From Volcanoes To Bathtubs: On the Many Uses and Forms of Pumice

You have been sharing your bath with a very dangerous material. Pumice, the bubbly, lighter-than-water rock used to attack hard skin on your feet, was once a foaming mass of gas and lava. Like an agitated champagne bottle, magma can carry great quantities of dissolved gases. They come out of solution violently once pressure lowers. Like a cork being popped, that pressure drops as the magma approaches the surface of the Earth, causing the release of a huge rush of gas around the vent of the volcano. Lava explodes out as molten froth, solidifying into pumice as it flies through the air.

Explosive eruptions can spew out such quantities of material at speed that they cause craters to collapse. Burning hot clouds of pumice, gas, and ash cascade across the surrounding landscape in a pyroclastic flow (a phenomenon helpfully explained by the rapper Ice Cube in the track “What is a Pyroclastic Flow?”—it’s what happens when volcanoes blow).

Borne like a hovercraft on a low-friction cloud of gas, a pyroclastic flow can reach speeds of over sixty miles per hour, and temperatures as high as 1300° F. The largest volcanic eruption in recorded history—at Mount Tambora, Indonesia, in 1815—spewed a pyroclastic flow that continued to travel twenty-five miles out across the Flores Sea, causing the water to boil. “Pyroclastic” is a coinage from a Greek root meaning fire and fragmented rocks. If the flow glows, it’s known by the French term nuée ardente—burning cloud.

Not all volcanoes behave this way. Effusive eruptions caused by less viscous magma with a lower gas content instead produce lava flows like those that created Hawaii. Volcanologists use an index grading eruptions from the most explosive to the most effusive (a term that sounds wonderfully as though the volcano is trying to ingratiate itself).

The most powerful explosive eruptions are categorized as “Plinian” after Gaius Plinius Caecilius Secundus—the Roman magistrate known as Pliny the Younger—whose description of Mount Vesuvius in 79 C.E. is our earliest detailed account of a volcano in action. From the port of Misenum on the Bay of Naples, Pliny observed a cloud of unusual size “more like an umbrella pine than any other tree, because it rose high up in a kind of trunk and then divided into branches.” The cloud sometimes “looked light colored, sometimes… mottled and dirty with the earth and ash it had carried up.” His uncle, the natural philosopher known as Pliny the Elder, ordered a boat so that he could study the phenomenon at close hand. The trip quickly turned into a rescue attempt.

The elder Pliny captained galleys in the direction of the flaming eruption and inhabitants whose only hope of escape was now by water. “The ash already falling became hotter and thicker as the ships approached the coast and it was soon superseded by pumice and blackened burned stones shattered by the fire.” Disembarking, he calmed his friends and went to bed, but was woken as the courtyard rapidly filled with hot ash and the buildings shook. “Outside, there was the danger from the falling pumice, although it was only light ad porous. After weighing up the risks, they… tied pillows over their heads with cloths for protection.” Shortly after walking down to the shore the elder Pliny collapsed and died, asphyxiated, his nephew presumed, by heat and sulfuric fumes.

Death came faster to those cities southeast of the volcano, as winds showered them with hot ash and pumice for eighteen hours. Herculaneum had been destroyed in a nuée ardente the previous evening. Vesuvius produced a sequence of six pyroclastic surges, of increasing power. The fourth reached the city of Pompeii, covering it in hot ash. The inhabitants were subjected to a momentary heat surge in excess of 400°F—hot enough to melt silverware but not glass—causing instant death.

Like obsidian and Pele’s hair, pumice is a kind of glass, formed from silica-rich lavas. Research at Pompeii led by Sissel Tolaas is exploring what the pumice might reveal about unseen aspects of the eruption, treating the foamy rocks as time capsules—a moment frozen in stone. Tolaas is, among other things, a scientist of smell. She has previously worked with MIT to identifying the smell of fear in men’s sweat, and developed an odor for a women’s organization in India, which is used to deter sexual assault. “Stone also has memory,” says Tolaas. “There are a lot of smells that are unique to this site, and if you don’t preserve them, one way or another, soon there will be none at all.”

Traces remain of the gas trapped during the rapid hardening of frothing magma. Studying the air pockets, Tolaas has also found deposits of powdery ash captured in the moment of the explosion and preserved for almost 2,000 years. Analyzing molecules harvested from the pumice, she has found traces of organic material, but most of the compounds she has discovered she has never smelled before.

They do not correspond to any of the 10,000 smells in her library, which includes samples gathered at mountains in Iceland, Norway, and South Africa. It is, she thinks, not a smell from the surface of the Earth, but from somewhere deeper: “It’s like the interior of the earth breathing. One nanosecond of that breath, captured in a bubble.” It’s something no human would ordinarily be able to smell and survive: Vesuvius, erupting.

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Excerpted from Lapidarium: The Secret Lives of Stones by Hettie Judah. Copyright © 2023. Published by Penguin Books, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC.