Mt Hverfjall is a gigantic tephra crater, a so-called Tuff Ring, created 2,500 years ago. It is one kilometre in diameter and the concave is 140 metres deep. Tephra has been carried from Hverfjall, all over the Lake Myvatn area. A landslide apparently occurred in the south part of the crater during the eruption, which accounts for the disruption of the round shape of the mountain.  During the Age of Settlement, lava flowed from Svortuborgir, at the southern end of Namafjall Mountain, around Mt. Hverfjall, which was nearly engulfed by the lava. At the same time an eruption occurred in the slopes above the valley of Hlidardalur.

The rim of the crater is only accessible by two trails, from the northwest and south.  It is strictly forbidden to use other routes up or down.