Iceland's Most Active Volcano Looks Like It's Getting Ready to Erupt Again

  The ice-covered Grímsvötn volcano on Iceland produced an oddly large and powerful eruption in 2011, sending ash 20 kilometres into the atmosphere, causing the cancellation of about 900 passenger flights. as compared, the much smaller 2010 eruption of Eyjafjallajökull led to the cancellation of about 100,000 flights.

Understandably, any mention of another explosive eruption from an Icelandic volcano will raise concerns within the aviation industry, which is currently reeling from the COVID-19 pandemic.

But there are clear signs that the Grímsvötn volcano is preparing to erupt again. As a result, the authorities have recently raised the threat level for this volcano.

Grímsvötn may be a peculiar volcano because it lies almost wholly beneath the ice, and therefore the only permanently visible part is an old ridge on its side which forms the sting of an outsized crater (a caldera). And it's along the bottom of this ridge, under the ice, that the majority recent eruptions have occurred.

Another peculiarity is that the warmth output from the volcano is awfully high (2000-4000 MW), and this melts the overlying ice and produces a hidden subglacial lake of meltwater. this is often up to 100 metres (328 feet) deep and has iced over to about 260 metres (850 feet) thick floating on that. Fresh ice is continually flowing into the caldera, where it melts, then the water level just keeps rising and rising.

This meltwater can escape suddenly, and after travelling southwards beneath the ice for about 45 kilometres it emerges at the ice margin as a flood, which within the past has washed away roads and bridges. Fortunately, the passage of meltwater beneath the ice to its outlet is tracked, then roads are closed blast to avoid travellers getting caught within the flood and killed.

Yet another important peculiarity of Grímsvötn is that it can have a hair-trigger response to pressure. This happens when the meltwater lake drains – removal of the water from across the highest of the volcano rapidly reduces the pressure.

This can trigger an eruption – it's like lifting the lid off an autoclave. This went on repeatedly at Grímsvötn.

Grímsvötn is Iceland's most often erupting volcano, and over the past 800 years, some 65 eruptions are known with some certainty. The time gaps between eruptions are variable – and, for instance, before the larger 2011 eruption there have been smaller eruptions in 2004, 1998 and 1983 with gaps of between four and 15 years.

Crucially, and with the subsequent eruption in mind, Grímsvötn appears to possess a pattern of infrequent larger eruptions that occur every 150-200 years (for example 2011, 1873, 1619), with smaller and more frequent eruptions occurring roughly once a decade in between.

The roughly 1.5 km wide hole melted in the ice by the 2011 eruption. (Dave McGarvie)

Signs of activity

A high frequency of eruptions at a volcano allows scientists to detect patterns that result in eruptions (precursors). And if these are repeated every time a volcano erupts then it becomes possible for scientists to be more confident that an eruption is probably going to happen within the near future.

It is, however, seldom possible to be precise about the precise day.

Icelandic scientists are carefully monitoring Grímsvötn since its 2011 eruption, and have seen various signals that suggest the volcano is preparing to erupt.

Old ridge of Grímsvötn. (Dave McGarvie)

For example, the volcano has been inflating as new magma moves into the utility beneath it (think of burying a balloon within the sand and so inflating it). The increasing thermal activity has been melting more ice and there has also been a recent increase in earthquake activity.

So what happens next? Again, supported the pattern observed at past eruptions, an intense swarm of earthquakes lasting some hours (one to 10 hours) will signal that magma is moving towards the surface which an eruption is imminent. In cases where the hidden subglacial lake drains and triggers the eruption, the earthquakes occur after the lake has drained and just before the eruption.

The smaller Grímsvötn eruptions expend plenty of energy once they interact with water and ice at the surface. which means the resulting ash gets wet and sticky so falls from the sky relatively quickly.

Ash clouds therefore only travel some tens of kilometres from the eruption site. this can be an honest scenario for Icelanders and also for travel because it prevents the formation of considerable ash clouds that would drift around and shut off airspace.

But will it be a little eruption? If Grímsvötn's past pattern of occasional large eruptions with more numerous smaller eruptions occurring in between continues into the longer term, then the subsequent eruption should be a little one (given there was an outsized one in 2011). and also the word "should" is vital here – Iceland's volcanoes are complex natural systems and patterns don't seem to be always followed faithfully. The Conversation