The Great Þjórsá Lava is the largest lava flow in Iceland

Iceland

Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

 both according to area and volume and the largest lava that is known to have erupted in a single eruption in the Holocene

Holocene

The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 time. It belongs to a group of lavas known as the Tungná lavas. It was erupted in the Veiðivötn region, Central Iceland, 8600 years BP (6600 BC). The crater area is covered by younger lavas and eruptives. The Þjórsá lava does not appear on the surface until 70 km downstream of its supposed crater area. In the lowlands the lava has overflown wide areas, covering the districts Landsveit, Gnúpverjahreppur, Skeið and Flói. The main rivers of S-Iceland Þjórsá

Þjórsá

Þjórsá is Iceland's longest river at 230 kilometers. It is in the southern region of the island.Þjórsá is a glacier river has its source on the glacier Hofsjökull. It flows out through narrow gorges in the highlands of Iceland. Further downstream, another river, the Tungnaá, flows into it , before...

 and Hvítá

Hvítá

Hvítá is a river whose source is in the glacier lake Hvítárvatn at Langjökull glacier in the highlands of Iceland at . The river flows for 40 km before dropping down into a narrow gorge at the waterfall Gullfoss....

/Ölfusá

Ölfusá

The Ölfusá is a river in Iceland. It is formed when the rivers Hvítá and Sog combine, just north of the town of Selfoss and flows for 25 km into the Atlantic ocean. It is Iceland's largest river with an average flow of 423 m³/s. Its discharge area is 5760 km². The Ölfusá has a strong salmon...

 stream along the borders of the lava in east and west and the 25 km long beach between the river mouths is formed by. Then the sea level seems to have been around 15 m lower than to day. Along with rising sea level the ocean has transgressed the lava front so its border line is submerged several hundreds of metres off-shore and an interesting littoral zone can be inspected along the beach. The towns of Selfoss

Selfoss

Selfoss can mean either of the following locations in Iceland:*Selfoss *Selfoss...

, Eyrarbakki

Eyrarbakki

Eyrarbakki is a fishing-town on the south-coast of Iceland with a population of about 570 people, not including inhabitants of the prison located there...

 and Stokkseyri

Stokkseyri

Stokkseyri, a small town in Southern Iceland, with a population of around 445 inhabitants.-Overview:Founded around 900 AD by the settler Hásteinn Atlason...

 are built on the lava. Its area is around 970 km², the thickness 26 m and volume therefore close to 26 km³. The Þjórsá lava is porphyritic

Porphyritic

Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

 with large light coloured feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 phenocryst

Phenocryst

thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

s sitting in a dark, fine grained ground mass.

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.