Scoria or 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. Scoria has relatively low density, as it is riddled with macroscopic ellipsoidal vesicles (gas bubbles), but in contrast to pumice, scoria usually has a specific gravity greater than 1 and sinks in water. Some scoria can have a specific gravity similar to pumice especially if the vesicles are large and abundant alongside the walls being thin causing it to float. Examples of floating scoria were observed at the Taal Caldera lake in 2023. 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.
Scoria may form as part of a lava flow, typically near its surface as a crust, or more commonly 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 when Taal erupted in 2020 which was of andesitic composition. 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).
