火山渣一词被地质学家用来形容含很多泡沫或空隙的火成岩。

火山渣的形成原因如下: 当含有很多空气的岩浆被喷出后，因为岩浆遇到比较低压的环境，被岩浆溶解的空气要离开岩浆而产生气泡，这些气泡被冷却成固态的岩浆困住，而成为有很多气孔的火山渣。

火山渣常见于火山锥上，通常火山渣会形成山，山顶为火山口。 像新西兰奥克兰的威灵顿山就是这样。

另一种火山渣则是从泥火山燃烧爆发时产生，被加热的泥会形成火山渣锥。

Scoria is a highly vesicular, dark colored volcanic rock that may or may not contain crystals (phenocrysts). It is typically dark in color (generally dark brown, black or purplish red), and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1, and sinks in water. The holes or vesicles form when gases that were dissolved in the magma come out of solution as it erupts, creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies. Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks and bombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassy fragments, and may contain phenocrysts. The word scoria comes from the Greek σκωρία, skōria, rust. An old name for scoria is cinder.