1. The fine-grained or glassy matrix of an igneous rock in which larger crystals are embedded; the base material of a porphyritic rock.
Usage: geology; plural of groundmass

Examples

• The groundmasses of these volcanic rocks contain microscopic crystals visible only under a microscope.
• Geologists examined the groundmasses to determine the cooling rate of the lava.
• The contrast between large phenocrysts and fine groundmasses is characteristic of porphyritic textures.
• Different groundmasses indicate varying conditions during the rock's formation.
• The groundmasses in basaltic rocks are typically darker than those in granitic rocks.