basalt is an aphanitic igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. Basalt features a glassy matrix interspersed with minerals.
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 rust. 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 are quite light coloured, superficially resembling rhyolite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it is often porphyritic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with aphaneritic (coarse) groundmass are generally referred to as diabase (also called dolerite)
In the Hadean and Archean (and the early Precambrian) eras of Earth's history the chemistry of erupted basalts was significantly different from today's, due to crustal and asthenosphere differentiation issues—so much so that there is an alternate (but less well known) name for this kind of basalt
|Stone Name||:||Basalt: Igneous Rock|
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