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groundmass The finer-grained material of igneous rocks in which larger crystals (phenocrysts) and xenoliths are enclosed. The groundmass of an igneous rock commonly represents that part of the magma system which has cooled rapidly. Rapid cooling initiates numerous crystal nucleation sites which limit the size to which crystals can grow. In lavas, the fine ground-mass is formed when the magma is erupted on to the Earth's surface and is cooled rapidly by air convection over the flow. Compare MATRIX.