A sample of pyroxene, which appears sharp and jagged

These crystal lava shards are ‘four dimensional videos’ of a volcano’s underworld

The 2021 volcanic blast in the Canary Islands entombed more than 2,800 buildings in lava and ash. Now scientists are searching for clues to understand what made La Palma’s first eruption in 50 years so powerful.

Crystallized bits of lava from the eruption in La Palma, such as this microscopic sample of pyroxene, contain details about what made the blast so destructive—and possible clues about how to help predict what types of eruptions will happen in the future. These samples are part of a research partnership between the University Complutense of Madrid and the Geological and Mining Institute of Spain.

For the Spanish island of La Palma—a natural castle built above the Atlantic Ocean by ancient volcanic eruptions—2021 marked 50 years without a mote of lava seeing the surface. But everything changed later that year, on September 19, when a pockmarked volcanic ridge named Cumbre Vieja cracked open, oozing molten rock across the island and putting many of La Palma’s 86,000 residents at risk.

By the time the last embers had gone out 85 days later, more than 2,800 buildings—including many homes—and 864 acres of farmland had been entombed under lithified lava. Thanks to the concerted efforts of scientists and emergency response teams, people were evacuated in time and no one was killed, but thousands of lives were indelibly and gravely changed by one of the most destructive European eruptions of the past century.

Within the lava, and inside rocky shards jettisoned from the volcano’s multiple breaches, hide crystals that preserve the chemical material of the magmatic source deep below. By looking at these geometric, iridescent prisms, “you get a snapshot of the interior of the Earth that otherwise won’t ever be possible,” says Esteban Gazel, a geologist at Cornell University.

An eruption’s crystals can reveal where the underground caches of magma are stored—and what type of magma it is, from the viscous, explosion-prone variety to a runnier type of liquid rock. By matching those depths with seismic data from earthquakes caused by moving magma, scientists can forecast not just that an eruption may be approaching, but what kind of eruption it could be. “It gives us a better idea of [which] system might be reactivating,” says Kyle Dayton, a geoscientist at Cornell University.

Hoping to fully decipher the 2021 La Palma eruption, scientists will spend years forensically examining the crystalline capsules within its hardened lava. Contained within are “four dimensional videos” of a volcano’s underworld, says Matthew Pankhurst, a geoscientist at the Institute of Technology and Renewable Energy on Tenerife—its past form, its present state, and even its future incarnations.

Urban volcanology

The volcanoes of La Palma—an island sitting atop a fountain of unfathomably hot rock rising from the depths of Earth’s mantle—are all idiosyncratic. Cumbre Vieja, meaning “old summit” in Spanish, is no exception. It isn’t a typical, cone-shaped mountain, but a ridge capable of squeezing or blasting out molten rock from any number of points along its spine and flanks.

The lava painting over La Palma was mostly of the ‘A’ā (pronounced ah-ah) type: less like flowing rivers, more like crumbling, rubbly serpents of melted stone that crawl and tumble forth. It “sounded like broken glass as it advanced,” says Pablo J. González, a physical volcanologist at the Institute of Natural Products and Agrobiology on Tenerife.

That’s because it was the sound of shattering glass. Those flows were filled with natural glass, the sort that forms as lava speedily cools when exposed to air. “As the lava advances, it breaks itself and those cracks and snaps produce loud audible sounds,” says González.

A new eruption, while hazardous, allows volcanologists to better understand this complex system. Scooping up fresh lava samples is a sufficiently surreal activity while studying eruptions far from population centers. But it took on a whole new dimension on La Palma, where after a half-century slumber, the volcano angrily awoke, pouring lava directly into populated neighborhoods.

“It’s very dramatic and awe-inspiring to see our infrastructure totaled by nature,” says Pankhurst. He describes this sort of work as “urban volcanology,” the practice of monitoring an erupting volcano and collecting red-hot samples of lava, ash, and noxious gas right in the middle of villages and towns, with molten rock slithering through the streets.

