Basalt

Basalt
Igneous rock
BasaltUSGOV.jpg
Composition
Mafic: amphibole and pyroxene, sometimes plagioclase, feldspathoids, and/or olivine.

Basalt (t/)[1] is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava[2] exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt.[3] Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Definition

Columnar basalt flows in Yellowstone National Park, USA

By definition, basalt is an aphanitic (fine-grained) igneous rock with generally 45-53% silica (SiO2)[4] and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase. This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme.[5][6][7] It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi. Basalt commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 g/cm3.

Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into hematite and other iron oxides and hydroxides. Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. These phenocrysts usually are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt.

Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is mostly solid; when the vesicles are over half the volume of a specimen, it is called scoria. This texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.

The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Gabbro is often marketed commercially as "black granite."

Columnar basalt at Szent György Hill, Hungary
Vesicular basalt at Sunset Crater, Arizona. US quarter for scale.

In the Hadean, Archean, and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica (SiO2) contents below 45% are usually classified as komatiites.

Etymology

The word "basalt" is ultimately derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης (basanites), from βάσανος (basanos, "touchstone") and perhaps originated in Egyptian bauhun "slate".[8] The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the Schloßberg (local castle hill) at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.[9]

Types

Large masses must cool slowly to form a polygonal joint pattern, as here at the Giant's Causeway in Northern Ireland
Other Languages
العربية: بازلت
aragonés: Basalto
asturianu: Basaltu
azərbaycanca: Bazalt
تۆرکجه: بازالت
Bân-lâm-gú: Hiân-bú-giâm
башҡортса: Базальт
беларуская: Базальт
беларуская (тарашкевіца)‎: Базальт
български: Базалт
bosanski: Bazalt
català: Basalt
čeština: Čedič
Cymraeg: Basalt
dansk: Basalt
Deutsch: Basalt
eesti: Basalt
Ελληνικά: Βασάλτης
español: Basalto
Esperanto: Bazalto
euskara: Basalto
فارسی: بازالت
français: Basalte
Gaeilge: Basalt
Gàidhlig: Basalt
galego: Basalto
한국어: 현무암
հայերեն: Բազալտ
हिन्दी: बेसाल्ट
hrvatski: Bazalt
Ido: Bazalto
Bahasa Indonesia: Basal
íslenska: Basalt
italiano: Basalto
עברית: בזלת
ქართული: ბაზალტი
қазақша: Базальт
Kreyòl ayisyen: Bazalt
Кыргызча: Базальт
Latina: Basaltes
latviešu: Bazalts
Lëtzebuergesch: Basalt
lietuvių: Bazaltas
Limburgs: Basalt
magyar: Bazalt
македонски: Базалт
मराठी: असिताश्म
Bahasa Melayu: Basalt
Nederlands: Basalt (gesteente)
नेपाल भाषा: बसाल्ट
日本語: 玄武岩
norsk: Basalt
norsk nynorsk: Basalt
occitan: Basalt
oʻzbekcha/ўзбекча: Bazalt
ਪੰਜਾਬੀ: ਬਸਾਲਟ
Plattdüütsch: Basalt
polski: Bazalt
português: Basalto
română: Bazalt
Runa Simi: Basaltu
русский: Базальт
shqip: Bazalti
Simple English: Basalt
slovenčina: Bazalt
slovenščina: Bazalt
српски / srpski: Базалт
srpskohrvatski / српскохрватски: Bazalt
Basa Sunda: Basalt
suomi: Basaltti
svenska: Basalt
Tagalog: Basalto
татарча/tatarça: Базальт
тоҷикӣ: Базалт
Türkçe: Bazalt
українська: Базальт
Tiếng Việt: Bazan
吴语: 玄武岩
粵語: 玄武岩
žemaitėška: Bazalts
中文: 玄武岩