Many basalts contain phenocrysts of olivine, plagioclase feldspar and pyroxene.
Basalts are divided into two main types, alkali basalts and tholeiites , with the tholeiites being subdivided into olivine tholeiites, tholeiites, and quartz tholeiites. They are formed by partial melting of mantle peridotite . Alkali basalts are typically found on oceanic islands and on the continental crust in regions of crustal upwarping and rifting. Tholeiites are typically found on the ocean floor and on the stable continental crust where they form large basalt plateaux such as the Deccan Traps of India. All rights reserved.