The Only Volcano on Earth That Erupts Black Carbonate Lava Bulged Briefly, and Virginia Tech Now Knows Why: A Million Cubic Meters of Magma, About 400 Olympic Pools, Poured Into a Chamber 3 Kilometers Under Tanzania's Ol Doinyo Lengai. It Is Not Rising Toward the Crater, and the Team Says That Is the Line to Watch.
Six GNSS stations installed in 2016 track the ground to within a millimeter, and two seismometers listen for the kind of magnitude-5.9 quake that preceded the last explosive eruption. The mountain blows up roughly every 10 to 15 years, and the researchers expect one 'within our lifetime.'
Ol Doinyo Lengai, the "Mountain of God" of the Maasai in northern Tanzania, is the only active volcano on Earth that erupts natrocarbonatite, a sodium-rich carbonate lava so cool it flows black and turns white within days as it reacts with the air. It is also a volcano that sits directly over the East African Rift, where the continent is slowly tearing apart. A study published Sept. 18 in Frontiers in Earth Science has now explained a short-lived swelling of the ground around it: about 1 million cubic meters of fresh magma pushed into a reservoir roughly 3 kilometers (1.9 miles) below the surface.
The work comes from a Virginia Tech team led by geophysicist D. Sarah Stamps, which has run the only continuous ground-based monitoring network on the volcano for a decade. "With as much certainty as we can get from numerical modeling, we found that the signal was due to an influx of magma into an existing magma reservoir about 3 kilometers underground," Stamps said. "The volume of the magma intrusion is about 1 million cubic meters. You could think of that as 400 Olympic-size swimming pools, which is a modest amount."
The point of the exercise is not this intrusion, which Stamps says is not a cause for concern, but the next one. Ol Doinyo Lengai alternates between gentle effusive eruptions, in which a lava lake bubbles in the crater, and explosive ones that hurl ash kilometers into the sky. "On average, there's an explosive eruption every 10 to 15 years," Stamps said. "We do expect one within our lifetime, and we hope that our monitoring and modeling efforts will be worthwhile." The team is trying to learn the volcano's particular time delay between what the ground does and what the crater does, so the Tanzania Geological Survey has time to order evacuations.
The tools are simple and precise. In 2016 the team installed six Global Navigation Satellite System instruments around the mountain that track horizontal and vertical motion of the surface to within 1 millimeter, plus two broadband seismic stations to catch earthquakes, supplemented by periodic benchmark surveys. Satellites, including NASA's, provide some coverage from above, but Stamps said her group is currently the only one doing consistent ground-based GNSS and seismic monitoring at the site. That decade of data is what let the modeling pin the bulge, described in the paper as the 2022–2023 uplift episode, to a specific volume of magma at a specific depth.
What would worry her is a different sequence. "The last time it erupted explosively, there was a magnitude 5.9 earthquake first," Stamps said. "I would be more concerned if there was a significant earthquake, and then we started to monitor observable surface deformation changes. Also, if we observe the magma moving from a deeper to a shallower location over time, that suggests the magma is moving up the volcanic vent. That would be concerning." In this case the magma stayed put. "There's a little more magma being injected into a system that we know exists, but the detected magma does not appear to be moving closer to the crater."
The paper, "Inflation at Ol Doinyo Lengai driven by shallow magma intrusion during the 2022–2023 uplift episode in an early-phase continental rift," carries the DOI 10.3389/feart.2026.1881885. Its larger contribution is a calibrated baseline: the team now has a worked example of what a modest, harmless intrusion looks like in its instruments, which is exactly what will make a dangerous one recognizable. "We've now collected over a decade of observations," Stamps said. "We have a better understanding of what magma movements underground match what we observe over this time period. This work can help us better understand when the volcano is getting closer to an explosive eruption based on what we're observing at the surface."
Originally reported by Phys.org.