Made of, containing, or resembling basalt; relating to or composed of the dark volcanic rock basalt.
From 'basalt' plus the adjectival suffix '-ic.' Basalt comes from Greek 'basanites' (test stone). The adjectival form became standard in geology during the 18th and 19th centuries.
The Moon's dark 'seas' are actually filled with basaltic lava flows from ancient lunar volcanoes—when astronauts landed there, they walked on the same type of rock that creates volcanic islands on Earth.
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