Dating igneous volcanic rocks
) and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase.
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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.
The majority of tholeiites are formed at approximately 50–100 km depth within the mantle.
Olivine may also be a phenocryst, and when present, may have rims of pigeonite.
The groundmass contains interstitial quartz or tridymite or cristobalite.
Augite is rich in titanium compared to augite in tholeiitic basalt.
Minerals such as alkali feldspar, leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.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.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.Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.