Scoria

Scoria is a textural term for macrovesicular volcanic rock. It is commonly, but not exclusively, basaltic or andesitic in composition. Scoria is light as a result of numerous macroscopic ellipsoidal vesicles, but most scoria has a specific gravity greater than 1, and sinks in water. The vesicularity results from the exsolution of magmatic volatiles prior to chilling. Scoria differs from pumice in having larger vesicles and thicker vesicle walls, and hence is typically darker in colour (generally dark brown, black or red) and denser. The textural difference is probably the result of lower magma viscosity, allowing rapid volatile diffusion, bubble growth, coalescence, and bursting. Scoria may form as part of a lava or as fragmental ejecta (lapilli, blocks and bombs) for example at Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassy fragments, and may contain phenocrysts. An old name for scoria is cinder.

The word comes from the Greek σκωρία, "skōria", rust.

How it is formed

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 cones of scoria can be left behind after eruptions, usually forming mountains with a crater at the summit. An example is Mount Wellington, Auckland in New Zealand, which like the Three Kings Mount 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, Australia.

The quarry of Puna Pau on Rapa Nui/Easter island was the source of a red coloured scoria which the Rapanui people used to carve the Pukao (or top knots) for their distinctive Moai statues, and to carve some Moai from.

Reticulite ("thread-lace scoria") differs from scoria in being considerably less dense. It is formed from a thin layer of froth occurring on some basaltic lava flows due to the bursting of vesicle walls. The thin glass threads are the intersections of burst vessicles. This is the lightest rock on earth with its specific gravity less than 0.3. The delicate framework of thread-lace scoria is so open that the average porosity is 98-99%.

See also

* Cinder
* Tuff
* Pumice
* Volcanoes
* List of rock types
* Rock (geology)