“No one can predict where, or with what force, an eruption might commence.”
Currently, some of the 30 volcanic systems in Iceland, most of them with a large central volcano, show signs of unrest. It is ever thus, but a greater or lesser number of dormant volcanoes are potentially active in any given decade or so. In some cases, as with Hekla and Grímsvötn, monitoring tells us that swelling of the volcano, and of the nearby crust (due to an influx of magma), has reached the same threshold as before the volcano’s last eruption. This, however, is not enough to permit a sound forecast.
In the subglacial Katla central volcano, frequent earthquakes (some at a depth of 15-25 km), increased geothermal activity and crustal uplift are telltale signs of something brewing. This swelling, however, is partly due to a lessening ice load and partly due to an influx of magma.
Currently, the glacier-covered Bárðarbunga central volcano also receives some magma, but it is impossible to predict what threshold the increased magma pressure and growing tensile stresses must attain before fissures in the crust become filled with magma (dyke formation, in geological terms) and, eventually, a volcanic eruption starts. Nor can anyone predict where or with what force such an eruption might commence. An eruption in Bárðabunga proper means interaction between magma and ice, producing ash and pumice plus a large and powerful flash flood. A more distant eruption within the vast, elongated Bárðarbunga volcanic system, and in an ice-free area, would mean a lava-producing event in the fashion of the well-documented Holuhraun event. That lasted six months, basically from the end of August 2014 to the end of February 2015, forming a lava flow with an area of almost 90 square kilometres.
Not over, yet
Lessons from former episodes of unrest in the large Bárðarbunga volcanic system are clear. Most episodes, with rifting and volcanic activity, tend to stretch over many years, even decades. Some of the rifting and eruption events during each episode have been powerful and productive, with a lot of ash and lava pouring out from fissures in ice-free areas, as for example in the late 15th century (the Veiðivötn Fires).
Elaborate monitoring now reveals that the central region of the volcanic system, including the Bárðarbunga central volcano with its ice-covered caldera, has not come to a rest. Magma rises into the deep plumbing system below the volcano. This is evident from GPS monitoring stations. A recent data interpretation map from the Icelandic Met Office and the University of Iceland shows horizontal movements that are a combination of general tectonic plate drift, and swelling due to magma rising from depth. A GPS station high up on the flanks of Bárðarbunga moved over 6.5 cm to the northwest between July 10th and mid-December 2015, and simultaneously rose some 3 cm in altitude. Earthquakes are being registered in abundance and quite many have attained magnitudes between 3.0 and 3.8 on the Richter Scale.
It remains to be seen what all of this will bring us in the near future. A renewed period of rifting within the Bárðarbunga system, possibly including a volcanic eruption, can start any time. The monitoring system is, however, capable of delivering a warning, hours or days prior to such an event.