Why do volcanic rocks float on water?

In this case the CO2 that is dissolved in the liquid comes bubbling out. Now in the case of magma since it solidifies at 900 degree C, the bubbles become frozen when the temp drops and we have volcanic foam. Some volcanic rocks that are very vesicular (called PUMICE) actually float on tap water. read more

Some volcanic rocks that are very vesicular (called PUMICE) actually float on tap water. If you visit a volcanic island (like Barren Island in the Andamans, or Anak Krakatau in Indonesia), you can see several acres of pumice around the shore, remnants of past eruptions. read more

(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. read more