Grimsvotn is not just another volcano; It’s a special beast that captivates both Mafic Maniacs and Caldera Crazies. This volcano plays many characters, producing everything from expansive lava flows to basaltic Plinian eruptions but mostly mundane lava flows or irrelevant explosions  For me, Grimsvotn holds a special place among other volcanoes, ranking among my top three candidates likely to produce a caldera-forming eruption within the next decade, alongside Ioto and Chiles-Cerro Negro.

What sets Grimsvotn apart from its peers? At first glance, it may appear to be the odd one out. Unlike Ioto and Chiles-Cerro Negro, which have longer dormancy periods and lacks felsic magma, Grimsvotn is believed to be in a phase of recovery following its substantial eruption in 1783. On the surface, it lacks the typical characteristics one might expect in discussions of caldera-forming volcanoes. However, beneath this façade lies compelling evidence suggesting a more vigorous and dynamic system than initially assumed.

To understand Grimsvotn’s potential for future eruptions, we must first delve into its plumbing. The volcano’s shallow magma chamber has been rapidly fed with exceptionally rich basaltic magma. This influx has been so significant that Grimsvotn managed to recover from its near VEI 5 eruption in a mere four years. In 2011, an eruption released approximately 0.2 km³ of magma, which implies a minimum annual magma supply rate of 0.05 km³. It is essential to note that this figure may be a minimal figure; the presence of the overlying ice sheet complicates the assessment of vertical deformation, making it challenging to determine the precise timing of the magma chamber’s recovery. Some GPS data suggests that Grimsvotn might have fully recovered by late 2013 or early 2014. Since 2011, a rough calculation indicates that about 0.65 km³ of magma has entered the system, enough to produce a VEI 5 eruption but not enough for a caldera-forming event.

Grimsvotn’s system is mature, characterized by a persistent shallow magma chamber that measures between 7 km to 8 km in width and 4 km to 5 km in height, with a depth of approximately 1 km. This results in a total volume estimate of 22-31.4 km³. The shallow chamber is being fed by a much larger deeper system. Unlike other volcanoes such as Fagradalsfjall, Grimsvotn has the potential to erupt more magma than it has accumulated post-eruption.

If Grimsvotn were to produce a caldera-forming eruption, it would likely involve reactivated magma, as the ratio of intruded magma to the chamber volume is remarkably high. Over the past 14 years, the shallow chamber has received about 2-3% of its total volume, significantly more than the ratios observed in other volcanoes experiencing similar intrusions, such as Laguna del Maule or Öræfajökull. In general, caldera-collapse eruptions can expel between 10-40% of the total chamber volume, which give us an eruption volume of 2.2 to 12.56 km³ of magma. Such an eruption would likely be sulfur-rich, and might lead to considerable climatic repercussions.

Currently, Grimsvotn is characterized by a large, shallow system undergoing significant magma intrusion, with no signs of a downturn in activity. This persistent state raises the question: when will Grimsvotn erupt again? Since the inception of this blog, this question has been posed annually.

The Challenge of Predicting Eruptions

Forecasting volcanic eruptions is notoriously complex, as exemplified by the predictions made by Albert and Carl. Using various methodologies including, Cumulative Seismic moment deformation monitoring, and pressure calculations, they anticipated potential eruption windows in late 2020-2021 and 2018-2019, respectively. However, nature often has its own agenda, and Grimsvotn promptly defied these predictions.

One critical aspect to consider is that the impediment to an eruption at Grimsvotn does not stem from a lack of pressure or magma. Instead, the volcano seems to struggle with forming a conduit robust enough to allow for magma ascent. Without a suitable medium for the magma to reach the surface, pressure builds without resulting in an eruption. Since 2004, Grimsvotn has been getting hotter and hotter while I don’t know the specific metrics. It is well-documented that when magma reservoirs reach a certain thermal threshold, the system becomes less capable of producing fracturing earthquakes. As the surrounding rock heats up, it transitions from a brittle state to a more elastic one, making it increasingly difficult for magma to fracture the rock and reach the surface. This change in thermal dynamics likely explains the deviation observed in the current CSM curve compared to the two previous eruptions. Notably, there have been significantly fewer small earthquakes, and most of the seismic activity recorded over the last 13 years has come from larger events. Grimsvotn’s elevated temperature means that despite having the pressure and magma, the volcanic system is unable to erupt.

The seismic activity around Grimsvotn is also noteworthy. The quakes have not concentrated in a single area but have instead spread out in an oval pattern surrounding the volcano. This distribution likely reflects the pressure exerting stress on the entire chamber roof rather than resulting from localized ruptures. The quakes are growing larger, with the most significant events reaching magnitude 4. If a conventional magma intrusion does not develop into an eruption, it is plausible that Grimsvotn may produce a ring dyke or a larger traditional dyke, setting the stage for a more significant eruption. In this way, Grimsvotn finds itself caught in a feedback loop: as long as the magma intrusion persists, the surrounding rock will continue to heat up and remain elastic, complicating the eruption process further. I am of the opinion that Grimsvotn will erupt in the next 2 years not based on any models or formulas but the oh-so-logical gut feeling. The last four years have seen a steep increase in accumulated seismic energy, indicating that Grimsvotn may be nearing its limit. While seismic activity has weakened over the past seven months, one should not be misled. The volcano is still accumulating strain, and the potential for an eruption remains present. If the caldera roof eventually succumbs to the built-up pressure, it could lead to an explosive event, or perhaps a style of eruption akin to that witnessed in 2011, albeit this is less likely according to my assessments.

In summary, Grimsvotn is a unique and complex volcano that offers much for us volcano fanatics. Its history, magma dynamics, and the ongoing interactions between temperature, pressure, and rock elasticity could provide a good case study for understanding volcanic behavior. Whether it erupts soon or remains dormant for a little longer, this volcano is not going to get boring for a while as it reveals more of its secrets as time goes on.

Tallis