Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock
Geochemical characterization
Alkali basalt is chemically classified as a rock in region B (basalt) of the total alkali versus silica (TAS) diagram that contains nepheline in its CIPW norm. Basalts that do not contain normative nepheline are characterized as sub-alkali basalts, which include tholeiitic basalts and calc-alkaline basalts.
Petrography
The groundmass of alkali basalt is mainly composed of olivine, titanium-rich augite and
plagioclase feldspar and may have
alkali feldspar or
feldspathoid interstitially, but is poor in
Silicon dioxide minerals, such as
hypersthene and
quartz.
Phenocrysts are ubiquitous in alkali basalt and, similarly to the groundmass, are usually made up of olivine and titanium-rich augite but can also have plagioclase and Iron oxide with lower frequency.
Geologic context
Alkali basalt can be found in areas associated with volcanic activity, such as oceanic islands (
Hawaii,
Madeira,
,
, etc.), continental rifts and volcanic fields.
Continental alkali basalt can be found in every continent, with prominent examples being the Rio Grande Rift (USA), the East African Rift and the Pali-Aike volcanic field.
The results from the gamma ray spectrometer on Venera 8 on Venus suggest it landed on alkali basalt.
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