift on a fault plane—is about 50–150 bars.

 In studying the deformation of rocks one can start with the assumption of ideal behaviour: elastic strain and homogeneous and isotropic stress and strain. In reality, on a microscopic scale there are grains and pores in sediments and a fabric of crystals in igneous and metamorphic rocks. On a large scale, rock bodies exhibit physical and chemical variations and structural features. Furthermore, conditions such as extended length of time, confining pressure, and subsurface fluids affect the rates of change of deformation. Figure 7—> shows the generalized transition from brittle fracture through faulting to plastic-flow deformation in response to applied compressional stress and the progressive increase of confining pressure.

Stress-strain relationships
      The deformation of materials is characterized by stress-strain relations. For elastic-behaviour materials, the strain is proportional to the load (i.e., the applied stress). The strain is immediate with stress and is reversible (recoverable) up to the yield point stress, beyond which permanent strain results. For viscous material, there is laminar (slow, smooth, parallel) flow; one must exert a force to maintain motion because of internal frictional resistance to flow, called the viscosity. Viscosity varies with the applied stress, strain rate, and temperature. In plastic behaviour, the material strains continuously (but still has strength) after the yield point stress is reached; however, beyond this point there is some permanent deformation. In elasticoviscous deformation, there is combined elastic and viscous behaviour. The material yields continuously (viscously) for a constant applied load. An example of such behaviour is creep, a slow, permanent, and continuous deformation occurring under constant load over a long time in such materials as crystals, ice, soil and sediment, and rocks at depth. In firmoviscous behaviour, the material is essentially solid but the strain is not immediate with application of stress; rather, it is taken up and released exponentially. A plasticoviscous material exhibits elastic behaviour for initial stress (as in plastic behaviour), but after the yield point stress is reached, it flows like a viscous fluid.

       Some typical values of elastic constants and propertiesSome representative values of elastic constants and properties are listed in Table 36 (Some typical values of elastic constants and properties). The coefficient of viscosity (η) is the ratio of applied stress to the rate of straining (change of strain with time). It is measured in units of poise; one poise equals one dyne-second per square centimetre.

       Rheidity threshold of fluidlike deformationRheology is the study of the flow deformation of materials. The concept of rheidity refers to the capacity of a material to flow, arbitrarily defined as the time required with a shear stress applied for the viscous strain to be 1,000 times greater than the elastic strain. It is thus a measure of the threshold of fluidlike behaviour. Although such behaviour depends on temperature, relative comparisons can be made. Some representative values of rheidity times are given in the Table (Rheidity threshold of fluidlike deformation).

 Typical stress-strain (deformation) curves for rock materials are shown in Figure 8—>. The stress σ, compression in the figure, is force per unit area. The strain ε is fractional shortening of the specimen parallel to the applied compression; it is given here in percent. The brittle material behaves elastically nearly until the point of fracture (denoted X), whereas the ductile (plastically deformable) material is elastic up to the yield point but then has a range of plastic deformation before fracturing. The ability to undergo large permanent deformation before fracture is called ductility. For plastic deformation, the flow mechanisms are intracrystalline (slip and twinning within crystal grains), intercrystalline motion by crushing and fracture (cataclasis), and recrystallization by solutioning or solid diffusion.

       Some typical values of elastic constants and propertiesIf the applied stress is removed while a ductile material is in the plastic range, part of the strain is recoverable (elastically), but there is permanent deformation. The ultimate strength is the highest point (stress) on a stress-strain curve, often occurring at fracture (which is the complete loss of cohesion). The strength of a material is its resistance to failure (destruction of structure) by flow or fracture; it is a measure of the stress required to deform a body. Typical compressive strengths (the stress required to cause failure under compression) are given in the Table (Some typical values of elastic constants and properties).

Effect of environmental conditions
      The behaviour and mechanical properties of rocks depend on a number of environmental conditions. (1) Confining pressure increases the elasticity, strength (e.g., yield point and ultimate fracture stress), and ductility. (2) Internal pore-fluid pressure reduces the effective stress acting on the sample, thus reducing the strength and ductility. The effective, or net, confining pressure is the external hydrostatic pressure minus the internal pore- fluid pressure. (3) temperature lowers the strength, enhances ductility, and may enhance recrystallization. (4) Fluid solutions can enhance deformation, creep, and recrystallization. (5) Time is an influential factor as well. (6) The rate of loading (i.e., the rate at which stress is applied) influences mechanical properties. (7) Compaction, as would occur with burial to depth, reduces the volume of pore space for sedimentary rocks and the crack porosity for crystalline rocks.

      Rocks, which are typically brittle at the Earth's surface, can undergo ductile deformation when buried and subjected to increased confining pressure and temperature for long periods of time. If stress exceeds their strength or if they are not sufficiently ductile, they will fail by fracture—as a crystal, within a bed or rock, on an earthquake fault zone, and so on—whereas with ductility they can flow and fold.

       Rock strengths, with varying temperature and pressureSome strengths for various rock types under different temperatures and confining pressures are listed in the Table (Rock strengths, with varying temperature and pressure). The plastic yield strength here is the stress at a 2 percent strain; the ultimate strength, as stated above, is the highest point (stress) on the stress-strain curve.

      An increase in confining pressure causes brittle fracture to become shear slippage and eventually causes flow (ductile) behaviour. This transition is also aided by higher temperature, decreased internal pore-fluid pressure, and slower strain rate.

       Variation of some elastic constants (in 10^6 bars) with rock type and confining pressureThe Table (Variation of some elastic constants (in 10^6 bars) with rock type and confining pressure) gives the values of some elastic constants—bulk modulus (k), Young's modulus (E), shear modulus (μ), and Poisson's ratio (σp)—at room pressure (1 bar) and high confining pressure (3,000 bars). The values for clastic sedimentary rocks would be particularly variable.

Thermal properties
      Heat flow (heat transfer) (or flux), q, in the Earth's crust or in rock as a building material, is the product of the temperature gradient (change in temperature per unit distance) and the material's thermal conductivity (k, the heat flow across a surface per unit area per unit time when a temperature difference exists in unit length perpendicular to the surface). Thus,

      The units of the terms in this equation are given below, expressed first in the centimetre-gram-second (cgs) system and then in the International System of Units (SI) system, with the conversion factor from the first to the second given between them.

      Thermal conductivity can be determined in the laboratory or in situ, as in a borehole or deep well, by turning on a heating element and measuring the rise in temperature with time. It depends on several factors: (1) chemical composition of the rock (i.e., mineral content), (2) fluid content (type and degree of saturation of the pore space); the presence of water increases the thermal conductivity (i.e., enhances the flow of heat), (3) pressure (a high pressure increases the thermal conductivity by closing cracks which inhibit heat flow), (4) temperature, and (5) isotropy and homogeneity of the rock.

       Typical values of thermal conductivityTypical values of thermal conductivities of rock materials are given in the Table (Typical values of thermal conductivity). For crystalline silicate rocks—the dominant rocks of the “basement” crustal rocks—the lower values are typical of ones rich in magnesium and iron (e.g., basalt and gabbro) and the higher values are typical of those rich in silica (quartz) and alumina (e.g., granite). These values result because the thermal conductivity of quartz is relatively high, while that for feldspars is low.

      The change in dimension—linear or volumetric—of a rock specimen with temperature is expressed in terms of a coefficient of thermal expansion. This is given as the ratio of dimension change (e.g., change in volume) to the original dimension (volume, V) per unit of temperature (T) change:

       Thermal expansion of rocksMost rocks have a volume-expansion coefficient in the range of 15–33 × 10-6 per degree Celsius under ordinary conditions. Quartz-rich rocks have relatively high values because of the higher volume expansion coefficient of quartz. Thermal-expansion coefficients increase with temperature. Table 41 (Thermal expansion of rocks) lists some linear-expansion coefficients,

      where L represents length. All data are based on at least three samples.

Radioactive (radioactivity) heat generation
      The spontaneous decay (partial disintegration) of the nuclei of radioactive elements provides decay particles and energy. The energy, composed of emission kinetic energy and radiation, is converted to heat; it has been an important factor in affecting the temperature gradient and thermal evolution of the Earth. Deep-seated elevated temperatures provide the heat that causes rock to deform plastically and to move, thus generating to a large extent the processes of plate tectonics—plate motions, seafloor spreading, continental drift, and subduction—and most earthquakes and volcanism.

      Some elements, or their isotopes (nuclear species with the same atomic number but different mass numbers), decay with time. These include elements with an atomic number greater than 83—of which the most important are uranium-235, uranium-238, and thorium-232—and a few with a lower atomic number, such as potassium-40.

