Lake Mývatn was created about 2300 years ago by a large fissure eruption pouring out basaltic lava.
The lava flowed down Laxárdalur Valley to the lowland plain of Aðaldalur where it entered the Arctic Ocean about 50 km away from Mývatn.
The crater row that  was formed on top of the eruptive fissure is called Þrengslaborgir (or Lúdentarborgir) and has often been used as a textbook example of this type of volcanic activity.
 There was a large lake in the area at the time, a precursor of the present-day Mývatn.
When the glowing lava encountered the lake some of the water-logged lake sediment was trapped underneath it.
The ensuing steam explosions tore the lava into small pieces which were thrown up into the air, together with some of the lake.

By repeated explosions in a number of locations, groups of craters built up and now dominate the landscape on the shore of Lake Mývatn and also form some of the islands in the lake.
 This type of lava formation is known as rootless cones or pseudocraters.
A group of such craters at Skútustaðir on the south shore of the lake is protected as a natural monument and is frequented by tourists.
 Other rootless cone groups in this lava field are in the Laxárdalur Valley and Aðaldalur.
The formation of rootless cones halted the advance of the lava in some places creating temporary lava lakes.
The lava eventually drained from the lakes, leaving behind a forest of rock pillars.
 The biggest of these formations is named Dimmuborgir.
 At another place, Höfði, the pillars stand in the lake water.
 The lava created by the Þrengslaborgir eruption is known as the Younger Laxá Lava.

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.
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 table mountains (Mounts Bláfjall, Sellandafjall, Búrfell, Gæsafjöll), those which didn't formed hyaloclastite 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 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 Laxárdalur 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 Reykjahlíð and Vogar.
Approximately 200 years later a vast lava flow, the Younger Laxá-lava, was erupted (see above).
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 1724–1729 which commenced with an explosion that formed the crater lake Víti.
Later lava flowed from Leirhnjúkur down to the north end of Lake Mývatn, destroying two farms.
The Mývatnseldar eruptions are quite similar in character to the recent volc
Lake Mývatn was created about 2300 years ago by a large fissure eruption pouring out basaltic lava. The lava flowed down Laxárdalur Valley to the lowland plain of Aðaldalur where it entered the Arctic Ocean about 50 km away from Mývatn. The crater row that  was formed on top of the eruptive fissure is called Þrengslaborgir (or Lúdentarborgir) and has often been used as a textbook example of this type of volcanic activity.  There was a large lake in the area at the time, a precursor of the present-day Mývatn. When the glowing lava encountered the lake some of the water-logged lake sediment was trapped underneath it. The ensuing steam explosions tore the lava into small pieces which were thrown up into the air, together with some of the lake.
By repeated explosions in a number of locations, groups of craters built up and now dominate the landscape on the shore of Lake Mývatn and also form some of the islands in the lake.  This type of lava formation is known as rootless cones or pseudocraters. A group of such craters at Skútustaðir on the south shore of the lake is protected as a natural monument and is frequented by tourists.  Other rootless cone groups in this lava field are in the Laxárdalur Valley and Aðaldalur. The formation of rootless cones halted the advance of the lava in some places creating temporary lava lakes. The lava eventually drained from the lakes, leaving behind a forest of rock pillars.  The biggest of these formations is named Dimmuborgir.  At another place, Höfði, the pillars stand in the lake water.  The lava created by the Þrengslaborgir eruption is known as the Younger Laxá Lava.
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. 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 table mountains (Mounts Bláfjall, Sellandafjall, Búrfell, Gæsafjöll), those which didn't formed hyaloclastite 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 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 Laxárdalur 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 Reykjahlíð and Vogar. Approximately 200 years later a vast lava flow, the Younger Laxá-lava, was erupted (see above). 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 1724–1729 which commenced with an explosion that formed the crater lake Víti. Later lava flowed from Leirhnjúkur down to the north end of Lake Mývatn, destroying two farms. The Mývatnseldar eruptions are quite similar in character to the recent volc
Mývatn