The Great Þjórsá Lava (Icelandic: Þjórsárhraunið mikla in Icelandic pronounced as /ˈθjour̥sˌaur̥ˌr̥œyːnɪð ˈmɪhkla/) is the largest lava flow in Iceland (by both area and volume) and the largest lava flow[1] that is known to have erupted in a single eruption in the Holocene. Þjórsá Lava has a total volume of more than, covering approximately .[2] The Þjórsá Lava does not appear on the surface until downstream of its identified eruptive area.
In the lowlands of South Iceland the lava has overflown wide areas, covering the districts Landsveit, Gnúpverjahreppur, Skeið and Flói. The main rivers of South Iceland, Þjórsá and Hvítá/Ölfusá, stream along the borders of the lava to the east and west and the 25km (16miles) long beach between the river mouths is formed by the lava. The sea level seems to have been around 15m (49feet) lower than today when the lava was erupted. Along with the rising sea level the ocean has transgressed the lava front so its border line is submerged several hundreds of metres off-shore and its littoral zone can be inspected along the beach.
The towns of Selfoss, Eyrarbakki and Stokkseyri are built on the lava.
The Þjórsá Lava was erupted in the Veiðivötn (Thjorsarhraun) region, Central Iceland, 8600 years BP (6650 BCE ± 50 BC).[1] It belongs to a group of lavas known as the Tungnaá lavas and is mostly covered by younger members of them. The crater area itself is also covered by younger lavas and eruptives. One calculation of its area is around 970km2, the thickness 26m (85feet) and volume therefore close to 26km3. The Þjórsá lava is plagioclase-porphyritic, with large light coloured feldspar phenocrysts sitting in a dark, fine grained ground mass. The plagioclase and the host lava are unrelated and developed from two distinct mantle origins, with the feldspar plagioclase xenocrysts coming from a magma chamber at least deep. The host lava is tholeiitic basalt. Composition is within the range of values observed for the lavas of the Bárðarbunga volcanic system.
The littoral part of the Great Þjórsá Lava can be inspected off the seawalls of Stokkseyri and Eyrarbakki. During low tide the Atlantic waves break at the submerged lava front far off-shore but closer to the beach small channels in between flat lava skerries, grown with seaweed, indicate the landscape.