      The heat generated within rocks depends on the types and abundances of the radioactive elements and their host minerals. Such heat production, A, is given in calories per cubic centimetre per second, or 1 calorie per gram per year = 4.186 × 107 ergs per gram per year = 1.327 ergs per gram per second. The rate of radioactive decay, statistically an exponential process, is given by the half-life, t1/2. The half-life is the time required for half the original radioactive atoms to decay for a particular isotope.

       Some radioactive decay series Heat productivitiesSome radioactive decay series are listed in the Table (Some radioactive decay series). The isotopic abundance is the percent of the natural element that exists as that particular radioactive isotope; for example, 99.28 percent of natural uranium is U-238, and 100 percent of thorium is the radioactive Th-232. The final product is the end result of the process (usually multistage) of disintegration. The Table (Heat productivities) gives the heat productivities of radioactive elements and rock types as reported by George D. Garland. For the rocks, the typical content is given for uranium and thorium (in parts per million [ppm] of weight) and for potassium (in weight percent). The heat production of natural uranium is close to that for the isotope U-238, since almost all natural uranium is of that isotopic species.

       Heat productivities of various rocksThe radioactive elements are more concentrated in the continental upper-crust rocks that are rich in quartz (i.e., felsic, or less mafic). This results because these rocks are differentiated by partial melting of the upper-mantle and oceanic-crust rock. The radioactive elements tend to be preferentially driven off from these rocks for geochemical reasons. A compilation of heat productivities of various rock types is given in the Table (Heat productivities of various rocks).

Electrical properties
      The electrical nature of a material is characterized by its conductivity (or, inversely, its resistivity) and its dielectric constant, and coefficients that indicate the rates of change of these with temperature, frequency at which measurement is made, and so on. For rocks with a range of chemical composition as well as variable physical properties of porosity and fluid content, the values of electrical properties can vary widely.

       resistance (R) is defined as being one ohm when a potential difference (voltage; V) across a specimen of one volt magnitude produces a current (i) of one ampere; that is, V = Ri. The electrical resistivity (ρ) is an intrinsic property of the material. In other words, it is inherent and not dependent on sample size or current path. It is related to resistance by R = ρL/A where L is the length of specimen, A is the cross-sectional area of specimen, and units of ρ are ohm-centimetre; 1 ohm-centimetre equals 0.01 ohm-metre. The conductivity (σ) is equal to 1/ρ ohm -1 · centimetre-1 (or termed mhos/cm). In SI units, it is given in mhos/metre, or siemens/metre.

       Typical resistivitiesSome representative values of electrical resistivity for rocks and other materials are listed in the Table (Typical resistivities). Materials that are generally considered as “good” conductors have a resistivity of 10-5–10 ohm-centimetre (10-7–10-1 ohm-metre) and a conductivity of 10–107 mhos/metre. Those that are classified as intermediate conductors have a resistivity of 100–109 ohm-centimetre (1–107 ohm-metre) and a conductivity of 10-7–1 mhos/metre. “Poor” conductors, also known as insulators, have a resistivity of 1010–1017 ohm-centimetre (108–1015 ohm-metre) and a conductivity of 10-15–10-8. Seawater is a much better conductor (i.e., it has lower resistivity) than fresh water owing to its higher content of dissolved salts; dry rock is very resistive. In the subsurface, pores are typically filled to some degree by fluids. The resistivity of materials has a wide range—copper is, for example, different from quartz by 22 orders of magnitude.

      For high-frequency alternating currents, the electrical response of a rock is governed in part by the dielectric constant, ε. This is the capacity of the rock to store electric charge; it is a measure of polarizability in an electric field. In cgs units, the dielectric constant is 1.0 in a vacuum. In SI units, it is given in farads per metre or in terms of the ratio of specific capacity of the material to specific capacity of vacuum (which is 8.85 × 10-12 farads per metre). The dielectric constant is a function of temperature, and of frequency, for those frequencies well above 100 hertz (cycles per second).

      Electrical conduction occurs in rocks by (1) fluid conduction—i.e., electrolytic conduction by ionic transfer in briny pore water—and (2) metallic and semiconductor (e.g., some sulfide ores) electron conduction. If the rock has any porosity and contained fluid, the fluid typically dominates the conductivity response. The rock conductivity depends on the conductivity of the fluid (and its chemical composition), degree of fluid saturation, porosity and permeability, and temperature. If rocks lose water, as with compaction of clastic sedimentary rocks at depth, their resistivity typically increases.

Magnetic properties
      The magnetic properties of rocks arise from the magnetic properties of the constituent mineral grains and crystals. Typically, only a small fraction of the rock consists of magnetic minerals. It is this small portion of grains that determines the magnetic properties and magnetization of the rock as a whole, with two results: (1) the magnetic properties of a given rock may vary widely within a given rock body or structure, depending on chemical inhomogeneities, depositional or crystallization conditions, and what happens to the rock after formation; and (2) rocks that share the same lithology (type and name) need not necessarily share the same magnetic characteristics. Lithologic classifications are usually based on the abundance of dominant silicate minerals, but the magnetization is determined by the minor fraction of such magnetic mineral grains as iron oxides. The major rock-forming magnetic minerals are iron oxides and sulfides.

      Although the magnetic properties of rocks sharing the same classification may vary from rock to rock, general magnetic properties do nonetheless usually depend on rock type and overall composition. The magnetic properties of a particular rock can be quite well understood provided one has specific information about the magnetic properties of crystalline materials and minerals, as well as about how those properties are affected by such factors as temperature, pressure, chemical composition, and the size of the grains. Understanding is further enhanced by information about how the properties of typical rocks are dependent on the geologic environment and how they vary with different conditions.

Applications of the study of rock magnetization
      An understanding of rock magnetization is important in at least three different areas: prospecting, geology, and materials science. In magnetic prospecting, one is interested in mapping the depth, size, type, and inferred composition of buried rocks. The prospecting, which may be done from ground surface, ship, or aircraft, provides an important first step in exploring buried geologic structures and may, for example, help identify favourable locations for oil, natural gas, and economic mineral deposits.

      Rock magnetization has traditionally played an important role in geology. Paleomagnetic work seeks to determine the remanent magnetization (remanent magnetism) (see below Types of remanent magnetization (rock)) and thereby ascertain the character of the Earth's field when certain rocks were formed. The results of such research have important ramifications in stratigraphic correlation, age dating, and reconstructing past movements of the Earth's crust. Indeed, magnetic surveys of the oceanic crust provided for the first time the quantitative evidence needed to cogently demonstrate that segments of the crust had undergone large-scale lateral displacements over geologic time, thereby corroborating the concepts of continental drift and seafloor spreading, both of which are fundamental to the theory of plate tectonics (see plate tectonics).

      The understanding of magnetization is increasingly important in materials science as well. The design and manufacture of efficient memory cores, magnetic tapes, and permanent magnets increasingly rely on the ability to create materials having desired magnetic properties.

Basic types of magnetization
      There are six basic types of magnetization: (1) diamagnetism, (2) paramagnetism, (3) ferromagnetism, (4) antiferromagnetism, (5) ferrimagnetism, and (6) superparamagnetism.

      Diamagnetism arises from the orbiting electrons surrounding each atomic nucleus. When an external magnetic field (electric field) is applied, the orbits are shifted in such a way that the atoms set up their own magnetic field in opposition to the applied field. In other words, the induced diamagnetic field opposes the external field. Diamagnetism is present in all materials, is weak, and exists only in the presence of an applied field. The propensity of a substance for being magnetized in an external field is called its susceptibility (magnetic susceptibility) (k) and it is defined as J/H, where J is the magnetization (intensity) per unit volume and H is the strength of the applied field. Since the induced field always opposes the applied field, the sign of diamagnetic susceptibility is negative. The susceptibility of a diamagnetic substance is on the order of -10-6 electromagnetic units per cubic centimetre (emu/cm3). It is sometimes denoted κ for susceptibility per unit mass of material.

       paramagnetism results from the electron spin of unpaired electrons. An electron has a magnetic dipole moment—which is to say that it behaves like a tiny bar magnet—and so when a group of electrons is placed in a magnetic field, the dipole moments tend to line up with the field. The effect augments the net magnetization in the direction of the applied field. Like diamagnetism, paramagnetism is weak and exists only in the presence of an applied field, but since the effect enhances the applied field, the sign of the paramagnetic susceptibility is always positive. The susceptibility of a paramagnetic substance is on the order of 10-4 to 10-6 emu/cm3.

