It has likely not escaped anyone’s attention that Öræfajökull is stirring after nearly three centuries of dormancy. The signs are threefold: significantly increased earthquake activity over the past year; land uplift; and clear indications of geothermal activity in recent weeks.
The first two factors, increased earthquake activity and land uplift, are typical precursors to volcanic eruptions, although such preludes often end without an eruption. However, it is worth carefully considering the geothermal activity in Öræfajökull and reflecting on what it may be telling us.
Öræfajökull is one of several massive stratovolcanoes that adorn Iceland, along with Eyjafjallajökull and Snæfellsjökull. Snæfell and Hekla could likely also be classified as stratovolcanoes. What these volcanoes have in common is that they rise significantly above their surroundings and lack any substantial geothermal systems, either in the mountains themselves or in their immediate vicinity. Most other central volcanoes in the country are accompanied by powerful high-temperature areas with fault systems that extend far beyond the central volcano itself, as seen in Hengill, the Torfajökull area, Katla, Grímsvötn, Askja, and Krafla.
High-temperature areas derive their energy from cooling intrusions or magma near the surface of the Earth’s crust, typically at depths of only a few kilometres, as is the case with Krafla. Cold groundwater seeps down to these intrusions, heats up upon contact with the hot rock or magma, and rises to the surface as hot water or steam. The formation of high-temperature areas depends on the presence of cooling magma intrusions or magma chambers close to the surface. From this, we can deduce that shallow magma chambers are absent in the stratovolcanoes, with the magma lying much deeper in the Earth’s crust, below the depth that the water cycle reaches. Thus, the stratovolcanoes are formed primarily by magma rising directly from deep within the Earth to the surface, with only a minimal portion remaining as shallow intrusions that would form high-temperature systems. Hence, there are no high-temperature systems associated with the stratovolcanoes.
The stratovolcano Öræfajökull is the highest mountain in the country. At the top of the mountain is a caldera about 5 kilometres wide, which is believed to have formed at some point during a major eruption. It is thought to be up to 540 meters deep and filled with glacial ice. There were no indications of significant geothermal activity in the area until November 2017. At that time, a sulfuric smell was detected near Kvíá river, and a subsidence bowl began to form in the middle of the caldera. Both of these signs clearly indicate that a new high-temperature area has developed in the middle of Öræfajökull’s caldera beneath a 400-500-meter-thick ice sheet.
Based on the melting of the ice, we can estimate the power of this new geothermal system. The subsidence bowl is estimated to be about 1 kilometer in diameter, and if we assume its average depth is 10 meters, the density of the ice is 900 kg/m³, the latent heat of ice is 335 kJ/kg, and that the melting occurred over one month, then we calculate that the power output of this new geothermal area is about 900 MW. This assumes that the melting began when seismic activity increased in mid-October. However, if we assume it started when seismic activity began to increase in May 2017, the estimated power output is 150 MW. By monitoring how quickly the subsidence bowl deepens over time, we can get a better idea of the rate of melting.
Due to the elevation and depth of the groundwater table, it can be assumed that this energy reaches the base of the glacier as hot steam rather than liquid water. If we assume that the steam is around 250°C and the heat content of such steam is about 2800 kJ/kg, more than 300 kg/second of 250°C hot steam is flowing up from the bedrock, melting the ice, and condensing into pure water. Along with this steam, some magmatic gases, such as sulfuric compounds and carbon dioxide, are also released.
The above calculations assume that the water melting at the base of the caldera can escape from there. It is unlikely that it can escape above ground under the glacial ice, as it would first need to rise from the 400-500-meter-deep caldera, and no deep breaches in the caldera rim are known that could provide an outlet. This means that the water must either flow back into the ground or simply accumulate at the bottom of the caldera under the ice. If the water flows easily back into the ground, part of it would come into contact with the scorching hot bedrock or magmatic intrusions, heat up, and rise to the surface again, thereby intensifying geothermal activity, while another part would manifest as increased flow in the rivers and streams on the slopes of the mountain. If the melting has been occurring for 1 month, the runoff from the melting would be 3-4 m³/second, but only about 0.5 m³/second if the melting has been ongoing for 6 months. Therefore, it is not certain that the increased water flow from the melting ice would be noticeably reflected in the rivers and streams.
In the case that the water does not escape, the reduction in volume, as indicated by the subsidence bowl, would be solely due to the volume reduction from the ice melting, minus the water that condenses from the geothermal steam. This means that the amount of ice that must melt to create the subsidence depression would be about ten times greater than the volume of the depression itself. In this scenario, the power of the geothermal system would be between 1,500 and 9,000 MW, depending on how long the melting has been occurring, assuming that no water escapes from the caldera. However, this seems unlikely because young, unaltered lava layers in the mountain are likely to be highly permeable, allowing most of the water to drain away as groundwater. Some of the meltwater would then be recirculated into the newly formed geothermal system, while some would appear in the rivers and streams on the slopes of the mountain.
It is not actually known when the melting began, but the calculations assume it has been ongoing for one month, starting when the last earthquake swarm began in late October 2017. If we assume that the increased seismic activity in May 2017 marks the start of the melting, then the melting has been occurring for six months, and the power of the system would be one-sixth of what has been calculated above. If we consider the uncertainty in these calculations, the minimum power of this new geothermal system in Öræfajökull would be around 150 MW if all the meltwater drained away immediately and the melting has been ongoing for six months, but up to 9,000 MW if no water has escaped and the melting has only been happening for one month. The truth is likely somewhere in between.
It is nigh inconceivable that such a powerful geothermal system could suddenly form in a place where there was previously no system, unless magma had risen very close to the surface, where there is sufficient access to cold groundwater to create a highly active water cycle. Although there have been no signs of volcanic tremors detected by seismometers, it seems overwhelmingly likely that volcanic activity has already begun in Öræfajökull, with viscous, silica-rich rhyolitic magma being pushed up under the bedrock beneath the caldera—similar to squeezing toothpaste from a tube.
From all this, it can be concluded that volcanic activity is already underway in Öræfajökull, and the question is whether, and if so, how soon the magma will reach the surface. In any case, there is a considerable likelihood that Öræfajökull will erupt in the near future.
Ólafur G. Flóvenz
