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Term used to describe [[volcana|volcanic]] rock or [[magma]] composed chiefly of dark-colored, iron- and magnesium-rich minerals. (Miller, 1989) <ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology]</ref>
Term used to describe [[sedimentary]] rock or [[magma]] composed chiefly of dark-colored, iron- and magnesium-rich minerals. (Miller, 1989) <ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology]</ref>
==References==
==References==