Lithology

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Stratigraphy as seen in southeastern Utah SEUtahStrat.JPG
Stratigraphy as seen in southeastern Utah

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]

Contents

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 ItalyPillowBasalt.jpg
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 GLMsed.jpg
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 Iddingsite.JPG
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. Olivine basalt.jpg
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 WaveRipple.JPG
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]

Related Research Articles

<span class="mw-page-title-main">Gneiss</span> Common high-grade metamorphic rock

Gneiss is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under pressures anywhere from 2 to 15 kbar, sometimes even more, and temperatures over 300°C (572°F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage.

<span class="mw-page-title-main">Sandstone</span> Type of sedimentary rock

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains, cemented together by another mineral. Sandstones comprise about 20–25% of all sedimentary rocks.

<span class="mw-page-title-main">Schist</span> Easily split medium-grained metamorphic rock

Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as mica, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Metamorphic rock</span> Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.

<span class="mw-page-title-main">Rock (geology)</span> Naturally occurring mineral aggregate

In geology, rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere. The study of rocks involves multiple subdisciplines of geology, including petrology and mineralogy. It may be limited to rocks found on Earth, or it may include planetary geology that studies the rocks of other celestial objects.

<span class="mw-page-title-main">Metamorphism</span> Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

<span class="mw-page-title-main">Basanite</span> Type of volcanic rock

Basanite is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and calcic plagioclase and forms from magma low in silica and enriched in alkali metal oxides that solidifies rapidly close to the Earth's surface.

<span class="mw-page-title-main">Amphibolite</span> Metamorphic rock type

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

<span class="mw-page-title-main">Quartzite</span> Hard, non-foliated metamorphic rock

Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of hematite. Other colors, such as yellow, green, blue and orange, are due to other minerals.

<span class="mw-page-title-main">Conglomerate (geology)</span> Sedimentary rock composed of smaller rock fragments

Conglomerate is a sedimentary rock made up of rounded gravel-sized pieces of rock surrounded by finer-grained sediments. The larger fragments within conglomerate are called clasts, while the finer sediment surrounding the clasts is called the matrix. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

<span class="mw-page-title-main">Intrusive rock</span> Magmatic rock formed below the surface

Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, such as batholiths, dikes, sills, laccoliths, and volcanic necks.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Foliation (geology)</span> Repetitive layering in metamorphic rocks

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

<span class="mw-page-title-main">Texture (geology)</span>

In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline, fragmental, aphanitic, and glassy. The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. A common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.

<span class="mw-page-title-main">Igneous textures</span> Property of igneous rocks

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. The six main types of textures are phaneritic, aphanitic, porphyritic, glassy, pyroclastic, and pegmatitic.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Igneous rock</span> Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.

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 (2): 41–43. Bibcode:1981GeoRu..70..794S. 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. 1 2 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. 1 2 "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
  11. 1 2 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. 1 2 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". 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.
  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. ↑ "McCarthy, T. & 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.
  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.
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