Þjórsárhraun lava field was formed for about 8,600 years ago due to a huge volcanic eruption in the area around Veiðivötn nearby the centre of the island, around 130 km from the coastline of South Iceland.

The lava flowed down the river path of Þjórsá river in the east and Hvítá and Ölfusá rivers in the west all the way down to the coastline by the two towns of Stokkseyri and Eyrarbakki. It is possible to see the lava field outside of the coastline. It is believed the sea level was about 15 meters lower at the time of the formation of Þjórsárhaun from where it is today.

Þjórsárhraun is one of the biggest lava fields that has flowed on earth since the last ice age about 10 thousands years ago. The lava field covers an area around 970 square kilometres and it is believed to be at least around 26 cubic kilometres. It is still easy to see the lava on the surface in many places between the rivers Þjórsá and Ölfusá.