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ROCKS AND THEIR CLASSIFICATION
452 ROCKS AND THEIR CLASSIFICATION
ROCKS AND THEIR CLASSIFICATION R C Selley, Imperial College London, London, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction: What on Earth is a Rock? To a geologist the term ‘rock’ is applied to all the solid materials of the Earth, whether they are hard enough to hit with a hammer or soft unconsolidated gravel, sand, or mud. The nomenclature and classification of rocks is a prerequisite to understanding them. One of several definitions of science is that it is organized knowledge. It is the organization and classification of knowledge that provides insight into processes and adds predictive value. The Periodic Table of the Elements illustrates these points admirably. Classification is an integral part of geology in all its forms. It extends from palaeontology, where the classification of fossils according to the binomial nomenclature of Linnaeus is a prerequisite to demonstrating evolution and understanding palaeoecology, to mineralogy, where minerals are grouped into like families according to their chemistry and crystallography. The object of this chapter is to review the fundamental classification of rocks as a foundation for discussing igneous, sedimentary, and metamorphic rock nomenclature and classification (see Sedimentary Rocks: Mineralogy and Classification, Metamorphic Rocks: Classification, Nomenclature and Formation). Rocks are traditionally grouped into three main classes, according to their genesis. These will now be defined and described. All generalizations are dangerous, however, including this one. Thus, a detailed examination of the classification (see Igneous Processes) of rocks soon reveals discrepancies and inconsistencies (see Metamorphic Rocks: Classification, Nomenclature and Formation). It may, therefore, be helpful to examine these subsequently.
discussed elsewhere in this encyclopaedia, magma may move up through the crust towards the surface. It may cool very slowly beneath the surface to form coarsely crystalline rocks. These include silica-rich rocks, such as granite (see Igneous Rocks: Granite), and ferromagnesian-rich rocks, such as gabbro. If the magma reaches the surface, it erupts in volcanoes (see Volcanoes), and the melt cools quickly to form microcrystalline or even glassy rocks. Depending on their chemistry and texture, volcanic rocks may be classified as silica-rich rhyolite or ferromagnesian-rich basalt. Thus, owing to their mode of formation, igneous rocks tend to be structureless when viewed at outcrop or in a hand specimen. Under the microscope they are seen to consist of a random arrangement of interlocking crystals of a range of different minerals, which formed when the melt crystallized. Porosity is generally absent. Sedimentary Rocks
Sedimentary rocks are formed from the detritus of pre-existing rocks, which may be igneous, metamorphic, or sedimentary. The way in which rock is
A Classification of Rocks Rocks are of three genetic types: igneous (formed from cooling magma), sedimentary (formed by the breakdown of pre-existing rock), and metamorphic (formed by the action of heat and pressure on pre-existing rock) (Figure 1 and Table 1). Igneous Rocks
Magma is molten material from deep beneath the surface of the Earth. Through a variety of processes
Figure 1 The Rocks Display’d. An illustration of the modes of formation of sedimentary, igneous, and metamorphic rocks, accompanied by their classical deities Neptune, Vulcan, and Pluto (together with his synonymous dog). Reproduced by permission of The Geologists’ Association from Proceedings of The Geologists’ Association, Wilson G, in Read HH, Meditations on Granite 2, volume 55, pp. 4–93. Fig. 1. ß 1944 The Geologists’ Association.
