Convening on caldera collapses
In February 2025, scientists from around the world met in Hilo, on the flanks of the Mauna Loa and Kīlauea volcanoes, for a scientific conference on basaltic caldera collapses. Organized by the USGS scientists and partners and attended by 155 researchers from 15 different countries, the goal of the meeting was to combine observations, discuss open questions, and plan new research.
The California Volcano Monitor is a weekly column written by scientists and collaborators of the California Volcano Observatory. This week's contribution is from Kyle Anderson, volcano geophysicist with the U.S. Geological Survey.
Eruptions aren't just explosive – they can reshape a landscape in dramatic ways. If a volcanic eruption is large enough, the ground above a volcano’s magma reservoir can collapse downwards, creating a depression called a caldera. These depressions can be multiple miles (kilometers) in diameter and hundreds of feet (meters) deep. Around the world, caldera collapses have had huge impacts on nearby communities, including long-term evacuations and the destruction of hundreds of homes. That's why scientists want to better understand how and why calderas form.
Calderas can be found all over the world. Famous examples include Yellowstone (Wyoming), Campi Flegrei (Italy), and Long Valley (California). While most of these, like the 765,000-year-old Long Valley Caldera, are very old, in the last half century scientists have observed the creation or growth of a handful of calderas in places like Miyakejima (Japan), Piton de la Fournaise (La Réunion, France), and Kīlauea (Hawaii). Most of the newer calderas formed at basaltic volcanoes, which tend to erupt more fluid and less explosive lavas. Some, as at Kīlauea volcano in 2018, were very closely documented using modern monitoring networks, satellite, photo, video, and field observations. These data are now inspiring new efforts to understand calderas and their hazards, in order to help local communities.
At the 2025 meeting, scientists found that comparing historic basaltic caldera collapses reveals intriguing similarities. All were triggered by magma traveling underground to points lower on volcano flanks, effectively draining magma from storage reservoirs at higher elevations. Most strikingly, collapse at most of these volcanoes took place in episodes where piston-like blocks of reservoir roof rock subsided bit by bit into the reservoir, pushing magma out and sustaining eruptions elsewhere on the volcano. These cycles sometimes produced surges of lava tens of kilometers (dozens of miles) away from the collapsing calderas!
Despite their hazardous impacts, caldera collapses offer scientists unique insights into the plumbing of volcanoes. We now have a better understanding of how magma is stored beneath calderas and in flank rift zones, how volcanic summits and flanks can be closely coupled, and how caldera (ring) faults slip during collapse. Yet, we still don't know why some eruptions lead to caldera collapse and others do not, why these eruptions end, and how lessons from basaltic caldera collapses can be applied to more explosive settings like Long Valley. Building on the lessons of observed collapses and momentum formed during the conference, the volcanology community is now working to address these questions. If you would like to read more about these efforts, please see the article "Lessons and Lingering Questions from Collapsing Basaltic Calderas."