Facts about Basalt
Rare feldspathoid-rich mafic rocks, kin to alkali basalts, may have Na2O plus K2O contents of 12 percent or more.
Many alkali basalts may be formed at greater depths, perhaps as deep as 150-200 km.
The crustal portions of oceanic tectonic plates are comprised predominantly of basalt, produced from upwelling peridotite in the mantle below ocean ridges.
Basalt generally has a composition of 45-55 wt percent SiO2, 2-6 wt percent total alkalis, 0.5-2.0 wt percent TiO2, 5-14 wt percent FeO and 14 wt percent or more Al2O3.
High alumina basalts have aluminium contents of 17-19 wt percent Al2O3; boninites have magnesium contents of up to 15 percent MgO.
Other famous accumulations of basalts include Iceland and the islands of the Hawaii volcanic chain, forming above a mantle plume.
Basaltic cinders are often red, colored by oxidized iron from weathered iron-rich minerals such as pyroxene.
Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200°C (liquidus) and near or below 1000°C (solidus); these values are higher than those of other common igneous rocks.
Various metamorphic facies are named after the mineral assemblages and rock types formed by subjecting basalts to the temperatures and pressures of the metamorphic event.
When basalt erupts underwater or flows into the sea, the cold water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow.
The Ming armies pursued and defeated them in Mongolia, but did not conquer Mongolia.
Basalts are important rocks within metamorphic belts, as they can provide vital information on the conditions of metamorphism within the belt.
Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, ash or cinder; breccia and lava flows.
The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows.
Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much-studied, so as to learn about evolution of the Earth's mantle.
The origin of high-alumina basalt continues to be controversial, with interpretations that it is a primary melt and that instead it is derived from other basalt types (e.g., Ozerov, 2000).
Basalt compositions are rich in MgO and CaO and low in SiO2 and Na2O plus K2O relative to most common igneous rocks, consistent with the TAS classification.
Basalt magmas form by decompression melting of peridotite in the Earth's mantle (see Igneous rock).
The island of Surtsey in the Atlantic is a basalt volcano which breached the ocean surface in 1963.
Pliny used the word basalt, which is said to have an Ethiopian origin, meaning a black stone.
Lunar basalts differ from their terrestrial counterparts principally in their high iron contents, which typically range from about 17 to 22 wt percent FeO.
Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism, lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration.
Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed.
Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock.
MORB basalt, a tholeiite particularly low in total alkalis and in incompatible trace elements, has a flat REE pattern normalized to mantle or chondrite values.
Pahoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and form lava lakes.
Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti.