{{short description|Dark vesicular volcanic rock}} {{other uses}} {{Refimprove|date=September 2010}} {{Infobox rock |name=Scoria |type=Igneous |image= Scoria edit.jpg |caption= |composition=Volcanic glass |composition_secondary= Plagioclase, amphibole, pyroxene, magnesium, and olivine |class=Mafic to intermediate |texture=Vesicular and glassy to aphanitic, sometimes porphyritic }}
'''Scoria''' ({{plural abbr|'''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.<ref name="FisherOthers1984a"/><ref name="NeuendorfOthers2005a">{{cite book |editor-first1=K. K. E.|editor-last1=Neuendorf |editor-first2=J. P.|editor-last2=Mehl, Jr. |editor-first3=J. A.|editor-last3=Jackson |year=2005 |title=Glossary of Geology |edition=5th |location=Alexandria, Virginia |publisher=American Geological Institute |page=779 |isbn=0-922152-76-4}}</ref> 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.<ref name="NeuendorfOthers2005a" /><ref name="McPhieOthers1993a">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. {{ISBN|9780859015226}}</ref>
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.<ref name="PNA2023FloatingRocks">Chavez, Pot (21 June 2023). “‘Floating’ rocks in Taal Lake not sign of volcanic eruption.” Philippine News Agency. Retrieved from https://www.pna.gov.ph/articles/1204022#:~:text=Rocks%20were%20found%20floating%20on%20lake%20water…</ref> 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<ref>{{cite web|title=Volcano Event Information – Event #754|url=https://www.ngdc.noaa.gov/hazel/view/hazards/volcano/event-more-info/754|website=NCEI/WDS Global Significant Volcanic Eruptions Database |publisher=National Centers for Environmental Information, NOAA|access-date=2 January 2026 |quote=Significant volcanic eruption event recorded in the NOAA NCEI Global Significant Volcanic EruptionsDatabase, with start date 15 May 1754, end date 4 December 1754, and a Volcanic Explosivity Index (VEI) of 4.}}</ref> and which later analysis found to be of basaltic composition.<ref>{{cite web|title=Attention focuses on Taal: Decade volcano of the Philippines|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/95EO00137|website=AGU Publications|publisher=Wiley on behalf of the American Geophysical Union|access-date=2 January 2026 |quote=Article discusses why Taal Volcano in the Philippines was selected as a Decade Volcano due to its complex eruptive history and significant hazard potential.}}</ref> 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.<ref name="Siebe_et_al_1995">{{cite journal |title=Submarine eruption near Socorro Island, Mexico: Geochemistry and scanning electron microscopy studies of floating scoria and reticulite |first1=C. |last1=Siebe |first2=J.-C. |last2=Komorowski |first3=C. |last3=Navarro |first4=J. |last4=McHone |first5=H. |last5=Delgado |first6=A. |last6=Cortés |journal=Journal of Volcanology and Geothermal Research |year=1995 |volume=68 |issue=4 |pages=239–271 |doi=10.1016/0377-0273(95)00029-1}}</ref>
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.<ref name="FisherOthers1984a" />
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).<ref>[https://www.researchgate.net/publication/330007091_Preliminary_Assessment_of_Utilization_of_Al-Jaif_Scoria_NW_Sana'a_Yemen_for_Cement_Production Preliminary Assessment of Utilization of Al-Jaif Scoria (NW Sana'a, Yemen) for Cement Production]</ref>
The word ''scoria'' comes from Ancient Greek word {{wikt-lang|grc|σκωρία}} ({{grc-transl|σκωρία}}), meaning 'rust'. It is also the Italian term for slag. In earlier terminology, scoria was usually defined with a size range, e.g. {{cvt|2|-|24|mm|in}} in diameter, corresponding to lapilli, but neither color nor composition was typically a part of the definition.<ref name="StokeOthers1955a">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.</ref><ref name="Wentworth1953a">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.</ref> 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.<ref name="FisherOthers1984a">Fisher, R.V. and Schmincke, H.U., 1984. ''Pyroclastic rocks.'' Springer Science & Business Media. New York, New York, Springer Science & Business Media. 472 pp. {{ISBN|978-3-642-74864-6}}</ref>
==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.<ref>[https://geologyscience.com/rocks/scoria/ Scoria from the Geology Science Website]</ref>
==Formation== [[File:Pine trees growing on the slope of Cinder Cone.jpg|thumb|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.
Volcanic cones of scoria can be left behind after eruptions, usually forming mountains with a crater at the summit. An example is Maungarei in Auckland, New Zealand, which like Te Tatua-a-Riukiuta in the south of the same city has been extensively quarried. Quincan, a unique variety of scoria, is quarried at Mount Quincan in Far North Queensland, Australia.
==Uses== [[File:Ahu Tahai.jpg|thumb|upright|Tuff moai with red scoria pukao on its head.|150px]]Scoria is used for a variety of purposes. It is commonly mined for use as loose construction aggregate in Europe, the Southwestern United States, Oregon<ref>Cascade_Lakes_Scenic_Byway</ref> and Japan. Another major use of cinders is in manufacture of concrete and cinder blocks.
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.<ref name="DehnOthers2015a">Dehn J., and McNutt, S.R., 2015. ''Volcanic materials in commerce and industry.'' In: Sigurdsson H., Houghton B., F., McNutt, S.R., Rymer, H., and Stix, J. (eds.) ''The encyclopedia of volcanoes.'' 2nd ed. England, Oxford, Elsevier. pp 1285-1294.</ref> 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.<ref name="DehnOthers2015a" /> Its striking colours and water-holding properties can make it attractive for landscaping and drainage works.<ref>[http://www.winstoneaggregates.co.nz/Auckland.php?location=Three%20Kings Three Kings Quarry] {{Webarchive|url=https://web.archive.org/web/20110724195217/http://www.winstoneaggregates.co.nz/Auckland.php?location=Three%20Kings |date=2011-07-24 }}, Winstone Aggregates.</ref>
Scoria can be used for high-temperature insulation, as in gas barbecue grills.<ref>[http://www.michigan.gov/documents/deq/GIMDL-GGRF_307777_7.pdf Rock Types and Rocks Found in Michigan]</ref>
The ancient Romans used cinders as construction aggregates, one of the earliest industrial uses of volcanic rocks.<ref name="DehnOthers2015a" /> 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.{{citation needed|date=January 2026}}
==See also== *{{annotated link|Cinder track}} *{{annotated link|Extrusive rock}} *{{annotated link|Igneous rock}} *{{annotated link|List of rock types}} *{{annotated link|Parícutin}} *{{annotated link|Tephra}} *{{annotated link|Tuff}} *{{annotated link|Ye Olde Cinder House}}
==References== {{Reflist}}
==External links== *{{commons-inline}} {{Volcanoes}} {{Rock type}} Category:Volcanic rocks Category:Glass in nature Category:Igneous petrology Category:Tephra