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Askja and Herðubreið, The Start of Our Exploration of the Northern Volcanic Zone, Iceland

Good Morning!

As the new volcano at Geldingadalur continues to grow, opening and closing new fissures, we have returned to our tour of Iceland.  We have now reached the Northern Volcanic Zone, where the plate boundary heads northwards from Kverkfjöll to meet the Tjörnes Fracture Zone.   Active volcanoes in the zone are Kverkfjöll, Askja, Fremrinámur, Heiðarsporðar, Krafla and Þeistareykir; Herðubreið, itself, is Pleistocene palagonite table-mountain.

We are starting with the currently most seismically active volcanoes, Askja and Herðubreið, located where the Eastern Volcanic Zone meets the Northern Volcanic Zone, north of the Vatnajökull ice-cap. The mantle plume, itself, is thought to be located to the north west of the Vatnajökull ice-cap.


Fig 1: Combined images of Askja, cropped from photos by Michael Ryan, 1984 (U.S. Geological Survey): Askja Shield (top) and Askja Caldera (bottom) from GVP

The Askja volcanic system comprises a 1,516 m high central volcano and 190 km long fissure system, the central volcano being the Dyngjufjöll massif. It has three nested calderas, the latest of which formed in a rhyolitic eruption in 1875.  The central volcano, itself, is made up of Pleistocene glacio-volcanic tuffs, hyaloclastites, pillow basalts and intercalated sub aerial lava and capped by Holocene sub aerial lavas and pumice.  The fissure system, itself, extends from beneath the Vatnajokull ice-cap to the north coast of Iceland and includes small shield volcanoes.

This volcanic system does not erupt frequently; GVP records 14 Holocene eruptions which range from VEI 0 to VEI 5, the VEI 5s occurred in c. 8910 BC and 1875.  Askja’s lava types are tholeiitic basalt / picro-basalt and rhyolite.  Her main eruption types are effusive basalt with occasion explosive basalt or rhyolite.  The 1875 eruption created a 4.5-km-wide caldera which is now filled by Öskjuvatn lake. The most recent eruption in 1961 was a VEI 2 effusive basalt one.

Fig 2:  The Askja volcanic system from Icelandic Volcanoes . The boundary of the fissure system is delineated with a dotted line, the central volcano with a black line and the calderas with bold lines.  The three letter abbreviations are other volcanic systems in the area: BAR is Barðarbunga, KVE is Kverkfjöll, SNF is Snæfell, ASK is Askja, FRE is Fremrinámur, HEI is Heiðarsporðar, KRA is Krafla and TEY is Þeistareykir.  The author has added the names Herðubreið and Herðubreiðartögl.

The Askja Fires, 1874 to 1929

Askja was little known before the Askja Fires.  The area is sparsely inhabited, sited in lava fields and ash deserts.   The Fires occurred during a volcano-tectonic episode between 1874 to 1929.

A steam column rising from the central volcano in February 1874 was the first observed sign that the volcano was active. Northern Iceland was rocked by many large earthquakes in December 1874.  Steam and ash were seen in early January 1875 and light ashfall was noted south of Öxarfjörður.  By 15 February 1875, 10m subsidence had occurred in the main caldera along with the formation of a crater erupting mud.  A basalt lava flow at Holuhraun to the south of Askja occurred around this time. 

On 18 February 1875, a fissure eruption started on the Sveinagjá fissure north of the volcano; this generated 0.2 to 0.3km3 of basaltic lava over the course of several months.

On 29 March 1975, the Plinian eruption at the central volcano started in earnest.  The initial output was a wet and sticky tephra.  Shortly after 05:30, pumice was erupted, reaching as far as Scandinavia; this phase lasted until the following day. The Víti crater was formed later in a short hydro-magmatic episode.  The caldera, itself, formed over a period of 40 to 50 years, is now filled by Öskjuvatn lake.  As the volume of the new caldera is greater than the calculated erupted volume of lava and ash, it is thought that the excess lava is stored in the fissure system.

In 1929 to 1930, five eruptions occurred on ring faults around the Öskjuvatn caldera, with a 6 km long fissure eruption occurring on the southern side of the volcano that created the Þorvaldsraun lava.

The 1875 eruption is not the first time Askja has erupted rhyolite. Two other instances have been occurred: the c.10ka Skolli eruption and one around 2.1ka; these deposited thick layers of tephra and ash from the latter reached as far afield as Scotland and Sweden.

Holuhraun, which should be familiar to those interested in volcanology, is the area where a fissure eruption occurred in 2014.  This time the central volcano responsible was Barðarbunga.  At the time there was some concern at the time that the activity in Holuhruan would extend to Askja, triggering a rhyolitic eruption.  Fortunately, that did not happen.


Fig 3: Image of Herðubreið, cropped from a photo by Icemuon, published under CC BY-SA 3.0

 Herðubreið is a 1,682m high Pleistocene palagonite table-mountain (tuya) made up of pillow lavas, hyaloclastite, capped by a 300m thick lava shield. Herðubreiðartögl, a small ridge extending from the south of Herðubreið, may be part of the same system.  Although Herðubreið is close to the Askja and Kverkfjöll volcanic systems, in the absence of any post glacial activity it not known if it belongs to either system.  We are including the volcano here as it is difficult to allocate the seismic activity in the area to each volcano without more local knowledge.

