- Igneous textures
Igneous textures include the rock textures occurring in
igneous rocks. Igneous textures are utilized by geologists in determining the mode of origin igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.
Phaneritic" ("phaner" = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below the Earth's surface. As a magmacools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystalwith the naked eye. Examples of phaneritic igneous rocks are gabbro, dioriteand granite.
"Aphanitic" ("a" = not, "phaner" = visible) rocks in contrast to phaneritic rocks, typically form from
lavawhich crystallize rapidly on or near the Earth' surface. Because extrusive rocks make contact with the atmospherethey cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguisable to the naked eye. Examples of aphanitic igneous rock include basalt, andesiteand rhyolite.
"Porphyritic" textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (
phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcanobut is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
"Glassy" textures occur during some volanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include
Pyroclastic" ("pyro" = igneous, "clastic" = fragment) testures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilliand volcanic bombs.
"Pegmatitic" texture occurs during magma cooling when some minerals may grow so large that they become massive (the size ranges from a few centimetres to several metres). This is typical of
*E.J. Tarbuck and F.K. Lutgens, "Earth An introduction to physical geology". seventh edition, Prentice Hall, 2002.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Igneous rock — Geologic provinces of the world (USGS) Shield … Wikipedia
igneous rock — Any of various crystalline or glassy, noncrystalline rocks formed by the cooling and solidification of molten earth material (magma). Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary … Universalium
List of rock textures — This page is intended to be a list of rock textural and morphological terms. A * Adcumulate * Agglomeritic * Adamantine a type of lustre * Amygdaloidal * Anhedral * Antitaxial veins * Aphanitic * Aplitic; aplite * Augen textured gneiss *… … Wikipedia
Rock microstructure — includes the texture of a rock and the small scale rock structures. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful… … Wikipedia
Narryer Gneiss Terrane — Stratigraphic range: > 3.3 Ga Satellite image of Jack Hills in the Narryer Gneiss Terrane Type Geological complex Sub units … Wikipedia
Texture (geology) — Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations… … Wikipedia
Jiao-Liao-Ji Belt — The paleoproterozoic Jiao Liao Ji Belt lies at the eastern margin of the Eastern Block (North China Craton), with its northern segment intervening between the Northern Liaoning Southern Jilin Complex and the Southern Liaoning Nangrim Complex and… … Wikipedia
Nininger Meteorite Award — The Nininger Meteorite Award recognizes outstanding student achievement in the “Science of Meteoritics” as embodied by an original research paper. In 1965, Dr. Harvey H. Nininger and Mrs. Addie D. Nininger endowed the Nininger Science of… … Wikipedia
Churchill craton — North America cratons and basement rock. The Churchill craton is colored in blue and labelled as the Churchill collision on this map. The Churchill craton is the northwest section of the Canadian Shield and stretches from southern Saskatchewan… … Wikipedia
Metamorphic rock — Metamorphic redirects here. For other uses, see Metamorphic (disambiguation). Quartzite, a form of metamorphic rock, from the Museum of Geology at University of Tartu collection. Metamorphic rock is the transformation of an existing rock type,… … Wikipedia