Grímsvötn is an active subglacial volcano that is located in southeast highlands of Iceland. This volcano reaches an elevation of 5,659 feet and is located in the northwestern side of the Vatnajökull ice-cap. It is comprised of basaltic lava and has the highest frequency of eruptions of any volcano in Iceland. The majority of this volcano resides under Vatnajökull, so its eruptions are primarily subglacial and often cause phreatomagmatic explosions.

Grímsvötn produced an eruption in 1998 that lasted one week, but it did not produce any glacial explosions. Another week-long eruption occurred in 2004 that produced enough ash to reach mainland Europe and disrupt air traffic for a short period. Although this volcano often produces explosive activity, these eruptions were minor and did not produce explosions.
In 2010, harmonic tremors were recorded from the area around Grímsvötn that indicated a future eruption. These coincided with movement detected from the magma within the volcano and were followed by a flow of melt-water from the Vatnajökull glacier, which further supported a pending eruption. The eruption began in May of 2011, producing earthquakes and a high plume of ash that was ten times larger than the ash cloud that occurred in 2004. This eruption was given a rating of VEI4 on the Volcanic Explosivity Index. The eruption began on the 21st of May and began to pause on the 25th, although major activity did not subside until the 26th.

The 2011 eruption did not produce a jökulhlaup, which often occurs during caldera eruptions from this volcano, but it did disturb air travel in many areas including Iceland, Denmark, Ireland, and Germany. This eruption, along with three others that have occurred in the last fifteen years, is thought to show the beginning of an active phase that will produce eruptions from Grímsvötn in Vatnajökull every two to seven years.

Image Caption: Grimsvötn in Vatna Jökull glacier in Iceland on 29 Jul 1972. Credit: Roger McLassus/Wikipedia (CC BY-SA 3.0)