Thousands of earthquakes affecting Portugal’s São Jorge Island in the Azores in March 2022 were triggered by a vast sheet of magma (molten rock) rising from more than 20km below Earth’s surface and stalling just 1.6km beneath the island, finds a new study led by UCL (University College London) researchers.
Much of this ascent occurred with little seismic activity, with most earthquakes occurring after the magma stopped ascending. The magma rose over just a few days – there was enough of it to fill 32,000 Olympic-sized swimming pools, the study suggested.
Lead author Dr Stephen Hicks, based at UCL Earth Sciences, said: “This was a stealthy intrusion. Magma moved quickly through the crust, but much of its journey was silent, making it difficult to forecast whether an eruption would occur.”
For the study, published in the journal Nature Communications , an international team reconstructed the detailed underground movement of magma using seismometers on land and on the Atlantic seafloor to precisely map where earthquakes were occurring, as well as data from satellites and GPS to see how the ground moved at the time.
Satellite observations showed that the volcano's surface rose by 6 cm, confirming that magma had entered the shallow crust. However, the intrusion stalled before reaching the surface, resulting in what scientists define as a “failed eruption”. Such intrusions help to grow islands and this study’s unprecedented sharp earthquake maps show how this happens.
The magma rose through one of the island’s main fault systems, the Pico do Carvão Fault Zone. By studying geological traces left by ancient earthquakes, scientists had previously found that this fault system has produced large earthquakes in the past. But instead of a single large earthquake, the unrest from rising magma produced many small earthquakes clustered along this fault.
The team concluded that the fault helped guide magma upward, and may also have allowed gases and fluids to escape sideways, lowering pressure in the magma and helping halt its ascent.
Lead author Dr Pablo J. González, from the Spanish National Research Council (IPNA-CSIC) in Tenerife, said: “The fault acted like both a highway and a leak. It helped magma rise, but may also have prevented an eruption.”
The findings show that large magma intrusions can occur rapidly and with limited warning, and that major geological faults can strongly influence whether magma erupts or stalls underground, key insights for improving volcanic hazard forecasting.
Dr Ricardo Ramalho, a co-author from Cardiff University, said: “This study supported local authorities in assessing a potential volcanic threat, highlighting the value of combining onshore and offshore geophysical data for accurate detection and localisation of seismic events and ground deformation.”
Professor Ana Ferreira, a co-author from UCL Earth Sciences, said: “Securing urgent NERC funding to access equipment from its Geophysical Equipment Facility (GEF), alongside additional support from Portugal, was a tremendous collective effort and a clear example of transnational cooperation between academic and civil institutions in Portugal, the UK, and Spain.”
The work was funded by research grants from the Natural Environment Research Council (NERC; UK), the European Research Council (ERC), Fundação para a Ciência e a Tecnologia (FCT; Portugal), Agencia Estatal de Investigación (Spain), and the Regional Government of the Azores, with field assistance for the offshore deployment provided by the Portuguese Navy (Marinha Portuguesa). Geophysical equipment was provided by NERC’s Geophysical Equipment Facility (GEF).
The following institutions were involved in the work: UCL, Spanish National Research Council (IPNA-CSIC), Cardiff University, University of Manchester, Universidade de Lisboa (Portugal), Instituto Politécnico de Lisboa (Portugal), University of Évora (Portugal), University of Beira Interior (Portugal), Centro de Informação e Vigilância Sismovulcânica dos Açores (CIVISA; Portugal), Research Institute for Volcanology and Risk Assessment (IVAR), University of the Azores (UAc), University of Algarve (Portugal), Instituto Português do Mar e da Atmosfera (IPMA; Portugal), AIR Centre (Portugal), C4G (Portugal).
Nature Communications