Dating volcanic rock
Feldspar phenocrysts typically are labradorite to andesine in composition.Augite is rich in titanium compared to augite in tholeiitic basalt.) and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase.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 Hawaii.The groundmass contains interstitial quartz or tridymite or cristobalite.Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass.These ultramafic volcanic rocks, with silica (Si O) contents below 45% are usually classified as komatiites.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".
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.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." 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.Ocean floor basalts, erupted originally at mid-ocean ridges, are known as MORB (mid-ocean ridge basalt) and are characteristically low in incompatible elements.Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
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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.