On a quiet April night in 2024, the sky over Ruang Island, North Sulawesi, suddenly turned ominous. Mount Ruang, one of the active volcanoes in the Sangihe Islands, erupted after more than two decades of dormancy.
Two powerful eruptions occurred within two weeks, on 17 and 30 April, categorized as sub-Plinian events, comparable to the 2014 Kelud eruption.
However, beyond the dramatic visuals, volcanologists discovered something even more remarkable.
This eruption was not just about smoke and ash—it involved magma so densely packed with crystals that it altered how the eruption unfolded.
This finding was published in a recent study by a multi-institutional research team, including Universitas Gadjah Mada, in the Journal of Volcanology and Geothermal Research on Apr. 14.
Dr. Indranova Suhendro, a lecturer at the Department of Environmental Geography, Faculty of Geography at UGM, who led the research team, explained that Mount Ruang presents a unique case that could be key to understanding future explosive eruptions.
“The decompression of Mount Ruang’s magma was extremely rapid—comparable to the ‘Pompeii’ eruption of Mount Vesuvius and the 1991 Pinatubo eruption—but the eruption column was much smaller. This is because the magma was too crystal-rich, making it too heavy to be propelled further into the atmosphere,” he said on Wednesday (Apr. 16).
The research team noted that the ash columns from the two eruptions reached only 9 to 19 kilometers high.
Sub-Plinian eruptions are considered relatively low, especially given that the magma surged to the surface at a decompression rate of 29 megapascals per second.
In many cases, such speed could result in a Plinian eruption with higher columns and broader impacts. But Mount Ruang’s columns were held back.
“This was puzzling at first until we discovered the cause: the magma was so densely filled with crystals that the eruption was internally hindered,” he explained.
Using microscopic analysis, geochemical methods, and pressure modeling, the researchers found that Mount Ruang’s magma contained an extraordinary number of large crystals, such as plagioclase and amphibole, comprising up to 37–87% of the pumice volume. This increased the magma’s density, limiting its upward thrust.
“It’s like an overfilled hot air balloon. It becomes too heavy to lift off,” Dr. Suhendro illustrated.
Interestingly, the study also identified signs of interaction between the magma and external water during the eruption.
This was visible in the blocky texture of volcanic ash, which bore glass-like fracture marks (hackle marks).
The water-magma interaction may have added explosive force to the eruption, though still not enough to propel the ash higher.
The team also noted that the eruption occurred only seven days after a magnitude 6.5 earthquake struck the Maluku Sea.
The spike in seismic activity leading up to the eruption suggests the quake may have acted as a trigger.
These findings offer new perspectives on how volcanic eruptions behave, especially in Indonesia, where many volcanoes have crystal-rich magma.
Until now, volcano monitoring has largely focused on seismic activity and gas emissions. However, this research indicates that magma’s high crystal content can affect eruption intensity and column height, even when the magma is under high pressure and moving rapidly.
In the context of disaster mitigation, the findings are significant.
If magma crystal content can be routinely measured and analyzed, it could serve as an additional indicator in predicting eruption hazards, including how high ash columns might rise, the spread of erupted material, and potential impacts on nearby communities.
“We hope that in the future, magma crystallinity analysis before eruptions, using geophysical tools, for instance, can be used as an additional predictive method, not just gas and tremor monitoring,” Dr. Suhendro added.
This study stands as a clear example of Indonesian scientists’ contribution to understanding extreme geological events that directly impact public safety.
The involvement of various institutions from both Indonesia and abroad also highlights the importance of interdisciplinary and international collaboration in disaster research, particularly in eruption-prone regions like Indonesia.
“I’m also grateful to have a dedicated and passionate team. Many of them were my classmates, and we’ve continued to work closely together to advance volcano research in Indonesia,” Dr. Suhendro concluded.
One year after the eruption, Mount Ruang leaves behind more than just ash; it offers new insights.
From the tiny crystals within its magma, we learn that natural disasters follow their own logic, and science remains our first line of defense.
Author: Triya Andriyani
Post-editor: Afifudin Baliya
Photographs: Dr. Indranova Suhendro