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Igneous Rocks

Igneous rocks

The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma . The word igneous comes from the Latin word ignis, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust ; the molten material rises, filling any available crevices, into the crust, and eventually hardens. These rocks are not visible until the earth above them has eroded away. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. A good example of intrusive igneous rock is granite . Extrusive igneous rocks form when the magma or molten rock pours out onto the earth's surface or erupts at the earth's surface from a volcano . Extrusive rocks are also called volcanic rocks. Basalt , formed from hardened lava , is the most common extrusive rock. Obsidian , a black glassy rock, is also an extrusive rock.

Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the cooling, the larger the crystal. Intrusive rock can take one million years or more to cool. Fast cooling results in smaller, often microscopic, grains. Some extrusive rocks solidify in the air, before they hit the ground. Sometimes the rock mass starts to cool slowly, forming larger crystals , and then finishes cooling rapidly, resulting in rocks that have crystals surrounded by a fine, grainy rock mass. This is known as a porphyritic texture.

Most of Earth's minerals are made up of a combination of up to ten elements. Over 99% of Earth's crust consists of only eight elements (oxygen , silicon , aluminum , iron , calcium, sodium, potassium, and magnesium). Most igneous rocks contain two or more minerals, which is why some rocks have more than one color. For example, the most common minerals in granite are quartz (white or gray), feldspar (white or pinks of varying shades), and mica (black). The amount of a specific element in a mineral can determine a color or intensity of color. Because of the way granite is formed, the different composition of minerals is easy to see. It is difficult to see the distinct composition of some extrusive rocks, like obsidian, due to their extremely fine texture. Igneous rocks contain mostly silicate minerals and are sometimes classified according to their silica content. Silica (SiO2) is a white or colorless mineral compound. Rocks containing a high amount of silica, usually more than 50%, are considered acidic (sometimes the term felsic is used), and those with a low amount of silica are considered basic (or mafic ). Acidic rocks are light in color and basic rocks are dark in color.

Essentially, Earth's continents are slabs of granite sitting on top of molten rock. The crustal plates of Earth are continually shifting, being torn open by faults, and altered by earthquakes and volcanoes. New igneous material is continually added to the crust, while old crust falls back into the earth, sometimes deep enough to be remelted. Igneous rocks are the source of many important minerals, metals , and building materials.

See also Magma chamber; Volcanic eruptions

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igneous rock

igneous rock One of three major classes of rock produced by the cooling and solidifying of molten magma. All igneous rocks are crystalline and most resist erosion. Extrusive, or volcanic, igneous rocks, such as basalt, form by the rapid cooling of molten material at the surface. Intrusive, or hyperbyssal, rocks, such as dolerite, form in sills or dykes at intermediate depth. Plutonic rocks, such as granite, form in batholiths at greater depth. The major chemical constituent is silica. Acid rocks contain high amounts of quartz, feldspar or mica, and are light coloured. Basic rocks are darker and contain 45–55% silica, including minerals such as hornblende. See also metamorphic rock; sedimentary rock

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spilite

spilite A low-grade metamorphic rock composed of albite, chlorite, actinolite, sphene, and calcite, with or without epidote, prehnite, and laumonite, and formed by sea-floor metasomatism of mid-oceanic-ridge basalts. Sea water circulating through the oceanic crust is heated by the cooling basalt dykes and lavas and reacts with them, introducing sodium and water into the rock system and converting the basalt mineral assemblage into a typical spilite assemblage.

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igneous

igneous XVII. f. L. igneus, f. ignis fire (rel. to OSl. ognĭ, Lith, ugnis, Skr. agni-); see -EOUS. ignite make intensely hot, spec. to the point of combustion or chemical change XVII; trans. set on fire XVIII; intr. take fire XIX. f. ignīt-, pp. stem of L. ignīre set on fire.
So ignition XVII.

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igneous

ig·ne·ous / ˈignēəs/ • adj. Geol. (of rock) having solidified from lava or magma. ∎  relating to or involving volcanic processes: igneous activity. ∎ rare of fire; fiery.

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igneous

igneous Applied to one of the three main groups of rock types (igneous, metamorphic, and sedimentary), to describe those rocks that have crystallized from a magma. The work is derived from the Latin ignis, meaning ‘fire’.

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igneous

igneous Applied to one of the three main groups of rock types (igneous, metamorphic, and sedimentary), to describe those rocks that have crystallized from a magma.

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igneous rock

igneous rock: see rock.

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igneous

igneousBierce, fierce, Pearce, Peirce, pierce, tierce •Fabius, scabious •Eusebius •amphibious, Polybius •dubious • Thaddeus • compendious •radius • tedious •fastidious, hideous, insidious, invidious, perfidious •Claudiuscommodious, melodious, odious •studious • Cepheus •Morpheus, Orpheus •Pelagius • callipygous • Vitellius •alias, Sibelius, Vesalius •Aurelius, Berzelius, contumelious, Cornelius, Delius •bilious, punctilious, supercilious •coleus • Julius • nucleus • Equuleus •abstemious •Ennius, Nenniuscontemporaneous, cutaneous, extemporaneous, extraneous, instantaneous, miscellaneous, Pausanias, porcellaneous, simultaneous, spontaneous, subcutaneous •genius, heterogeneous, homogeneous, ingenious •consanguineous, ignominious, Phineas, sanguineous •igneous, ligneous •Vilnius •acrimonious, antimonious, ceremonious, erroneous, euphonious, felonious, harmonious, parsimonious, Petronius, sanctimonious, Suetonius •Apollonius • impecunious •calumnious • Asclepius • impious •Scorpius •copious, Gropius, Procopius •Marius • pancreas • retiarius •Aquarius, calcareous, Darius, denarius, gregarious, hilarious, multifarious, nefarious, omnifarious, precarious, Sagittarius, senarius, Stradivarius, temerarious, various, vicarious •Atreus •delirious, Sirius •vitreous •censorious, glorious, laborious, meritorious, notorious, uproarious, uxorious, vainglorious, victorious •opprobrious •lugubrious, salubrious •illustrious, industrious •cinereous, deleterious, imperious, mysterious, Nereus, serious, Tiberiuscurious, furious, injurious, luxurious, penurious, perjurious, spurious, sulphureous (US sulfureous), usurious •Cassius, gaseous •Alcaeus • Celsius •Theseus, Tiresias •osseous, Roscius •nauseous •caduceus, Lucius •Perseus • Statius • Propertius •Deo gratias • plenteous • piteous •bounteous •Grotius, Photius, Proteus •beauteous, duteous •courteous, sestertius •Boethius, Prometheus •envious • Octavius •devious, previous •lascivious, niveous, oblivious •obvious •Vesuvius, Vitruviusimpervious, pervious •aqueous • subaqueous • obsequious •Dionysius

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