Scoria is a vesicular igneous rock that can be basaltic or andesitic in composition. These rocks cooled rapidly on the surface of the earth, meaning they are aphanitic and have a fine-grained matrix. Scoria is formed when magma containing dissolved gas is erupted or flows out of a volcano reducing the pressure, causing the dissolved gas to escape. The rapid cooling process of the rock causes it to solidify as the gas is escaping, leaving vacant cavities behind. Scoria can contain phenocrysts, as seen in one of theses samples with the presence of olivine crystals.