       ferromagnetism also exists because of the magnetic properties of the electron. Unlike paramagnetism, however, ferromagnetism can occur even if no external field is applied. The magnetic dipole moments of the atoms spontaneously line up with one another because it is energetically favourable for them to do so. A remanent magnetization can be retained. Complete alignment of the dipole moments would take place only at a temperature of absolute zero (0 kelvin [K], or -273.15° C). Above absolute zero, thermal motions begin to disorder the magnetic moments. At a temperature called the Curie temperature (Curie point), which varies from material to material, the thermally induced disorder overcomes the alignment, and the ferromagnetic properties of the substance disappear. The susceptibility of ferromagnetic materials is large and positive. It is on the order of 10 to 104 emu/cm3. Only a few materials—iron, cobalt, and nickel—are ferromagnetic in the strict sense of the word and have a strong residual magnetization. In general usage, particularly in engineering, the term ferromagnetic is frequently applied to any material that is appreciably magnetic.

       antiferromagnetism occurs when the dipole moments of the atoms in a material assume an antiparallel arrangement in the absence of an applied field. The result is that the sample has no net magnetization. The strength of the susceptibility is comparable to that of paramagnetic materials. Above a temperature called the Néel temperature, thermal motions destroy the antiparallel arrangement, and the material then becomes paramagnetic. Spin-canted (anti)ferromagnetism is a special condition which occurs when antiparallel magnetic moments are deflected from the antiferromagnetic plane, resulting in a weak net magnetism. Hematite (α-Fe2O3) is such a material.

       ferrimagnetism is an antiparallel alignment of atomic dipole moments which does yield an appreciable net magnetization resulting from unequal moments of the magnetic sublattices. Remanent magnetization is detectable (see below). Above the Curie temperature the substance becomes paramagnetic. Magnetite (Fe3O4), which is the most magnetic common mineral, is a ferrimagnetic substance.

      Superparamagnetism occurs in materials having grains so small (about 100 angstroms) that any cooperative alignment of dipole moments is overcome by thermal energy.

Types of remanent magnetization
      Rocks and minerals may retain magnetization after the removal of an externally applied field, thereby becoming permanent weak magnets. This property is known as remanent magnetization and is manifested in different forms, depending on the magnetic properties of the rocks and minerals and their geologic origin and history. Delineated below are the kinds of remanent magnetization frequently observed.

      CRM (chemical, or crystallization, remanent magnetization) can be induced after a crystal is formed and undergoes one of a number of physicochemical changes, such as oxidation or reduction, a phase change, dehydration, recrystallization, or precipitation of natural cements. The induction, which is particularly important in some (red) sediments and metamorphic rocks, typically takes place at constant temperature in the Earth's magnetic field.

      DRM (depositional, or detrital, remanent magnetization) is formed in clastic sediments when fine particles are deposited on the floor of a body of water. Marine sediments, lake sediments, and some clays can acquire DRM. The Earth's magnetic field aligns the grains, yielding a preferred direction of magnetization.

      IRM (isothermal remanent magnetization) results from the application of a magnetic field at a constant (isothermal) temperature, often room temperature.

      NRM (natural remanent magnetization) is the magnetization detected in a geologic in situ condition. The NRM of a substance may, of course, be a combination of any of the other remanent magnetizations described here.

      PRM (pressure remanent, or piezoremanent, magnetization) arises when a material undergoes mechanical deformation while in a magnetic field. The process of deformation may result from hydrostatic pressure, shock impact (as produced by a meteorite striking the Earth's surface), or directed tectonic stress. There are magnetization changes with stress in the elastic range, but the most pronounced effects occur with plastic deformation when the structure of the magnetic minerals is irreversibly changed.

      TRM (thermoremanent magnetization) occurs when a substance is cooled, in the presence of a magnetic field, from above its Curie temperature (Curie point) to below that temperature. This form of magnetization is generally the most important, because it is stable and widespread, occurring in igneous and sedimentary rocks. TRM also can occur when dealing exclusively with temperatures below the Curie temperature. In PTRM (partial thermoremanent magnetization) a sample is cooled from a temperature below the Curie point to yet a lower temperature.

      VRM (viscous remanent magnetization) results from thermal agitation. It is acquired slowly over time at low temperatures and in the Earth's magnetic field. The effect is weak and unstable but is present in most rocks.

hysteresis and magnetic susceptibility
 The concept of hysteresis is fundamental when describing and comparing the magnetic properties of rocks. Hysteresis is the variation of magnetization with applied field and illustrates the ability of a material to retain its magnetization, even after an applied field is removed. Figure 9—> illustrates this phenomenon in the form of a plot of magnetization (J) versus applied field (Hex). Js is the saturation (or “spontaneous”) magnetization when all the magnetic moments are aligned in their configuration of maximum order. It is temperature-dependent, reaching zero at the Curie temperature. Jr,sat is the remanent magnetization that remains when a saturating (large) applied field is removed, and Jr is the residual magnetization left by some process apart from IRM saturation, as, for example, TRM. Hc is the coercive field (or force) that is required to reduce Jr,sat to zero, and Hc,r is the field required to reduce Jr to zero.

      Magnetic susceptibility is a parameter of considerable diagnostic and interpretational use in the study of rocks. This is true whether an investigation is being conducted in the laboratory or magnetic fields over a terrain are being studied to deduce the structure and lithologic character of buried rock bodies. Susceptibility for a rock type can vary widely, depending on magnetic mineralogy, grain size and shape, and the relative magnitude of remanent magnetization present, in addition to the induced magnetization from the Earth's weak field. The latter is given as Jinduced = kHex, where k is the (true) magnetic susceptibility and Hex is the external (i.e., the Earth's) magnetic field. If there is an additional remanent magnetization with its ratio (Qn) to induced magnetization being given by

      then the total magnetization is

      where kapp, the “apparent” magnetic susceptibility, is k(1 + Qn).

Magnetic minerals and magnetic properties of rocks
      The major rock-forming magnetic minerals are the following iron oxides: the titanomagnetite series, xFe2TiO4 · (1 - x)Fe3O4, where Fe3O4 is magnetite, the most magnetic mineral; the ilmenohematite series, yFeTiO3 · (1 - y)Fe2O3, where α-Fe2O3 (in its rhombohedral structure) is hematite; maghemite, γ-Fe2O3 (in which some iron atoms are missing in the hematite structure); and limonite (hydrous iron oxides). They also include sulfides—namely, the pyrrhotite series, yFeS · (1 - y)Fe1 - xS.

       Approximate apparent susceptibilities for rock typesThe Table (Approximate apparent susceptibilities for rock types) gives some typical values of the apparent susceptibility for various rock types, which usually include some remanent as well as induced magnetization. Values are higher for mafic igneous rocks, especially as the content of magnetite increases.

       Measured susceptibilities for rock typesA distribution of measured (true) susceptibilities for various rock types is shown in the Table (Measured susceptibilities for rock types). Basic refers to those rocks high in iron and magnesium silicates and magnetite, extrusive means formed by cooling after extruding onto the land surface or seafloor. The data in each category are based on at least 45 samples.

       Typical magnetic properties of rocksThe Table (Typical magnetic properties of rocks) lists representative values for the magnetic properties Jn (natural remanent magnetization), k (susceptibility), and ratio Qn. Natural remanent magnetization is some combination of remanences; typically TRM in an igneous rock, perhaps DRM or CRM or both in a sedimentary rock, and all with an additional VRM. The ratio Qn is typically higher for rocks with a strong, stable remanence—e.g., magnetite-rich and fine-grained extrusive rocks such as seafloor basalts.

Robert S. Carmichael

Additional Reading
Standard mineralogical reference works include W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 2nd ed. (1997); and Annibale Mottana, Rodolfo Crespi, and Giuseppe Liborio, Simon and Schuster's Guide to Rocks and Minerals (also published as The Macdonald Encyclopedia of Rocks and Minerals, 1978; originally published in Italian, 1977). Useful texts and monographs include Harvey Blatt, Sedimentary Petrology (1992); Richard V. Dietrich and Brian J. Skinner, Rocks and Rock Minerals (1979); Anthony Hall, Igneous Petrology, 2nd ed. (1996); Cornelis Klein, Minerals and Rocks: Exercises in Crystallography, Mineralogy, and Hand Specimen Petrology (1989); Cornelis Klein and Cornelius S. Hurlbut, Jr., Manual of Mineral Science (After James D. Dana), 22nd ed. (2002); and Harvey Blatt and Robert J. Tracy, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed. (1996).Cornelis Klein D.H. Griffiths and R.F. King, Applied Geophysics for Geologists and Engineers: The Elements of Geophysical Prospecting, 2nd ed. (1981); Robert S. Carmichael (ed.), Handbook of Physical Properties of Rocks, 3 vol. (1982–84), also available in a 1-vol. abridged ed., Practical Handbook of Physical Properties of Rocks and Minerals (1989); Edgar W. Spencer, Introduction to the Structure of the Earth, 3rd ed. (1988); and D.R. Bowes (ed.), The Encyclopedia of Igneous and Metamorphic Petrology (1989), may also be consulted.Robert S. Carmichael

also called  rock and roll,  rock & roll , or  rock 'n' roll 

      form of popular music that emerged in the 1950s.