ROCKS AND THEIR CLASSIFICATION 453
Table 1 The three main classes of rock, their origins, and examples Class of rock
Origin
Examples
Sedimentary
Deposition of particles of pre-existing rocks Chemical replacement and precipitation Heated by igneous intrusions Heated and subjected to high pressure by deep burial Volcanic eruptions Deep cooling of magma
Gravel, sand, and mudstone Limestone dolomite and salt Hornfels Slate, schist, and gneiss Basalt, andesite, and rhyolite Granite, diorite, and gabbro
Metamorphic Igneous
weathered, eroded, transported, and deposited is discussed in detail elsewhere (see Weathering, Sedimentary Processes: Erosional Sedimentary Structures, Sedimentary Environments: Depositional Systems and Facies). Sediments possess a wide range of particle sizes, ranging from boulders to clay, and of chemical compositions, including silica, lime, and ferromagnesian volcanic detritus. These parameters of particle size and composition are used to classify sedimentary rocks (see Sedimentary Rocks: Mineralogy and Classification). Sedimentary rocks commonly exhibit two properties that may be used to differentiate them from igneous and metamorphic rocks. Where they crop out at the surface of the Earth, sedimentary rocks generally show stratification. The strata indicate successive episodes of deposition. Layering is usually absent from igneous rocks, as discussed above, but is found in some metamorphic rocks, as discussed below. When examined under the microscope, sedimentary rocks are generally seen to consist of particles. Void space (porosity) is commonly present between the constituent grains. If there are interconnected pores, these give the rock permeability. Permeability allows fluids to migrate through the rock and enables rock and soil to drain. Fossils are found only in sedimentary rocks, some of which are, indeed, made up of nothing else. Sedimentary rocks include sands, including terrigenous silica-rich sandstone, and carbonates, termed limestone. Sediments composed of rounded pebbles are termed conglomerates, while those composed of angular clasts (fragments) are termed breccias. Metamorphic Rocks
Metamorphic rocks are so termed from the Greek meta-, meaning altered, and morphos-, meaning shape. The word is applied to rocks, of whatever previous origin, that have been changed as a result of being exposed to high temperature, high pressure, or both. Two types of metamorphism are recognized, regional metamorphism, and thermal or contact metamorphism. When magma moves up through the crust it creates a metamorphic aureole in the adjacent cover rocks. This is termed contact or thermal metamorphism. By contrast, rocks that are subjected to
high temperatures and pressures during deep burial are said to have undergone regional metamorphism. Common thermally metamorphosed rocks include hornfels and quartzite. Regionally metamorphosed rocks include schist and gneiss. Thermally metamorphosed rocks are often crystalline and devoid of porosity. They may be massive or layered, according to the character of the original rock. Regionally metamorphosed rocks are also commonly crystalline and devoid of porosity. They are, however, often banded. The banding seldom resembles the regular stratification of sediments, but often takes the form of curvaceous foliation and cleavage (see Regional Metamorphism).
Rock Classification: A Simple Summary Igneous and metamorphic rocks are both normally composed of a mosaic of interlocking crystals that formed at high temperatures and pressures; they thus usually lack porosity and permeability. In the petroleum industry igneous and metamorphic rocks are generally grouped together under the term ‘basement’, indicating that it is pointless to continue drilling deeper into such tight rocks. Igneous rocks are generally massive and structureless, while metamorphic rocks are often banded. Sediments, on the other hand, are deposited with space between the constituent particles. Thus, most sedimentary rocks, by their very nature, tend to be porous and permeable. Sediments are deposited episodically and thus characteristically exhibit stratification. They are also the only one of the three main classes of rock to contain fossils. Igneous, metamorphic, and sedimentary rocks may therefore be differentiated from one another using macroscopic structure and microscopic texture as shown in Figure 2.
Complications and Anomalies When the tripartite classification of rocks was introduced at the beginning of this article it was pointed out that, like all classifications, it has complications
454 ROCKS AND THEIR CLASSIFICATION
Figure 2 A rough guide to how igneous, metamorphic, and sedimentary rocks may be differentiated from one another by macroscopic structure (left) and microscopic texture (right). As discussed in the text, however, there are exceptions to these simple guidelines. (A) Igneous rocks are generally structureless in the field, with a random mosaic of crystals when viewed in thin section under the microscope. (B) Metamorphic rocks are generally banded (often curvaceous) in the field, with a linear arrangement of crystals when viewed in thin section under the microscope. (C) Sedimentary rocks are generally stratified in the field and exhibit rounded grains with intergranular pores when viewed in thin section under the microscope.
