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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.
Get closer to Geology
Discover the unique landscape of Great Britain and Ireland with our interactive Geology Toolkit, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips.
The Geology Toolkit helps explain some of the UK's familiar landscapes
One of the most important maps of the UK ever made – described as the ‘Magna Carta of geology’ – is to go on permanent public display in Cambridge after being restored to its former glory.
Study a free course
In this free course, An introduction to geology, you will explore basic geological processes, focusing on how, where and why different rocks and natural resources form across the Earth.
In this free course, An introduction to minerals and rocks under the microscope, you will experience the study of minerals using a polarising microscope. While the study of minerals can involve electron or ion beam chemical analysis, the polarising microscope remains the prime tool for the study of rock thin sections and is the foundation of ...
Have you ever wondered how scientists analyse the environment? This free course, Practising science: Reading the rocks and ecology, introduces you to the techniques used by science students at residential schools. You will learn how to determine where rocks have come from and how they were made. You will also examine the processes involved in ...