Geology of Iceland
Iceland is a volcanic island located in the North Atlantic Ocean between Greenland and Norway between 63°23’N to 66°30’N and between 13°30’W to 24.30’W. The island is part of a much larger landmass situated at the junction of two large submarine physiographic structures, the Mid-Atlantic Ridge and the Greenland-Iceland-Faeroes Ridge.
It is thought to have been formed about 24 million years ago from magma welling up through the Mid-Atlantic Ridge and is the only place on earth where the ridge rises above sea level. Iceland is sitting on the junction of the North American Plate and the Eurasian Plate and is slowly being pulled apart by them at approximately 2 cm per year, that’s about 1 cm in each direction. However, it is also sitting over the Iceland mantle plume or hotspot which has been continuously forcing magma up from the earth’s interior for some 65 million years and fuels the island’s volcanoes. This view has, however, been challenged by Foulger and Anderson who suggest “processes related to plate tectonics that are sourced in the shallow upper mantle”.
Although the birth of Iceland dates from around 24 million years ago, the oldest rock on land is a relatively young 18 my and this has been continually added to by frequent volcanic eruptions. Approximately 11.05% of the land area of the island (about 11,400 km² out of the total area of 103,125 km²) is covered by glaciers, and the majority of the present landscape was fully developed by the beginning of the Holocene period, about 9,700 years ago, with the exception of the river courses and canyons which were formed after the melting of the Younger Dryas glacier, and the active volcanic zones which are being actively modified by volcanic activity.
There are 30 active volcanic zones and 20 central volcanoes that have been active in the recent geological past.The volcanic zones consist of a central volcano, a fissure swarm or a combination of both, The fissure swarms which are elongated strips between 5-20 km wide can extend for between 50 to 200 km, whilst the eruptions produced can be either short-lived (days to weeks) or long-lived (months to years).
Volcanic eruptions can be divided into three types:
- explosive, with little or no lava, but a large volume of tephra (all fragments of rock including ash, ejected into the air by an eruption)
- effusive, mainly consisting of lava flows and
- mixed, which, as its name suggests, has properties of both the previous types.
Around midmorn on Whitsun, June 8th of 1783, in clear and calm weather,a black haze of sand appeared to the north of the mountains nearest the farms of the Siða area. The cloud was so extensive that in a short time it had spread over the entire Siða area and part of Fljótshverfi as well, and so thick that it caused darkness indoors and coated the earth so that tracks could be seen. The powder which fell to earth looked like burnt ash from hard coal. A light drizzle, which fell from that black cloud that day over the Skaftártunga region, turned this powder into black, inky liquid.
Thus began, in the words of Jón Steingrimsson, the second largest basaltic lava flow eruption in recorded history, the first being that of Eldgjá in 934-940. During 8 months of activity, Laki spewed out 14.7 ± 1 km3 = 4.0 x 1013 kg of lava, 0.4 km3 = 1.1 x 1012 kg of tephra, 122 Mt. i.e. 122,000,000 tonnes of sulphur dioxide, 235 Mt. water vapour, 15 Mt. of fluorine and 7 Mt. of chlorine. To put this into perspective, the total tephra released was double the amount of the 1980 Mt. St. Helens eruption, whilst the volume of lava ejected was great enough to have been able to bury the square mile (2.5898 km2) of the City of London to a depth of ~5.7km.
Laki is not a single volcanic cone, but a 27 km long vent system comprising 10 volcanic fissures or cones and over 140 vents, extending in a north easterly direction from the Ulfarsdalur Valley in the west towards the Sidujökull glacier in the east. In its turn, Laki is part of the Grimsvôtn volcanic system which consists of the Grimsvötn central volcano and a volcanic fissure swarm approximately 100 km long and 15 km wide.
To be strictly accurate, the Laki eruption of 1783-1784, which commenced on the 8th of June, 1783 and ceased on the 7th February, 1784, represents the main volcanic event of a two-year-long volcano-tectonic episode within the Grimsvôtn system lasting from May 1783 to 26th May 1785. In Iceland, this is often called the Lakagígar or Skaftáreldar (Skaftár fires) eruption.
Most of the eruptions began with an earthquake swarm of increasing intensity, leading to the emergence of a new fissure segment. This was followed by an explosive eruption and a massive increase in the outpouring of lava. The remaining two earthquake swarms do not appear to be directly connected to either the Laki or Grimsvôtn eruptions. By the end of October, nearly 90% of the total volume of tephra had been emitted
Large explosive volcanic eruptions, for example, Semlas, 1278, Tambora, 1815, and Pinatubo, 1991, inject large amounts of sulphur dioxide, other volatiles and volcanic dust directly into the stratosphere, and this may significantly affect climate worldwide, whereas fissure eruptions such as Laki, seem only to have sufficient power to send the majority of their ejecta into the middle and upper troposphere and a much smaller quantity into the lower stratosphere. It has been estimated that Laki’s explosive eruption columns ejected gases and volcanic dust from between 9km to 13km into the atmosphere, and of these, 2/3 remained in the troposphere and were rapidly transported by the westerly jet stream towards the high pressure zone positioned over Europe, whilst the remainder persisted in the stratosphere for 12 months or more
A trough of low pressure existed over Iceland, whilst there was a zone of high pressure over Europe. Any volatiles and dust which were confined to the troposphere would have been drawn to high altitude and transported by the ambient air currents which thus facilitated the transference of the eruption cloud to the high pressure cell. The gases would then have been concentrated near the surface by the descending air column, where they would, in some instances, have been strong enough to damage vegetation and possibly be the cause of abnormal weather conditions.
The following passage, again from Steingrimsson, gives one a very vivid idea of how frightening the eruption must have appeared.
“On the 12th the weather was clear, with a wind from the south. Now the flood of lava spilled out of the canyon of the River Skaftá and poured forth with frightening speed, crashing, roaring and thundering. When the molten lava ran into wet-lands or streams of water the explosions were as loud as if many cannon were fired at one time. At first this fiery flood followed the main course of the river, and then spread over the banks and out over the older lava fields which stretch out on both sides.” (STEINGRIMSSON, Jón, Fires of the Earth: The Laki eruption1783-1784,pp 25-26.)
The consequences of this event were catastrophic for Iceland. The despoliation of pasture and farm land by volcanic ash and polluted rainfall, resulted in the death of more than 60% of the grazing livestock, mainly through chronic fluorosis. Over 10,000 cattle, 27,000 horses and 190,000 sheep died between 1783 and 1784. It is estimated that these figures represent approximately 50% of the total stock of cattle, 75% of the horses and nearly 80% of the sheep. As mutton made up a large proportion of the people’s diet, these losses were devastating, and widespread starvation ensued, killing over 20% of the population, i.e. approximately 10,000 people.
After 1786 the population rapidly began to recover, having an annual growth rate of some 1.6%, and by 1824 the population had surpassed pre-eruption levels.
To be continued…..
Related article: Volcanic Ash Shrouds the Islands of the Kingdom of Tonga