basic rock Rock with a relatively high concentration of iron, magnesium, and calcium, and with 45–53% of silica by weight. Examples include gabbro, which is a coarse-grained basic intrusive rock, and basalt, which is a fine-grained basic volcanic (extrusive) rock. Compare ACID ROCK; and INTERMEDIATE ROCK. See also ALKALINE ROCK.

basic rock Rock that has a relatively high concentration of iron, magnesium, and calcium, and with 45–53 per cent of silica by weight. Examples include gabbro, which is a coarse-grained basic intrusive rock, and basalt, which is a fine-grained basic volcanic (extrusive) rock. See also alkaline rock. Compare acid rock and intermediate rock.