1878 J. Le Conte iii. 84 The whole liquid mass may swell into a rock-froth, which rises to the lip of the crater.
1919 iii. 93 These are called pumice when the bubbles are very small, forming a light rock-froth.
2005 (Nexis) 11 Apr. c1 Rock froth is shattered and hurled high into the air as volcanic cinders ash and dust.