Morro Grande, caldera on the island of Sao Jorge, in the Azores
A series of earthquakes on the island of São Jorge, in the Azores, revealed magma accumulated just a mile deep. According to a new study, underground faults prevented the eruption. It was a near-eruption, which never materialized.
In March 2022, the small Azorean island of São Jorge was suddenly hit by an earthquake, after almost 60 years without significant seismic activity.
The tremors continued for months, with micro-earthquakes persisting throughout two yearsbut what was happening deep underground on the Azorean island continued to be a mystery.
In a new study, an international team of researchers believes they have clarified the chain of events that occurred at the time.
O , published on Thursday in the magazine Nature Communicationsdescribes how magma burst towards the surface in just two days, in a volume equivalent to approximately 32,000 Olympic-sized swimming pools.
By analyzing seismic data collected on land and under the sea, along with satellite images, researchers concluded that magma rose from more than 19 km deep, ending up stagnating at about 1.6 km below the surface.
In short, it was a near-eruption that never came to fruition, say the study authors.
“This was a stealth intrusion,” he said. Stephen Hicksresearcher at University College London and lead author of the study, in a statement published on .
“Magma moved quickly through the crustbut much of its route was silent, which made it difficult to predict about whether or not an eruption would occur”, adds the researcher.
Underground accumulation of magma made the island grow
When magma pushes upward through layers of the Earth’s crust, it often causes volcanic eruptions. But not always. Sometimes, stagnates at different depths, without being able to break the surface, explains .
That’s exactly what happened under Saint George. The Azores are located along the Terceira Riftwhere the Eurasian and African tectonic plates slowly move away from each other, making the relatively common seismic activity.
The island itself, only about 56 km long and about 6.4 km widehas a history of powerful earthquakesincluding an event of magnitude 7.5 in 1757, one of the biggest earthquakes recorded in the Azores.
After the 2022 earthquakes began, researchers reconstructed underground activity using a combination of seismic records, GPS measurements, and satellite data.
These instruments revealed that the ground had risen approximately 6.35 cm — strong evidence that magma had penetrated the crust superficial. On long time scales, this type of uplift it’s actually one of the ways islands gain altitude.
The study authors further found that the magma moved along an important fault system that runs through the island, known as the Pico do Carvão Fault.
Faults can guide magma and reduce pressure
Faults and fractures in the Earth’s crust can act as passage ways for rising magma, although scientists continue to work to fully understand this relationship. In the case of São Jorge, the fault system appears to have played a surprisingly complex role.
Previous studies had already demonstrated that this fault zone was capable of produce large earthquakes in the past. During the 2022 seismic crisis, instead of one large earthquake, researchers observed numerous smaller magnitude earthquakes, clustered along the faulttriggered by the movement of magma at depth.
According to researchers, the failure acted as a kind of guidefacilitating the eruption of magma. At the same time, it may have allowed gases and fluids to escape laterally, reducing the pressure inside the magma and ultimately preventing the eruption.
“The fault worked simultaneously like a highway and like an escape“explains Pablo Gonzalezresearcher at Spain’s Superior Council for Scientific Investigations and the study’s co-author, in the statement. “It helped the magma rise, but it may also have prevented an eruption.”
Understanding how magma moves below the surface is critical to interpreting volcanic unrest and predict what might happen next.
The Azores are a rare natural laboratorywhere active magmatic systems intersect with large earthquake-generating faults, which makes it easier for scientists to study the real-time interaction between tectonic structures and molten rock.
Observations of the São Jorge events suggest that massive magma eruptions can develop quickly and without prior warningbut they also reveal how geological faults influence whether this magma reaches the surface or is retained in depth.
Both conclusions are crucial to making volcanic activity more predictable.
“This study supported local authorities in assessing a potential volcanic threat”, explains Ricardo Ramalhoresearcher at Cardiff University and also co-author of the study, in the statement.
“The study highlights the value of combining geophysical data on land and at sea for precise detection and location of seismic events and ground deformation”, concludes the Portuguese researcher
