Hljóðaklettar are elongated volcanic plugs arranged in the dominant fault trend of the area. Through time the Jökulsá river has removed the scoria from the craters leaving the crystalline rock pillars behind. They are part of a crater row which extends for about 3 km along the river. At the northern half, the craters are preserved, although somewhat eroded by the pounding of the river and overrun by a glacier from the east. Columnar basalt characterizes Hljóðaklettar, being arranged in tiers of irregularly curved formations. Lava from the craters can only be tracked over a short distance to the north resting upon the Stóravíti lava. The age of the Hljóðaklettar lava is uncertain but it is thought to be from a similar time as Randarhólar by Hafragil which would make it about 11-12,000 years old. This eruption may well be related to the Krafla volcanic system.