Iceland’s Pompeii

Öræfajökull is Iceland’s tallest (2110 m) and largest volcano, with a caldera 5 km across. This caldera is filled with ice and glaciers spill down its valley sides. There is evidence that ice has been present for a long period during the volcano’s formation, probably during the last 780,000 years when the bulk of the volcano was built. When a volcano erupts under ice there is explosive volcanism because of the creation of steam. The flanks of the volcano consist of lava flows, dikes (intruded sheets of magma), glassy tuff (volcanic ash airfall), breccia (fragmented blocks), and tillites (glacial sediments turned to rock). The relationship between these various layers is complex because of erosion and deposition through time. One unusual feature of this volcano is that it erupts both mafic (rocks with dark, heavy minerals) and felsic (rocks with pale, light minerals) magma. This occurs because magma can sit beneath the volcano for a long period of time and the minerals separate out. A bit like vegetable soup left on the hotplate too long, froth can accumulate on the top and the heavier pieces sink to the bottom. After a long period of quiescence, the “frothy” magma can cause violent explosive eruptions.

The two most recent eruptions of Öræfajökull (originally known as Knappafell) have occurred since the settlement of Iceland by people (~874 CE). In a previous post I described the site where the first settlers arrived. Ingólfshöfði lies on the coast south of Öræfajökull. The area around Öræfajökull was known originally as Herad, which has the best weather in Iceland. The population of Herad in the 14th century was at least 220, a high number in the sparsely populated Iceland. Ancient documents record that the volcano erupted early in Iceland’s history “…causing desolation for a distance of some 100 miles”. The incredible written record of Iceland allows it to be concluded that the eruption occurred in mid June, 1362 CE. The explosion produced some 2 km3 of explosive material. The present caldera has an area of about 20 km2 so this presents a small component of the eruptions which have formed the volcano. Nevertheless, it is probably the third biggest explosive eruption in Iceland since the last ice age. One account describes that “No living creature survived except one old woman and a mare”. The farms around Öræfajökull that were not destroyed by floods were buried in volcanic ash. After this date, the area was renamed Öræfi (meaning wasteland) and the volcano was renamed Öræfajökull.

The section above is cut into the back of a moraine near Skaftafell. The position on a slope means that slope erosion makes the section much thicker than it ordinarily would be. The section records multiple tephra layers. Tephra is the general term for all rock fragments ejected during a volcanic eruption, regardless of size. The term was coined by the father of the study of volcanic ash, Sigurður Thoraninsson. Sigurður was an accomplished Icelandic geologist, glaciologist and volcanologist and had great foresight into the value of tephra to explore the age and evolution of the landscape. He established tephrochronology as a means of using the history of volcanism to establish the timing and rates of other events, such as soil erosion or climate change. Our knowledge of the Öræfajökull eruption and the distribution of its tephra was established by Sigurður in 1958.

The section records the eruption of Öræfajökull soon after the moraine was formed. The lowest layer begins in washed gravel. This is the sediment deposited by a glacier flood or jökulhlaup (Icelandic for “glacier run/burst”). The enormous release of heat during an eruption melts the ice at the base of the ice cap. After the caldera fills with water, it drains down the valleys around the volcano as floods. These floods are very destructive and cause significant erosion of sediments and rock on the mountain side. The layer above the gravel is made of fine ash and pumice. Pumice is a glassy, porous, and consequently very light, rock formed when the gas-rich froth of lava cools quickly. Within this layer is a band of fragmented basalt. This wasn’t lava from the eruption but is part of the caldera which exploded. Overlying the tephra is another bed of jökulhlaup gravel, indicating a second flood. This is overlain by a layer of clay which represents standing water after the flood swept through. Over this there is another return to tephra fallout, including a very fine bed representing a break in the eruption.

Over the top of this rather thick sequence of flood gravel and tephra is another sequence containing tephra. There are no palaeosols (old soils), indicating that deposition was more or less continuous from 1362 CE up until today. The sediment is mostly composed of loess (windblown dust, from German, meaning “drop”). This sediment originates from the gigantic Skeiðarársandur (sand plain of the Skeiðarár river). This sandur is the largest glacial outwash plain on Earth, and produces vast quantities of finely ground dust from the skeiðarárjökull (skeiðarár glacier). Within the loess there are four black tephra bands. These tephras are common in soil profiles on the east side of Öræfajökull. Identifying them is rather difficult, because the colour and geochemistry tends to be quite similar. However, the relative position of each allows some educated guesses to be made. The oldest tephra is probably from the great volcano Katla in 1416. The next one is either from the neighbouring volcano Grímsvötn in 1650 or from the fissure eruption at Veiðivötn in 1477. The third could be one of 3 eruptions; Laki 1783-4, Katla again in 1755, or from Öræfajökull again in 1727. The last tephra is probably Katla, once again, in 1918.

Tephrochronology reveals that Iceland’s volcanoes are regularly erupting. It’s not a matter of if Öræfajökull will erupt explosively again, wiping out all life on its flanks, its a matter of when.

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