Volcanic eruptions can produce a rock that floats on water

Saturday, April 05, 2014 by: David Gutierrez, staff writer
Tags: volcanic eruptions, pumice, low-density rocks

(NaturalNews) Only one type of rock on Earth is light enough to float on water: pumice, produced from explosive volcanic eruptions. Pumice forms either when super-heated, highly pressurized lava erupts violently from a volcano or when lava strikes water and rapidly cools. In both cases, the lava cools and depressurizes quickly. The falling pressure inside the lava lowers the solubility of trapped gases like carbon dioxide, causing them to separate out as bubbles. At the same time, the rapid cooling causes the lava to solidify before these gases can escape. The result is pumice: a rock full of trapped gas bubbles that is up to 90 percent porous. The high proportion of trapped gas makes pumice remarkably low in density, thus accounting for its unique ability to float on water.

Pumice is typically pale or gray in color but can range from white or cream to blue, green-brown or even black. It is easily recognized by its distinctive, holey appearance. A similar-looking type of rock, scoria, is usually darker in color and is more dense than water.

Pumice actually plays an important ecological role, with massive "rafts" of pumice ejected by volcanoes helping to support and disperse numerous marine species. Underwater volcanic eruptions that took place near the archipelago of Tonga in 1979, 1984 and 2006 actually crated pumice rafts as large as 30 kilometers (20 miles) across that floated 500 miles or more to the island of Fiji. Pumice islands produced by the Krakatoa eruption of 1883 were found floating in the ocean as much as 20 years later.

Due to its low density and abrasive surface, pumice is used in a wide variety of industrial applications. It is ground up to be used as a component of lightweight cement that was used as far back as Roman times, and as a growing medium for some garden crops. As an abrasive, it is used in cosmetics, pencil erasers, toothpastes and heavy-duty hand cleaners. Actual pumice stones are used as exfoliating beauty products (to remove calluses from the feet, for example) and to remove mineral deposit rings from toilet bowls.

Sources:

http://www.livescience.com

http://web.maths.unsw.edu.au

http://www.int-res.com