Basaltic Scoria

The word scoria comes from the Greek "skoria"= rust; 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 mass 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.

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Scoria from a cinder cone in Tenerife, Canary Islands. From Sand Atlas.

Red scoria. From James St. John.

Dark gray scoria. From James St. John.

Scoria cones, Etna, Sicily, Italy. From Liberamente.

Scoria cone in the Voragine crater, Etna, February 25, 2020. From INGV.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Olivine and Pyroxene in a Basaltic Scoria. Etna (Italy). PPL image, 2x (Field of view = 7mm)	Olivine and Pyroxene in a Basaltic Scoria. Etna (Italy). XPL image, 2x (Field of view = 7mm)	Olivine and PlagioclaSe in a Basaltic Scoria. Etna (Italy). XPL image, 10x (Field of view = 2mm)
Olivine with Chromite inclusions in a Basaltic Scoria. Etna (Italy). PPL image, 10x (Field of view = 2mm)	Olivine and Pyroxene in a Basaltic Scoria. Etna (Italy). PPL image, 2x (Field of view = 7mm)	Olivine and Pyroxene in a Basaltic Scoria. Etna (Italy). XPL image, 2x (Field of view = 7mm)
Olivine in a Basaltic Scoria. Etna (Italy). XPL image, 10x (Field of view = 2mm)	Plagioclase in a Basaltic Scoria. Etna (Italy). XPL image, 10x (Field of view = 2mm)	Pyroxene in a Basaltic Scoria. Etna (Italy). PPL image, 10x (Field of view = 2mm)
Pyroxene in a Basaltic Scoria. Etna (Italy). XPL image, 10x (Field of view = 2mm)	Olivine in a Basaltic Scoria. Etna (Italy). PPL image, 10x (Field of view = 2mm)	Olivine in a Basaltic Scoria. Etna (Italy). XPL image, 10x (Field of view = 2mm)