The Difference between Pumice and Cinder

Volcanic rocks are useful in many ways, but it can be difficult to tell them apart. Pumice, for instance, is that light-colored rock that’s full of holes and floats on water. Scoria, the geologist’s name for cinder rock, is dark-colored, also full of holes, and makes great lava rocks in a barbecue grill. They both look so much alike – are there really any differences? There certainly are.

♦ Different uses of scoria and pumice

One big difference is that the US Geology Survey counts pumice as a mineral resource, but not scoria. You might not have known that America even has a national pumice resource, but it exists and the Survey monitors it. They also keep track of the international pumice trade and publish yearly commodity sheets (link is to PDF file) and yearbook reports (link is to PDF file) on pumice.

Pumice is very important economically not only for its service in beauty tools and as the “lava” in Lava® soap. It is also the stone used to wash jeans. Gardeners mix it up in soil to improve aeration and drainage. Powdered, it is used to make construction blocks, concrete and aggregate. It works well in filters, is a good absorbent, and has many more uses.

Scoria contains a lot of iron and so can be quite dark or even rusty in appearance. It comes in many shades, some attractive enough to use in landscaping. It is the lava rock in your barbecue grill, where it holds in heat and absorbs grease and liquids. Despite its holes, this volcanic rock is also sturdy enough to use for road bases or as ballast to support railroad ties. Overall, though, scoria has nowhere near the economic importance of pumice.

♦ Does it float?

Both pumice and scoria form as molten lava cools and hardens, but scoria generally comes from the iron-rich “red lava” type of volcano like Kilauea in Hawaii, where the big cinder cone of Pu’u O’o, like all cinder cones, is made of scoria. Scoria doesn’t weigh as much as rocks of a similar size, in spite of its high iron content. This is because it has a lot of air pockets that once were gas bubbles that were trapped when the lava “froze,” giving it a vesicular texture. However, these pockets have pretty thick walls, and so scoria is usually denser than water and will sink.

Pumice, on the other hand, forms during very explosive eruptions of silica-rich “gray lava.” Volcanoes with this type of lava, like Mount St. Helens, are often found near subduction zones. When they erupt, some of the lava cools so quickly that the gas inside it bubbles out all at once, leaving behind something that looks more like frozen froth than a true rock. With so many air pockets, and relatively thin walls between them, most pumice is less dense than water and can float for a few months, until all the air spaces have filled with water; only then will pumice sink.

Pumice and scoria are very different materials, even though they both come from lava and bear a strong resemblance to each other. To tell the difference between the two, look at how each is used or simply toss a piece into water and see if it floats. While humans have many more uses for pumice than for scoria, their lives would be much less pleasant if either one of these valuable volcanic rocks ever vanished.