alkali basalt

A Dictionary of Earth Sciences | 1999 | 632 words | Copyright

alkali basalt 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 Na₂O and K₂O than other basalt types such as tholeiites. They are also characterized by the development of modal nepheline in their groundmass (only seen with the highest powered lens on a petrological microscope) and normative nepheline (Ne) in their CIPW norms. Alkali basalts are typically found on updomed and rifted continental crust, and on oceanic islands such as Hawaii and Ascension Island.

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AILSA ALLABY and MICHAEL ALLABY. "alkali basalt." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>.

AILSA ALLABY and MICHAEL ALLABY. "alkali basalt." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O13-alkalibasalt.html

AILSA ALLABY and MICHAEL ALLABY. "alkali basalt." A Dictionary of Earth Sciences. 1999. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-alkalibasalt.html

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