When scientists peered into magma from La Palma's 2021 Tajogaite eruption, they were watching molten rock tell the story of how volcanoes decide to rage or simmer. An international team led by The University of Manchester has discovered that superheated magma—lava heated above the temperature at which crystals normally form—can delay the formation of crystals by hours, fundamentally changing whether an eruption will be a dramatic lava fountain or a gentle flow.
This matters because volcanoes are notoriously unpredictable. Two volcanoes with nearly identical chemical compositions can erupt in completely different ways, and scientists have long puzzled over why. The new research, published in Nature Communications, reveals that a magma's thermal history acts like a hidden switch, controlling how quickly crystals form and ultimately determining the character of the eruption itself.
The researchers recreated the conditions inside La Palma's volcano in laboratory experiments using real magma samples and cutting-edge technology. Using synchrotron X-ray microtomography at Diamond Light Source in the UK, they could watch crystallization happen in real time, while longer experiments in Prague allowed them to observe the process over extended periods. What they found was striking: magma that hadn't been superheated began crystallizing within about 20 minutes, but magma exposed to strong superheating delayed crystal formation for more than eight hours.
The mechanism is elegant. When magma is superheated, the extreme temperatures dissolve tiny pre-existing crystal "seeds" that normally act as starting points for new crystals to grow. Superheating also makes the magma's internal structure more uniform, making it harder for new crystals to form at all. This has cascading consequences. When crystallization is delayed, magma can rise rapidly through Earth's crust while remaining relatively fluid, allowing gases to build pressure and creating the conditions for explosive, fountaining eruptions. Conversely, magma that crystallizes earlier becomes more viscous, rises more slowly, and allows gases to escape gradually, producing gentler, effusive behavior.
Dr. Barbara Bonechi, the lead researcher at The University of Manchester, emphasized the importance of their novel observational technique: "The history of crystal and bubble growth can dramatically control how a magma erupts." Using an X-ray transparent pressure vessel they designed specifically for this work, the team could actually observe these processes happening inside the magma, something scientists couldn't do before.
When the researchers incorporated their experimental findings into numerical models simulating magma ascent, the results validated the mechanism. Long crystallization delays predicted rapid, fluid magma rise and dramatic fountaining—exactly what was observed during the Tajogaite eruption.
The implications extend beyond pure science. Dr. Margherita Polacci, a senior volcanologist at The University of Manchester, notes that current volcanic hazard models focus on magma chemistry, gas content, and pressure changes, but typically overlook pre-eruptive thermal history. "This work suggests that pre-eruptive thermal history and crystallization kinetics may also play an important role in controlling magma ascent and eruptive behavior, with implications for volcanic hazard assessment." As volcanoes become increasingly important to monitor in a crowded world, understanding these hidden thermal dynamics could help scientists better forecast how—and how violently—a volcano will erupt.