 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Scoria" – news ·newspapers ·books ·scholar ·JSTOR(September 2010) (Learn how and when to remove this message) |
Scoria orcinder is apyroclastic, highlyvesicular, dark-coloredvolcanic rock formed by ejection from avolcano as a molten blob and cooled in the air to form discrete grains calledclasts.[1][2] It is typically dark in color (brown, black or purplish-red), andbasaltic orandesitic in composition. Scoria has relatively lowdensity, as it is riddled with macroscopic ellipsoidal vesicles (gas bubbles), but in contrast topumice, scoria always has aspecific gravity greater than 1 and sinks in water.
Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks, and bombs), for instance inStrombolian eruptions that form steep-sidedscoria cones, also calledcinder cones.[1] Scoria's holes or vesicles form when gases dissolved in the originalmagma 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 containphenocrysts. A sample fromYemen was mainly composed ofvolcanic glass with a fewzeolites (e.g.,clinoptilolite).[3]
Thegeological termcinder is synonymous and interchangeable withscoria, thoughscoria is preferred in scientific literature.[2][4] The word comes fromGreek σκωρία,skōria, rust. In earlier terminology, scoria was usually defined with a size range, e.g. 2 to 24 mm (0.079 to 0.945 in) in diameter, but neither color nor composition was typically a part of the definition.[5][6] 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 in composition.[1]
Scoria differs frompumice, 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.
As rising magma encounters lower pressures, dissolved gases are able toexsolve 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 isMaungarei inAuckland,New Zealand, which likeTe Tatua-a-Riukiuta in the south of the same city has been extensively quarried. Quincan, a unique variety of scoria, is quarried atMount Quincan inFar North Queensland,Australia.
Scoria is used for a variety of purposes. It is commonly mined for use as looseconstruction aggregate inEurope, theSouthwestern United States, andJapan. Another major use of cinders is in manufacture of concrete andcinder blocks.
Scoria is also used in the construction of flexible, long-lastingroadbeds, due to its high strength and high angles of internal friction. Because of its good insulating properties, such roadbeds protect the ground beneath them fromfrost heave and heat deformation.[7] 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.[7] Its striking colours and water-holding properties can make it attractive for landscaping and drainage works.[8]
Scoria can be used for high-temperature insulation, as in gas barbecue grills.[9]
Theancient Romans used cinders as construction aggregates, one of the earliest industrial uses of volcanic rocks.[7] OnRapa Nui/Easter Island, the quarry ofPuna Pau was the source of reddish scoria used to carve thepukao (topknots) for the famousmoai statues, and even for the main bodies of somemoai.
Media related toScoria at Wikimedia Commons| Types |  | |
|---|---|---|
| Volcanic rocks | ||
| Lists and groups | ||