and anomalies. These will now be examined for igneous, metamorphic, and sedimentary rocks in turn. Igneous Rock Anomalies
When describing igneous rocks it was stated that they form from cooling magma and are generally composed of a mosaic of interlocking randomly arranged crystals. Thus, in hand specimen, and at outcrop, igneous rocks present a massive appearance. They generally lack the foliation of metamorphic rocks and the stratification of sediments. Now for the exceptions, which occur in both volcanic lavas and major intrusions. Lava is extruded from a volcano episodically, on time-scales ranging from thousands of years to minutes. Major sequences of lava flows give rise to a very typical landform, with plateaus of lava bounded by cliffs in which successive lava flows can be discerned,
separated from one another by weathered soil horizons that formed between individual flow events. Superficially this gives rise to landscapes analogous to those created by stratified sedimentary rocks. Plateau basalts are widespread around the world, from the Deccan Traps of India to the Tertiary volcanic province of the Scottish Hebridean islands. When examined closely, lavas commonly exhibit flowbanding on a variety of scales. This includes the ‘ropy’ structure of some basalt flows, the so-called ‘pahoehoe’ lavas of Hawaii, and a diverse range of structures that are morphologically similar to the post-depositional structures of sedimentary rocks (see Sedimentary Processes: Post-Depositional Sedimentary Structures). For example the Devonian rhyolites of the Glen Coe cauldron subsidence in Scotland exhibit excellent examples of slumps, slides, load-casts, and flame structures. It was also noted earlier that igneous rocks generally lack porosity. Some gassy lavas, however, consolidate with contained bubbles of gas, giving rise to what are termed vesicular lavas. Vesicular lavas thus exhibit porosity, but, unless the bubbles are abundant and interconnected, there is no significant permeability. Deep-seated igneous intrusions also sometimes show features indicative of sedimentation. These are most commonly found in ultrabasic ferromagnesianrich intrusives, such as the gabbroic Skaergaard complex of Greenland and the Bushveld intrusion of South Africa. These intrusions show a gravity segregation of minerals, with the densest, such as chromite and iron ores, at the base, overlain by ferromagnesian minerals, and then feldspars above. It is clear that the minerals have crystallized out of a melt, and have settled under gravity. There is more. This gravity separation may occur not only on an intrusion-wide scale, but also on a scale of metres or centimetres, resulting in size-graded ‘beds’ analogous to those deposited by sediment gravity flows. The evidence suggests that convection currents within the magma chamber permit successive episodes of deposition from density flows. Inclined bedding analogous to that produced by traction currents in running water is also known (see Unidirectional Aqueous Flow). Sedimentary Rock Anomalies
Sedimentary rocks are defined as those that are composed of fragments of pre-existing rocks, of a wide range of particle sizes, that are deposited under ice or water or in the atmosphere. They are thus commonly stratified and contain fossils, and usually exhibit porosity between the constituent sediment grains. This definition is adequate for gravels, sands, and muds and for the conglomerates, sandstones, and shales that are their lithified equivalents. There is,
ROCKS AND THEIR CLASSIFICATION 455
however, a large group of sedimentary rocks where these characters are largely absent. These are the chemical sediments. The chemical sediments are a diverse group that includes limestones, dolomites, ironstones, evaporites, chert, hydrocarbons (peat, lignite, and coal), and phosphate (see Sedimentary Rocks: Chert; Dolomites; Evaporites; Ironstones; Limestones; Phosphates). Limestones are frequently composed of clasts and exhibit sedimentary structures analogous to those of sandstones. The other chemical rocks, however, owe their peculiar properties very largely to post-depositional chemical changes during diagenesis (see Diagenesis, Overview). In the extreme case of the so-called evaporites, it is now recognized that they should be more properly termed ‘replacementites’ since their original chemical composition has been completely changed, though primary sedimentary features may be discerned as ghostly relics. Indeed it could be argued that the evaporites/replacementites are really metamorphic rocks. Stratification is sometimes absent in sedimentary rocks. Massive structureless sediments include ‘loess’ (wind-blown silt) and grain-flow and mass-flow deposits such as debrites and olistostromes. Metamorphic Rock Anomalies
The previous paragraph on anomalous sedimentary rocks provides a useful introduction to problems of metamorphic-rock classification. Just as the boundary between chemical sediments and metamorphic rocks is gradational, so too is the boundary between detrital sediments and metamorphic rocks. During burial gravel lithifies to form conglomerate, sand forms sandstone, and mud forms shale. With increasing burial temperature and pressure, porosity is destroyed, and texture and mineralogy are modified. There is therefore a gradation between highly cemented sedimentary rocks and low-grade metamorphic rocks. Similar gradations occur when superficial igneous rocks undergo deep burial, with concomitant mineralogical and textural changes as they metamorphose. Historically nowhere has this problem been more acute than in the controversial theories about the origin of granite. Granites were believed to have formed from the slow cooling of magma that had intruded shallower rocks. It was pointed out, however, that, when considering the vast granite masses at the eroded roots of mountains, there was a space problem. This was resolved when it was realized that wholesale replacement of preexisting rock had taken place. Intrusive granites were found only at shallow depths, having moved upwards from deep parent batholiths.
