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Scoria is a type of volcanic rock that is full of tiny holes formed by bubbles of gas. These bubbles are called vesicles and are trapped in the rock as it hardens after a volcanic eruption. Scoria is a class of igneous rock but is categorized by its dense matrix of vesicles rather than the chemical or mineralogical makeup of the rock, which can vary. It is often dark in color, with a grey or reddish hue and is usually glassy as well. The name comes from the name of a waste material with a similar texture that results from the smelting of ores.
During volcanic eruptions, molten rock is ejected from the vent of the volcano and may take the form of lava flows or individual masses of molten rock, called volcanic bombs, that are flung into the air. Often, lava is permeated with gases that become trapped within the lava as it cools into rock. These gasses form bubbles or pockets within the hardened rock, giving it a porous, spongy aspect, although the rocks are not physically soft and compressible like a sponge. Scoria rocks can be found in large masses, random chunks, or even as crumbly layers of small particles laid down on the sides of a cinder cone volcano.
For this reason, scoria is sometimes confused with pumice, a similar volcanic rock that is riddled with many small holes and pockets of air and gas. The main difference between the two is that pumice generally has a density less than that of water due to the amount of trapped gas and will actually float. Scoria is denser than water and will sink. The air or gas bubbles in scoria tend to be larger than those in pumice, which tends to have very large numbers of smaller pockets. These differences are due to differences in the density and viscosity of the molten rock that produces them.
While it has few uses, scoria, which is sometimes called cinders, does have some value. They are sometimes crushed and used for roadbeds and for fill and to provide traction for wheeled vehicles during wintry conditions. Chunks of scoria are also sometimes sold for landscaping and gardening purposes or for use in cooking grills. When marketed in such a way, it is commonly sold as "lava rock".