Grímsvötn is Iceland’s most historically active volcano by eruption frequency. It lies beneath the Vatnajökull ice cap in southeastern Iceland, where a central volcano and a southwest–northeast fissure system produce frequent eruptions. Because much of the system is subglacial, its activity can be difficult to observe directly.
The volcano’s eruptions are often basaltic and explosive. When hot magma meets glacier ice and meltwater, it can generate phreatomagmatic explosions, ash clouds, and sudden glacial floods called jökulhlaups. Grímsvötn has erupted repeatedly during the historical period, and geological studies indicate an average prehistoric frequency of roughly seven explosive eruptions per century, though the rate has varied considerably.
The figure “over 100 eruptions” depends on how researchers define an eruption, the time span counted, and whether associated fissures such as Laki are included. The safer established fact is that Grímsvötn has the highest eruption frequency among Icelandic volcanoes. That is why it is commonly contrasted with Hekla and Katla, which are also highly active but have lower documented frequencies.
Its most recent confirmed surface eruption began in May 2011. Earlier notable events include eruptions in 2004 and 1998, while the enormous 1783–1784 Laki eruption occurred within the broader Grímsvötn-Laki volcanic system.