Rocks and their Classification: Conclusion Rocks are of three genetic types: igneous, formed from cooling magma; sedimentary, formed mainly by the breakdown of pre-existing rock; and metamorphic, formed by the action of heat and pressure on pre-existing rock. Igneous and metamorphic rocks are both normally composed of a mosaic of interlocking crystals that formed at high temperatures and pressures; they thus usually lack porosity and permeability. In the petroleum industry igneous and metamorphic rocks are generally grouped together under the term ‘basement’, indicating that it is pointless to continue drilling deeper into such tight rocks. Igneous rocks are generally massive and structureless, while metamorphic rocks are often banded. Sediments, on the other hand, are deposited with space between the constituent particles. Thus, sedimentary rocks, by their very nature, tend to be porous and permeable. Sediments are deposited episodically, and thus characteristically exhibit stratification. They are also the only one of the three main classes of rock to contain fossils. All classifications have their illogicalities and inconsistencies. Thus sedimentary rocks grade with increasing burial into metamorphic rocks, and metamorphic rocks ultimately melt to form magma, which cools to form igneous rocks, which are weathered, eroded, transported, and redeposited as sediments.
See Also Diagenesis, Overview. Igneous Processes. Igneous Rocks: Granite. Metamorphic Rocks: Classification, Nomenclature and Formation. Regional Metamorphism. Sedimentary Environments: Depositional Systems and Facies. Sedimentary Processes: Erosional Sedimentary Structures; Post-Depositional Sedimentary Structures. Sedimentary Rocks: Mineralogy and Classification; Chert; Dolomites; Evaporites; Ironstones; Limestones; Phosphates. Unidirectional Aqueous Flow. Volcanoes. Weathering.
Further Reading Norton WH (2002) The Elements of Geology. Indypublish.com, Cybercity. Peters KE (1997) No Stone Unturned. San Francisco: WH Freeman. Rothery D, Turner D, and Wilson RCL (2001) Geology. Milton Keynes: Open University Worldwide.
ROCKS AND THEIR CLASSIFICATION R C Selley, Imperial College London, London, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction: What on Earth is a Rock? To a geologist the term ‘rock’ is applied to all the solid materials of the Earth, whether they are hard enough to hit with a hammer or soft unconsolidated gravel, sand, or mud. The nomenclature and classification of rocks is a prerequisite to understanding them. One of several definitions of science is that it is organized knowledge. It is the organization and classification of knowledge that provides insight into processes and adds predictive value. The Periodic Table of the Elements illustrates these points admirably. Classification is an integral part of geology in all its forms. It extends from palaeontology, where the classification of fossils according to the binomial nomenclature of Linnaeus is a prerequisite to demonstrating evolution and understanding palaeoecology, to mineralogy, where minerals are grouped into like families according to their chemistry and crystallography. The object of this chapter is to review the fundamental classification of rocks as a foundation for discussing igneous, sedimentary, and metamorphic rock nomenclature and classification (see Sedimentary Rocks: Mineralogy and Classification, Metamorphic Rocks: Classification, Nomenclature and Formation). Rocks are traditionally grouped into three main classes, according to their genesis. These will now be defined and described. All generalizations are dangerous, however, including this one. Thus, a detailed examination of the classification (see Igneous Processes) of rocks soon reveals discrepancies and inconsistencies (see Metamorphic Rocks: Classification, Nomenclature and Formation). It may, therefore, be helpful to examine these subsequently.
