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.[1][2] 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.[3] 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.[4]
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.[1] 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).[5]
The word scoria comes from Greek σκωρία, skōria, rust. It is also the Italian term for slag. In earlier terminology, scoria was usually defined with a size range, e.g. 2 to 24mm (0.079 to 0.945in) in diameter, corresponding to lapilli, but neither color nor composition was typically a part of the definition.[6][7] 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.[1]
The geological term cinder is synonymous and interchangeable with scoria, though scoria is preferred in scientific literature.[2][8]
Comparisons
Scoria differs from pumice, another vesicular volcanic rock, in having larger vesicles and thicker vesicle walls, and hence is denser. The difference is probably the result of lower magma viscosity, allowing rapid volatile diffusion, bubble growth, coalescence, and bursting.[9]
Formation
Pine trees growing on the slope of Cinder Cone, a volcanic cone of scoria in Lassen Volcanic National Park, California
As rising magma encounters lower pressures, dissolved gases are able to exsolve and form vesicles. Some of the vesicles are trapped when the magma chills and solidifies. Vesicles are usually small, spheroidal and do not impinge upon one another; instead, they open into one another with minimal distortion.
Scoria is also used in the construction of flexible, long-lasting roadbeds, due to its high strength and high angles of internal friction. Because of its good insulating properties, such roadbeds protect the ground beneath them from frost heave and heat deformation.[10] It is also spread as a traction aid on ice- and snow-covered roads, and around oil wells to firm up mud produced by heavy truck traffic.
Scoria is used in horticulture. Because it can hold water in its vesicles and in the pore space between grains in aggregates, it can improve the water-holding capacity of planting soils. When sorted to specific sizes and tightly packed, it is also an effective barrier against tunneling pests such as termites.[10] Its striking colours and water-holding properties can make it attractive for landscaping and drainage works.[11]
Scoria can be used for high-temperature insulation, as in gas barbecue grills.[12]
The ancient Romans used cinders as construction aggregates, one of the earliest industrial uses of volcanic rocks.[10] On Rapa Nui/Easter Island, the quarry of Puna Pau was the source of reddish scoria used to carve the pukao (topknots) for the famous moai statues, and even for the main bodies of some moai.
See also
Cinder track– Race track whose surface is composed of cinders
123Fisher, R.V. and Schmincke, H.U., 1984. Pyroclastic rocks. Springer Science & Business Media. New York, New York, Springer Science & Business Media. 472 pp. ISBN978-3-642-74864-6
12Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN0-922152-76-4
↑Siebe, C.; Komorowski, J.-C.; Navarro, C.; McHone, J.; Delgado, H.; Cortés, A. (1995). "Submarine eruption near Socorro Island, Mexico: Geochemistry and scanning electron microscopy studies of floating scoria and reticulite". Journal of Volcanology and Geothermal Research. 68 (4): 239–271. doi:10.1016/0377-0273(95)00029-1.
↑Stokes, W. L., and Varnes, D. J., 1955. Glossary of selected geologic terms, with special reference to their use in engineering: Denver.Proceedings of the Colorado Scientific Society. vol. 15, pp 1-165.
↑Wentworth, C.K. and Macdonald, G.A., 1953. Structures and forms of basaltic rocks in Hawaii.United States Geological Survey Bulletin. no. 944, pp 1-98.
↑McPhie, J., M. Doyle, and R. Allen (1993) Volcanic Textures A guide to the interpretation of textures in volcanic rocks Centre for Ore Deposit and Exploration Studies, University of Tasmania, Hobart, Tasmania..198 pp. ISBN9780859015226
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