In March 2022, the residents of São Jorge Island in Portugal’s Azores archipelago felt the earth tremble beneath them as thousands of earthquakes rattled the landscape. While the seismic activity suggested a looming disaster, the expected eruption never arrived. Instead, a massive volume of molten rock had staged a “stealth” ascent, stopping just short of the surface in what geologists now call a failed eruption.
A new study published in Nature Communications reveals the sheer scale of this underground event. Researchers found that a magma surge beneath São Jorge Island involved enough molten rock to fill approximately 32,000 Olympic-sized swimming pools. The magma traveled rapidly from depths of more than 20 kilometers, surging upward to within 1.6 kilometers of the surface before stalling.
For the scientific community, the event is a cautionary tale about the unpredictability of volcanic systems. Much of the magma’s journey was nearly silent, producing very little seismic activity until the molten rock had already reached the shallow crust. This lack of early warning signals made it exceptionally hard for authorities to forecast whether the island would face a full-scale eruption.
Dr. Stephen Hicks of UCL Earth Sciences described the event as a stealthy intrusion. He noted that while the magma moved quickly through the crust, much of its journey was silent, which complicated the ability to predict an eruption.
The ‘Highway and Leak’ Mechanism
The research team, comprising experts from the UK, Spain and Portugal, identified a critical geological feature that dictated the magma’s path: the Pico do Carvão Fault Zone. This major fault system acted as a guide, directing the magma upward through the Earth’s crust. However, the fault also played a paradoxical role in preventing the eruption.
Rather than acting solely as a conduit for the magma to reach the surface, the fault zone likely allowed gases and fluids to escape sideways. This “leaking” effect reduced the internal pressure of the magma surge, effectively robbing the system of the drive needed to break through the surface.
Dr. Pablo J. González of the Spanish National Research Council (IPNA-CSIC) explained that the fault acted like both a highway and a leak, facilitating the magma’s rise while simultaneously helping to prevent an eruption.
The interaction between the magma and the fault zone resulted in a specific seismic pattern. Instead of a few large, destructive earthquakes, the movement triggered thousands of smaller quakes clustered along the fault, providing a detailed map of the magma’s underground trajectory.
Mapping a Hidden Threat
To reconstruct the event, scientists utilized a combination of onshore and offshore geophysical data. This included seismometers placed across the Atlantic seafloor and on land, as well as high-precision GPS measurements and satellite observations. These tools allowed the team to track ground deformation in real-time.
Satellite data confirmed that the ground above the volcano rose by approximately 6 centimeters, a clear indicator that a significant volume of magma had entered the shallow crust. This combination of data provided one of the clearest views to date of how underground magma intrusions shape and expand volcanic islands over time.
| Metric | Observation |
|---|---|
| Magma Volume | Equivalent to 32,000 Olympic pools |
| Maximum Ascent | From >20km to 1.6km depth |
| Surface Uplift | Approximately 6 centimeters |
| Seismic Impact | Thousands of small-magnitude earthquakes |
Improving Volcanic Hazard Forecasting
The findings from São Jorge Island have significant implications for how scientists monitor volcanic threats globally. The discovery that large magma intrusions can develop rapidly with minimal seismic warning suggests that current forecasting models may need to be adjusted to account for “silent” movements.
Dr. Ricardo Ramalho of Cardiff University emphasized that the study helped local authorities assess potential threats by demonstrating the necessity of combining onshore and offshore data for accurate detection. By understanding how fault systems influence magma propagation, volcanologists can better predict whether an intrusion will result in a surface eruption or remain trapped underground.
The project was a massive transnational effort, requiring urgent funding from the Natural Environment Research Council (NERC) and cooperation between academic and civil institutions across Portugal, the UK, and Spain. Professor Ana Ferreira of UCL Earth Sciences highlighted this cooperation as essential for accessing the specialized equipment needed to monitor the Atlantic seafloor.
As monitoring technology evolves, the data from the 2022 unrest will serve as a baseline for future activity in the Azores. The next phase of research will likely focus on whether these “failed” eruptions prime the volcano for future events by weakening the crust or if they represent a stabilizing release of pressure.
For residents and authorities in the Azores, official updates and seismic monitoring are maintained by the Centro de Informação e Vigilância Sismovulcânica dos Açores (CIVISA).
Do you think current early-warning systems are enough for volcanic regions? Share your thoughts in the comments or share this story with your network.
