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Lithology

February 04, 2023

For the study of rocks and their formation, see Petrology.

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition.[1][2][3] Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.[4]

Stratigraphy as seen in southeastern Utah

Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

Rock type

 
A basalt, showing the 'pillow' lava shape characteristic of underwater eruptions, Italy

The naming of a lithology is based on the rock type. The three major rock types are igneous, sedimentary, and metamorphic. Igneous rocks are formed directly from magma, which is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified. Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure.[5]

Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by volcanic processes (tephra) are classified as pyroclastic rock. Pyroclastic rocks are further classified by average fragment (clast) size and whether the fragments are mostly individual mineral crystals, particles of volcanic glass, or rock fragments.[6] Further classifications, such as by chemical composition, may also be applied.[7][8] Igneous rocks that have visible mineral grains (phaneritic rocks) are classified as intrusive, while those that are glassy or very fine-grained (aphanitic) are classified as extrusive rock. Intrusive igneous rocks are usually classified using the QAPF classification, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid. Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or carbonatites. Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition is impractical, they may be classified chemically using the TAS classification. This is based on the total content of silica and alkali metal oxides and other chemical criteria.[9][10][11]

Sedimentary rocks are further classified by whether they are siliciclastic or carbonate. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments.[12] Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock.[13]

Metamorphic rock naming can be based on protolith, mineral composition, texture, or metamorphic facies. Naming based on texture and a pelite (e.g., shale, mudrock) protolith can be used to define slate and phyllite. Texture-based names are schist and gneiss. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism.[14] Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form.[15] Additional metamorphic rock names exist, such as greenschist (metamorphosed basalt and other extrusive igneous rock) or quartzite (metamorphosed quartz sand).[16]

Grain/clast size

 
A claystone, the finest-grained sedimentary rock, deposited in Glacial Lake Missoula, Montana

In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in the rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive.[17] Metamorphism of rock composed of mostly a single mineral, such as quartzite or marble, may increase grain size (grain growth), while metamorphism of sheared rock may decrease grain size (syntectonic recrystallization).[18]

In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate, sandstone, or mudstone). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are "pebble conglomerate" and "fine quartz arenite".[19]

Mineralogy

 
An ultramafic mantle xenolith with olivine and pyroxene (altering brown to iddingsite) in a matrix of mafic basalt scoria

In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description. In the case of sequences possibly including carbonates, calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate) using dilute hydrochloric acid and looking for effervescence.[20]

The mineralogical composition of a rock is one of the major ways in which it is classified. Igneous rocks are classified by their mineral content whenever practical, using the QAPF classification or special ultramafic or carbonatite classifications.[9][10][11] Likewise metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in a sample.[15]

Colour

The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system.[21]

Fabric

The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main visible fabric is normally bedding, and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, forming cumulates.

Texture

 
The lithology of this porphyritic basalt is characterized by olivine and augite phenocrysts.

The texture of a rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting, grading, shape and roundness of the clasts.[22] Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation—before deformation porphyroclast—after deformation porphyroblast.[23] Igneous textures include such properties as grain shape, which varies from crystals with ideal crystal shapes (euhedral) to irregular crystals (anhedral), whether the rock shows highly nonuniform crystal sizes (is porphyritic), or whether grains are aligned (which is described as trachytic texture).[24]

Small-scale structures

 
Ripple marks from Mongolia

Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks this may include sole markings, ripple marks, mudcracks and cross-bedding. These are recorded as they are generally characteristic of a particular depositional environment and may provide information on paleocurrent directions.[25] In metamorphic rocks associated with the deeper levels of fault zones, small scale structures such as asymmetric boudins[26] and microfolds are used to determine the sense of displacement across the zone.[27] In igneous rocks, small-scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻā basaltic flows,[28] and pillows showing eruption within a body of water or beneath ice.[29][30][31]

