pumice - teaching student specimens of glassy dark gray pumice UNIT OF 5 SPECIMENS
Forcefully ejected from a volcanic vent, pumice hardens as it flies through the air, trapping bubbles of gas. It is light enough to float on water - these specimens will all float. The composition of this silica rich volcanic rock is similar to that of obsidian or granite. 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.
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, the sea was coated with floating pumice for miles around.
Scoria and basalt are silica poor, so lava readily fountains from vents. Loose pieces of 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 scoria as "McDonald's flower bed rock." Pumice is used as an abrasive.
Not all pumice will float. The large holes in this pumice fill with water almost instantly and it sinks.
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