Forcefully ejected from a volcanic vent, pumice hardens as it flies through the air, trapping bubbles of gas. It is normally light enough to float on water, but some pumice has large vesicles that rapidly fill with water, so it soon sinks. The composition of this silica rich volcanic rock is similar to that of obsidian, with which it was associated.
This came from one of a chain of pumice cinder cones, shown here, that are associated with the Long Valley Caldera in Mono County, California, which erupted and then collapsed about 700,000 years ago. Cinder cones composed of pumice are not common, since silica-rich glassy lava is sticky and eruptions tend to be explosive, causing wide distribution of the pumice. After the eruption of Krakatoa, the sea was coated with floating pumice for miles around. Pumice is mined for use as an abrasive and occasionally as a decorative rock.
The silica poor equivalent is scoria or volcanic cinder. Also full of entrapped gas bubbles, scoria's composition is the same as basalt, the lava that makes up the Hawaiian and other island chains as well as the oceanic crust. Silica poor lava such as basalt is not viscous and readily fountains from vents. Loose pieces of basalt scoria form any number of small cinder cone volcanoes in the western U.S. Both pumice and scoria are mined as decorative rock. Students will recognize oxidized reddish scoria as "McDonald's flower bed rock."
Not all pumice floats. The large holes in this pumice fill almost instantly with water and it sinks. The pencil is 5 1/2 inches long, for scale.
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