Scoria is the vesicular ejecta of mafic to intermediate magmas such as basalt and andesite. Scoria is generally a dark brownish black or red. Its specific gravity ranges from 0.8 to 2.1. Scoria is generally thought of as the mafic version of pumice. It forms when magma rich in dissolved gases is vented. 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. 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 (this will lead to a dark colour), and is dense.

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%.

See also

• Cinder
• Tuff

スコリア | 火山渣