Surficial lithology

Unconsolidated surficial materials may also be given a lithology. This is defined by grain size and composition and is often attached to an interpretation of how the unit formed. Surficial lithologies can be given to lacustrine, coastal, fluvial, aeolian, glacial, and recent volcanic deposits, among others. Examples of surficial lithology classifications used by the U.S. Geological Survey are, "Glacial Till, Loamy", "Saline Lake Sediment", and "Eolian Sediment, Coarse-Textured (Sand Dunes)".[32]

References

  1. ^ "Lithology". Earthquake Glossary. US Geological Survey. Retrieved 29 October 2010.
  2. ^ Bates, R. J.; Jackson, J. A., eds. (1984). Dictionary of Geological Terms (3 ed.). American Geological Institute. p. 299. ISBN 0-385-18101-9.
  3. ^ Allaby, Ailsa; Allaby, Michael (1999). Oxford Dictionary of Earth Sciences (2 ed.). Oxford University Press. p. 320. ISBN 0-19-280079-5.
  4. ^ American Heritage Dictionary, ed. (2005). The American heritage science dictionary. Houghton Mifflin Harcourt. p. 364. ISBN 978-0-618-45504-1.
  5. ^ Levin, Harold L. (2010). The earth through time (9th ed.). Hoboken, N.J.: J. Wiley. p. 57. ISBN 9780470387740.
  6. ^ Schmidt, R. (1981). "Descriptive nomenclature and classification of pyroclastic deposits and fragments: recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks". Geology. 9: 41–43. doi:10.1007/BF01822152. S2CID 128375559. Retrieved 27 September 2020.
  7. ^ Fisher, Richard V.; Schmincke, H.-U. (1984). Pyroclastic rocks. Berlin: Springer-Verlag. pp. 98–99. ISBN 3540127569.
  8. ^ Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. p. 138. ISBN 9783540436508.
  9. ^ a b Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. CiteSeerX 10.1.1.692.4446. doi:10.1144/gsjgs.148.5.0825. S2CID 28548230.
  10. ^ a b "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
  11. ^ a b Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143. ISBN 9780521880060.
  12. ^ Blatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 217–220, 257–258. ISBN 0716724383.
  13. ^ Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 169–173. ISBN 0131547283.
  14. ^ Yardley, B. W. D. (1989). An introduction to metamorphic petrology. Harlow, Essex, England: Longman Scientific & Technical. pp. 21–27. ISBN 0582300967.
  15. ^ a b Yardley 1989, pp. 49–51.
  16. ^ Yardley 1989, pp. 21, 26.
  17. ^ Levin 2010, pp. 58–59.
  18. ^ Yardley 1989, pp. 154–155.
  19. ^ Blatt & Tracy 1996, p. 241-242.
  20. ^ Geology.com. "The Acid Test for Carbonate Minerals and Carbonate Rocks". Geology.com. Retrieved 28 November 2016.
  21. ^ "4 Classification of rocks and description of physical properties of rock". (PDF). Vol. 1. US Bureau of Reclamation, Technical Service Center Engineering Geology Group. 1998. pp. 57–90. Archived from the original (PDF) on 17 June 2015. Retrieved 7 June 2010.
  22. ^ Boggs 2006, p. 130.
  23. ^ Yardley 1989, pp. 154–170.
  24. ^ Blatt & Tracy 1996, pp. 39–44.
  25. ^ Boggs 2006, pp. 74–118.
  26. ^ Fossen, H. (2010). Structural Geology. Cambridge University Press. p. 272. ISBN 978-0-521-51664-8.
  27. ^ Carreras, J.; Druguet E.; Griera A. (2005). "Shear zone-related folds". Journal of Structural Geology. 27 (7): 1229–1251. Bibcode:2005JSG....27.1229C. doi:10.1016/j.jsg.2004.08.004. Retrieved 2009-10-31.
  28. ^ James Furman Kemp: A handbook of rocks for use without the microscope : with a glossary of the names of rocks and other lithological terms. 5. Aufl., New York: D. Van Nostrand, 1918, pp. 180, 240: C. E. Dutton, 4th Annual Report U.S. Geological Survey, 1883, S. 95; Bulletin of the Geological Society of America, Volume 25 / Geological Society of America. 1914, p. 639
  29. ^ (PDF). Web.wits.ac.za. Archived from the original (PDF) on 2009-04-07. Retrieved 2014-03-10.
  30. ^ Walker, George P L. (1992-08-01). "Walker, G.P.L. 1992. Morphometric study of pillow-size spectrum among pillow lavas". Bulletin of Volcanology. 54 (6): 459–474. Bibcode:1992BVol...54..459W. doi:10.1007/BF00301392. S2CID 129797887.
  31. ^ Harmon, Russel S.; Rapela, Carlos W. (1991). Andean Magmatism and Its Tectonic Setting. Geological Society of America. p. 24. ISBN 978-0-8137-2265-8.
  32. ^ USGS Rocky Mountain Geographic Science Center. "Surficial Lithology: Attribute information". US Geological Survey. Retrieved 15 September 2011.

