Þjórsárhraun
The great lava expanse of Þjórsárhraun is the largest lava field in Iceland, both in terms of area and volume, and is the largest known lava flow on Earth from a single eruption since the end of the last ice age around 10,000 years ago. The lava ran between Hekla and Búrfell and through the lowlands along the course of the river Þjórsá and flowed 1 km out to sea at Eyrar. The lava is pāhoehoe and originated from volcanoes in the Veiðivötn area around 8,600 years ago. This eruption was presumably part of the Torfajökull volcanic system.
Vatnaöldur 870
Just as the first settlers were thought to have been arriving in Iceland, a major eruption was coming to an end at Vatnaöldur in the Fjallabak area. A fissure at least 10 km long opened and ejected 3.3 km3 of tephra and a small amount of lava. This eruption and a simultaneous eruption in the Torfajökull system produced what is known as the “settlement layer,” a bicoloured layer of tephra. This layer of ash is often referred to when scientists need to date archaeological remains in the ground. Major eruptions in the Veiðivötn area often appear to have accompanied unrest in the Torfajökull system. The eruption produced substantial ash fall across Iceland, except the West Fjords.
Eldgjá 934 – 940
Iceland certainly didn’t give its first settlers a warm welcome. Only 65 years after the huge eruption at Vatnaöldur came to an end, the largest eruption since settlement began at Katla and Eldgjá. This eruption produced massive glacial outbursts and the volcanic fissure grew in both directions for weeks and months. It stretched under the Katla caldera and produced a huge volume of tephra in a cataclysmic eruption. The fissure then extended to the north and created the canyon Eldgjá. The northernmost part of Eldgjá is around 60 km from the glacier which speaks volumes about the scale of the eruption, even though the eruption did not occur along its entire length at the same time.
An enormous amount of lava flowed out of Eldgjá and reached all the way to the sea at Álftaver. Recent studies indicate that the lava from Eldgjá has a volume of 18 km3 and covers an area of 800 km2, making it larger than the Skaftáeldar eruption. Lava from the latter eruption actually flowed over part of the Eldgjá lava. It is thought that the tephra from the eruption has a volume of 5-7 km3 and this amount, irrespective of the lava, is sufficient to classify it as a major eruption. The tephra layer from the Eldgjá eruption is also used to date archaeological finds and is about 65-70 years younger than the settlement layer.
The Eldgjá lava created two well-known pseudocrater clusters, Álftaversgígar and Landsbrotshólar. Álftaversgígar are protected pseudocraters and Landbrotshólar is the most extensive area of pseudocraters in Iceland, covering an area of 50 km2. Pseudocraters are formed when lava flows into or over rivers, lakes or marshes. When 1100°C hot lava comes into contact with water it causes huge explosions as the water vaporizes immediately. This rips apart the lava, hurling tephra into the air and forming hummocks and pseudocraters. Pseudocraters are found in many parts of Iceland, a particularly well-known cluster being at Mývatn. The rock formations Rauðhólar near Reykjavík are the remains of pseudocraters.
Skaftáreldar – Lakagígar
The Skaftáreldar eruption (“Skaftá Fires”) began on Whitsunday, 8 June 1783, in a 27 km long fissure which eventually became the craters known as Lakagígar, and lasted until February 1784. Skaftáreldar produced the largest flow of lava from a single eruption in the last one thousand years, spewing out a total volume of 12-14 km³ and covering an area of 580 km². However, the eruption at Eldgjá might in fact rank ahead of Skaftáreldar as the largest eruption in historical times. Lakagígar form part of the Grímsvötn volcanic system.
The eruption also produced a great amount of ash and poisonous gases which enveloped the country. A thick volcanic haze, rich in sulfurous gases, filled the atmosphere and its effects could be felt throughout the northern hemisphere. The following winter was harsh across Europe, and it is believed that famine in France following the eruption in Iceland may have precipitated the French Revolution. The winter was particularly severe in Iceland. Livestock died and famine raged across the country. The period of hardships following the Skaftáreldar eruption is known in Iceland as móðuharðindin or the famine of the mists, named after the volcanic haze. It lasted until 1785, by which time an estimated 80% of sheep had died, 60% of horses and 50% of cattle. More than 10,000 people in Iceland died in this disaster, more than 20% of the population of Iceland.