pumice - teaching hand specimen of typical white pumice
Ejected in a blast from a volcanic vent, pumice hardens as it flies through the air, trapping bubbles of gas and steam. 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. Pumice is commercially used as an abrasive.
This pumice is from the Coso Range in California. The area is geologically active, with a nearby geothermal plant on the China Lake Naval Air Weapons Station producing electricity from steam. The Navy sells surplus electricity into the commercial grid.
This pumice is closely associated with rhyolite porphyry, blocks of which are buried in a pumice and ash flow. All have a similar silica-rich composition - rhyolite lava flows reluctantly because the silica makes it viscous. This same viscosity prevents easy fountaining of lava, so pressure builds up and a silica-rich eruption is explosive.
The silica poor equivalent of pumice would be 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 Island chain as well as the oceanic crust. 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 varies in appearance. See also our student specimens of glassy gray pumice and of pink pumice. Students should see a variety of pumice types.
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