February 04, 2023
lithology, study, rocks, their, formation, petrology, lithology, rock, unit, description, physical, characteristics, visible, outcrop, hand, core, samples, with, magnification, microscopy, physical, characteristics, include, colour, texture, grain, size, compo. For the study of rocks and their formation see Petrology The lithology of a rock unit is a description of its physical characteristics visible at outcrop in hand or core samples or with low magnification microscopy Physical characteristics include colour texture grain size and composition 1 2 3 Lithology may refer to either a detailed description of these characteristics or a summary of the gross physical character of a rock Examples of lithologies in the second sense include sandstone slate basalt or limestone 4 Stratigraphy as seen in southeastern Utah Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas In certain applications such as site investigations lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7 Contents 1 Rock type 2 Grain clast size 3 Mineralogy 4 Colour 5 Fabric 6 Texture 7 Small scale structures 8 Surficial lithology 9 ReferencesRock type Edit A basalt showing the pillow lava shape characteristic of underwater eruptions Italy The naming of a lithology is based on the rock type The three major rock types are igneous sedimentary and metamorphic Igneous rocks are formed directly from magma which is a mixture of molten rock dissolved gases and solid crystals Sedimentary rock is formed from mineral or organic particles that collect at the Earth s surface and become lithified Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure 5 Igneous rocks are further broken into three broad categories Igneous rock composed of broken rock fragments created directly by volcanic processes tephra are classified as pyroclastic rock Pyroclastic rocks are further classified by average fragment clast size and whether the fragments are mostly individual mineral crystals particles of volcanic glass or rock fragments 6 Further classifications such as by chemical composition may also be applied 7 8 Igneous rocks that have visible mineral grains phaneritic rocks are classified as intrusive while those that are glassy or very fine grained aphanitic are classified as extrusive rock Intrusive igneous rocks are usually classified using the QAPF classification which is based on the relative content of quartz alkali feldspar plagioclase and feldspathoid Special classifications exist for igneous rock of unusual compositions such as ultramafic rock or carbonatites Where possible extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification but when determining the mineral composition is impractical they may be classified chemically using the TAS classification This is based on the total content of silica and alkali metal oxides and other chemical criteria 9 10 11 Sedimentary rocks are further classified by whether they are siliciclastic or carbonate Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz feldspar and lithic rock fragments 12 Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock 13 Metamorphic rock naming can be based on protolith mineral composition texture or metamorphic facies Naming based on texture and a pelite e g shale mudrock protolith can be used to define slate and phyllite Texture based names are schist and gneiss These textures from slate to gneiss define a continually increasing extent of metamorphism 14 Metamorphic facies are defined by the pressure temperature fields in which particular minerals form 15 Additional metamorphic rock names exist such as greenschist metamorphosed basalt and other extrusive igneous rock or quartzite metamorphosed quartz sand 16 Grain clast size Edit A claystone the finest grained sedimentary rock deposited in Glacial Lake Missoula Montana In igneous and metamorphic rocks grain size is a measure of the sizes of the crystals in the rock In igneous rock this is used to determine the rate at which the material cooled large crystals typically indicate intrusive igneous rock while small crystals indicate that the rock was extrusive 17 Metamorphism of rock