Volcanic Activity in Iceland
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Why are volcanoes found in Iceland?
Iceland is a volcanic island sited on the mid oceanic Atlantic Ridge, which is the boundary between the North American and Eurasian plates. Known as a constructive plate margin, new crust continually forms in volcanic activity along the ridge as the plates drift apart at a rate averaging one centimetre per year. Places where the activity is frequent and intense are known as hot spots and one such hot spot lies under Iceland, which explains why it is much larger than many other volcanic islands. Where hot spots occur, the earth's crust is thin. In Iceland it can be just a few kilometres thick as opposed to 40-60 km thick in much of Europe. Tectonic movements and mounting pressure cause the thin crust to crack, forming swarms of fissures below and at the surface. Magma enters the fissures and when it reaches the surface, it causes a volcanic eruption.
How many volcanoes are there in Iceland?
Within Iceland, volcanic activity in the last 10,000 years has been centred on two active zones, the first running from Reykjanes to the Langjökull icecap and the second from the Westman Islands to the Melrakkaslétta Peninsula in the north east. Outside these main zones are two further areas of activity: the Snæfellsnes Peninsula in the west and the volcanic massif of Öræfajökull, in the south-east of the island.
There are around 30 volcanic systems in Iceland, broadly divided into three types:
- Strato volcanoes, e.g. Eyjafjallajökull, Öræfajökull, Snæfellsjökull. These are volcanic systems which erupt repeatedly over thousands, even millions of years and build up substantial mountains, topped by a crater. Hekla is a strato volcano, but differs from the others in that it is formed along a ridge.
- Central volcanoes, with an underlying magma chamber, which feeds surrounding fissure swarms, e.g. Krafla, Askja, Grímsvötn, Katla. Calderas are often present, forming when subsidence follows sudden emptying of the magma chamber, whose roof collapses.
- Fissure swarms and crater rows, e.g. Eldvörp on Reykjanes – these are often associated with central volcanoes.
Each volcanic system can be as many as tens of kilometres in length and several kilometres wide.
How often do you get an eruption in Iceland and how long do they last?
On average there is one eruption in Iceland every 5 years but during the last 100 years, eruptions have been more frequent. Most eruptions in Iceland are rather short-lived, lasting only a matter of days or at most a few weeks, but occasionally like that of Surtsey 1963-67, they can last several years.
One hundred years of eruptions in Iceland
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1910 Grímsvötn |
1945 Grímsvötn |
1983 Grímsvötn |
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Almost all types of volcanic activity found on earth occur in Iceland and the range of volcanic features is extensive and of great interest to study groups as well as general enthusiasts. What's more, the lack of tree cover and vegetation means that they are easy to recognize and photograph.
Why do some eruptions produce ash, some lava and others both?
Volcanic products are either lava or explosive pyroclastic material, which includes pumice, ash and tephra. Most eruptions in Iceland are a mix of both types, the ash and tephra usually erupted in the earlier stage of the eruption, which is often explosive and gassy, while lava issues in the later, calmer stages. Grænavatn on Reykjanes and the tephra crater Hverfjall at Mývatn are examples of craters from explosive activity. Flood eruptions produce only lava and are quite unusual in Iceland, forming shield volcanoes like Skjáldbreiður and spatter ring craters like Eldborg on Snæfellsnes. Eruptions that occur under the icecaps are explosive and produce almost exclusively ash and tephra, because of the contact between magma and water from melting ice. Lavas erupted in the last 10,000 years in Iceland cover about one-tenth of the island's surface and their total volume amounts to about 400 km³.
What is the most common type of eruption in Iceland?
Fissure eruptions are very common in Iceland and are responsible for most of the crater rows that dot the landscape within the volcanic zone – you can even see them from the plane as you come into land at Keflavík Airport. Rows of cinder or scoria craters form when lava fountains concentrate activity at several vents along a fissure and the material builds up around the vent. Such craters are not usually very high. The longest crater row in Iceland is the Laki crater row where 100 craters are strung out along a 25 km long fissure.
Are there different types of lava in Iceland?
Volcanoes produce acidic (rhyolite) or basic (basalt) rocks and sometimes both types. The acidity is determined by the origin of the magma and its silica content – more than 65% produces acidic rocks, which are typically light in colour, whilst basalt is dark. The best known rhyolite mountains in Iceland are those around Landmannalaugar.
Thin, fast-moving basalt lava is usually low in gas and typically forms shield volcanoes as the flows are layered one on top of another. As the surface hardens, lava continues to flow underneath and the surface becomes wrinkled, forming ropes, giving it the name ropy lava. When the lava cools its surface is quite smooth and easy to walk on. Such lava is called 'helluhraun' meaning 'plate lava' in Icelandic but the Hawaiian name 'pahoe-hoe' is often used. Ropy lava sometimes flows from the crater in submerged channels and when these empty at the end of an eruption, they form lava tubes, also known as lava tunnels. Scoria lava is also basalt but gassy, more viscous and slower flowing and its movement is a bit like earth or rocks being pushed by a bulldozer. The Hawaiian name 'aa' is often used, while in Icelandic it is known as 'apalhraun'. On cooling, the surface is rough, irregular and very difficult to walk over. Acidic or rhyolite lava is rather rare in Iceland and is even more viscous and slow-moving. It cools to a shiny glass-like finish.
Why does new lava look different from old lava?
New lava is black and shiny when it hardens. Minerals coat the surface giving it a colourful 'oily' sheen. As time passes, weathering removes this and the rock become more subdued in colour. At the same time steam rising from the lava allows moss and lichen to begin growing and in time these turn the lava grey.
Where is geothermal activity found in Iceland?
Geothermal fields are divided into high and low temperature areas. High temperature areas are found within the volcanic zone and typically produce steam vents and solfataras, while low temperature areas are found in geologically older rocks either side of the volcanic zone and consist mostly of hot springs. Most of Iceland's domestic heating needs are met by geothermal hot water but the east and south-east of the country are least endowed with this valuable resource. Geothermal activity is usually prolific and intense at eruption sites following the end of an eruption.
Is it dangerous to visit Iceland when an eruption is happening?
Generally speaking no but there may be a small area closed off to visitors around the eruption site, depending on what stage the eruption is at and where it takes place. The early stages of the current eruption were a perfect 'Tourist Eruption' in that access to the site was possible and the eruption itself was very beautiful. Curiously, even when the gas plume from the most recent eruption was at its most productive, 95% of the island, including the entire capital area, was completely unaffected by it. Icelanders have a sophisticated surveillance of seismic and volcanic activity and predictions of the likelihood of an eruption are accurate.
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