The two craters of the lava-ring type are the southwesternmost of five craters on a NE-SW trending crater row, of which (Stóra) Eldborg is the largest. Lava rings build up as ramparts from the spatter of lava fountains, in this case from the two erupting chimneys. The ramparts steepen upwards. They consist of 5-10 cm, rarely up to 20 cm, thick sheets, each resembling a flow unit increasing in vesicularity upwards. The lava flowed out through openings in the crater rims into lava channels showing the same shelly pahoehoe structure as the crater ramparts.
The lava of Eldborg is an olivine-rich pahoehoe. The age is not well known, but judging from the soil cover, gravel deposits covering it and the effects of weathering on its surface, it might be 7000-8000 years old.
Kristján Sæmundsson, 2010