Scoria is a textural term for macrovesicular volcanic rock. It is commonly, but not exclusively, basaltic or andesitic in composition. Scoria is light as a result of numerous macroscopic ellipsoidal vesicles, but most scoria has a specific gravity greater than 1, and sinks in water. The vesicularity results from the exsolution of magmatic volatiles prior to chilling. Scoria differs from pumice in having larger vesicles and thicker vesicle walls, and hence is typically darker in colour (generally dark brown, black or red) and denser. The textural difference is probably the result of lower magma viscosity, allowing rapid volatile diffusion, bubble growth, coalescence, and bursting. Scoria may form as part of a lava or as fragmental ejecta (lapilli, blocks and bombs) for example at Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassy fragments, and may contain phenocrysts. Cinder is an archaic name for scoria.
