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Pumice

Pictures of Igneous Rock Types

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Pumice is basically lava froth, an extrusive rock frozen as its dissolved gases come out of solution. It looks solid but often floats on water. (more below)
A fluffy stone
Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)
This pumice specimen is from the Oakland Hills in northern California and reflects the high-silica (felsic) magmas that form when subducted marine crust mixes with granitic continental crust. Pumice may look solid, but it's full of small pores and spaces and weighs very little. Pumice is easily crushed and used for abrasive grit or soil amendments.

Pumice is much like scoria in that both are frothy, lightweight volcanic rocks, but the bubbles in pumice are small and regular and its composition is more felsic than scoria's. Also, pumice is generally glassy whereas scoria is a more typical lava with microscopic crystals.

For photos of related rocks, see the volcanic rocks gallery.

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