Scoria

Scoria

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Scoria 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. Scoria has relatively low density, as it is riddled with macroscopic ellipsoidal vesicles (gas bubbles), but in contrast to pumice, scoria always has a specific gravity greater than 1 and sinks in water.

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, also called cinder cones. 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 geological term cinder is synonymous and interchangeable with scoria, though scoria is preferred in scientific literature. The word comes from Greek σκωρία, skōria, rust. In earlier terminology, scoria was usually defined with a size range, e.g. 2 to 24 mm (0.079 to 0.945 in) in diameter, 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 in composition.

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