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

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

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

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

. Þjórsá Lava has a total volume of more than , covering approximately . The Þjórsá Lava does not appear on the surface until 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 Þjórsá () is Iceland's longest river at . It is in the south of the island. Þjórsá is a glacier river and has its source on the glacier Hofsjökull. It flows out through narrow gorges in the highlands of Iceland. Further downstream, a ...

and Hvítá/

Ölfusá The Ölfusá () is a river in Iceland. It begins at the junction between the Hvítá, Árnessýsla, Hvítá and Sog River, Sog rivers, just north of the town of Selfoss (town), Selfoss, and flows for 25 km into the Atlantic Ocean. It is Ice ...

, 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 its littoral zone can be inspected along the beach. The towns of Selfoss,

Eyrarbakki Eyrarbakki () is a fishing village on the south coast of Iceland with a population of about 570 people, not including inhabitants of the prison located there. The village is founded on the Great Þjórsá Lava. History For centuries, the harbou ...

and

Stokkseyri Stokkseyri () is a small in Southern Iceland, with a population of around 445. Overview Founded around 900 AD by the settler Hásteinn Atlason, it was an important fishing and trading village in previous times. The town is founded on the Great ...

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 , the thickness and volume therefore close to . The Þjórsá lava is

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

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 ...

, with large light coloured

feldspar Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s 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 Bárðarbunga (, alternative name Veiðivötn), is an active and productive stratovolcano located under Vatnajökull in Vatnajökull National Park which is Iceland's most extensive glacier. The second highest mountain in Iceland, above sea le ...

volcanic system.

Approach

The littoral part of the Great Þjórsá Lava can be inspected off the seawalls of

and

. 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 List of townlands in County Armagh#S, townland in Coun ...

, grown with seaweed, indicate the landscape.

References

Sources

* * * 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