When Spanish conquistadors first laid eyes on the Masaya volcano’s lava lake in the early 1520s, some believed it must be the entrance to hell. Recent activity from Nicaragua’s busiest volcano has mostly consisted of passive degassing, lava lake formation, and vent-clearing blasts, but Masaya has also produced at least four massively explosive Vesuvian eruptions throughout its history.
That’s unusual for a basaltic shield volcano like Masaya. These volcanoes tend to produce slow, effusive lava flows, but Masaya can transition between effusive eruptions and more dangerous explosive behavior. Today, understanding what drives those transitions is critical to assessing the threat the volcano poses to millions living within 13 miles (21 kilometers) of it.
To that end, researchers recently used satellite data to investigate how the ground around Masaya has changed over a six-year study period. Their findings, published in the journal Geophysical Research Letters, suggest this volcano is fed by not one, but two active magma reservoirs.
“Having a thorough understanding of Masaya’s volcanic activity and its related hazards is critical, as around 2 million people live near Masaya, and the caldera is a national park and popular tourist destination,” Lizzie Johnson, who led the research as an undergraduate in Penn State’s geosciences program and is now pursuing her PhD in Earth sciences at the University of Hawaii at Manoa, said in a statement.
“Although Masaya last produced a major lava flow in 1772, its persistent degassing often produces dangerously high concentrations of sulfur dioxide volcanic gas,” Johnson added.
Uncovering Masaya’s secret
Masaya Volcano is a complex caldera system that includes a nested series of cones and craters with an active vent in the Santiago crater. Its buried magma systems are equally complex, and this has made it difficult for researchers to understand how they work, according to Johnson.
She and her colleagues aimed to improve this understanding to aid eruption forecasting and assess potential risks to the public, particularly residents of nearby Managua. The researchers analyzed ground deformation at Masaya using a time series of observations from the European Space Agency’s Sentinel-1 satellite from January 2018 to February 2024.
The analysis revealed that the ground above the Masaya Central Reservoir—a magma source situated north of the Santiago crater and about 2 miles (3 km) beneath the surface—sank roughly 3 centimeters between 2018 and mid-2022, then rose again between mid-2022 and 2024. Over the course of the entire six-year period, the area around the active Santiago crater consistently sank, hinting at a second, shallower magma reservoir located about 656 feet (200 meters) underground.
“When it’s subsiding, that means that there’s probably magma draining away from the volcano, but once the ground starts uplifting again, that can mean that there’s potentially new magma moving in,” Johnson said. “At the same time, the area directly around the active Santiago crater deflated over the entire study period independently of the transition to inflation of the main reservoir. This indicates that the plumbing system is more complex and could be due to decreasing magma in a second, shallower reservoir.”
The discovery of this second magma reservoir does not directly explain why Masaya produces both effusive and highly explosive eruptions, but it does give scientists a clearer picture of the underground plumbing system they need to understand to answer that question.
Johnson and her colleagues note that their study did not identify any imminent threat to communities surrounding Masaya, though it’s worth noting that the continuous release of noxious volcanic gases does pose a public health threat. It’s been centuries since Masaya last produced a violent eruption, but it could certainly do so again one day.
“Further long-term geodetic and modeling studies will provide a more thorough understanding and allow for effective hazard monitoring and mitigation at Masaya and other lava-lake hosting basaltic volcanoes,” Johnson said.