discussed elsewhere in this encyclopaedia, magma may move up through the crust towards the surface. It may cool very slowly beneath the surface to form coarsely crystalline rocks. These include silica-rich rocks, such as granite (see Igneous Rocks: Granite), and ferromagnesian-rich rocks, such as gabbro. If the magma reaches the surface, it erupts in volcanoes (see Volcanoes), and the melt cools quickly to form microcrystalline or even glassy rocks. Depending on their chemistry and texture, volcanic rocks may be classified as silica-rich rhyolite or ferromagnesian-rich basalt. Thus, owing to their mode of formation, igneous rocks tend to be structureless when viewed at outcrop or in a hand specimen. Under the microscope they are seen to consist of a random arrangement of interlocking crystals of a range of different minerals, which formed when the melt crystallized. Porosity is generally absent. Sedimentary Rocks
Sedimentary rocks are formed from the detritus of pre-existing rocks, which may be igneous, metamorphic, or sedimentary. The way in which rock is
A Classification of Rocks Rocks are of three genetic types: igneous (formed from cooling magma), sedimentary (formed by the breakdown of pre-existing rock), and metamorphic (formed by the action of heat and pressure on pre-existing rock) (Figure 1 and Table 1). Igneous Rocks
Magma is molten material from deep beneath the surface of the Earth. Through a variety of processes
Figure 1 The Rocks Display’d. An illustration of the modes of formation of sedimentary, igneous, and metamorphic rocks, accompanied by their classical deities Neptune, Vulcan, and Pluto (together with his synonymous dog). Reproduced by permission of The Geologists’ Association from Proceedings of The Geologists’ Association, Wilson G, in Read HH, Meditations on Granite 2, volume 55, pp. 4–93. Fig. 1. ß 1944 The Geologists’ Association.
ROCKS AND THEIR CLASSIFICATION 453
Table 1 The three main classes of rock, their origins, and examples Class of rock
Origin
Examples
Sedimentary
Deposition of particles of pre-existing rocks Chemical replacement and precipitation Heated by igneous intrusions Heated and subjected to high pressure by deep burial Volcanic eruptions Deep cooling of magma
Gravel, sand, and mudstone Limestone dolomite and salt Hornfels Slate, schist, and gneiss Basalt, andesite, and rhyolite Granite, diorite, and gabbro
Metamorphic Igneous
weathered, eroded, transported, and deposited is discussed in detail elsewhere (see Weathering, Sedimentary Processes: Erosional Sedimentary Structures, Sedimentary Environments: Depositional Systems and Facies). Sediments possess a wide range of particle sizes, ranging from boulders to clay, and of chemical compositions, including silica, lime, and ferromagnesian volcanic detritus. These parameters of particle size and composition are used to classify sedimentary rocks (see Sedimentary Rocks: Mineralogy and Classification). Sedimentary rocks commonly exhibit two properties that may be used to differentiate them from igneous and metamorphic rocks. Where they crop out at the surface of the Earth, sedimentary rocks generally show stratification. The strata indicate successive episodes of deposition. Layering is usually absent from igneous rocks, as discussed above, but is found in some metamorphic rocks, as discussed below. When examined under the microscope, sedimentary rocks are generally seen to consist of particles. Void space (porosity) is commonly present between the constituent grains. If there are interconnected pores, these give the rock permeability. Permeability allows fluids to migrate through the rock and enables rock and soil to drain. Fossils are found only in sedimentary rocks, some of which are, indeed, made up of nothing else. Sedimentary rocks include sands, including terrigenous silica-rich sandstone, and carbonates, termed limestone. Sediments composed of rounded pebbles are termed conglomerates, while those composed of angular clasts (fragments) are termed breccias. Metamorphic Rocks
Metamorphic rocks are so termed from the Greek meta-, meaning altered, and morphos-, meaning shape. The word is applied to rocks, of whatever previous origin, that have been changed as a result of being exposed to high temperature, high pressure, or both. Two types of metamorphism are recognized, regional metamorphism, and thermal or contact metamorphism. When magma moves up through the crust it creates a metamorphic aureole in the adjacent cover rocks. This is termed contact or thermal metamorphism. By contrast, rocks that are subjected to
high temperatures and pressures during deep burial are said to have undergone regional metamorphism. Common thermally metamorphosed rocks include hornfels and quartzite. Regionally metamorphosed rocks include schist and gneiss. Thermally metamorphosed rocks are often crystalline and devoid of porosity. They may be massive or layered, according to the character of the original rock. Regionally metamorphosed rocks are also commonly crystalline and devoid of porosity. They are, however, often banded. The banding seldom resembles the regular stratification of sediments, but often takes the form of curvaceous foliation and cleavage (see Regional Metamorphism).
Rock Classification: A Simple Summary Igneous and metamorphic rocks are both normally composed of a mosaic of interlocking crystals that formed at high temperatures and pressures; they thus usually lack porosity and permeability. In the petroleum industry igneous and metamorphic rocks are generally grouped together under the term ‘basement’, indicating that it is pointless to continue drilling deeper into such tight rocks. Igneous rocks are generally massive and structureless, while metamorphic rocks are often banded. Sediments, on the other hand, are deposited with space between the constituent particles. Thus, most sedimentary rocks, by their very nature, tend to be porous and permeable. Sediments are deposited episodically and thus characteristically exhibit stratification. They are also the only one of the three main classes of rock to contain fossils. Igneous, metamorphic, and sedimentary rocks may therefore be differentiated from one another using macroscopic structure and microscopic texture as shown in Figure 2.
Complications and Anomalies When the tripartite classification of rocks was introduced at the beginning of this article it was pointed out that, like all classifications, it has complications
454 ROCKS AND THEIR CLASSIFICATION
Figure 2 A rough guide to how igneous, metamorphic, and sedimentary rocks may be differentiated from one another by macroscopic structure (left) and microscopic texture (right). As discussed in the text, however, there are exceptions to these simple guidelines. (A) Igneous rocks are generally structureless in the field, with a random mosaic of crystals when viewed in thin section under the microscope. (B) Metamorphic rocks are generally banded (often curvaceous) in the field, with a linear arrangement of crystals when viewed in thin section under the microscope. (C) Sedimentary rocks are generally stratified in the field and exhibit rounded grains with intergranular pores when viewed in thin section under the microscope.
and anomalies. These will now be examined for igneous, metamorphic, and sedimentary rocks in turn. Igneous Rock Anomalies
When describing igneous rocks it was stated that they form from cooling magma and are generally composed of a mosaic of interlocking randomly arranged crystals. Thus, in hand specimen, and at outcrop, igneous rocks present a massive appearance. They generally lack the foliation of metamorphic rocks and the stratification of sediments. Now for the exceptions, which occur in both volcanic lavas and major intrusions. Lava is extruded from a volcano episodically, on time-scales ranging from thousands of years to minutes. Major sequences of lava flows give rise to a very typical landform, with plateaus of lava bounded by cliffs in which successive lava flows can be discerned,
separated from one another by weathered soil horizons that formed between individual flow events. Superficially this gives rise to landscapes analogous to those created by stratified sedimentary rocks. Plateau basalts are widespread around the world, from the Deccan Traps of India to the Tertiary volcanic province of the Scottish Hebridean islands. When examined closely, lavas commonly exhibit flowbanding on a variety of scales. This includes the ‘ropy’ structure of some basalt flows, the so-called ‘pahoehoe’ lavas of Hawaii, and a diverse range of structures that are morphologically similar to the post-depositional structures of sedimentary rocks (see Sedimentary Processes: Post-Depositional Sedimentary Structures). For example the Devonian rhyolites of the Glen Coe cauldron subsidence in Scotland exhibit excellent examples of slumps, slides, load-casts, and flame structures. It was also noted earlier that igneous rocks generally lack porosity. Some gassy lavas, however, consolidate with contained bubbles of gas, giving rise to what are termed vesicular lavas. Vesicular lavas thus exhibit porosity, but, unless the bubbles are abundant and interconnected, there is no significant permeability. Deep-seated igneous intrusions also sometimes show features indicative of sedimentation. These are most commonly found in ultrabasic ferromagnesianrich intrusives, such as the gabbroic Skaergaard complex of Greenland and the Bushveld intrusion of South Africa. These intrusions show a gravity segregation of minerals, with the densest, such as chromite and iron ores, at the base, overlain by ferromagnesian minerals, and then feldspars above. It is clear that the minerals have crystallized out of a melt, and have settled under gravity. There is more. This gravity separation may occur not only on an intrusion-wide scale, but also on a scale of metres or centimetres, resulting in size-graded ‘beds’ analogous to those deposited by sediment gravity flows. The evidence suggests that convection currents within the magma chamber permit successive episodes of deposition from density flows. Inclined bedding analogous to that produced by traction currents in running water is also known (see Unidirectional Aqueous Flow). Sedimentary Rock Anomalies
