Revised age of the Hallmundarhraun lava, West Iceland

For decades, the Hallmundarhraun lava has been categorized as a historic lava, i.e. postdating the Landnám tephra layer (LNL) from AD 877. In the summer of 2024, the LNL was found on top of the lava, somewhat unexpectedly. In 2025, approximately twenty test pits were excavated to corroborate this initial finding. In all cases, the presence of the LNL was confirmed.

In connection with this study, a sample of barren plant remains was collected from beneath the lava and submitted for radiocarbon (¹⁴C) dating in Aarhus, Denmark.

The LNL is a widespread, two-coloured tephra, consisting of a lower light-coloured unit (c. 0.5 cm thick) of fine silicic pumice and an upper olive-green unit (1.5–2 cm thick) composed of basaltic glass shards. The LNL is one of the most important marker tephra layers in Iceland. It was found close to the lava surface, commonly separated from it by a 1–3 cm thick soil layer, although in some cases the soil cover was thinner.

In total, six distinct tephra layers were identified within the soil cover of the Hallmundarhraun lava. Samples from all layers were analysed chemically using an electron microprobe. The tephra layers younger than the LNL are, in descending order, H-1766, K-1721, H-1693, and H-1104. The oldest tephra layer identified is a black Katla tephra lying directly on top of the lava, with no intervening soil layer. This suggests that the Katla tephra and the lava are close in age.

A literature review was conducted to identify information that might constrain the age and distribution of this Katla tephra. Although the results were not conclusive, a possible correlation was identified with a widespread Katla tephra known as Hrafnkatla. This tephra has been identified in ice cores from the Greenland Ice Sheet and dated to AD 763 based on annual layer counting. The Katla tephra overlying the Hallmundarhraun lava may correlate with the Hrafnkatla tephra; however, as two other Katla tephra layers of similar age have been identified in soils and lake sediments, this correlation remains uncertain.

Taking all available evidence into account, the results indicate that the Hallmundarhraun lava most likely formed during the period AD 760–780. The radiocarbon dating supports this interpretation. Previously, the lava was thought to have formed between AD 910 and 950. The Hallmundarhraun eruption therefore predates the Norse settlement of Iceland in the mid-to-late 9th century, effectively excluding the possibility of eyewitness observations or contemporaneous written accounts.