A cataclysmic subglacial eruption in Iceland gave scientists a unique opportunity to witness how volcanoes directly feed CO2 into our atmosphere and oceans.
In late September 1996, scientists in Iceland detected ominous seismic rumbles beneath Europe's largest glacier, Vatnajökull. A volcanic fissure named Gjálp was tearing open between two major subglacial volcanoes, beginning what would become one of the most studied geological events in modern history 2 5 . The resulting eruption unleashed a Hiroshima-scale nuclear weapon's worth of energy every minute for a week, melting cubic kilometers of ice 2 .
Europe's largest glacier by volume, covering approximately 8% of Iceland's land area.
A subglacial volcanic fissure that erupted between Bárðarbunga and Grímsvötn volcanoes.
What made this event particularly remarkable to scientists wasn't just the eruption itself, but where it occurred—beneath hundreds of meters of glacial ice. This created a unique natural laboratory to answer a crucial question: What happens to volcanic CO2 when it's released in such massive quantities? The findings would challenge our understanding of how Earth's geological forces contribute to the planet's carbon cycle .
Seismic activity detected beneath Vatnajökull glacier
Subglacial eruption at Gjálp fissure continues
Jökulhlaup (glacial outburst flood) occurs
While we often picture volcanoes as conical mountains spewing lava, Iceland hosts a different kind of volcanic activity—subglacial eruptions that occur beneath massive ice sheets. When fire meets ice at this scale, the results are dramatic. The 1996 eruption at Vatnajökull provided a perfect case study of this phenomenon 2 .
The eruption generated an immense amount of meltwater—approximately 3 cubic kilometers of ice transformed into water—that was temporarily trapped beneath the 600-800-meter-thick glacier 2 . This set the stage for a catastrophic glacial flood, known by the Icelandic term "jökulhlaup." On November 5, 1996, the ice dam containing this water finally burst, unleashing a violent flood that at its peak reached a flow rate of over 50,000 cubic meters per second—briefly creating the second largest river on Earth 2 .
Volcanoes influence climate through multiple mechanisms, primarily by emitting various gases and particles. During eruptions, they release sulfur dioxide, which forms sulfuric acid aerosols in the atmosphere that reflect sunlight and can cause temporary global cooling 7 . They also emit greenhouse gases, including carbon dioxide, which have a warming effect 7 .
However, the climate impact of volcanic CO2 is often misunderstood. While volcanoes do contribute to atmospheric CO2, their emissions are dwarfed by human activities. NASA estimates that human activities release more than 100 times the CO2 that all the world's volcanoes combined emit annually 9 . The 1996 Vatnajökull eruption provided an exceptional opportunity to quantify exactly how much CO2 a major subglacial eruption actually contributes to the global carbon cycle .
The 1996 eruption offered a rare opportunity for scientists to measure volcanic CO2 release in real-time. Recognizing this, Icelandic researchers implemented comprehensive monitoring of the glacial rivers surrounding Vatnajökull 5 . Their experimental approach was both methodical and adaptive to the unfolding natural disaster.
The research team established stations along rivers likely to be affected by the eruption, particularly Jökulsá á Fjöllum to the north and Skeidará to the south 5 .
They conducted sampling both during and after the eruption, with particular focus on the jökulhlaup event in November 5 .
The scientists employed analysis measuring not just chemical composition but also conductivity, pH, sediment load, and discharge rates 5 .
Data collection occurred under challenging conditions. The team had to work around infrastructure damage caused by the flood, which destroyed bridges and roads 2 . They implemented frequent sampling schedules, sometimes 2-3 times per week at key locations, to track changes in water chemistry 5 . Perhaps most importantly, they established baseline measurements for comparison, using pre-eruption data where available and continuing monitoring for months after the event to understand long-term effects 5 .
| Parameter | Measurement Method | Significance |
|---|---|---|
| Conductivity | Regular field measurements | Indicator of dissolved solid content |
| pH | Water sampling and laboratory analysis | Determined acidity/alkalinity of floodwaters |
| Discharge rate | Continuous recording at monitoring stations | Measured water volume and flow speed |
| Chemical composition | Laboratory analysis of water samples | Identified elements of magmatic origin |
| Sediment load | Sampling and filtering | Quantified material transport by flood |
The scientific monitoring produced striking results. Water samples from Jökulsá á Fjöllum showed abnormal chemical composition with unusually high conductivity readings reaching 200 μS/cm during the eruption—far above normal levels 5 . Analysis revealed that the minimum concentration of resolved CO2 in the erupted magma was 516 mg per kilogram of magma .
