Þjórsá Lava

The Great Þjórsá Lava (Icelandic: Þjórsárhraunið mikla ) is the largest lava flow in Iceland (by both area and volume) and the largest lava flow that is known to have erupted in a single eruption in the Holocene. Þjórsá Lava has a total volume of more than 25 km3, covering approximately 900 km2. The Þjórsá Lava does not appear on the surface until 70 km downstream of its identified eruptive area.

Geography
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 25 km long beach between the river mouths is formed by the lava. The sea level seems to have been around 15 m 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.

Geology
The Þjórsá Lava was erupted in the Veiðivötn (Thjorsarhraun) region, Central Iceland, 8600 years BP (6650 BCE ± 50 BC). 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 970 km2, the thickness 26 m and volume therefore close to 26 km3. 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 9 km deep. The host lava is tholeiitic basalt. Composition is within the range of values observed for the lavas of the Bárðarbunga volcanic system.

Approach
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.