A VERY POROUS, froth-like volcanic glass, pumice is created when gas-saturated liquid magma erupts like a fizzy drink released from a shaken bottle, and cools so rapidly that the resulting foam solidifies into a glass full of gas bubbles. The hollows in the froth (called vesicles) can be rounded, elongated, or tubular, depending on the flow of the solidifying lava. The glassy material that forms pumice can be in threads, fibers, or thin partitions between the vesicles. Pumice has a very low density due to its large number of air-filled pores. As a result, it can easily float in water, and it frequently does when erupted into the sea. After the eruption of Krakatau in Indonesia in 1883, banks of pumice covered the surface of the sea up to a depth of 5ft (1.5m). Pumice differs from tuff in that it seldom forms thick deposits, but instead tends to form thin layers on the surface.
Although pumice is mainly composed of glass, small crystals of various minerals do occur, the most common being feldspar, augite, hornblende, and zircon. In older pumices, the vesicles are more often than not filled with deposits of minerals such as zeolites brought in by water percolating through them. Under favourable conditions, most types of lava can form pumice – but silica-rich lavas are more prone to froth than silica-deficient ones because they are more viscous and tend to hold their volatiles until they are erupted. Pumices are frequently accompanied by obsidian, which forms under similar conditions but results from greater pressure. Pumices formed from silica-rich lavas are white, while those from lavas with intermediate silica content are often yellow or brown, and the rarer silica-poor pumices (like those in the Hawaiian Islands) are black. Pumice is important economically as an abrasive in cleaning, polishing, and scouring compounds.
- Rock type Volcanic, igneous
- Origin Extrusive
- Major minerals Glass
- Minor minerals Feldspar, augite, hornblende, zircon
- Color White, yellow, brown, black
- Texture Fine
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