Found in the chilly region of Iceland, Grímsvötn at its core is a rather peculiar volcano.
For one, it is a volcano that is wholly covered by ice and the only visible part of the mountain is an old ridge on its south side which forms the edge of a larger crater — also known as a caldera.
It is here, along the base of the ridge and under the ice, that most recent eruptions (the most recent dating back to 2011) have occurred.
Grímsvötn Volcano In Iceland
Aside from that, another peculiarity of this unique volcano is its extraordinarily high output of heat which can range from 2,000 to 4,000 MW. This heat melts the enclosing ice to produce a rather hidden subglacial lake of meltwater that may be up to 100 metres deep and has thick ice (up to 260 metres thick) floating on it.
Once the ice melts, fresh ice flows into the caldera which then melts, and so the water level of this hidden subglacial lake continually rises over time. More importantly, the rising meltwater may overflow at any given time. After travelling southwards beneath the ice for around 45 kilometres, the overflowing water emerges at the ice margin as a flood — a disaster which has washed away roads and bridges in the past.
Fortunately, the passage of meltwater and the resulting flood can be easily predicted, so roads are closed well ahead of time to avoid any unfortunate incidents.
Moreover, another important (and potentially) deadly quirk of Grímsvötn volcano is that it can be very sensitive to pressure. This typically happens when the aforementioned meltwater overflows and is drained, resulting in a rapid decrease in pressure across the top of the volcano. Such instances will likely trigger an eruption — akin to lifting the lid off a pressure cooker. A situation like this has happened countless times before at Grímsvötn.
Over a span of 800 years, about 65 eruptions have been identified with some certainty to have occurred at Grímsvötn. And the time gaps between each eruption have not followed a particular time patter. For instance, prior to the massive 2011 Grímsvötn eruption, there were smaller eruptions happening in 2004, 1998, and 1983 with gaps of between four and 15 years.
Crucially, with what we know so far, Grímsvötn appears to have a pattern of infrequent larger eruptions which may occur every 150 to 200 years (the timeline of larger eruptions are 2011, 1873m 1619, etc.). On the other hand, the minuscule and more frequent eruptions look to be occurring roughly once every decade.