Scoria
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Scoria is the vesicular ejecta of mafic to intermediate magmas, such as basalt and andesite. Generally a dark brownish black or red, scoria is generally thought of as the mafic version of pumice, forming when magma rich in dissolved gases is vented.
Its specific gravity ranges from 0.8 to 2.1, making it a relatively light form of rock - sometimes light enough to float (at less than 1.0 specific gravity).
[edit] Creation
As the magma encounters lower pressures, the gases are able to escape and form vessicles. These vessicles are trapped when the magma cools and solidifies. Vessicles 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.
Another material can be formed during burning eruption of a mud volcano, when heated mud forms scoria cones. Natural coal fires have also created large deposits of material which have erroneously been called scoria. Scoria contains large amounts of iron and magneseium, contributing to its dark colour.Scoria rocks are igneous rocks which were formed when lava cooled quickly above ground. You can see where little pockets of air had been. Scoria is actually a kind of glass and not a mixture of minerals.
An uncommon form of scoria is thread-lace scoria also known as reticulites. These are formed from a thin layer of froth occurring on some basaltic lava flows due to the bursting of vessicle 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%.
