Geology
Iceland is located on the Mid-Atlantic Ridge where two tectonic plates diverge, or what is known as a constructive plate boundary. Iceland is also located above a mantle plume, centred on Vatnajökull. The interaction between the constructive plate boundary and the mantle plume results in complex and diverse volcanic activity in Southern Iceland. The Mid-Atlantic Ridge passes through Iceland and splits into two branches in Southern Iceland. The West Volcanic Zone enters Iceland at the Reykjanes Peninsula and traverses the western part of Southern Iceland. The East Volcanic Zone cuts across the central part of Southern Iceland and extends into Vatnajökull. The majority of the southern lowlands is located between the two zones.
Volcanoes in Katla UNESCO Global Geopark and Vestmannaeyjar
Vestmannaeyjar
Activity in the Vestmannaeyjar has been limited in the Holocene. There was a series of eruptions from 1963 to 1973, first around Surtsey (1963-67) and then at Heimaey in 1973. The system is 30-35 km in length and 20-25 km wide and forms an archipelago 10-30 km off the south coast of Iceland, whose highest point is Heimaklettur at 283 m. The most recent eruption before the last series of eruptions was believed to have been at Helgafell, 5,900 years ago. The archipelago comprises 15 islands and 30 skerries which are the remains of craters and islands formed during the Holocene.
The Heimaey eruption
The Heimaey eruption was a fissure eruption that opened up at the edge of the town and then became concentrated in a single crater, Eldfell. Almost the entire population of the town, around 5,000 people, was evacuated to the mainland on the night the eruption began. Around 400 houses were buried under tephra and lava. The eruption lasted for almost five and half months. Only one person died in the eruption, a victim of poisonous gases in the basement of a house. There was some concern that the lava would close off the harbour, but in fact, it improved it, providing more shelter. Hot lava was used to heat houses in the town for around 15 years after the eruption ended, probably the only case of lava-heated homes in the world.
Eyjafjallajökull
The central volcano Eyjafjallajökull has been relatively active over the last 8,000 years. The last eruption occurred in 2010 when 0.27 km3 of tephra and 0.023 km3 of lava was ejected. Eyjafjallajökull is 1651 m high and is located in the East Volcanic Zone and the central volcano is considered to be an independent volcanic system. The summit contains a 2.5 km wide ice-filled caldera. The upper part of the volcano is largely covered by a glacier that is up to 200 metres thick.
The ash plume from the 2010 eruption consisted of such fine ash that it posed a major threat to jet engines. As a result flight routes and airports across much of Europe were closed for a week in mid-April. Routes from Keflavík to North America did remain open but they nevertheless represented the most wide-reaching restrictions on global flights since the Second World War.
Katla
The Katla volcanic system, which is partly covered by Mýrdalsjökull, has been highly active in the Holocene and the past 11 centuries have seen at least 21 eruptions. The last eruption to break through the ice cap was in 1918 and it was considered a major eruption. People have been waiting for an eruption in Katla for some time; there have been some minor eruptions but they have not penetrated the ice.
Katla is a major central volcano, one of the largest and most active in the country. It is about 30 km in diameter and reaches a height of 1480 m. In the centre of the volcano is the Katla caldera which covers an area of 100 km2 and is up to 700 metres deep. The ice in the caldera is in many places 400-700 m thick. The caldera is divided into three drainage areas: Kötlujökull, Sólheimajökull and Entujökull.
Grímsvötn
The Grímsvötn volcanic system has erupted more frequently than any other volcanic system in Iceland in the Holocene. The system comprises a central volcano and a fissure swarm which is around 100 km long and up to 20 km across. The highest point is Grímsfjall at 1722 m. The most recent eruption was in 2011 which produced approximately 0.8 km3 of tephra. The Grímsvötn system is part of the East Volcanic Zone and much of it lies beneath the thick ice of Vatnajökull. The Grímsvötn caldera is mostly covered by the glacier. Another central volcano in the Grímsvötn system is Þórðarhyrna to the south-west. This has seen little recent activity, erupting most recently in 1903.
Hrómundartindur, Hofsjökull, Tindafjallajökull and Esjufjöll are volcanoes in Southern Iceland which have only seen limited eruptions in the Holocene.
Major eruptions outside or on the edge of central volcanoes (in chronological order)
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 class 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.