Grímsvötn

Grímsvötn (vötn = "waters", singular: vatn) is an active volcano with a (partially subglacial) fissure system located in Vatnajökull National Park, Iceland. The volcano itself is completely subglacial and located under the northwestern side of the Vatnajökull ice cap. The subglacial caldera is at 64.41667°N, -17.33333°W, at an elevation of 1725 m. Beneath the caldera is the magma chamber of the Grímsvötn volcano.

Grímsvötn is a basaltic volcano which has the highest eruption frequency of all the volcanoes in Iceland and has a southwest-northeast-trending fissure system. The massive climate-impacting Laki fissure eruption of 1783–1784 was a part of the same Grímsvötn-Laki volcanic system. Grímsvötn was erupting at the same time as Laki during 1783, but continued to erupt until 1785. Because most of the volcanic system lies underneath Vatnajökull, most of its eruptions have been subglacial and the interaction of magma and meltwater from the ice causes phreatomagmatic explosive activity. Within the Grímsvötn-Laki volcanic system is a second central volcano called Thordarhyrna (Þórðarhyrna).

Jökulhlaup
Eruptions in the caldera regularly cause glacial outbursts known as jökulhlaup. Eruptions or geothermal activity, melt enough ice to fill the Grímsvötn caldera with water, and the pressure may be enough to suddenly lift the ice cap, allowing huge quantities of water to escape rapidly. Earthquakes and seismic tremor may occur. Jökulhlaup can occur independent of eruptions or be followed by eruptions. Jökulhlaup independent of eruptions occurred in November, December 2021 and October 2022. Jökulhlaup which were followed by eruptions occurred in 1922, 1934 and 2004. Consequently, the Grímsvötn caldera is monitored very carefully.

When a large eruption occurred in 1996, geologists knew well in advance that a glacial burst was imminent. It did not occur until several weeks after the eruption finished, but monitoring ensured that the Icelandic ring road (Hringvegur) was closed when the burst occurred. A section of road across the Skeiðará sandur was washed away in the ensuing flood, but no one was hurt.

Gjálp 1996
(See also the main article: 1996 eruption of Gjálp

The Gjálp fissure vent eruption in 1996 revealed that an interaction may exist between Bárðarbunga and Grímsvötn. A strong earthquake in Bárðarbunga, about magnitude 5, is believed to have triggered the eruption in Gjálp. On the other hand, because the magma erupted showed strong connections to the Grímsvötn Volcanic System according to petrology studies, the 1996 as well as a former eruption there in the 1930s are thought to have taken place within Grímsvötn Volcanic system.

1998 and 2004 eruptions
A week-long eruption occurred at Grímsvötn starting on 28 December 1998, but no glacial burst occurred. In November 2004, a week-long eruption occurred. Volcanic ash from the eruption fell as far away as mainland Europe and caused short-term disruption of airline traffic into Iceland, but again no glacial burst followed the eruption.

2011 eruption
Harmonic tremors were recorded twice around Grímsvötn on 2 and 3 October 2010, possibly indicating an impending eruption. At the same time, sudden inflation was measured by GPS in the volcano, indicating magma movement under the caldera. On 1 November 2010 meltwater from the Vatnajökull glacier was flowing into a lake, suggesting that an eruption of the underlying volcano could be imminent.



On 21 May 2011 at 19:25 UTC, an eruption began, with 12 km high plumes accompanied by multiple earthquakes,  Until 25 May, the eruption scale had been larger than that of the 2010 eruption of Eyjafjallajökull. The ash cloud from the eruption rose to 20 km, and was so far 10 times larger than the 2004 eruption, and the strongest in Grímsvötn in the last 100 years.

Disruption to air travel in Iceland commenced on 22 May, followed by Greenland, Scotland, Norway, Svalbard and a small part of Denmark on subsequent days. On 24 May the disruption spread to Northern Ireland and to airports in northern England. The cancellation of 900 out of 90,000 European flights in the period 23–25 May was much less widespread than the 2010 disruption after the Eyjafjallajökull eruption.

The eruption stopped at 02:40 UTC on 25 May 2011, although there was some explosive activity from the eruptive vents affecting only the area around the crater.

