Hallmundarhraun is the largest lava field of the Borgarfiord district. It was probably created in the beginning of the tenth century (Haukur Johannesson 1989), about half a century after the first Nordic settlers came to the country. The lava came through a few craters in the northwestern slopes of The Langjokull glacier.

Two of them are still standing, but the rest has obviously been covered with ice, because a few lava flows appear at the edge of the ice. The lava flows are clearly of "The Shield Volcano Type". The lava flowed across the heaths between the glaciers and the Nordlingafljot River, down through the valley along the mountain Strutur, all the way to the Hvita River.

The total length of the lava is approximately 52 km and it hardly exceeds 7 km in width. It is mostly flat, of the Pa-hoe-hoe type, and naked but for a few spots, where vegetation has started growing near the end of the lava and where it is sanded, close to the place of origin. The big caves, Stefanshellir, Surtshellir and Vidgelmir, are situated in the narrowest stretch of the lava tongue, close to its end.