Compared to the violent explosions of Mount Vesuvius or Mount St. Helens, Hawaiian volcanic eruptions are relatively calm, characterized by flowing rivers and fountains of lava. This “Hawaiian style” of eruption is made possible by fluid, low-viscosity magma with low gas content and high temperatures.
However, researchers have now discovered that even low-viscosity magma sometimes behaves less like stretchy taffy and more like brittle glass that shatters into fine particles (pyroclasts). The fine-grained ash produced by brittle fragmentation can get into and damage lungs, jet engines, water supplies, and other important infrastructure.
“Smaller pyroclasts that spread to a broader area can be a potential disaster,” said Atsuko Namiki, an associate professor in the Department of Earth and Environmental Sciences at Nagoya University, Japan. “In this work, we aimed to reveal the fragmentation process—the generation of small pyroclasts from magma—by analyzing the detailed structure of pyroclasts.” The results were published in the journal Nature Geoscience.
Using x-ray microtomography at Advanced Light Source (ALS) Beamline 8.3.2, Namiki and her collaborators from the University of Hawaii, UC Berkeley, and the U.S. Geological Survey (USGS) studied the internal structures of highly porous pyroclastic rocks that erupted in May 2018 from what was then called “fissure 8” (now officially named Ahuʻailāʻau) on the lower East Rift Zone of Kīlauea Volcano in Hawai’i.
The analysis showed that the pyroclasts had a honeycomb structure—vestiges of gas bubbles trapped in the magma—covered by a dense crust, or rind. Based on these and other observations, the researchers proposed a mechanism in which rapidly expanding volcanic gas efficiently cools pyroclast surfaces, while the expansion of gas trapped inside promotes brittle fragmentation. Repeated over time, the process generates progressively smaller fragments, leading to more widespread pyroclast dispersal and a more distributed hazard for local communities.
Brittle fragmentation is a key factor in determining eruptive style. This work indicates that, for low-viscosity magma fragments within a rising fountain, brittle fragmentation may be a widespread phenomenon, and its incorporation into computer models could improve simulations of eruption plumes as well as highly destructive pyroclastic flows.
A. Namiki, M.R. Patrick, M. Manga, and B.F. Houghton, “Brittle fragmentation by rapid gas separation in a Hawaiian fountain,” Nat. Geosci. 14, 242 (2021), doi:10.1038/s41561-021-00709-0.