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pumice - display specimen of banded glassy gray pumice from Mono Craters, California

$ 7.50

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Forcefully ejected from a volcanic vent, pumice hardens as it flies through the air, trapping bubbles of gas. It is often light enough to float on water. The composition of this silica-rich volcanic rock is similar to that of obsidian or granite. The silica poor equivalent would form 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.

This particularly interesting specimen is banded due to mixing of compositionally different magma. The vesicles in the upper bands show stretching, likely from differential decompression. Cinder cone volcanoes composed of pumice are not common. Silica-rich glassy lava is sticky and eruptions tend to be explosive, causing wide distribution of the pumice. After the eruption of Krakatoa, for example, the sea was coated with floating pumice for miles around. Mono Craters is a small chain of five pumice cones south of mono Lake, Mono County, California.

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