1. the fine-grained material in which larger crystals are embedded in an igneous rock
Usage: geology

Examples

• The porphyry contained large feldspar crystals set in a dark groundmass.
• Geologists examined the groundmass under a microscope to identify the mineral composition.
• The volcanic rock’s groundmass was too fine-grained to see individual crystals with the naked eye.
• In this basalt sample, the groundmass makes up about 80% of the rock’s volume.
• The groundmass cooled rapidly, preventing large crystal formation.
• Students learned to distinguish between phenocrysts and groundmass in igneous rocks.