Coloured scanning electron micrograph (SEM) of Fractured igneous volcanic rock (pumice) from Mt. St. Helens (Washington State). Pumice is a light-coloured, highly porous igneous rock that forms during explosive volcanic eruptions. The pore spaces in pumice are known as vesicles. The vesicles are actually gas bubbles that were trapped in the rock during the rapid cooling of a gas-rich frothy magma. The material cools so quickly that atoms in the melt are not able to arrange themselves into a crystalline structure. Thus, pumice is an amorphous volcanic glass known as a mineraloid. It is used as aggregate in lightweight concrete, as landscaping aggregate, and as an abrasive in a variety of industrial and consumer products. It is also used as an abrasive, especially in polishes, pencil erasers, cosmetic exfoliants. Magnification: x460 when shortest axis printed at 25 millimetres.
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