A dark-coloured, fine-grained, extrusive, igneous rock composed of plagioclase feldspar, pyroxene, and magnetite, with or without olivine, and containing not more than 53 wt. % SiO₂. 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. Petrographically (see petrography), alkali basalts have as their groundmass pyroxene titanaugite (an augite rich in titanium), whereas tholeiites have pigeonite (a calcium-poor pyroxene). Also, for similar concentrations of SiO₂, alkali basalts have a higher content of Na₂O and K₂O than tholeiites. Basalt flows cover about 70% of the Earth’s surface and huge areas of the terrestrial planets, and are therefore arguably the most important of all crustal rocks. 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.