      It is certainly arguable that by the end of the 20th century rock was the world's dominant form of popular music. Originating in the United States in the 1950s, it spread to other English-speaking countries and across Europe in the '60s, and by the '90s its impact was obvious globally (if in many different local guises). Rock's commercial importance was by then reflected in the organization of the multinational recording industry, in the sales racks of international record retailers, and in the playlist policies of music radio and television. If other kinds of music—classical, jazz, easy listening, country, folk, etc.—are marketed as minority interests, rock defines the musical mainstream. And so over the last half of the 20th century it became the most inclusive of musical labels—everything can be “rocked”—and in consequence the hardest to define. To answer the question, What is rock?, one first has to understand where it came from and what made it possible. And to understand rock's cultural significance one has to understand how it works socially as well as musically.

What is rock?

The difficulty of definition
      Dictionary definitions of rock are problematic, not least because the term has different resonance in its British and American usages (the latter is broader in compass). There is basic agreement that rock “is a form of music with a strong beat,” but it is difficult to be much more explicit. The Collins Cobuild English Dictionary, based on a vast database of British usage, suggests that “rock is a kind of music with simple tunes and a very strong beat that is played and sung, usually loudly, by a small group of people with electric guitars and drums,” but there are so many exceptions to this description that it is practically useless.

      Legislators seeking to define rock for regulatory purposes have not done much better. The Canadian government defined “rock and rock-oriented music” as “characterized by a strong beat, the use of blues forms and the presence of rock instruments such as electric guitar, electric bass, electric organ or electric piano.” This assumes that rock can be marked off from other sorts of music formally, according to its sounds. In practice, though, the distinctions that matter for rock fans and musicians have been ideological. Rock was developed as a term to distinguish certain music-making and listening practices from those associated with pop; what was at issue was less a sound than an attitude. In 1990 British legislators defined pop music as “all kinds of music characterized by a strong rhythmic element and a reliance on electronic amplification for their performance.” This led to strong objections from the music industry that such a definition failed to appreciate the clear sociological difference between pop (“instant singles-based music aimed at teenagers”) and rock (“album-based music for adults”). In pursuit of definitional clarity, the lawmakers misunderstood what made rock music matter.

Crucial rock musicians
      For lexicographers and legislators alike, the purpose of definition is to grasp a meaning, to hold it in place, so that people can use a word correctly—for example, to assign a track to its proper radio outlet (rock, pop, country, jazz). The trouble is that the term rock describes an evolving musical practice informed by a variety of nonmusical arguments (about creativity, sincerity, commerce, and popularity). It makes more sense, then, to approach the definition of rock historically, with examples. The following musicians were crucial to rock's history. What do they have in common?

      Elvis Presley (Presley, Elvis), from Memphis, Tennessee, personified a new form of American popular music in the mid-1950s. Rock and roll was a guitar-based sound with a strong (if loose) beat that drew equally on African-American and white traditions from the southern United States, on blues, church music, and country (country music) music. Presley's rapid rise to national stardom revealed the new cultural and economic power of both teenagers and teen-aimed media—records, radio, television, and motion pictures.

      The Beatles (Beatles, the), from Liverpool, England (via Hamburg, Germany), personified a new form of British popular music in the 1960s. Merseybeat was a British take on the black and white musical mix of rock and roll: a basic lineup of lead guitar, rhythm guitar, bass guitar, and drums (with shared vocals) provided local live versions of American hit records of all sorts. The Beatles added to this an artistic self-consciousness, soon writing their own songs and using the recording studio to develop their own—rather than a commercial producer's—musical ideas. The group's unprecedented success in the United States ensured that rock would be an Anglo-American phenomenon.

      Bob Dylan (Dylan, Bob), from Hibbing, Minnesota (via New York City), personified a new form of American music in the mid-1960s. Dylan brought together the amplified beat of rock and roll, the star imagery of pop, the historical and political sensibility of folk, and—through the wit, ambition, and obscurity of his lyrics—the arrogance of urban bohemia. He gave the emerging rock scene artistic weight (his was album, not Top 40, music) and a new account of youth as an ideological rather than a demographic category.

      Jimi Hendrix (Hendrix, Jimi), from Seattle, Washington (via London), personified the emergence of rock as a specific musical genre in the late 1960s. Learning his trade as a guitarist in rhythm-and-blues bands and possessing a jazzman's commitment to collective improvisation, he came to fame leading a trio in London and exploring the possibilities of the amplifier as a musical instrument in the recording studio and on the concert stage. Hendrix established versatility and technical skill as a norm for rock musicianship and gave shape to a new kind of event: the outdoor festival and stadium concert, in which the noise of the audience became part of the logic of the music.

      Bob Marley (Marley, Bob), from Kingston, Jamaica (via London), personified a new kind of global popular music in the 1970s. Marley and his group, the Wailers, combined sweet soul vocals inspired by Chicago groups such as the Impressions with rock guitar, a reggae beat, and Rastafarian (Rastafari) mysticism. Marley's commercial success established Jamaica as a major source of international talent, leaving a reggae imprint not just on Western rock but also on local music makers in Africa, Asia, and Australia.

       Madonna, from suburban Detroit, Michigan (via New York City), personified a new sort of global teen idol in the 1980s. She combined the sounds and technical devices of the New York City disco-club scene with the new sales and image-making opportunities offered by video promotion—primarily by Music Television (MTV) (Schäfer, Karl), the music-based cable television service. As a star Madonna had it both ways: she was at once a knowing American feminist artist and a global sales icon for the likes of Pepsi-Cola.

       Public Enemy, from New York City, personified a new sort of African-American music in the late 1980s. Rap, the competitive use of rhyming lines spoken over an ever-more-challenging rhythmic base, had a long history in African-American culture; however, it came to musical prominence as part of the hip-hop movement. Public Enemy used new digital technology to sample (use excerpts from other recordings) and recast the urban soundscape from the perspective of African-American youth. This was music that was at once sharply attuned to local political conditions and resonant internationally. By the mid-1990s rap had become an expressive medium for minority social groups around the world.

      What does this version of rock's history—from Presley to Public Enemy—reveal? First, that rock is so broad a musical category that in practice people organize their tastes around more focused genre labels: the young Presley was a rockabilly, the Beatles a pop group, Dylan a folkie, Madonna a disco diva, Marley and the Wailers a reggae act, and Public Enemy rappers. Even Hendrix, the most straightforward rock star on this list, also has a place in the histories of rhythm and blues and jazz. In short, while all these musicians played a significant part in the development of rock, they did so by using different musical instruments and textures, different melodic and rhythmic principles, different approaches to song words and performing conventions.

Musical eclecticism and the use of technology
      Even from a musicological point of view, any account of rock has to start with its eclecticism. Beginning with the mix of country and blues that comprised rock and roll (rock's first incarnation), rock has been essentially a hybrid form. African-American musics were at the centre of this mix, but rock resulted from what white musicians, with their own folk histories and pop conventions, did with African-American music—and with issues of race and race relations.

 Rock's musical eclecticism reflects (and is reflected in) the geographic mobility of rock musicians, back and forth across the United States, over the Atlantic Ocean, and throughout Europe. Presley was unique as a rock star who did not move away from his roots; Hendrix was more typical in his restlessness. And if rock and roll had rural origins, the rock audience was from the start urban, an anonymous crowd seeking an idealized sense of community and sociability in dance halls and clubs, on radio stations, and in headphones. Rock's central appeal as a popular music has been its ability to provide globally an intense experience of belonging, whether to a local scene or a subculture. Rock history can thus be organized around both the sound of cities ( Philadelphia and Detroit, New York City and San Francisco, Liverpool and Manchester) and the spread of youth cults (rock and roll, heavy metal, punk, and grunge). (See Rock Music Creative Centres map—>.)

