Pumice is whipped glass. Whipped in the sense that it incorporates bubbles that make it light enough to float on water, and glass in the sense that its chemical composition is the same as that of obsidian, volcanic glass. Pumice is thrown from erupting volcanoes in blocks and bombs, or forms as a crust on certain lava flows.
Pumice is formed when gas bubbles are trapped in viscous (gluey) molten rock. The bubbles form when pressures on the rock suddenly decrease, as when magma flows upward to areas of lower pressure, or when an erupting volcano ejects rock. A similar decrease in pressure lets bubbles appear in champagne. The bubbles are then trapped in the rock as it quickly solidifies. Pumice may also form when molten rock converts water to penetrating steam.
The small bubbles are known as vesicles, and the rock that surrounds them is called matrix. Viewed through a magnifying glass, the structure of pumice appears as silky strands of glass fiber, molded around small pores. The fibers may be parallel or tangled, and either way, they sometimes give the rock a silky luster.
Pumice may be off white, yellowish, gray, brown, gray blue, or, rarely, a dull red. It tends to be pale. Chemically, it has the same composition as granite, rhyolite, or obsidian. Physically, it is as different from obsidian as that shiny mineraloid is from granite.
Pumice generally comes from felsic lavas, which are high in feldspar and silicon. Felsic rocks are relatively light colored, and relatively light. The most common felsic rock is granite. At the other end of the spectrum are the mafic lavas. They contain magnesium and iron, and tend to be darker and have a higher specific gravity. A common mafic rock is basalt.
Scoria is a coarser grained, darker volcanic rock formed when vesicles form in molten rock. The vesicles are larger than those in pumice are, with thicker walls between them. Heavier than pumice, it does not float. It usually forms in mafic lava. Probably, basaltic lava that becomes scoria is less viscous than lava that becomes pumice, and so the gases it holds are able to form larger bubbles, and to escape before lightening the rock as much.
Reticulite, once called thread-lace scoria, is the lightest rock in the world. It forms out of burst vesicles. It is essentially a network of thin glassy threads and air. Reticulite sinks in water, because the water easily penetrates it.
Pumice is an igneous rock formed by volcanoes. Its light weight and airy texture are the result of bubbles, vesicles, left by gases that had been dissolved in the molten rock. It is similar to scoria, which also has vesicles, and to obsidian, which is another form of volcanic glass.