São Jorge Island as viewed by drone. Part of the island rose by 6cm, confirming that magma had entered the shallow crust. Credit: Ricardo Ramalho. CC BY-SA.
A Portuguese island in the Atlantic Ocean narrowly escaped a so-called ‘stealthy’ eruption as magma rose close to its surface.
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.
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 of a new study, 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.”
‘Failed eruption’
For the study, 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 6cm, 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.”
Hazard forecasting
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.”
The study is published in Nature Communications.
