What is the coarse-grained intrusive equivalent of a fine-grained extrusive rock that has a plagioclase-dominated matrix with phenocrysts of calcium-rich plagioclase and pyroxene?

What is the coarse-grained intrusive equivalent of a fine-grained extrusive rock that has a plagioclase-dominated matrix with phenocrysts of calcium-rich plagioclase and pyroxene?

• A. Gabbro
• B. Diorite
• C. Granite
• D. Peridotite

Answer: (A)

Cooling rate controls crystal size: magma cooling slowly underground allows crystals to grow large, producing a coarse-grained plutonic rock that has nearly the same mineral makeup as the fine-grained volcanic rock that crystallized at the surface. The described fine-grained extrusive rock has a groundmass dominated by plagioclase with phenocrysts of calcium-rich plagioclase and pyroxene, which is characteristic of basalt, a mafic volcanic rock. Its coarse-grained intrusive counterpart, formed from the same magma at greater depth, is gabbro, sharing the same mineral assemblage—calcium-rich plagioclase and pyroxene—just in larger crystals. The other options don’t fit: granite is felsic and quartz-rich; diorite is intermediate; peridotite is ultramafic and lacks that plagioclase-dominated groundmass.

Cooling rate controls crystal size: magma cooling slowly underground allows crystals to grow large, producing a coarse-grained plutonic rock that has nearly the same mineral makeup as the fine-grained volcanic rock that crystallized at the surface. The described fine-grained extrusive rock has a groundmass dominated by plagioclase with phenocrysts of calcium-rich plagioclase and pyroxene, which is characteristic of basalt, a mafic volcanic rock. Its coarse-grained intrusive counterpart, formed from the same magma at greater depth, is gabbro, sharing the same mineral assemblage—calcium-rich plagioclase and pyroxene—just in larger crystals. The other options don’t fit: granite is felsic and quartz-rich; diorite is intermediate; peridotite is ultramafic and lacks that plagioclase-dominated groundmass.