[ ground-mas ]
/ ˈgraʊndˌmæs /


the crystalline, granular, or glassy base or matrix of a porphyritic or other igneous rock, in which the more prominent crystals are embedded.

Origin of groundmass

First recorded in 1875–80; ground1 + mass Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for groundmass

  • Pyroxene, quartz and augite form the groundmass, as seen in section.

  • These lavas have groundmass textures that vary from almost holo-crystalline to glassy.

  • In a basal flow in Moraine Park, the slaggy and compact phases show differences in phenocrysts as well as in groundmass.

British Dictionary definitions for groundmass


/ (ˈɡraʊndˌmæs) /


the matrix of igneous rocks, such as porphyry, in which larger crystals (phenocrysts) are embedded
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012