The Great Þjórsá Lava ( Icelandic: ''Þjórsárhraunið mikla'' ) is the largest lava flow in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...

(by both area and volume) and the largest lava that is known to have erupted in a single eruption in the

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

. It belongs to a group of lavas known as the Tungnaá 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 of South Iceland the lava has overflown wide areas, covering the districts Landsveit, Gnúpverjahreppur, Skeið and Flói. The main rivers of South Iceland,

Þjórsá Þjórsá () is Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykja ...

and Hvítá/

Ölfusá The Ölfusá () is a river in Iceland. It begins at the junction between the Hvítá and Sog rivers, just north of the town of Selfoss, and flows for 25 km into the Atlantic ocean. It is Iceland's largest river by volume with an averag ...

, stream along the borders of the lava to the east and west and the long beach between the river mouths is formed by the lava. The sea level seems to have been around 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 an interesting littoral zone can be inspected along the beach. The towns of

Selfoss Selfoss may refer to: *Selfoss (town), Iceland *UMF Selfoss, a football club based in Selfoss **Selfoss men's football **Selfoss women's football *Selfoss (waterfall), Iceland *Selfoss Airport Selfoss Airport is an airport serving Selfoss, a to ...

, Eyrarbakki and Stokkseyri are built on the lava. Its area is around , the thickness and volume therefore close to . The Þjórsá lava is

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning a ...

with large light coloured

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...

phenocryst 300px, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter 2.3 cm) for scale. A phenocryst is an early forming, relati ...

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 A skerry is a small rocky island, usually defined to be too small for habitation. Skerry, skerries, or The Skerries may also refer to: Geography Northern Ireland * Skerries, County Armagh, a townland in County Armagh * Skerry, County Antrim, a ...

, grown with seaweed, indicate the landscape.

References

* Árni Hjartarson 1988: „Þjórsárhraunið mikla - stærsta nútímahraun jarðar“. ''Náttúrufræðingurinn'' 58: 1-16. * Árni Hjartarson 1994: „Environmental changes in Iceland following the Great Þjórsá Lava Eruption 7800 14C years BP“. In: J. Stötter og F. Wilhelm (ed.) ''Environmental Change in Iceland'' (Munchen): 147-155. {{DEFAULTSORT:Thjorsa Lava Bárðarbunga East Volcanic Zone of Iceland Holocene volcanism Lava fields Lava flows