Composition: A fine-grained, igneous rock formed when lava flows or erupts onto the surface of the earth and cools rapidly. Contains less than 53% silica. Commonly contains olivine, plagioclase, and pyroxene.
Industrial Uses: Crushed stone.
Additional Notes: Basalt is the most common rock type in the Earth's crust. The magma from which basalt forms is most frequently produced by direct melting of the Earth's mantle. Shield volcanos, such as those that make up the Hawaiian islands, are composed almost entirely of basalt.
Afars & Issas : Scott 349 : 10 francs : 22 Nov 1971
Diabase (doleritic basalt) [thin-section under polarized light]
Afars & Issas : Scott 350 : 15 francs : 8 Oct 1971
Olivine basalt [thin-section under polarized light]