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Scoria is a
pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
, highly vesicular, dark-colored
volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcan ...
that was ejected from a
volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates ...
as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, 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. It is typically dark in color (generally dark brown, black or purplish-red), and
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
ic or
andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained ( aphanitic) to porphyritic in texture, and is composed predomi ...
in composition. Scoria is relatively low in
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to
pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...
, all scoria has a
specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
greater than 1, and sinks in water. The holes or vesicles form when gases that were dissolved in the
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...
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 Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range ...
, 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. Analysis of scoria found in
Yemen Yemen (; ar, ٱلْيَمَن, al-Yaman), officially the Republic of Yemen,, ) is a country in Western Asia. It is situated on the southern end of the Arabian Peninsula, and borders Saudi Arabia to the north and Oman to the northeast an ...
showed that it was mainly composed of
volcanic glass Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the closely packed, highly ordered array of a crystal and the highly disordered array of liqu ...
with a few
zeolites Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These ...
(e.g. clinoptilolite). The word ''scoria'' comes from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
σκωρία, ''skōria'', rust. The term cinder is now considered to be synonomous and interchangeable with the term ''scoria'', and ''scoria'' is the preferred term used in the scientific literature.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. Eruption of scoria frequently form volcanic cones, called either
cinder cone A cinder cone (or scoria cone) is a steep conical hill A conical hill (also cone or conical mountain) is a landform with a distinctly conical shape. It is usually isolated or rises above other surrounding foothills, and is often of volcanic ...
s or scoria cones. Prior to the 1980s, neither color nor composition was typically a part of the definition and a size range of the particles, e.g. in diameter, was usually specified.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. During the 1980s, the size range disappeared from the definition and a requirement that scoria be black or reddish in color, mafic in composition, or combination of both was added.Fisher, R.V. and Schmincke, H.U., 1984. ''Pyroclastic rocks.'' Springer Science & Business Media. New York, New York, Springer Science & Business Media. 472 pp.


Comparisons

Scoria differs from
pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...
, 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.


Formation

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 little distortion.
Volcanic cone Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature ...
s of scoria can be left behind after eruptions, usually forming mountains with a crater at the summit. An example is Maungarei in
Auckland Auckland (pronounced ) ( mi, Tāmaki Makaurau) is a large metropolitan city in the North Island of New Zealand. The most populous urban area in the country and the fifth largest city in Oceania, Auckland has an urban population of about I ...
,
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...
, which like Te Tatua-a-Riukiuta in the south of the same city has been extensively quarried. Quincan, a unique form of Scoria, is quarried at Mount Quincan in
Far North Queensland Far North Queensland (FNQ) is the northernmost part of the Australian state of Queensland. Its largest city is Cairns and it is dominated geographically by Cape York Peninsula, which stretches north to the Torres Strait, and west to the Gulf ...
, Australia.


Uses

Scoria has several useful characteristics that influence how it is used. It is somewhat porous, has a high surface area and strength for its weight, and often has striking colours. Consequently, it is often used in landscaping and drainage works. It is also commonly used in gas barbecue grills.Rock Types and Rocks Found in Michigan
/ref> Scoria can be used for high-temperature insulation. It is also used on oil well sites to limit mud problems resulting from heavy truck traffic. The quarry of Puna Pau on
Rapa Nui Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearly ...
/
Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...
was the source of a red-coloured scoria which the Rapanui people used to carve the pukao (or topknots) for their distinctive
moai Moai or moʻai ( ; es, moái; rap, moʻai, , statue) are monolithic human figures carved by the Rapa Nui people on Rapa Nui in eastern Polynesia between the years 1250 and 1500. Nearly half are still at Rano Raraku, the main moai quarry, but ...
statues, and even to carve some moai themselves. It is also used as a traction aid on ice- and snow-covered roads. Scoria is mined and used for a variety of purposes. It commonly mined for use as loose
construction aggregate Construction aggregate, or simply aggregate, is a broad category of coarse- to medium-grained particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregate ...
in
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...
, the
Southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural region of the United States that generally includes Arizona, New Mexico, and adjacent portions of California, Colorado, N ...
, and Japan. Another major use of cinders is in manufacture of concrete and
cinder block A concrete masonry unit (CMU) is a standard-size rectangular block used in building construction. CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them. Th ...
s. In fact, the widespread use of cinders as construction aggregates by
ancient Romans In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 B ...
was one of earliest industrial uses of volcanic rocks.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. Because of its high strength, high angles of internal friction, and with good insulating properties, scoria is also used in the construction of flexible, long-lasting roadbeds. Because of insulating properties of scoria, such roadbeds protect the ground beneath them from extreme temperatures, and the associated deformation. Also, it can be used to minimize the effect of annual frost heave and
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surfac ...
on roads. Scoria has horticultural uses. Because it can hold water in its vesicles and in the pore space between grains in aggregates, it is used to improve the capacity of soils surrounding plants to hold moisture. When sorted to specific sizes, it is also an effective barrier for tunneling pests, such as termites. Its ragged surfaces allow it to be tightly packed and interlocked enough such that termites and other pests cannot fit between individual particles.


See also

* * *
Igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...
* * * * * * * *


References


External links

*{{commons-inline Volcanic rocks Glass in nature Igneous petrology Tephra