Geology: an overview
The Mývatn district lies on the western border of the volcanic zone which cuts across north-eastern Iceland from north to south and is an extension of the Mid Atlantic Ridge. All geological formations are quite recent, dating from the Ice Age and postglacial times.
The bedrock of the moors west of Lake Mývatn is made up of interglacial lava flows (=lava flowing during warm epochs of the Ice Age). Most of the mountains in the vicinity of the lake were formed by eruptions under the ice sheet in the glacial periods of the Ice Age. Eruptions that melted their way up through the ice formed tablemountains (Mounts Bláfjall, Sellandafjall, Búrfell, Gæsafjöll), those which didn't formed palagonite (or tuff) ridges (Mounts Vindbelgjarfjall, Námafjall, Dalfjall, Hvannfell).
At the close of the Ice Age, about 10,000 years ago, the Mývatn basin was covered by a glacier which pushed up huge end-moraines which can still be seen at the north end of the lake. After the glacier started melting, a glacial lake was dammed up in the Mývatn depression until the glacier retreated from the present course of Laxá river.
Postglacial volcanism in the Mývatn district may be divided into three distinct cycles. The Lúdent cycle commenced shortly after the close of the Ice Age. The explosion crater (tephra ring) Lúdent dates from this cycle. Its eruption was followed by a number of small fissure eruptions. About 3800 years ago the shield volcano Ketildyngja was formed about 25 km south-east of Mývatn, and from it a huge lava flow, the Older Laxá-lava, spread over the southern part of the district, plunged down the Laxßrdalur valley and flowed almost to the sea. This lava dammed up the first Mývatn, which was about as large as the present lake.
The second volcanic cycle, the Hverfjall cycle, began 2500 years ago with a gigantic but brief eruption, which formed the explosion crater (tephra ring ) Hverfjall (also named Hverfell). An eruption in Jardbadshólar followed, producing the lavafield between Reykjahlí and Vogar. Approximately 200 years later a vast lava flow, the Younger Laxá-lava, was erupted in the Threngslaborgir and Lúdentarborgir crater rows east of the lake. This lava covered the southern part of the district, including the old Mývatn and followed the course of the older lava down Laxárdalur and Aaldalur valleys all the way to the ocean. The pseudocraters around Mývatn were formed through steam explosions when the lava plunged into the lake. Dimmuborgir and the lava pillars at Kálfaströnd are remnants of emptied lava lakes that formed during the eruption. The lava dammed up the present Lake Mývatn and also the lakes Sandvatn, Grænavatn and Arnarvatn.
The third volcanic cycle began with the Mývatnseldar eruptions in 1724-29 which commenced with an explosion that formed the crater lake Víti. Later lava flowed from Leirhnj˙kur down to the north end of Lake Mývatn, destroying two farms. The Mývatnseldar eruptions are quite similar in character to the recent volcanic activity near Krafla in 1975-84. The source of both is a central volcano lying between Krafla and Gæsafjöll. Inside the volcano resides a magma chamber from which molten magma periodically bursts into a swarm of fissures that cut through the volcano from north to south. The recent activity was characterized by periods of slow land rise, interspersed by shorter periods of rapid subsidence, underground magma bursts, rifting, earthquakes and eruptions. This is an excellent example of the process of continental drift in Iceland. A central volcano and its associated fissure swarm is called a volcanic system. The Krafla volcanic system is one of several such systems which together form the volcanic zone of Iceland.
A few rhyolite mountains border the Krafla central volcano (Mounts Hlíarfjall, Jörundur, Hrafntinnuhryggur). The Dyngjufjöll mountains with the Askja caldera, about 50 km south of Lake Mývatn, are another central volcano visible from the lake.