Along the western flanks of the Central Andes, million-year periods of quietude were followed by moments of enormous activity. Magma lingered deep in the crust for a million years or longer, allowing layers of granite to form above it. The magma then surged upward over just a few decades and erupted as a supervolcano—an act that played out four times in 3 million years.
That scenario, published in Nature and developed through a study of crystals embedded in the layers of volcanic rock, provides a revised model of how supervolcanoes erupt.
“Prior studies of supervolcanoes favored a model where crystals were stored for hundreds of thousands of years in a magma mush—a material that contains melt, yet is too rich in crystals to erupt,” said Catherine Annen, a geologist at the Institute of Geophysics of the Czech Academy of Sciences who was not involved in the new study but has previously collaborated with some of its scientists. Instead, the study showed that crystals are stored in solid rock and mix with the rising magma shortly before eruption.
“We wanted to understand the timescales and dynamics for the assembly of magma reservoirs prior to supereruptions,” said Stephen Sparks, a study author and professor emeritus at the University of Bristol School of Earth Sciences. “Our study is part of an emerging understanding of the conditions that lead to supereruptions, which may ultimately help find sites with high risk of future supereruptions.”
Volcanic Crystals in Cold Storage
Sparks and his coauthors examined material extracted from oil company drill holes in the Oxaya Formation in northern Chile. The region includes four previously known supervolcanoes (defined as volcanoes with eruptions that deposit at least 1,000 cubic kilometers of rock): Cardones, Milonos, Oxaya, and Poconchile. They erupted at roughly million-year intervals, beginning with Poconchile about 22.6 million years ago.
In the laboratory at the Scottish Universities Environmental Research Centre, the team isolated crystals of zircon and sanidine, a potassium-rich form of feldspar, from a layer of volcanic rock known as ignimbrite. Among other techniques, researchers used a laser to melt sanidine crystals that averaged about 2 millimeters in diameter, extracted argon from the resulting gas, then compared the ratio of two isotopes, argon-40 and argon-39.
“The headline is, we got argon-argon ages from these samples, spanning several million years,” said Marissa Tremblay, a noble gas geochemist at Purdue University who participated in the analysis. “That tells us a lot about how these magmatic systems that lead to supervolcanoes had to evolve.”
In particular, the dating techniques revealed that the crystals, which formed in magma, spent a long time in “cold storage,” at temperatures no higher than 470°C. “We interpret that to mean they were part of a pluton,” a mass of granite in the middle to upper layer of the crust that built up over a period of a million years or longer, Tremblay said. “But at some point [the crystals] had to be reentrained with a magma and then quickly erupt with that magma—over a timescale of years to decades.”
“The study of these crystals shows that the magma chamber that directly feeds the eruption had a short lifetime of no more than a few centuries, so that large volumes of magma must have been emplaced very rapidly in the upper crust,” said Annen.
Recipe for a Supervolcano Eruption
The study team verified the findings with new mathematical models, then devised a scenario for the eruption of a supervolcano.
The zircon and sanidine crystals began forming more than 4 million years before the initial Poconchile eruption. They were emplaced at fairly shallow depths, where they cooled with the surrounding rock to form granite, supplemented by drizzles of fresh magma from a chamber far below.
Instabilities in the overlying rock led to the formation of wide dikes that allowed the magma to flow upward to form a chamber near the surface, melting and incorporating some of the granite along the way. “We propose rapid assembly of the magma bodies in the upper crust, which destabilized plutonic roof rocks just prior to and during the supereruptions,” said Sparks.
The shallow magma chamber formed quickly, then erupted, creating a wide, relatively shallow depression known as a caldera. “A supervolcano doesn’t have the typical cone shape like you’d see if you asked a kid to draw a volcano,” explained Tremblay. “A supervolcano is so catastrophic that it leaves a big hole in the ground.”
After each eruption, with the shallow chamber empty, the process began again, with magma creating a new pluton, then quickly forming a new shallow chamber and erupting. The sequence ended with a final eruption 19.6 million years ago.
No one knows when to expect the next big hole in the ground. Supervolcanoes appear to occur once every 20,000 or so years; the most recent, Taupō in New Zealand, occurred about 27,000 years ago. The new study’s findings could help guide geologists as they hunt for the likely sites of these future catastrophes.
—Damond Benningfield, Science Writer