Different teams had various ways to sample the lava. Some used shovels or pincer-like metal claws, waiting for part of the lava flow to break off and roll toward them before scooping up the incandescent rock. Others, like Pankhurst, used an ersatz sampling arm: a lengthy instrument made from pronged poles originally designed to prop up bunches of bananas on trees. “It looked like some ninja fighting instrument,” he says.

No matter how it was obtained, this molten matter was subsequently dumped into a water-filled steel bucket, quenching it quickly to try and preserve its subterranean chemistry.

Lava wasn’t the researchers’ only target. Pyroclastic matter, the stuff that a volcano jettisons from its vents or fissures, is equally coveted. The 2021 eruption primarily squeezed out gloopy, rubbly lava, but also occasionally mixed in a few explosions, sending ash and larger lava bombs into the air.

Both Gazel and Dayton hoped to collect as much of this debris as possible during the eruption. They found themselves frequently covered in it as small blasts flung thousands of volcanic fragments skyward. “It was definitely a unique experience to be in a rock rain,” says Dayton.

Throughout those eruptive months, the ground frequently jolted and shifted beneath everyone’s feet. “Every time there were earthquakes, there was magma rising,” says Gazel.

Clues in the crystals

Eruptions change in real time. “In any eruption, it’s important to have continuous sampling of the material—both lava flows and pyroclastic material,” says Stavros Meletlidis, a volcanologist at Spain’s National Geographic Institute.

By doing so, scientists can track even the most mercurial eruption, allowing them to update the emergency services on the fly. Perhaps the lava is starting to become runnier and faster flowing. Maybe it’s becoming more viscous—something capable of trapping more gas, accumulating pressure, and turning explosive.

The crystals within also reveal parts of the lava’s origin story. The chemistry, appearance, quantity, and textures tip scientists off to the types of rocks that originally melted to produce such dangerous lava.

But some of the most vital clues are hidden inside the crystals themselves. “These minerals work as pressure vessels,” says Gazel. They can imprison fluids from great depths (such as carbon dioxide and water), and scientists can extract or otherwise examine these materials.

The crystals don’t just identify some of the key ingredients that went into cooking up that batch of eruptible magma. They also act as barometers, telling scientists what pressures they were once subjected to—and pressure is connected to depth. That means these inclusions can pinpoint the locations of a volcano’s magmatic kitchens, allowing scientists to connect the eruptive chaos at the surface with its origins far below.

Some of the pockets of magma feeding the 2021 eruption were found dozens of miles deep, suggesting the eruption was drawing fuel from the mantle below—an inscrutable part of the planet that is difficult to study.

Working with the fluid-filled crystals can be troublesome work, however. “Sometimes they explode,” says Gazel.

And it isn’t always easy to locate fluid inclusions. They can be one-tenth as wide as a strand of hair, not much bigger than a red blood cell. “You have to get lucky on this treasure hunt,” says Dayton.

Fires of the future

Petrology, or the science of the origin and composition of rocks, is often conducted post-eruption, providing a retrospective understanding of the volcano. But this research is starting to become more prospective.

Volcanoes can now be monitored in real time myriad ways. Listening to the sounds of volcanic tremors, for example, can reveal movements below. Certain sorts of seismic waves are made by the rock-breaking migration of magma, and a seismic cacophony may suggest that the volcano is attempting to erupt.

Using a volcano’s seismic rumblings to see through solid rock is comparable to a doctor giving a patient a CT scan. But this health check can be augmented with some volcanological bloodwork.

To some, studying these crystals is like learning how to speak a new language—one that can be used to not only interrogate volcanoes about past or ongoing eruptions, but to help divine future paroxysms.

The crystals are like “Rosetta stones,” says Pankhurst. With them, you can decipher each type of eruption and write books about them. And when a new outburst begins, scientists can use these tomes to forecast what is likely to happen. “We can say: We’ve seen this before,” Pankhurst says, and emergency responders can act accordingly.

By studying enough eruptions, researchers can create an abundance of life-saving books. That is the power of petrology; with it, says Pankhurst, “we can build libraries.”

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