composed of mostly a single mineral such as quartzite or marble may increase grain size grain growth while metamorphism of sheared rock may decrease grain size syntectonic recrystallization 18 In clastic sedimentary rocks grain size is the diameter of the grains and or clasts that constitute the rock These are used to determine which rock naming system to use e g a conglomerate sandstone or mudstone In the case of sandstones and conglomerates which cover a wide range of grain sizes a word describing the grain size range is added to the rock name Examples are pebble conglomerate and fine quartz arenite 19 Mineralogy Edit An ultramafic mantle xenolith with olivine and pyroxene altering brown to iddingsite in a matrix of mafic basalt scoria In rocks in which mineral grains are large enough to be identified using a hand lens the visible mineralogy is included as part of the description In the case of sequences possibly including carbonates calcite cemented rocks or those with possible calcite veins it is normal to test for the presence of calcite or other forms of calcium carbonate using dilute hydrochloric acid and looking for effervescence 20 The mineralogical composition of a rock is one of the major ways in which it is classified Igneous rocks are classified by their mineral content whenever practical using the QAPF classification or special ultramafic or carbonatite classifications 9 10 11 Likewise metamorphic facies which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks are determined by observing the mineral phases that are present in a sample 15 Colour EditThe colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded sometimes against standard colour charts such as that produced by the Rock Color Chart Committee of the Geological Society of America based on the Munsell color system 21 Fabric EditThe fabric of a rock describes the spatial and geometric configuration of all the elements that make it up In sedimentary rocks the main visible fabric is normally bedding and the scale and degree of development of the bedding is normally recorded as part of the description Metamorphic rocks apart from those created by contact metamorphism are characterised by well developed planar and linear fabrics Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation forming cumulates Texture Edit The lithology of this porphyritic basalt is characterized by olivine and augite phenocrysts The texture of a rock describes the relationship between the individual grains or clasts that make up the rock Sedimentary textures include the degree of sorting grading shape and roundness of the clasts 22 Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation before deformation porphyroclast after deformation porphyroblast 23 Igneous textures include such properties as grain shape which varies from crystals with ideal crystal shapes euhedral to irregular crystals anhedral whether the rock shows highly nonuniform crystal sizes is porphyritic or whether grains are aligned which is described as trachytic texture 24 Small scale structures Edit Ripple marks from Mongolia Rocks often contain small scale structures smaller than the scale of an individual outcrop In sedimentary rocks this may include sole markings ripple marks mudcracks and cross bedding These are recorded as they are generally characteristic of a particular depositional environment and may provide information on paleocurrent directions 25 In metamorphic rocks associated with the deeper levels of fault zones small scale structures such as asymmetric boudins 26 and microfolds are used to determine the sense of displacement across the zone 27 In igneous rocks small scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻa basaltic flows 28 and pillows showing eruption within a body of water or beneath ice 29 30 31 Surficial lithology EditUnconsolidated surficial materials may also be given a lithology This is defined by grain size and composition and is often attached to an interpretation of how the unit formed Surficial lithologies can be given to lacustrine coastal fluvial aeolian glacial and recent volcanic deposits among others