When people think of rocks, they think of something hard and heavy: we even say that something that doesn’t float “sinks like a rock.” That’s usually true, but there is one fascinating rock type that simply doesn’t follow the rules: that rock is pumice.
Pumice (the word rhymes with “dumb sis”) is one of the extrusive igneous rocks. “Extrusive” means that, like basalt, it’s a rock deposited on the Earth’s surface by lava that flows from volcanoes and hardens. Extrusive igneous rocks are usually made of mineral crystals so small that you can’t see them with the naked eye. Small grains mean that the rock cooled quickly, as you would expect of a rock that forms at the surface. Instead of small crystals, however, pumice is made of volcanic glass – this suggests that the rock cooled even more quickly than basalt.
This glass-like texture of pumice is not the property that makes this rock type so interesting, though. What surprises people the first time they encounter pumice is that even large chunks of this rock are so light they can float!
Yes, pumice is the rock that floats – but how can that be? If you turn a chunk of this lightweight stuff around in your hands, you will see that it seems to have a spongy or bubble-like appearance. That’s because pumice has what is called a vesicular texture; which just means that a sample is composed of thousands of tiny, air-filled chambers separated by thin walls of volcanic glass. You’ve probably seen a texture like this before when you poured a warm soda into a glass: the foam at the top has the same vesicular texture, except that the bubble walls are liquid soda instead of solid rock.
The analogy of foam atop a glass of soda goes further than just appearance: it also helps explain how this frothy, bubbly rock forms. Bubbles form on your soda when trapped carbon dioxide expands and rises to the surface. This happens because the liquid in the can or bottle is under pressure until it’s opened. When that happens, the carbon dioxide has enough “room” to form bubbles; a process called exsolution.
Soda makers dissolve carbon dioxide in your drink to give it fizz. Magma, the molten rock that causes volcanoes to form, also contains dissolved gas and water vapor. These gases are trapped by both underground pressure and the extreme temperature that keeps the rock molten. When gas-filled molten rock is suddenly exposed to the low temperature and pressure at the surface, the trapped gases act just like the carbon dioxide in your soda: they exsolve to form bubbles. In cases, where the lava rapidly rises to the surface, all those bubbles can whip the molten rock into a foam; the hardened foam is pumice.
Not all magmas can form pumice. Besides requiring enough dissolved gas to form a vesicular texture, the magma also needs to be thick and gooey enough to keep the bubbles trapped. Basaltic lavas are usually too runny to form pumice, so most samples have the same distribution of elements as the extrusive igneous rock rhyolite and its coarse-grained equivalent, granite. There is a vesicular form of basalt, called scoria, but the walls are thicker and not glassy, and specimens will not float.
Besides being interesting because of its low density, pumice is also useful for other purposes. Deposits of pumice are mined and then crushed for use in cleaning products – crushed pumice is the secret ingredient in Lava soap – and in other abrasives. The volcanic glass has a hardness of about 6 on the Mohs Scale, so abrasives made with pumice powder will scratch glass and softer metals.
Compared to other igneous rocks, pumice is fragile and not stable in the surface environment. Most deposits are quite young, and are found in areas that are undergoing rhyolitic volcanism. In North America, this is mainly along the west coast, from Baja California up to the Cascade Range.
