Magma Does Not Shatter Once on the Way Out of a Volcano. Microscopes Show It Breaking, Healing and Breaking Again From Deep in the Conduit to the Open Air.

Textbook volcanology treats fragmentation as an event: at some depth in the conduit, gas bubbles expand faster than the surrounding melt can stretch, the magma tears apart, and what was a liquid becomes a spray of ash and pumice hurtling toward the surface. New microscopic evidence from three of Europe's most active volcanoes says it is not an event at all. Magma breaks, then partially heals, then breaks again, over and over, from deep inside the volcano to well outside it. The findings were published Monday in the journal Geology.

Jacopo Taddeucci of Italy's Istituto Nazionale di Geofisica e Vulcanologia led a team that examined pyroclasts, the fragments thrown out during eruptions, from Mount Etna and Stromboli in Italy and from Cumbre Vieja on La Palma in the Canary Islands, whose 2021 eruption destroyed more than 1,000 homes. Using scanning electron microscopy and X-ray microtomography, which builds a three-dimensional picture of a rock's interior without cutting it, the researchers identified and counted microtextures that a single break could not produce: crystals snapped and then re-cemented by fresh melt, suture lines where two once-separate fragments welded together, and older clasts fully enclosed inside younger glass.

"This process is not occurring at a single point in space and time, but it's protracted," Taddeucci said. Each suture and each embedded clast is a record of a moment when the magma had already fragmented, then came back into contact with itself while still hot enough to fuse. The abundance of those textures across all three volcanoes, which differ in size, plumbing and eruptive style, suggests the cycle of breaking and welding is a general feature of basaltic explosive eruptions rather than a quirk of one system.

The practical stakes are grain size. How far a particle travels once it leaves the vent, and therefore whether it lands on the flank of the volcano or in a jet engine 300 kilometers downwind, is set by how big it is. If the final size of pyroclasts is determined by a tug of war between breaking and healing rather than by breaking alone, then the eruption models used to forecast ash dispersal are missing half the physics. Etna made the point this summer, when ash from its eruption closed Catania's international airport for weeks.

Until now, the interior of an erupting conduit has been effectively unobservable. Instruments cannot survive there, and laboratory experiments can reproduce only slices of the process. The preserved microtextures offer a way around that: a fragment that reaches the surface carries, in its crystals and sutures, a timeline of what happened to it on the way up. Taddeucci's group argues that quantifying those textures could be used both to improve computer models of eruptions and to reconstruct the conditions inside past eruptions from the deposits they left behind.

The work adds to a run of recent findings that volcanic conduits are more dynamic than the steady-pipe picture allows. Earlier this year, researchers showed that superheating a magma can suppress crystal growth for hours and may explain why some volcanoes fountain rather than explode. The new study points the other way, toward a process that is messier and more repetitive than expected, and toward the possibility that the ash cloud over an airport was assembled, not simply blown apart.