verfjall (also known as Hverfell) is a tephra cone or tuff ring volcano in northern Iceland, to the east of Mývatn.
It erupted in 2500 BP in the southern part of the Krafla fissure swarm. The crater is approximately 1 km in diameter.
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 to the round shape of the mountain. During the Age of Settlement, lava flowed from Svortuborgir, at the southern end of Namafjall, around Hverfjall, which was nearly engulfed by the lava. At the same time an eruption occurred in the slopes above the valley of Hlidardalur
http://en.wikipedia.org/wiki/Hverfjall
