Alkali basalt

Alkali basalt or alkali olivine basalt is a fine-grained, dark-coloured, volcanic rock characterized by phenocrysts of olivine, titanium-rich augite, plagioclase feldspar and iron oxides. For similar SiO₂ concentrations, alkali basalts have a higher content of the alkalis, Na₂O and K₂O, than other basalt types such as tholeiites. They are also characterized by the development of modal nepheline in their groundmass (visible at highest magnification on a petrographic microscope) and normative nepheline in their CIPW norms. Alkali basalts are typically found on updomed and rifted continental crust, and on oceanic islands such as Hawaii, Madeira and Ascension Island.

References

Imperial College Rock Library

Types of basalts

Basalts by tectonic setting	Flood basalt Mid-ocean ridge basalt Ocean island basalt Volcanic arc basalt

Basalts by form and flow	Columnar basalt Pillow basalt Aa lava Pahoehoe lava

Basalts by chemistry	Tholeiitic basalt Calc-alkaline basalt Alkali basalt Trachybasalt Hawaiite Mugearite Picrite

Important minerals	Olivine Pyroxene Plagioclase Amphibole Magnetite Ilmenite Quartz Cristobalite Trydimite

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