Grimsvotn refers to subglacial lakes and a volcano of the same name that lie in the western part of Vatnajokull, Europe's largest glacier, in the Icelandic highlands. This volcano is the most active in Iceland.
The volcano is of the the basaltic type and is mostly covered by ice. It measures around 900 m in diameter and up to 1000 m above sea level.
The volcano in the same fissure system as Lakagigar. Lakagigar's' eruption of 1783-4 was one of the greatest in historic times. Grimsvotn erupted at the same time and this eruption lasted until 1785.
Most of Grimsvotn's eruptions have been subglacial and the area has produced great glacier bursts, mostly from the glacier rivers Skeidara, Sula and Nupsvotn, with enormous amounts of water flooding south to the sea.
The name Grimsvotn alludes to the subglacial lakes, but is used for the whole volcano. A part of Grimsvotn volcano that protrudes through the glacier is known as Grimsfjall mountain. There the Icelandic Glacier Research Society has set up a cabin, but access to the cabin is severely limited. A prior agreement with the association has to be reached if one wishes to stay there, and in any case one should never travel the glacier unless veing highly experienced with it or accompanied by seasoned guides.
Grimsvotn has had several eruptions in the recent past. Week-long eruptions occurred in 1998 and 2004, the latter disrupting air traffic for a short term though no glacier floods occured at either eruption.
Some tremor was recorded in 2010 and 2011 saw an eruption that was Grimsvotn's strongest in a 100 years and lasted from May 21st to May 28th. While there were no glacier floods this time, the ash rose as high as 20 km and disrupted air travel widely in Europe, resulting in 900 flights being cancelled.
It is believed that the recent eruptions may mark the beginning of an active period where an eruption may be expected every 2-7 years. Activity in the Bardarbunga stratovolcano has also been associated with an increased activity in Grimsvotn.