2020 onward threats of eruption
In June 2020, the Icelandic Meteorological Office (IMO) issued a warning that an eruption might take place in the coming weeks or months, following scientists reporting high levels of sulfur dioxide, which is indicative of the presence of shallow magma. IMO warned that a glacial flood as a result of melting ice could trigger an eruption. No eruption occurred.

In September 2021, an increase in water outflow from under the Vatnajökull ice cap was reported. The water contains elevated levels of dissolved hydrogen sulfide, suggesting increased volcanic activity under the ice. Jökulhlaup (glacial lake flooding) can occur before or after an eruption.

On 4 December 2021, a jökulhlaup occurred from Grímsvötn into the Gígjukvísl river, with an average flow of 2600 m3/s. Two days later, the Icelandic Meteorological Office increased the alert level for Grímsvötn from yellow to orange, after a series of earthquakes was detected. On 7 December, the alert level was lowered back to yellow, after seismic activity decreased and no signs of eruptive activity were detected.

On 11 December 2023, a jökulhlaup followed in time, a earthquake.

Eruption history before 1990
Tephra studies on soil samples from around the Vatnajökull ice-cap, show that the Grímsvötn volcanic system has high activity for Iceland, and had between 4 to 14 explosive eruptions every 100 years (mean about 7 eruptions every 100 years) between 7600 years ago and 870 AD. For technical reasons only the last 10,200 years of explosive eruptions have been characterised locally, and the record gets more inaccurate with time especially prior to 1598, which is first eruption timed to the day. The Laki eruptions which were both effusive and explosive between June 1783 and February 1784, produced the Skaftáreldahraun lava flows, which cover a large part of south-east Iceland. Before this the tip of the Rauðhólar-Eldgígur fissure system was active with the production of the 4550 BCE Botnahraun lava flow which extends beyond the furtherest extent to the south of the Laki lava field. The Rauðhólar-Eldgígur fissure system also formed the Núpahraun lava flows around 4000 BP that extend from the Vatnajökull glacier towards the south east coast and are covered in part by the northern Laki Skaftáreldahraun lava flow. There are two recent pre-historic but undated Bergvatnsárhraun lava flows just on the south-eastern edge of the Vatnajökull glacier, related to the line of the Rauðhólar-Eldgígur fissures.

Bacteria in the subglacial lakes
In 2004, a community of bacteria was detected in water of the Grímsvötn lake under the glacier, the first time that bacteria have been found in a subglacial lake. The lakes never freeze because of the volcanic heat. The bacteria can also survive at low concentrations of oxygen. The site is a possible analogue for life on the planet Mars, because there are also traces of volcanism and glaciers on Mars and thus the findings could help identify how to look for life on Mars.

Geology
There is the potential for mechanical interaction such as dyke propagation between Grímsvötn and Thordarhyrna. Interactions can also occur with the nearby Bárðarbunga volcano, which is part of a separate volcanic system. The volcano erupts predominantly tholeiitic basalt, and a close chemical affinity exists with the other lavas of the Grímsvötn-Laki volcanic system. The compositional separation from nearby systems has been studied in some detail, and is used to assign lava and tephra deposits to the volcanic system. Some of the tephra's erupted have transpired to have mixed compositions with nearby volcanic systems and it is not known if this is due to dual eruptions or intusions crossing magma reserviors. The volcanic system is part of the Eastern volcanic zone of Iceland, and is directly over the Iceland mantle plume. The volcanic system has crater rows extending to the south east; the 25 km long Laki–Grímsvötn fissure system and the 30 km long Rauðhólar-Eldgígur fissure system. The Rauðhólar-Eldgígur fissure system has been related by some authors to an alignment with the Thordarhyrna central volcano with the Laki–Grímsvötn more in alignment with the Grímsvötn central volcano.

Future trends
Studies indicate that volcanic activity in Iceland rises and falls so that the frequency and size of eruptions in and around the Vatnajökull ice cap varies with time. It is believed that the four eruptions between 1996 and 2011 could mark the beginning of an active period, during which an eruption in Grímsvötn in Vatnajökull may be expected every 2–7 years. Parallel volcanic activity in nearby Bárðarbunga is known to be associated with increased activity in Grímsvötn. Seismic activity has been increasing in the area in recent years, indicating the entry of magma.