The Leirhnjúkur area is a central volcano in Krafla caldera. One hundred thousand years ago, there was a volcanic cone here that caved in after erupting. The caldera thus formed is now filled with younger volcanic material. Below at a depth of about 3 km, however, is a magma chamber. Volcanic activity in the area occurs at intervals of several centuries.
The Mývatnseldar eruptions (the “Mývatn fires”) in 1724 began with a great volcanic explosion which formed the crater Stóra - Víti. In the following years, a series of earthquakes and eruptions occurred in the vicinity of Mt. Krafla. The greatest eruption took place in 1729, when lava flowed from Leirhnjúkur down to Mývatn lake. Eldhraun, the lava field formed during the eruptions. A new series of eruptions (the "Krafla-fires") began in Krafla in 1975, after an intermission of about 250 years. In the following nine years, nine eruptions occurred.
With a good eyes you can see the last fissure from which lava poured out. It is dotted with small fractures from which steam rises to the sky. Best view is from rim of a small crater near the walking path though it is prohibited to climb on it.