Satellite Data Reveal Nicaragua's Masaya Volcano Has Two Separate Magma Reservoirs

Beneath one of Central America's most restless volcanoes, geologists have found evidence of two separate pockets of molten rock moving independently of each other, a discovery that complicates the picture of how Masaya, in western Nicaragua, might behave the next time it stirs. The findings, published this week in Geophysical Research Letters, come from six years of satellite tracking of the ground's surface and suggest the volcano's plumbing is more layered, and less predictable, than previously assumed.

Masaya has rarely gone quiet in modern times. Since a lava lake reappeared in its Santiago crater in December 2015, the volcano has released a steady plume of sulfur dioxide and occasionally thrown ash and rock from vent-clearing explosions. It has not produced a major lava flow since 1772. What it lacked, until now, was a clear picture of how magma is actually supplied to it from below.

What the satellites recorded

The research team, led by Penn State geoscientist Lizzie Johnson along with colleague Young Cheol Kim, analyzed radar images captured by the European Space Agency's Sentinel-1 satellite constellation between 2018 and 2024. The technique, known as interferometric synthetic aperture radar, measures tiny shifts in the ground's elevation by comparing radar signals bounced off the same patch of terrain on different passes, roughly every 12 days. Unlike optical imagery, it works through cloud cover, a critical advantage over a tropical volcano frequently wreathed in its own gas plume.

The pattern that emerged was not uniform. Ground just north of the active Santiago crater sank steadily from 2018 through mid-2022, then reversed and rose through 2024. Over that same span, the ground immediately around Santiago crater itself did something different: it deflated continuously, without pause, for the entire six years. Two signals moving on two different schedules, the researchers concluded, point to two separate magma reservoirs rather than one connected tank of melt, with a deeper body apparently being replenished even as a shallower one steadily lost pressure.

A shield volcano with a complicated plumbing system

Masaya is technically a basaltic shield volcano, the same broad category that includes Hawaii's Kīlauea, where fluid lava normally oozes out gradually rather than exploding. Yet its geologic record includes at least four Plinian, caldera-forming eruptions over the past several thousand years, evidence that calm, effusive activity can give way abruptly to something far more violent. A separate line of research at Kīlauea has already documented a comparable arrangement of isotopically distinct magma bodies feeding a single summit, which the Penn State team cites as a useful analogue for what may be happening beneath Masaya.

The new study builds on earlier satellite work at the same volcano, including a 2018 analysis that first identified a magma reservoir roughly two miles from the crater feeding the system from the side rather than directly below it. That research helped explain how a volcano can erupt with little outward warning: the visible crater shows almost no deformation even while magma accumulates nearby. The new six-year record extends that picture, suggesting the plumbing is not just offset but layered into at least two distinct storage zones that behave independently.

Who lives in the shadow of the plume

Masaya's caldera sits about 13 miles, or 21 kilometers, from Managua, Nicaragua's capital, a metropolitan area of roughly 2 million people. The volcano is also the centerpiece of a national park that draws tourists to look directly down into the glowing lava lake, one of the few places in the world where that is possible. Its persistent degassing is the more immediate hazard for residents: the plume regularly carries sulfur dioxide concentrations that, according to figures cited in coverage of the study, exceed the 24-hour air-quality guideline value for sulfur dioxide set by the World Health Organization, a chronic exposure risk for communities downwind rather than the sudden threat of an eruption.

"These observations help scientists better understand how magma moves beneath Masaya and may improve monitoring and eruption forecasting in the future," said Lizzie Johnson, the study's lead author.

Kim, a co-author on the paper, framed the value of the record in terms of simply knowing what normal looks like. "Continuous monitoring allows us to understand baseline activity and see if things are changing," he said, according to the researchers' own account of the findings, cited in a summary of the study distributed by Penn State's research communications office. That baseline did not exist in this level of detail before the current analysis, which is part of why the researchers describe the finding as useful even though it points to no immediate change in hazard level.

What happens next

The authors are careful to say the two-reservoir pattern does not, by itself, signal an impending eruption. Masaya's recent history has been dominated by passive gas release and lava-lake activity rather than the explosive events its geologic record shows it is capable of, and the paper does not predict a shift in that behavior. What it does offer is a clearer baseline against which future satellite passes can be judged. Because Sentinel-1 continues to image the volcano roughly every 12 days, scientists monitoring Masaya, including Nicaragua's national geological survey, now have a model for distinguishing ordinary fluctuation in the two reservoirs from a departure that might warrant closer attention.

As other summaries of the research have noted, the broader significance extends beyond Masaya itself. Many of the world's shield volcanoes near dense population centers, including several across Central America and East Africa, have historically been monitored with the assumption of a single, simple magma source. If layered, independently behaving reservoirs turn out to be common rather than unusual, longstanding models used to forecast eruptions at similar volcanoes may need to be revisited. The Penn State team's next step, consistent with the approach taken in the current paper, is expected to be extending the satellite record forward in time to see whether the reservoir beneath the caldera continues to inflate, and what, if anything, that eventually means for the crater above it.