Volcano near which 2 million people live tracked by scientists for 2 years. They found hidden underground movements

A study of Nicaragua's active Masaya volcano, near which almost 2 million people live, has revealed that its underground magma system may be considerably more complex than previously understood. Scientists at Penn State analysed six years of satellite data to track subtle changes in the ground around the volcano between 2018 and 2024. They found that one part of the volcano initially sank before beginning to rise, possibly indicating that fresh magma was entering a deep reservoir. Meanwhile, the ground around its active Santiago crater and lava lake continued to sink, suggesting that a separate, shallower magma reservoir was behaving differently.

The findings, published in Geophysical Research Letters, could help scientists better understand how magma moves beneath the volcano and improve long-term monitoring of volcanic hazards.

"These observations help scientists better understand how magma moves beneath Masaya and may improve monitoring and eruption forecasting in the future," said lead author Lizzie Johnson, who earned her bachelor's degree in geosciences from Penn State in May.

Why Masaya Volcano Is Closely Monitored

Located near Managua, Nicaragua's capital, Masaya is famous for its glowing lava lake. The volcano is also part of a national park and a popular tourist destination. Masaya is a geologically complex volcano featuring multiple overlapping craters and calderas. Calderas are large depressions that generally form when the ground collapses after magma is removed from an underground chamber.

Although scientists have studied Masaya for years, the movement of magma through its underground reservoirs and passageways remains poorly understood. These systems can change over time, making it difficult to determine exactly what is happening beneath the volcano.

Although Masaya last produced a major lava flow in 1772, its persistent release of volcanic gases can produce dangerously high concentrations of sulfur dioxide.

Sulfur dioxide is a potentially harmful gas released by volcanic systems. Elevated concentrations can create hazardous conditions for visitors and surrounding communities, even without a major eruption.

Young Cheol Kim, who completed his doctorate in geosciences at Penn State this summer, explained that continuous monitoring is essential for distinguishing normal volcanic activity from changes that could signal increasing hazards.

"Doing continuous monitoring allows us to understand the baseline activity and identify see if things are changing," he said, explaining that certain shifts in volcanic behaviour could indicate a period of heightened activity or a possible eruption. "And then if there is a need to warn people, we can do that in a timely manner."

Satellites Reveal Hidden Movements Beneath the Volcano

To investigate Masaya's underground activity, the researchers analysed measurements collected by the Sentinel-1 satellite, which observes the volcano approximately once every 12 days.

The satellite measurements revealed an intriguing pattern between 2018 and 2024. During the first part of the study, the ground within Masaya's caldera gradually subsided, meaning it moved downward. Later, the same general area began to rise.

Scientists believe this transition could reflect changes in the amount of magma stored underground. When magma drains away from a reservoir, the overlying ground can sink. When additional magma enters and increases pressure within a reservoir, the surface may rise.

The most surprising finding was that the area surrounding Santiago crater did not follow the same pattern. Instead, it continued to sink throughout the six-year study, even as the broader caldera began rising.

"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."

Together, these observations suggest that Masaya may have at least two distinct parts of its magma storage system undergoing different changes. A deeper source could be supplying additional magma to the main reservoir, while a shallower reservoir near Santiago crater may be losing magma.

This could explain why different sections of the same volcano moved in opposite directions.

What the Discovery Means for Future Eruptions

Understanding these underground movements is important because changes in the ground can provide clues about where magma is accumulating, where it is moving and how it might eventually reach the surface.

The researchers emphasised that their observations did not indicate any immediate danger to communities surrounding Masaya. However, identifying the volcano's normal behaviour and understanding its magma supply system could make it easier to recognise unusual changes in the future.

Continued satellite observations, combined with more detailed computer models, could also help scientists better understand the connections between underground magma reservoirs and volcanic activity at the surface.

"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.

The findings may therefore have implications beyond Nicaragua, particularly for other active volcanoes that contain persistent lava lakes and complex underground magma systems.