Hverfjall is a tuff ring near Lake Mývatn that was created ca. 2500 years ago in a massive phreatomagmatic eruption (hot magma reacting with groundwater). It is part of the Krafla volcanic system.

de.wikipedia.org/wiki/Hverfjall

On the left in the PiP and just SE of Hverfjall is another tuff ring, 6600 years old Lúdent.. The big mountain in the background is Búrfjell, a 959m high tuya (table mountain created by volcanic eruptions under a glacier)