      Rock is better defined, then, by its eclecticism than by reference to some musical essence, and it is better understood in terms of its general use of technology rather than by its use of particular instruments (such as the guitar). Early rock-and-roll stars such as Presley and Buddy Holly (Holly, Buddy) depended for their sound on engineers' trickery in the recording studio as much as they did on their own vocal skills, and the guitar became the central rock instrument because of its amplified (amplifier) rather than acoustic qualities. Rock's history is tied up with technological shifts in the storage, retrieval, and transmission of sounds: multitrack tape recording made possible an experimental composition process that turned the recording studio into an artist's studio; digital recording made possible a manipulation of sound that shifted the boundaries between music and noise. Rock musicians pushed against the technical limits of sound amplification and inspired the development of new electronic instruments, such as the drum machine. Even relatively primitive technologies, such as the double-deck turntable, were tools for new sorts of music making in the hands of the “scratch” deejay, and one way rock marked itself off from other popular musical forms was in its constant pursuit of new sounds and new sound devices.

Rock and youth culture
      This pursuit of the new can be linked to rock's central sociological characteristic, its association with youth. In the 1950s and early 1960s this was a simple market equation: rock and roll was played by young musicians for young audiences and addressed young people's interests (quick sex and puppy love). It was therefore dismissed by many in the music industry as a passing novelty, “bubblegum,” akin to the yo-yo or the hula hoop. But by the mid-1960s youth had become an ideological category that referred to a particular kind of hedonism, individualism, and modernism. Whereas youth once referred to high-school students, it came to include college students. Moreover, rock became multifunctional—dance and party music on the one hand, a matter of serious attention and intimate expression on the other. As rock spread globally this had different implications in different countries, but in general it allowed rock to continue to define itself as youthful even as its performers and listeners grew up and settled down. And it meant that rock's radical claim—the suggestion that the music remained somehow against the establishment even as it became part of it—was sustained by an adolescent irresponsibility, a commitment to the immediate thrills of sex 'n' drugs 'n' outrage and never mind the consequences. The politics of rock fun has its own power structure, and it is not, perhaps, surprising that Madonna was the first woman to make a significant splash in rock history. And she did so by focusing precisely on rock's sexual assumptions.

Authenticity and commercialism
  Madonna can be described as a rock star (and not just a disco performer or teen idol) because she articulated rock culture's defining paradox: the belief that this music—produced, promoted, and sold by extremely successful and sophisticated multinational corporations—is nonetheless somehow noncommercial. It is noncommercial not in its processes of production but in the motivations of its makers and listeners, in terms of what, in rock, makes a piece of music or a musician valuable. The defining term in rock ideology is authenticity. Rock is distinguished from pop as the authentic expression of a performer's or composer's feelings and the authentic representation of a social situation. Rock is at once the mainstream of commercial music and a romantic art form, a voice from the social margins. Presley's first album for RCA in 1956 was just as carefully packaged to present him as an authentic, street-credible musician (plucking an acoustic guitar on the album cover) as was Public Enemy's classic It Takes a Nation of Millions to Hold Us Back, issued by the CBS-backed Def Jam in 1988; Madonna was every bit as concerned with revealing her artifice as art in the 1980s as Dylan was in the '60s.

      Rock, in summary, is not just an eclectic form musically but also a contradictory form ideologically. In making sense of its contradictions, two terms are critical. The first is presence. The effect of rock's musical promiscuity, its use of technology, and its emphasis on the individual voice is a unique sonic presence. Rock has the remarkable power both to dominate the soundscape and to entice the listener into the performers' emotional lives. The second is do-it-yourself (DIY). The credibility of this commercial music's claim to be noncommercial depends on the belief that rock is pushed up from the bottom rather than imposed from the top—hence the importance in rock mythology of independent record companies, local hustlers, managers, and deejays, fanzines, and broadcasters. Even as a multimillion-dollar industry, rock is believed to be a music and a culture that people make for themselves. The historical question becomes, What were the circumstances that made such a belief possible?

Rock in the 1950s

The development of the new vocal pop star
      If rock music evolved from 1950s rock and roll, then rock and roll itself—which at the time seemed to spring from nowhere—evolved from developments in American popular music that followed the marketing of the new technologies of records, radio, motion pictures, and the electric microphone. By the 1930s their combined effect was an increasing demand for vocal rather than instrumental records and for singing stars such as Bing Crosby (Crosby, Bing) and Frank Sinatra (Sinatra, Frank). Increasingly, pop songs were written to display a singer's personality rather than a composer's skill; they had to work emotionally through the singer's expressiveness rather than formally as a result of the score (it was Sinatra's feelings that were heard in the songs he sang rather than their writers'). By the early 1950s it was clear that this new kind of vocal pop star needed simpler, more directly emotional songs than those provided by jazz or theatre-based composers, and the big publishers began to take note of the blues and country numbers issued on small record labels in the American South. While the major record companies tried to meet the needs of Hollywood, the national radio networks, and television, a system of independent record companies (such as Atlantic, Sun, and Chess), local radio stations, and traveling deejays emerged to serve the music markets the majors ignored: African-Americans, Southern whites, and, eventually, youth.

Rural music in urban settings (rural society)
      Selling rural American musics (blues, folk, country, and gospel (gospel music)) had always been the business of small rather than corporate entrepreneurs, but World War II changed the markets for them—partly because of the hundreds of thousands of Southerners who migrated north for work, bringing their music with them, and partly because of the broadening cultural horizons that resulted from military service. Rural music in urban settings became, necessarily, louder and more aggressive (the same thing had happened to jazz in the early 1920s). Instruments, notably the guitar, had to be amplified to cut through the noise, and, as black dance bands got smaller (for straightforward economic reasons), guitar, bass, and miked-up voice replaced brass and wind sections, while keyboards and saxophone became rhythm instruments used to swell the beat punched out by the drums. Country dance bands, emerging from 1940s jazz-influenced western swing, made similar changes, amplifying guitars and bass, giving the piano a rhythmic role, and playing up the personality of the singer.

      Such music— rhythm and blues and honky tonk—was developed in live performance by traveling musicians who made their living by attracting dancers to bars, clubs, and halls. By the late 1940s it was being recorded by independent record companies, always on the lookout for cheap repertoire and aware of these musicians' local pulling power. As the records were played on local radio stations, the appeal of this music—its energy, humour, and suggestiveness—reached white suburban teenagers who otherwise knew nothing about it. Rhythm-and-blues record retailers, radio stations, and deejays (most famously ) became aware of a new market—partying teenagers—while the relevant recording studios began to be visited by young white musicians who wanted to make such music for themselves. The result was rock and roll, the adoption of these rural-urban, black and white sounds by an emergent teenage culture that came to international attention with the success of the film Blackboard Jungle in 1956.

Marketing rock and roll
      Rock and roll's impact in the 1950s reflected the spending power of young people who, as a result of the '50s economic boom (and in contrast to the prewar Great Depression), had unprecedented disposable income. That income was of interest not just to record companies but to an ever-increasing range of advertisers keen to pay for time on teen-oriented, Top 40 radio stations and for the development of teen-aimed television shows such as . For the major record companies, Presley's success marked less the appeal of do-it-yourself musical hybrids than the potential of teenage idols: singers with musical material and visual images that could be marketed on radio and television and in motion pictures and magazines. The appeal of live rock and roll (and its predominantly black performers) was subordinated to the manufacture of teenage pop stars (who were almost exclusively white). Creative attention thus swung from the performers to the record makers—that is, to the songwriters (such as those gathered in the in New York City) and producers (such as Phil Spector (Spector, Phil)) who could guarantee the teen appeal of a record and ensure that it would stand out on a car radio.

Rock in the 1960s

A black and white hybrid
      Whatever the commercial forces at play (and despite the continuing industry belief that this was pop music as transitory novelty (novelty song)), it became clear that the most successful writers and producers of teenage music were themselves young and intrigued by musical hybridity and the technological possibilities of the recording studio. In the early 1960s teenage pop ceased to sound like young adult pop. Youthful crooners such as Frankie Avalon (Avalon, Frankie) and Fabian were replaced in the charts by vocal groups such as the Shirelles (Shirelles, the). A new rock-and-roll hybrid of black and white music appeared: Spector derived the mini-dramas of girl groups such as the Crystals and the Ronettes from the vocal rhythm-and-blues style of doo-wop, the Beach Boys (Beach Boys, the) rearranged Chuck Berry (Berry, Chuck) for barbershop-style (barbershop quartet singing) close harmonies, and in Detroit Berry Gordy (Gordy, Berry, Jr.)'s Motown label drew on gospel music (first secularized for the teenage market by Sam Cooke (Cooke, Sam)) for the more rhythmically complex but equally commercial sounds of the Supremes (Supremes, the) and Martha and the Vandellas. For the new generation of record producer, whether Spector, the Beach Boys' Brian Wilson, or Motown's Smokey Robinson (Robinson, Smokey, and the Miracles) and the team of Holland-Dozier-Holland, the commercial challenge—to make a record that would be heard through all the other noises in teenage lives—was also an artistic challenge. Even in this most commercial of scenes (thanks in part to its emphasis on fashion), success depended on a creative approach to technological DIY.

