TERM OF THE WEEK
scoria: [geology] Vesicular, rock fragments (no size restrictions) with a specific gravity > 2.0, or a cindery crust of such material on the surface of andesitic or basaltic lava; the vesicular nature is due to the escape of volcanic gases before solidification; it is usually heavier, darker, and more crystalline than pumice (Schoeneberger and Wysocki, personal communication, 2013). Compare cinder, pumice, tephra.