Most significantly, researchers calculated that the total dissolved carbon flux from the eruption equaled approximately 0.6 million tonnes of CO2 . This massive CO2 release occurred through the meltwater that eventually drained into the ocean during the jökulhlaup, creating what amounted to a natural carbon injection experiment on a gigantic scale.
Beyond the sheer volume of CO2 released, the research uncovered several unexpected findings. The data revealed rapid chemical reactions between the volcanic gases and meltwater, with volatile elements dissolving into the water at an astonishing rate . Isotopic analysis showed that while most of the floodwater was of surface origin (meteoric water), the carbon and sulfur were clearly magmatic , providing a definitive geochemical fingerprint of the eruption's contribution.
About half of the released carbon would be permanently added to the atmospheric and oceanic CO2 reservoirs, while the other half would eventually precipitate out with calcium and magnesium .
| Element/Compound | Minimum Concentration in Erupted Magma | Eventual Destination |
|---|---|---|
| Carbon Dioxide (CO2) | 516 mg/kg | Atmosphere and oceans |
| Sulfur (S) | 98 mg/kg | Atmosphere and water |
| Chlorine (Cl) | 14 mg/kg | Meltwater and oceans |
| Fluorine (F) | 2 mg/kg | Meltwater and oceans |
This net addition to the active carbon cycle provides crucial insight into how subglacial volcanic activity contributes to long-term atmospheric CO2 levels.
Understanding volcanic contributions to the carbon cycle requires specialized approaches and methodologies. The 1996 Vatnajökull investigation employed several key techniques that have since become standard in similar research.
| Tool/Method | Primary Function | Application in 1996 Study |
|---|---|---|
| Seismic Monitoring | Detect and locate underground activity | Identified eruption start and fissure formation 2 |
| Water Chemistry Analysis | Determine dissolved elements | Quantified magmatic volatile release 5 |
| Isotopic Tracing | Identify element origins | Distinguished magmatic from atmospheric carbon |
| Conductivity Monitoring | Measure dissolved solid content | Tracked volcanic influence in rivers 5 |
| Aerial Observation | Document surface changes | Observed ice depression and eruption breakthrough 2 |
The research demonstrated that multi-disciplinary approaches are essential when studying complex geological events. By combining seismology, hydrology, geochemistry, and atmospheric science, researchers could build a comprehensive picture of how carbon moves from magma through ice and water into the atmosphere and oceans 5 .
Satellite imagery and aerial photography documented surface changes and flood extent.
River gauges and sampling stations tracked water chemistry and flow rates.
Laboratory techniques identified magmatic signatures in water samples.
The 1996 Vatnajökull eruption provided more than just a spectacular geological display—it offered fundamental insights into Earth's carbon cycle. The research demonstrated that single subglacial volcanic events can release substantial amounts of CO2 directly into our atmosphere and oceans, with approximately half of that carbon remaining in active circulation .
While the 0.6 million tonnes of CO2 released during the eruption seems massive, it's dwarfed by the 37.4 billion tons of carbon dioxide humans emitted in 2024 alone 6 . This puts into perspective NASA's observation that human activities release more than 100 times the CO2 of all volcanoes combined 9 .
"When what took millennia to form comes apart in mere days, it's little wonder folks give the event a name of its own." 2
Perhaps the most enduring lesson from the 1996 eruption is how much we still have to learn about Earth's complex systems. The jökulhlaup of 1996—and the scientists who raced to study it—revealed that even in an age of human-dominated climate change, natural forces continue to shape our planet in profound ways.