The fissure system of the Dyngjufjöll central volcano, with the Askja caldera, extends from the north margin of Vatnajökull some 100 km to the north. Volcanic activity in Dyngjufjöll is associated with two fissure systems, with slightly different strikes. These swarms are tangential to the east and west boarders of the present Askja caldera but intersect south of Dyngjufjöll.

The Askja lake is bounded in the south and east by precipitous cliffs and to the north and west by cliffs cutting the lavas that cover the bottom of the Askja caldera. The present level of the lake is 50 m below the bottom of the caldera. Soundings of the Askja lake reveal a roughly circular crater-like depression with a maximum depth of 224 m. Vigorous geothermal activity is manifest on the eastern and southern boarders of the lake and in wintertime small patches west of the lake remain ice free. A large crater, Víti, just northeast of the subsistence, contains fumaroles and a small pond with warm water.

The 1875 eruption

The first eruption in historical time which is well documented is the explosive eruption of 1875, which resulted in the formation of the nested caldera Öskjuvatn. At the time, however, Askja was practically unknown. Located within a large desert it had never been described.

In summary the sequence of events was as follows:

1. Injection of basaltic magma to shallow depth beneath the Askja caldera as a result of a rifting event affecting a large part of the 100 km long Dyngjufjöll fissure system. The rifting caused intense seismic activity and opening of fissures in the northern part of the system. Beneath Askja the basaltic intrusions were quenched by a large ground water body confined within the structural boundaries of the caldera with a resulting increase in geothermal activity as observed already in the beginning of 1874. Boiling of the ground water body continued for 13 months gradually exhausting the water reserve.
2. By January 1^st 1875 continued basaltic injection finally broke through the ground water barrier resulting in a short lived eruption of Surtseyjan tephra.
3. On February 15^th a significant subsidence was observed et the location of the present Askja lake. This subsidence was caused by the depletion of the water reservoir, a phenomena commonly observed in heavily exploited geothermal areas.
4. On February 18 basaltic eruptions started on the fissure system 40 km to the North of Askja.
5. The basltic intrusions beneath Askja engulfed a small body of silicic magma without intruding it. A slight heating of the silicic magma (not detectable in mineral equilibria) upset the volatile balance and initiated partial degassing and shift in oxygen fugacities of the magma which is displayed in the color stratification of the tephra..
6. With the water barrier removed the silicic magma was mobilised first by injection of dikes along the eastern caldera wall, which served as feeders to a dacitic lava flow.
7. This led to pressure release and explosive boiling of the silicic magma with opening of a ENE-WSW striking fissure(s) and production of fine-grained nonvesicular tephra for about one hour in a plinian eruption. After this initial phase the intensity of the eruption was somewhat reduced for about one hour before the beginning of a main plinian phase, which lasted for about 6 hours.
8. The phreatic explosion crater Víti was formed after the plinian eruption came to an end and phreatic activity continued for several months or years after the eruption.

In the period between 1920 and 1930 four basaltic eruption occurrred in and around the Askja lake subsidence producing small lava fields and building an island in the Askja lake. A fifth eruption occured on the southern flank of the Dyngjufjöll complex during this period.

The latest volcanic event in Dyngjufjöll occurred in 1961.