The British (United Kingdom) reaction
      Rock historians tend to arrange rock's past into a recurring pattern of emergence, appropriation, and decline. Thus, rock and roll emerged in the mid-1950s only to be appropriated by big business (for example, Presley's move from the Memphis label Sun to the national corporation RCA (RCA Corporation)) and to decline into teen pop; the Beatles then emerged in the mid-1960s at the front of a British Invasion that led young Americans back to rock and roll's roots. But this notion is misleading. One reason for the Beatles' astonishing popularity by the end of the 1960s was precisely that they did not distinguish between the “authenticity” of, say, Chuck Berry and the “artifice” of the Marvelettes.

      In Britain, as in the rest of Europe, rock and roll had an immediate youth appeal—each country soon had its own Elvis Presley—but it made little impact on national music media, as broadcasting was still largely under state control. (The connection between was still to come.) Local rock and rollers had to make the music onstage rather than on record. In the United Kingdom musicians followed the skiffle group model of the folk, jazz, and blues scenes, the only local sources of American music making. The Beatles were only one of many provincial British groups who from the late 1950s played American music for their friends, imitating all kinds of hit sounds—from Berry to the Shirelles, from Carl Perkins (Perkins, Carl) to the Isley Brothers (Isley Brothers, the)—while using the basic skiffle format of rhythm section, guitar, and shouting to be heard in cheap, claustrophobic pubs and youth clubs.

      In this context a group's most important instruments were their voices—on the one hand, individual singers (such as John Lennon and Paul McCartney) developed a new harshness and attack; on the other hand, group voices (vocal harmonies) had to do the decorative work provided on the original records by producers in the studio. Either way, it was through their voices that British beat groups, covering the same songs with the same lineup of instruments, marked themselves off from each other, and it was through this emphasis on voice that vocal rhythm and blues made its mark on the tastes of “mod” culture (the “modernist” style-obsessed, consumption-driven youth culture that developed in Britain in the 1960s). Soul (soul music) singers such as Ray Charles (Charles, Ray) and Sam Cooke were the model for beat group vocals and by the mid-1960s were joined in the British charts by more intense African-American singers such as Aretha Franklin (Franklin, Aretha) and Otis Redding (Redding, Otis). British guitarists were equally influenced by this expressive ideal, and the loose rhythm guitar playing of rock and roll and skiffle was gradually replaced by more ornate lead playing on electric guitar as local musicians such as Eric Clapton (Clapton, Eric) sought to emulate blues artists such as B.B. King (King, B.B.).

      Clapton took the ideal of authentic performance from the British jazz scene, but his pursuit of originality—his homage to the blues originals and his search for his own guitar voice—also reflected his art-school education (Clapton was one of many British rock stars who engaged in music seriously while in art school). By the end of the 1960s, it was assumed that British rock groups wrote their own songs. What had once been a matter of necessity—there was a limit to the success of bands that played strictly cover versions, and Britain's professional songwriters had little understanding of these new forms of music—was now a matter of principle: self-expression onstage and in the studio was what distinguished these “rock” acts from pop “puppets” like Cliff Richard (Richard, Cliff). (Groomed as Britain's Elvis Presley in the 1950s—moving with his band, the Shadows (Shadows, the), from skiffle clubs to television teen variety shows—Richard was by the end of the 1960s a family entertainer, his performing style and material hardly even marked by rock and roll.)

folk rock, the hippie movement, and “the rock paradox”
      The peculiarity of Britain's beat boom—in which would-be pop stars such as the Beatles turned arty while would-be blues musicians such as the Rolling Stones (Rolling Stones, the) turned pop—had a dramatic effect in the United States, not only on consumers but also on musicians, on the generation who had grown up on rock and roll but grown out of it and into more serious sounds, such as urban folk. The Beatles' success suggested that it was possible to enjoy the commercial, mass-cultural power of rock and roll while remaining an artist. The immediate consequence was folk rock. Folk musicians, led by Bob Dylan (Dylan, Bob), went electric, amplified their instruments, and sharpened their beat. Dylan in particular showed that a pop song could be both a means of social commentary (protest) and a form of self-expression (poetry). On both the East and West coasts, bohemia started to take an interest in youth music again. In San Francisco, for example, folk and blues musicians, artists, and poets came together in loose collectives (most prominently the Grateful Dead (Grateful Dead, the) and the Jefferson Airplane (Jefferson Airplane, the)) to make acid rock as an unfolding psychedelic (psychedelic rock) experience, and rock became the musical soundtrack for a new youth culture, the hippies (hippie).

 The hippie movement of the late 1960s in the United States—tied up with Vietnam War service and anti-Vietnam War protests, the civil rights movement, and sexual liberation—fed back into the British rock scene. British beat groups also defined their music as art, not commerce, and felt themselves to be constrained by technology rather than markets. The Beatles made the move from pop to rock on their 1967 album, Sgt. Pepper's Lonely Hearts Club Band, symbolically identifying with the new hippie era, while bands such as Pink Floyd and Cream (Clapton's band) set new standards of musical skill and technical imagination. This was the setting in which Hendrix became the rock musician's rock musician. He was a model not just in his virtuosity and inventiveness as a musician but also in his stardom and his commercial charisma. By the end of the 1960s the great paradox of rock had become apparent: rock musicians' commitment to artistic integrity—their disdain for chart popularity—was bringing them unprecedented wealth. Sales of rock albums and concert tickets reached levels never before seen in popular music. And, as the new musical ideology was being articulated in magazines such as Rolling Stone, so it was being commercially packaged by emergent record companies such as Warner Brothers in the United States and Island in Britain. Rock fed both off and into hippie rebellion (as celebrated by the Woodstock festival of 1969), and it fed both off and into a buoyant new music business (also celebrated by Woodstock). This music and audience were now where the money lay; the Woodstock musicians seemed to have tapped into an insatiable demand, whether for “progressive” rock (art rock) and formal experiment, heavy metal and a bass-driven blast of high-volume blues, or singer-songwriters and sensitive self-exploration.

Rock in the 1970s

Corporate rock
      The 1970s began as the decade of the rock superstar. Excess became the norm for bands such as the Rolling Stones (Rolling Stones, the), not just in terms of their private wealth and well-publicized decadence but also in terms of stage and studio effects and costs. The sheer scale of rock album sales gave musicians—and their ever-growing entourage of managers, lawyers, and accountants—the upper hand in negotiations with record companies, and for a moment it seemed that the greater the artistic self-indulgence the bigger the financial return. By the end of the decade, though, the 25-year growth in record sales had come to a halt, and a combination of economic recession and increasing competition for young people's leisure spending (notably from the makers of video games) brought the music industry, by this point based on rock, its first real crisis. The Anglo-American music market was consolidated into a shape that has not changed much since, while new sales opportunities beyond the established transatlantic route began to be pursued more intently.

Challenges to mainstream rock
      The 1970s, in short, was the decade in which a pattern of rock formats and functions was settled. The excesses of rock superstardom elicited both a return to DIY rock and roll (in the roots sounds of performers such as Bruce Springsteen (Springsteen, Bruce) and in the punk movement of British youth) and a self-consciously camp take on rock stardom itself (in the glam rock of the likes of Roxy Music, David Bowie (Bowie, David), and Queen). The continuing needs of dancers were met by the disco movement (originally shaped by the twist phenomenon in the 1960s), which was briefly seized by the music industry as a new pop mainstream following the success of the film Saturday Night Fever in 1977. By the early 1980s, however, disco settled back into its own world of clubs, deejays, and recording studios and its own crosscurrents from African-American, Latin-American, and gay subcultures. African-American music developed in parallel to rock, drawing on rock technology sometimes to bridge black and white markets (as with Stevie Wonder (Wonder, Stevie)) and sometimes to sharpen their differences (as in the case of funk).

