Askja is a caldera in a central volcano in the Dyngjufjöll Mountains. It is the center of a volcanic system with multiple fissures.
The mountains emerged in sub glacial eruptions in a Ice Age glacier cap. Askja caldera was formed, for the most part, at the end of the Ice Age in a major ash eruption which caused the roof of the magma chamber at the heart of the central volcano to subside. A deep circular depression remained that gradually began to fill with lava from subsequent eruptions around the rim of the depression. The bottom of Askja is currently at an altitude of 1100 m while the rim is 1300-1500 m above sea level. The term askja(the Icelandic word for caldera) is used in the names of many similar formations in other locations.
Lake Askja is the deepest lake in Iceland with a depth of more than 200 m. It was formed in 1875 when a powerful eruption occurred in the south of the caldera. Almost 2.5 cubic kilometres of volcanic material surged up from the vent in just a few hours. The process was similar to the sequence of events that originally created Askja. After the eruption, the magma chamber ceiling began to subside, eventually stopping almost 250 metres below its initial level. The depression filled with ground water and Lake Askja was formed.
Between 1922 and 1929, several small eruptions occurred around the edge of the new depression.
Víti – warm lake in a crater
The Askja caldera contains several volcanoes, including Víti, a maar (explosive volcanic crater) formed at the end of the eruption in 1875. Water has accumulated in the crater. It´s temperature is variable, depending on how much melt water is discharged into it in springtime – it is around 30°C on average. The depth of the water is greatest at the centre, more than 8 metres. Víti is a popular bathing site, but if you intend taking a dip, please be aware that the sloping path is very slippery in wet weather and the mud at the bottom is quite hot, especially on the eastern bank. There is also a danger of rocks falling from the edges.
Eruptions and lava
Askja has erupted several times in recorded history. It played a significant part in driving people away from East Iceland after 1875. The most recent eruption in Askja was in 1961 when lava flowed across Vikrahraun. American astronauts trained in this area – the landscape was thought to be similar to that on the moon.
Vikraborgir was the site of further eruptions in 1961. Askja is still active, and its base is still gradually sinking. This unique natural phenomenon is certainly alive and kicking, and it will continue to remind people from time to time that Iceland is still in a state of formation.
The lower slopes of Askja are covered with rough lava.
Askja stands in the Ódáðahraun lava field, a vast wilderness covering 5,000 km2, bounded by the marshland of Bárdardalsdrög in the west, the River Jökulsá á Fjöllum in the east and the barren plains of Mývatnsöræfi in the north. The desolate landscape is broken up by a number of small fertile areas.
The Dyngjufjöll volcanic zone cuts through the region, and the lava fields are the results of many eruptions there at different times during the past.