Pumice is a fine-grained volcanic rock. It is very light grey to medium grey in colour. It contains a lot of empty gas bubbles, so it is very light and looks rather like a sponge. Sometimes pumice is so light that it will float on water.
How is it formed?
Pumice is formed when volcanoes erupt explosively. It comes from the same kind of magma which would form granite or rhyolite, that is, a magma that contains lots of silica (quartz).
Magma with lots of silica is usually thick and sticky. Some of the gases which cause the explosive eruption get trapped in the magma and form gas bubbles. These are preserved as holes when the rock cools as it comes flying out of the volcano.
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