Sedimentary rocks are defined as those that are composed of fragments of pre-existing rocks, of a wide range of particle sizes, that are deposited under ice or water or in the atmosphere. They are thus commonly stratified and contain fossils, and usually exhibit porosity between the constituent sediment grains. This definition is adequate for gravels, sands, and muds and for the conglomerates, sandstones, and shales that are their lithified equivalents. There is,
ROCKS AND THEIR CLASSIFICATION 455
however, a large group of sedimentary rocks where these characters are largely absent. These are the chemical sediments. The chemical sediments are a diverse group that includes limestones, dolomites, ironstones, evaporites, chert, hydrocarbons (peat, lignite, and coal), and phosphate (see Sedimentary Rocks: Chert; Dolomites; Evaporites; Ironstones; Limestones; Phosphates). Limestones are frequently composed of clasts and exhibit sedimentary structures analogous to those of sandstones. The other chemical rocks, however, owe their peculiar properties very largely to post-depositional chemical changes during diagenesis (see Diagenesis, Overview). In the extreme case of the so-called evaporites, it is now recognized that they should be more properly termed ‘replacementites’ since their original chemical composition has been completely changed, though primary sedimentary features may be discerned as ghostly relics. Indeed it could be argued that the evaporites/replacementites are really metamorphic rocks. Stratification is sometimes absent in sedimentary rocks. Massive structureless sediments include ‘loess’ (wind-blown silt) and grain-flow and mass-flow deposits such as debrites and olistostromes. Metamorphic Rock Anomalies
The previous paragraph on anomalous sedimentary rocks provides a useful introduction to problems of metamorphic-rock classification. Just as the boundary between chemical sediments and metamorphic rocks is gradational, so too is the boundary between detrital sediments and metamorphic rocks. During burial gravel lithifies to form conglomerate, sand forms sandstone, and mud forms shale. With increasing burial temperature and pressure, porosity is destroyed, and texture and mineralogy are modified. There is therefore a gradation between highly cemented sedimentary rocks and low-grade metamorphic rocks. Similar gradations occur when superficial igneous rocks undergo deep burial, with concomitant mineralogical and textural changes as they metamorphose. Historically nowhere has this problem been more acute than in the controversial theories about the origin of granite. Granites were believed to have formed from the slow cooling of magma that had intruded shallower rocks. It was pointed out, however, that, when considering the vast granite masses at the eroded roots of mountains, there was a space problem. This was resolved when it was realized that wholesale replacement of preexisting rock had taken place. Intrusive granites were found only at shallow depths, having moved upwards from deep parent batholiths.
Rocks and their Classification: Conclusion Rocks are of three genetic types: igneous, formed from cooling magma; sedimentary, formed mainly by the breakdown of pre-existing rock; and metamorphic, formed by the action of heat and pressure on pre-existing rock. Igneous and metamorphic rocks are both normally composed of a mosaic of interlocking crystals that formed at high temperatures and pressures; they thus usually lack porosity and permeability. In the petroleum industry igneous and metamorphic rocks are generally grouped together under the term ‘basement’, indicating that it is pointless to continue drilling deeper into such tight rocks. Igneous rocks are generally massive and structureless, while metamorphic rocks are often banded. Sediments, on the other hand, are deposited with space between the constituent particles. Thus, sedimentary rocks, by their very nature, tend to be porous and permeable. Sediments are deposited episodically, and thus characteristically exhibit stratification. They are also the only one of the three main classes of rock to contain fossils. All classifications have their illogicalities and inconsistencies. Thus sedimentary rocks grade with increasing burial into metamorphic rocks, and metamorphic rocks ultimately melt to form magma, which cools to form igneous rocks, which are weathered, eroded, transported, and redeposited as sediments.
See Also Diagenesis, Overview. Igneous Processes. Igneous Rocks: Granite. Metamorphic Rocks: Classification, Nomenclature and Formation. Regional Metamorphism. Sedimentary Environments: Depositional Systems and Facies. Sedimentary Processes: Erosional Sedimentary Structures; Post-Depositional Sedimentary Structures. Sedimentary Rocks: Mineralogy and Classification; Chert; Dolomites; Evaporites; Ironstones; Limestones; Phosphates. Unidirectional Aqueous Flow. Volcanoes. Weathering.
Further Reading Norton WH (2002) The Elements of Geology. Indypublish.com, Cybercity. Peters KE (1997) No Stone Unturned. San Francisco: WH Freeman. Rothery D, Turner D, and Wilson RCL (2001) Geology. Milton Keynes: Open University Worldwide.