Pumice and Scoria: The Frothy Rocks of Explosive Eruptions
Among the strangest products of volcanoes are rocks so full of holes that they can float on water. Pumice and its darker cousin scoria are frozen volcanic froth, formed when gas-charged magma is blasted apart in an eruption and the bubbles are preserved as the rock solidifies in mid-air. Light, abrasive, and full of holes, these rocks are a direct record of the gas that drives explosive volcanism, and they have a surprising range of uses.
How frothy rocks form
Pumice and scoria form when magma rich in dissolved gas erupts explosively. As the magma rises and the pressure drops, the dissolved gas comes out of solution and forms bubbles, just as bubbles form when a bottle of fizzy drink is opened. If the lava is torn apart and cools quickly enough, these bubbles are frozen in place, leaving a rock riddled with holes, or vesicles.
Pumice: light enough to float
Pumice is a pale, frothy volcanic rock, usually formed from silica-rich magma. It is so full of gas bubbles that it has a very low density, and many pieces of pumice can float on water. Great rafts of floating pumice have been produced by submarine and island eruptions, drifting across oceans for months and even carrying marine life to distant shores.
Scoria: the darker cousin
Scoria, also called cinder, is a darker, denser frothy rock, usually formed from basaltic magma. Like pumice, it is full of holes, but its bubbles are typically larger and fewer, and it does not usually float. Scoria is the material that builds cinder cones, piling up around vents during mildly explosive eruptions to form the steep, symmetrical cones found in volcanic fields worldwide.
A record of volcanic gas
The holes in pumice and scoria are a direct record of the gas dissolved in the magma. By studying the size, shape, and abundance of these vesicles, scientists can learn how much gas the magma contained and how it behaved as it erupted. These frothy rocks are thus valuable clues to the explosive processes that produced them.
The uses of pumice
Pumice has been used by people for thousands of years. Its abrasive, lightweight nature makes it ideal for scrubbing and polishing, and it is still used in cosmetics, cleaning products, and as an abrasive. In the building trade, lightweight pumice is used in concrete and as an aggregate, while in horticulture it improves soil drainage. Few volcanic rocks have so many everyday uses.
Pumice rafts and ocean travel
One of the most remarkable phenomena involving pumice is the pumice raft, a vast mass of floating pumice produced by an eruption. These rafts can cover huge areas of ocean and drift for thousands of kilometres. They can even transport marine organisms across oceans, acting as floating habitats and playing a role in the dispersal of species to new shores.
Frothy rocks and explosive volcanoes
The presence of pumice and scoria in a volcanic deposit is a clear sign of explosive eruption. The amount and type of these frothy rocks help geologists reconstruct the violence and style of past eruptions, distinguishing the explosive activity that produces them from the quieter, effusive eruptions that produce solid lava flows.
Explore on the map
Pumice and scoria are found at explosive volcanoes and cinder cones around the world, from the volcanic islands of the Pacific to the cinder cone fields of many continents. Explore these volcanoes on the interactive map — filter by type to find the cones and explosive volcanoes that produce these remarkable frothy rocks.