This amazing, floating rock called Pumice is created by a process similar to how foam is created when you open a shaken soda bottle. When you open the lid on a soda bottle, the pressure quickly drops, and all the dissolved CO2 gas rushes to escape, creating soda foam. The same thing happens when superheated, highly pressurized rock violently erupts out of a volcano (usually between 1300 deg F and 2400 deg F). All that pressure of millions of tons of rock is suddenly gone, and the dissolved gasses in the rock rush to escape, creating rock foam. If this rock foam cools down fast enough, the foamy texture stays.
The density of Pumice varies, depending on how it was formed, but the more foamy pieces can be as low as ¼ the density of water, which means Pumice floats very well. On 3 occasions over the last 33 years, underwater volcanic eruptions near Tonga created giant pumice rafts, some as large as 30 km, that floated hundreds of miles all the way to Fiji.
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