      Rock, in other words, was routinized, as both a moneymaking and a music-making practice. This had two consequences that were to become clearer in the 1980s. First, the musical tension between the mainstream and the margins, which had originally given rock and roll its cultural dynamism, was now contained within rock itself. The new mainstream was personified by Elton John (John, Sir Elton), who developed a style of soul (soul music)-inflected rock ballad (pop ballad) that over the next two decades became the dominant sound of global pop music. But the 1970s also gave rise to a clearly “alternative” rock ideology (most militantly articulated by British punk musicians), a music scene self-consciously developed on independent labels using “underground” media and committed to protecting the “essence” of rock and roll from commercial degradation. The alternative-mainstream, authentic-fake distinction crossed all rock genres and indicated how rock culture had come to be defined by its own contradictions.

      Second, sounds from outside the Anglo-American rock nexus began to make their mark on it (and in unexpected ways). In the 1970s, for example, Europop began to have an impact on the New York City dance scene via the clean, catchy Swedish sound of Abba, the electronic machine music of Kraftwerk, and the American-Italian collaboration (primarily in West Germany) of Donna Summer (Summer, Donna) and Giorgio Moroder. At the same time, Marley's success in applying a Jamaican sensibility to rock conventions meant that reggae became a new tool for rock musicians, whether established stars such as Clapton and the Rolling Stones' Keith Richards or young punks like the Clash (Clash, the), and played a significant role (via New York City's Jamaican sound-system deejays) in the emergence of hip-hop.

Rock in the 1980s and '90s

Digital technology (digital sound recording) and alternatives to adult-oriented rock
      The music industry was rescued from its economic crisis by the development in the 1980s of a new technology, digital recording. Vinyl records were replaced by the compact disc (CD), a technological revolution that immediately had a conservative effect. By this point the most affluent record buyers had grown up on rock; they were encouraged to replace their records, to listen to the same music on a superior sound system. Rock became adult music; youthful fads continued to appear and disappear, but these were no longer seen as central to the rock process, and, if rock's 1970s superstars could no longer match the sales of their old records with their new releases, they continued to sell out stadium concerts that became nostalgic rituals (most unexpectedly for the Grateful Dead). For new white acts the industry had to turn to alternative rock. A new pattern emerged—most successfully in the 1980s for R.E.M. and in the '90s for Nirvana—in which independent labels, college radio stations, and local retailers developed a cult audience for acts that were then signed and mass-marketed by a major label. Local record companies became, in effect, research and development divisions of the multinationals.

 The radical development of digital technology occurred elsewhere, in the new devices for sampling and manipulating sound, used by dance music engineers who had already been exploring the rhythmic and sonic possibilities of electronic instruments and blurring the distinctions between live and recorded music. Over the next decade the uses of digital equipment pioneered on the dance scene fed into all forms of rock music making. For a hip-hop act such as Public Enemy, what mattered was not just a new palette of “pure” sound but also a means of putting reality—the actual voices of the powerful and powerless—into the music. Hip-hop, as was quickly understood by young disaffected groups around the world, made it possible to talk back to the media.

The global market and fragmentation
      The regeneration of DIY paralleled the development of new means of global music marketing. The 1985 Live Aid event, in which live television broadcasts of charity concerts taking place on both sides of the Atlantic were shown worldwide, not only put on public display the rock establishment and its variety of sounds but also made clear television's potential as a marketing tool. MTV, the American cable company that had adopted the Top 40 radio format and made video clips (music video) as vital a promotional tool as singles, looked to satellite technology to spread its message: “One world, one music.” And the most successful acts of the 1980s, Madonna and Michael Jackson (Jackson, Michael) (whose 1982 album, Thriller, became the best-selling album of all time by crossing rock's internal divides), were the first video acts, using MTV brilliantly to sell themselves as stars while being used, in turn, as global icons in the advertising strategies of companies such as Pepsi-Cola.

      The problem with this pursuit of a single market for a single music was that rock culture was fragmenting. The 1990s had no unifying stars (the biggest sensation, the Spice Girls, were never really taken seriously). The attempt to market a global music was met by the rise of world music, an ever-increasing number of voices drawing on local traditions and local concerns to absorb rock rather than be absorbed by it. Tellingly, the biggest corporate star of the 1990s, the Quebecois Céline Dion (Dion, Céline), started out in the French-language market. By the end of the 20th century, hybridity meant musicians playing up divisions within rock rather than forging new alliances. In Britain the rave scene (fueled by dance music such as house and techno, which arrived from Chicago and Detroit via Ibiza, Spain) converged with “indie” guitar rock in a nostalgic pursuit of the rock community past that ultimately was a fantasy. Although groups like Primal Scream and the Prodigy seemed to contain, in themselves, 30 years of rock history, they remained on the fringes of most people's listening. Rock had come to describe too broad a range of sounds and expectations to be unified by anyone.

Rock as a reflection of social and cultural change
      How, then, should rock's contribution to music history be judged? One way to answer this is to trace rock's influences on other musics; another is to attempt a kind of cultural audit (What is the ratio of rock masterworks to rock dross?). But such approaches come up against the problem of definition. Rock does not so much influence other musics as colonize them, blurring musical boundaries. Any attempt to establish an objective rock canon is equally doomed to failure—rock is not this sort of autonomous, rule-bound aesthetic form.

      Its cultural value must be approached from a different perspective. The question is not How has rock influenced society? but rather, How has it reflected society? From the musician's point of view, for example, the most important change since the 1950s has been in the division of music-making labour. When Elvis Presley became a star, there were clear distinctions between the work of the performer, writer, arranger, session musician, record producer, and sound engineer. By the time Public Enemy was recording (music recording), such distinctions had broken down from both ends: performers wrote, arranged, and produced their own material; engineers made as significant a musical contribution as anyone else to the creation of a recorded sound. Technological developments—multitrack tape recorders, amplifiers, synthesizers, and digital equipment—had changed the meaning of musical instruments (musical instrument); there was no longer a clear distinction between producing a sound and reproducing it.

      From a listener's point of view, too, the distinction between music and noise changed dramatically in the second half of the 20th century. Music became ubiquitous, whether in public places (an accompaniment to every sort of activity), in the home (with a radio, CD player, or cassette player in every room), or in blurring the distinction between public and private use of music (a Walkman, boom box, or karaoke machine). The development of the compact disc only accelerated the process that makes music from any place and any time permanently available. Listening to music no longer refers to a special place or occasion but, rather, a special attention—a decision to focus on a given sound at a given moment.

      Rock is the music that has directly addressed these new conditions and kept faith with the belief that music is a form of human conversation, even as it is mediated by television and radio and by filmmakers and advertisers. The rock commitment to access—to doing mass music for oneself—has survived despite the centralization of production and the ever-increasing costs of manufacture, promotion, and distribution. Rock remains the most democratic of mass media—the only one in which voices from the margins of society can still be heard out loud. Yet, at the beginning of the 21st century, rock and the music industry faced a new crisis. The development of digital technology meant that music could now be stored on easy-to-use digital files, which could, in turn, be transferred from personal computer to personal computer via the Internet. The resulting legal and corporate disputes about new digital formats such as MP3 and services such as Napster reflected both new commercial opportunities (musical rights holders had visions of making money every time a song was downloaded) and fears (that their songs would be exchanged without any money changing hands at all).

      Beginning in late 1999, the Recording Industry Association of America, Bertelsmann AG, and some artists sued Napster, an Internet company whose "peer-to-peer" file-sharing program allowed users to download music for free. Other artists lined up on either side of the issue. By the end of the century, however, Bertelsmann had become the majority owner of Napster, anxious to provide a fee-based service.

      While the issues here were new, the story line was not. Again, an emergent technology meant new commercial opportunities that were explored and developed by fledgling entrepreneurs before being absorbed and reordered by the major music companies; again, new ways of making and using music became subject to new laws and licenses. But what was most striking about the battle over Napster was the assumption that what was at issue was rock music. (Napster was not seen as a threat to classical or country music.) Significantly, the first widely successful use of MP3 technology, Napster, involved a global network of home "tapers" and drew on the rock ideology of DIY, community, and anti-commerce. Whatever Napster's fate, it ensured that rock music would be central to 21st-century ways of doing things. Rock, in short, not only reflects (and reflects on) social and cultural change; it is also a social force in its own right.

Simon Frith

Representative Works
(See author's description of how this list was compiled.)

