Scoria

Scoria (pl.scoriae), also called volcanic cinder, is a pyroclastic, highly vesicular, dark-colored volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air to form discrete grains called clasts. It is typically dark in color (brown, black, or purplish-red), and basaltic or andesitic in composition. The geological term cinder is synonymous and interchangeable with scoria, though scoria is preferred in scientific literature.

Scoriae have relatively low densities, as they are riddled with macroscopic ellipsoidal vesicles (gas bubbles), but in contrast to pumice, most scoriae usually have specific gravities of at least 1 and sink or remain in place in water rather than float at the surface. However, some scoriae can have densities similar to that of pumice, especially if they are unusually porous and have small and abundant vesicles, and will float in water.

Scoria with larger vesicles and thick walls can also float if the vesicles are not interconnected. Fragments of floating scoria were observed at Taal Lake in 2023. A priest who witnessed Taal Volcano's 1754 eruption noted in the lake a large quantity of floating rock which had been ejected by the volcano and which later analysis found to be of basaltic composition. Scoria from a 1993 undersea eruption near Socorro Island in the Pacific Ocean was observed to float on the ocean surface for up to 15 minutes before it sank.

If the crust of a lava flow contains a large amount of vesicles, it is often called "scoria" or vesicular basalt. This form of scoria often has fewer vesicles and a higher specific gravity than the more typical scoria ejected as tephra. It is also formed as fragmental ejecta (lapilli, volcanic blocks, and volcanic bombs), for instance in Strombolian eruptions that form steep-sided scoria cones, also called cinder cones.

Basaltic to andesitic plinian eruptions can also form scoria like the basaltic 1085 eruption of Sunset Crater and the andesitic 2020 eruption of Taal. Scoria's holes or vesicles form when gases dissolved in the original 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. Most scoria is composed of glassy fragments and may contain phenocrysts. A sample from Yemen was mainly composed of volcanic glass with a few zeolites (e.g. clinoptilolite).

The word scoria comes from Ancient Greek word (skōría), meaning 'rust'. It is also the Italian term for slag. In earlier terminology, scoria was usually defined with a size range, e.g. 2–24 mm (0.079–0.945 in) in diameter, corresponding to lapilli, but neither color nor composition was typically a part of the definition. During the 1980s, the size range disappeared from the definition, and a requirement was added that scoria be black or reddish in color and/or mafic to intermediate in composition.