Case 8: Basalts

Pasamonte eucrite, a basalt (6.5 cm across)

The most common type of volcanic rock on solar-system bodies is basalt, a dark, fine-grained igneous rock composed mainly of the minerals plagioclase and calcium pyroxene. Basalts occur on Earth, the Moon, Mars and differentiated asteroids.

Eucrites are basaltic meteorites that come from a differentiated asteroid, perhaps Vesta. Four eucrites are on the top shelf: Palo Blanco Creek [found in New Mexico in 1954], NWA 6694 (polymict eucrite) [found in Northwest Africa in 2011], NWA 999 [found in Morocco in 2000] and Pasamonte [fell in New Mexico in 1933]. Also on the top shelf is Los Angeles, a martian basalt (shergottite) [found in Los Angeles, County, California in 1999]. The second shelf contains basalts and related rocks from different parent bodies: Millbillillie (from the eucrite parent body), D’Orbigny (an angrite from a different differentiated asteroid), Zagami (a basalt from Mars) and a lunar impact melt rock (not a basalt) – NWA 482.

The bottom of the case includes a variety of terrestrial basalts – vesicular basalt, an alkali basalt with olivine inclusions, pahoehoe lava (i.e., ropy lava) and a fine-grained basalt.

For more-comprehensive definitions, please see the Glossary.