Is Iceland’s Tallest Volcano Awakening?
Dave McGarvie, John A Stevenson, Peter Nicholls, 14 Nov 17

Öræfajökull. Dave McGarvie, Author provided

Two women and a boy took refuge on the roof … but it was carried away by the deluge of water, and as far as the eye could reach, the three unfortunate persons were seen clinging to the roof. One of the women was afterwards found among the mud of the jökulhlaup [Icelandic term for meltwater flood], but burnt, and as it were parboild; her body was so damaged and tender, but it could scarcely be touched.

This is an eyewitness account of three fatalities during the last eruption (1727) of Iceland’s highest volcano, Öræfajökull. It was a relatively small eruption. The previous eruption in 1362, however, remains Iceland’s largest explosive eruption since the island was settled about 1,100 years ago.

This time, thick deposits of pumice and ash (also known as tephra) covered the volcano, while sailors at the time reported pumice floating “in such masses that ships could hardly make their way through it”. Ash from the 1362 eruption has been found in Greenland and western Europe recently.

Destructive meltwater floods from both the 1362 and 1727 eruptions travelled down these valley glaciers. Author provided

Since June 2017, there have been “swarms” of small earthquakes in the region. Earthquakes are rare at Öræfajökull, so these have prompted meetings between locals, scientists and civil protection authorities. The unusual activity may indicate a reawakening of Öræfajökull, so it is timely to review previous eruptions and the potential effects of a future eruption.

Towering over 2km above coastal plains, Öræfajökull is a majestic sight. Its upper half is covered in ice that feeds valley glaciers that can be easily accessed. Tourists and filmmakers love it.

We first studied the volcano in 2001-2002. Our two main findings were that it has a variety of eruption styles and a surprising abundance of lavas known as rhyolites. We know that this can erupt very explosively – as it did in 1362.

These glacier-scoured cliffs contain a surprising abundance of old evolved lavas. Image courtesy of Snaevarr Guðmundsson., Author provided

To better understand the 1362 eruption, we have also mapped deposits preserved on the volcano. We found that this explosive eruption was surprisingly variable. It was not just a simple plume that gets gradually weaker, as is common at other volcanoes such as Hekla.

The eruption started by depositing a tephra blanket from a relatively low plume. Next, sticky ash/hail carpeted the volcano. Then the main phase of the eruption began with pyroclastic flows (fast-moving current of hot gas and volcanic matter) racing down the flanks, before a tall plume was established and rained huge pumices down on the land while ash clouds drifted away. As the eruption ended, its explosive energy fluctuated rapidly.

The main implication of our study is that explosive eruptions at Öræfajökull can be complicated. The most powerful phase with the highest plume and widest ash dispersal was probably short-lived (lasting from a few hours to a few days), but there were other stages before and after it with varying degrees of explosivity.

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