1. 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. 2018

Examples from the Web for groundmass

Historical Examples of 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


  1. 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