● Bill Haley and His Comets, “Rock Around the Clock” (1955)
● Elvis Presley, Elvis Presley (1956)
● Chuck Berry, One Dozen Berry's (1958)
● Elvis Presley, Elvis' Golden Records (1958)
● Buddy Holly, The Buddy Holly Story (1959)
● Muddy Waters, Muddy Waters at Newport (1960)
● Ray Charles, The Genius Sings the Blues (1961)
● Sam Cooke, The Best of Sam Cooke (1962)
● The Beatles, With the Beatles (1963 [different version released in the United States as Meet the Beatles in 1964])
● James Brown, Live at the Apollo (1963)
● Bob Dylan, The Freewheelin' Bob Dylan (1963)
● The Ronettes et al., A Christmas Gift for You (1963)
● The Shirelles, The Shirelles'Hits (1963)
● The Beach Boys, All Summer Long (1964)
● Bob Dylan, Bringing It All Back Home (1965)
● B.B. King, Live at the Regal (1965)
● Otis Redding, Otis Blue/Otis Redding Sings Soul (1965)
● Various artists, A Package of 16 Big Tamla Motown Hits (1965)
● Cream, Fresh Cream (1966)
● The Beatles, Sgt. Pepper's Lonely Hearts Club Band (1967)
● Aretha Franklin, I Never Loved a Man the Way I Love You (1967)
● The Jefferson Airplane, Surrealistic Pillow (1967)
● Jimi Hendrix Experience, Are You Experienced? (1967)
● Van Morrison, Astral Weeks (1968)
● The Rolling Stones, Beggars Banquet (1968)
● Various artists, Woodstock (1970 [film soundtrack])
● David Bowie, Hunky Dory (1971)
● Carole King, Tapestry (1971)
● Led Zeppelin, the album usually referred to as “Zoso,” “Runes,” or “Four Symbols” (1971)
● Jimmy Cliff et al., The Harder They Come (1972 [film soundtrack])
● The Wailers, Catch a Fire (1973)
● Stevie Wonder, Innervisions (1973)
● Joni Mitchell, Court and Spark (1974)
● Elton John, Captain Fantastic and the Brown Dirt Cowboy (1975)
● Patti Smith, Horses (1975)
● Fleetwood Mac, Rumours (1977)
● Funkadelic, One Nation Under a Groove (1978)
● Kraftwerk, The Man-Machine (1978)
● Neil Young, Rust Never Sleeps (1979)
● The Clash, London Calling (1980)
● Talking Heads, Remain in Light (1980)
● Michael Jackson, Thriller (1982)
● Bruce Springsteen, Born in the U.S.A. (1984)
● Paul Simon, Graceland (1986)
● Madonna, You Can Dance (1987)
● Public Enemy, It Takes a Nation of Millions to Hold Us Back (1988)
● Nirvana, Nevermind (1991)
● Primal Scream, Screamadelica (1991)
● Portishead, Dummy (1994)

Additional Reading

There is an extensive literature on rock that ranges from academic musicology and sociology through every kind of journalism to disposable gossip and poster books. Peter van der Merwe, Origins of the Popular Style (1989, reissued 1992), a scholarly study of pre-20th-century popular music, helps explain why a music first appearing at the margins of Western culture so quickly became the mainstream. Charlie Gillett, The Sound of the City: The Rise of Rock and Roll, 2nd ed., newly illustrated and expanded (1996), is still the best account of how rock and roll was first shaped in a variety of local American settings. Rock and roll's roots in black and white music are covered in Country: The Music and the Musicians: From the Beginnings to the '90s, 2nd ed. (1994), an informative overview of country music history published by the Country Music Foundation; and Charles Keil, Urban Blues (1966, reissued 1991), an illuminating anthropological study of African-American musical culture in the late 1950s and early 1960s.The development of rock out of rock and roll was as much an ideological as a musical process, and the classic description of that ideology—of why and how rock drew from and came to articulate the contradictory impulses of American popular culture—is Greil Marcus, Mystery Train, 4th rev. ed. (1997), which, in its studies of particular musicians, was the first work to reveal the possibilities of rock criticism; Greil Marcus, Invisible Republic: Bob Dylan's Basement Tapes (1997), fills the biggest gap in Mystery Train. Simon Frith and Howard Horne, Art into Pop (1987), studies how British rock sensibility was shaped by art school ideas and practices. Simon Frith and Andrew Goodwin (eds.), On Record: Rock, Pop, and the Written Word (1990), is a useful anthology of 30 years of scholarly writing on rock, from a variety of disciplinary perspectives. The best studies of the rock music industry are Geoffrey Stokes, Star-Making Machinery (1976), a fine and undated piece of reportage on the making and marketing of a Commander Cody LP; Andrew Goodwin, Dancing in the Distraction Factory: Music Television and Popular Culture (1992), a lucid and thoughtful analysis of MTV's impact on rock culture; and Paul Théberge, Any Sound You Can Imagine: Making Music/Consuming Technology (1997), a comprehensive history of the effects of technology on music making, paying particular attention to digital technology.

Peter Guralnick, Last Train to Memphis (1994), is the definitive work on the young Elvis Presley and his influences; Peter Guralnick, Careless Love (1999), provides all one needs to know about Presley's subsequent career—its triumphs and tragedies. Good accounts of the ways in which musicians have tried to make sense of rock's confusion of art, commerce, and politics can be found in the biographies of four musicians who died young: Marc Eliot, Death of a Rebel (1979, reissued 1995), on the muddled life of folk-rock singer-songwriter Phil Ochs; Charles Shaar Murray, Crosstown Traffic (1989), a biography of Jimi Hendrix focusing on issues of race and identity; Dr. Licks, Standing in the Shadows of Motown: The Life and Music of Legendary Bassist James Jamerson (1989), a loving account of the origins and influence of one of rock's most significant rhythmic stylists; and Armond White, Rebel for the Hell of It (1997), on rap star Tupac (2pak) Shakur, an important reflection on music and the state of the American nation at the end of the 20th century.

The most enlightening books on particular musical genres are Andrew Holleran, Dancer from the Dance (1978, reissued 1990), a disco novel that captures the disco experience better than any other writing; Dick Hebdige, Cut 'n' Mix: Culture, Identity, and Caribbean Music (1987, reissued 1990), a suggestive application of cultural theory to the remarkable mobility of reggae music; Jon Savage, England's Dreaming: Sex Pistols and Punk Rock (1991; also published as England's Dreaming: Anarchy, Sex Pistols, Punk Rock, and Beyond, 1992), on music, suburbia, and boredom; David Toop, Rap Attack 2: African Rap to Global Hip Hop, rev. ed. (1991), a well-informed history of hip-hop; Robert Walser, Running with the Devil: Power, Gender, and Madness in Heavy Metal Music (1993), the most convincing of all the musicological rock studies; Sarah Thornton, Club Cultures: Music, Media, and Subcultural Capital (1995), an intelligent sociology of British dance clubs in the early 1990s; and Simon Reynolds, Generation Ecstasy: Into the World of Techno and Rave Culture (1998), a helpful map of a confused music scene. Finally, Evelyn McDonnell and Ann Powers (eds.), Rock She Wrote (1995), is an instructive anthology of rock writing from a female perspective; Mark Slobin, Subcultural Sounds: Micromusics of the West (1993), is an ethnomusicological study which makes clear that all popular musics, rock included, remain local even as they become global, just as in the first days of rock and roll.

* * *

Universalium. 2010.


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  • rock — (del inglés; pronunciamos roc ) adjetivo 1. Área: música De un género musical derivado del rock and roll: cantante rock. música rock. sustantivo masculino 1 …   Diccionario Salamanca de la Lengua Española

  • rock — Voz inglesa que se usa como abreviación de rock and roll (→ rock and roll) y designa también cada uno de los estilos diversos derivados de ese género musical. Por tratarse de un extranjerismo crudo, debe escribirse con resalte tipográfico. Para… …   Diccionario panhispánico de dudas

  • rock — s.m.inv., agg.inv. ES ingl. {{wmetafile0}} TS mus. 1a. s.m.inv. → rock and roll 1b. s.m.inv., denominazione collettiva del genere musicale sviluppatosi dal rock and roll, in cui si individuano vari filoni a seconda dello stile prevalente e degli… …   Dizionario italiano

  • rock — Ⅰ. rock [1] ► NOUN 1) the hard mineral material of the earth s crust, exposed on the surface or underlying the soil. 2) a mass of rock projecting out of the ground or water. 3) a boulder. 4) Geology any natural material with a distinctive… …   English terms dictionary

  • rock — rock1 [räk] n. [ME rokke < OFr roche < ML rocca] 1. a large mass of stone forming a peak or cliff 2. a) a large stone detached from the mass; boulder b) broken pieces of any size of such stone c) any stone, large or small …   English World dictionary

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