Examples of surficial lithology classifications used by the U S Geological Survey are Glacial Till Loamy Saline Lake Sediment and Eolian Sediment Coarse Textured Sand Dunes 32 References Edit Lithology Earthquake Glossary US Geological Survey Retrieved 29 October 2010 Bates R J Jackson J A eds 1984 Dictionary of Geological Terms 3 ed American Geological Institute p 299 ISBN 0 385 18101 9 Allaby Ailsa Allaby Michael 1999 Oxford Dictionary of Earth Sciences 2 ed Oxford University Press p 320 ISBN 0 19 280079 5 American Heritage Dictionary ed 2005 The American heritage science dictionary Houghton Mifflin Harcourt p 364 ISBN 978 0 618 45504 1 Levin Harold L 2010 The earth through time 9th ed Hoboken N J J Wiley p 57 ISBN 9780470387740 Schmidt R 1981 Descriptive nomenclature and classification of pyroclastic deposits and fragments recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks Geology 9 41 43 doi 10 1007 BF01822152 S2CID 128375559 Retrieved 27 September 2020 Fisher Richard V Schmincke H U 1984 Pyroclastic rocks Berlin Springer Verlag pp 98 99 ISBN 3540127569 Schmincke Hans Ulrich 2003 Volcanism Berlin Springer p 138 ISBN 9783540436508 a b Le Bas M J Streckeisen A L 1991 The IUGS systematics of igneous rocks Journal of the Geological Society 148 5 825 833 Bibcode 1991JGSoc 148 825L CiteSeerX 10 1 1 692 4446 doi 10 1144 gsjgs 148 5 0825 S2CID 28548230 a b Rock Classification Scheme Vol 1 Igneous PDF British Geological Survey Rock Classification Scheme 1 1 52 1999 a b Philpotts Anthony R Ague Jay J 2009 Principles of igneous and metamorphic petrology 2nd ed Cambridge UK Cambridge University Press pp 139 143 ISBN 9780521880060 Blatt Harvey Tracy Robert J 1996 Petrology igneous sedimentary and metamorphic 2nd ed New York W H Freeman pp 217 220 257 258 ISBN 0716724383 Boggs Sam 2006 Principles of sedimentology and stratigraphy 4th ed Upper Saddle River N J Pearson Prentice Hall pp 169 173 ISBN 0131547283 Yardley B W D 1989 An introduction to metamorphic petrology Harlow Essex England Longman Scientific amp Technical pp 21 27 ISBN 0582300967 a b Yardley 1989 pp 49 51 Yardley 1989 pp 21 26 Levin 2010 pp 58 59 Yardley 1989 pp 154 155 Blatt amp Tracy 1996 p 241 242 Geology com The Acid Test for Carbonate Minerals and Carbonate Rocks Geology com Retrieved 28 November 2016 4 Classification of rocks and description of physical properties of rock Engineering Geology Field Manual PDF Vol 1 US Bureau of Reclamation Technical Service Center Engineering Geology Group 1998 pp 57 90 Archived from the original PDF on 17 June 2015 Retrieved 7 June 2010 Boggs 2006 p 130 Yardley 1989 pp 154 170 Blatt amp Tracy 1996 pp 39 44 Boggs 2006 pp 74 118 Fossen H 2010 Structural Geology Cambridge University Press p 272 ISBN 978 0 521 51664 8 Carreras J Druguet E Griera A 2005 Shear zone related folds Journal of Structural Geology 27 7 1229 1251 Bibcode 2005JSG 27 1229C doi 10 1016 j jsg 2004 08 004 Retrieved 2009 10 31 James Furman Kemp A handbook of rocks for use without the microscope with a glossary of the names of rocks and other lithological terms 5 Aufl New York D Van Nostrand 1918 pp 180 240 C E Dutton 4th Annual Report U S Geological Survey 1883 S 95 Bulletin of the Geological Society of America Volume 25 Geological Society of America 1914 p 639 McCarthy T amp Rubidge B 2008 The story of earth and life Chapter 3 The first continent 60 91 Struik Publishers PDF Web wits ac za Archived from the original PDF on 2009 04 07 Retrieved 2014 03 10 Walker George P L 1992 08 01 Walker G P L 1992 Morphometric study of pillow size spectrum among pillow lavas Bulletin of Volcanology 54 6 459 474 Bibcode 1992BVol 54 459W doi 10 1007 BF00301392 S2CID 129797887 Harmon Russel S Rapela Carlos W 1991 Andean Magmatism and Its Tectonic Setting Geological Society of America p 24 ISBN 978 0 8137 2265 8 USGS Rocky Mountain Geographic Science Center Surficial Lithology Attribute information US Geological Survey Retrieved 15 September 2011 Retrieved from https en wikipedia org w index php title Lithology amp oldid 1137326608, wikipedia, wiki, book, books, library,

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