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The Rock - Pumice

  • Igneous Rock Type: Extrusive
  • Related to:  Rhyoliteobsidianscoria
  • Chemistry: Acidic to intermediate and rarely basic
  • Color: White,  gray, off-white, light black or green
  • Texture: Glassy
  • Origins: Volcanic arcs generally
  • Common Minerals: N/A
  • Accessory Minerals: N/A
  • Uses: Decorative stones, abrasive or scouring stones, filter media and as a polish, cement and soap additive
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Pumice is a textural rock and not a rock that is classified by mineralogy or chemistry.  It is basically the solidified foam that forms from a lava rich in volatiles or gases.  When the molten rock is effervescing to the point of forming a froth and then solidifies, pumice is the result.  Pumice has numerous open spaces or vesicles in the form of round bubbles, linear tubes or irregular cavities.  There is so much porosity in pumice that there is often more empty space than actual rock and in fact some pumice can float on water.

Some ocean related volcanoes have produced what are known as pumice rafts, which are actual floating mini islands made of rock.  These pumice islands can exist for years floating along the ocean currents.  Some may have been responsible for the distribution of island hopping animals and plants of the Pacific Ocean.  Some pumice islands have been found with plants actually growing on them.

Pumice differs from obsidian in that obsidian is all glass and lacks the extensive vesicles of pumice. Most pumice is acidic/felsic in composition associated with rhyolite since those lavas tend to have more volatiles, but intermediate and basic varieties are known to occur. Scoria is a much heavier ropey volcanic rock with larger but less prolific vesicles than pumice.

There are many uses for pumice as it is light weight, relatively hard and can have very sharp edges. It is used in the cosmetics industry as an exfoliating pad, scouring stones and as an additive in creams and lotions. Pumice's hardness makes it useful as an abrasive and its light weight gives it an advantage for use as large decorative rocks for landscaping. Its porous nature makes it a natural for filters.

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