Herðubreið has been studied as an indicator of climate change during the last glacial periods. Werner et al, (1996) proposed that Herðubreið developed in stages from initial sub-aerial, sub-aqueous, subglacial to sub-aerial.  The first sub-aerial activity occurred during an interglacial, creating an olivine tholeiitic shield volcano in the vicinity of Herðubreiðartögl.  A lull in volcanic activity coincided with the onset of the last ice-age. Activity resumed with the deposition of olivine tholeiites, followed by hyaloclastites in a lake environment until the volcano breached the lake surface to produce subaerial lavas. The tuya, itself, was formed during the last glacial maximum when the volcano erupted pillow lavas under hyaloclastite deposits in the ice-cap; these were later topped by subaerial lavas when the volcano broke through the ice-sheet.  At the end of the last ice-age, activity at Herðubreið had ceased, however, Herðubreiðartögl produced some later olivine tholeittic lava flows and ash deposits.

Recent Seismicity

We plotted the area between 64.95°N,17.2°W and 65.3°N,16.0°W, a total of 45,899 earthquakes.  As you can see from Fig 4, the area is very active (although perhaps we should not have used green dots in retrospect– Askja looks very unwell as a result).

Fig 4: Geoscatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for the period 31.12.2007 to 31.03.2021. Green dots denote earthquake epicentres; red stars denote those of 3.0 or more M. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.

The latitude v longitude scatter plot shows that activity follows a NE-SW pattern around Herðubreið, with a swarm to the south east; activity around Askja is focussed on the SE section of the caldera with some further east.  The plots are data-heavy so we have broken these down by year.

Fig 5: Lat v Lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for the period 31.12.2007 to 31.03.2021. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.
Fig 6: Depth v Lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for the period 31.12.2007 to 31.03.2021. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.

The years with most seismic activity in the sequence are: 2007, 2008, 2014 and 2019. 

Fig 7: Earthquakes in the plotted area by year by the author.  The years highlighted in green have above average seismicity. © Copyright remains with the author; all rights reserved, 2021.

In 2007 and 2008, there was a swarm that started in Upptyppingar and progressed to Álftadalsdyngja; this is thought to be due to magma movement. 2014 is the same year as the Barðarbunga eruption at Holuhraun; perhaps some of the seismicity is the result of the crust accommodating magma movement in the region, although the swarm here preceded the swarms at Barðarbunga.   In 2019, there was a swarm to the east of the Askja caldera.

The earthquake density plots and depth v longitude plots for these years are set out in Figs 8 to 11 below.

Fig 8: Earthquake density plot and depth v lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for 2007. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.
Fig 9: Earthquake density plot and depth v lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for 2008. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.
Fig 10: Earthquake density plot and depth v lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for 2014. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.
Fig 11: Earthquake density plot and depth v lon scatter plot by the author of earthquakes between 64.95°N,17.2°W and 65.3°N,16.0°W for 2019. Colours denote year of occurrence. Blue triangles denote volcanoes. © Copyright remains with the author; all rights reserved, 2021.
Fig 12: Video of earthquake density plots by the author for the period 2006 to 2021(3m) for the area 4.95°N,17.2°W and 65.3°N,16.0°W . © Copyright remains with the author; all rights reserved, 2021.

Let’s see what the scientists have said. Greenfield et al (2016) have noted from seismic studies that there is considerable melt storage and transportation (movement) under the lower crust in the region (which may or may not be typical of Icelandic volcanoes – more study would be needed); there is likely to be a magma intrusive complex in the shallow crust round Askja; and, the activity round Herðubreið is caused by fracturing in the region.

The region is well monitored due to the risk of another rhyolitic eruption from Askja; this time around one may cause some disruption to aviation and communication systems, by how much would depend on the size and length of the eruption.   In the light of the reawakening of Fagradalsfjall on the Reykjanes Peninsula, perhaps the Pleistocene volcanoes should be added to the watch list, although the monitoring of Holocene volcanoes is likely to pick up unusual activity. 

La Soufrière St. Vincent

We have not forgotten La Soufrière St. Vincent; our thoughts are still with the islanders.  We will do a fuller update soon. In the meantime, the volcano is still erupting and a new lava dome is forming in the crater.  The island has lost up to 50% of its GDP.  More aid is now reaching the island.  For updates, we use News 784 (link below).

Barbados continues to clear up the volcanic ash; this is putting strain on local water supplies. For updates, we use Nation News Barbados.

The Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2021.

Sources and Further Reading:

R. Werner, H. U. Schmincke, G. Sigvaldason ,“A new model for the evolution of table mountains: volcanological and petrological evidence from Herðubreið and Herðubreiðartögl volcanoes (Iceland)”, Geologische Rundschau 85, Article number: 390 (1996). https://link.springer.com/article/10.1007/BF02422244

T. Greenfield, R. S. White, S. Roecker, “The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography”, Journal of Geophysical Reseach: Solid Earth, AGU Publications https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JB013163

Thor Thordarson (Faculty of Earth Sciences, University of Iceland) and Al Margaret Hartley (University of Manchester (November 2019). Askja. In: Oladottir, B., Larsen, G. & Guðmundsson, M. T. Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from http://icelandicvolcanoes.is/?volcano=ASK

“Classic Geology in Europe 3: Iceland”, Thor Thordarson & Armann Hoskuldsson, Terra Publishing, Third Edition, 2009.

The Smithsonian Institution’s Global Volcanism Program (GVP): https://volcano.si.edu/

Earthquake raw data: IMO:  https://en.vedur.is/

For updates on La Soufriere St Vincent:

News 784: https://news784.com/

Nation News Barbados: https://www.nationnews.com/

For updates on the new volcano at Geldingadalur:

Icelandic Met Office: https://en.vedur.is/ (English site)

Icelandic Met Office: https:// vedur.is/ (Icelandic site)

Reykjavik Grapevine: https://grapevine.is/

Department of Civil Protection and Emergency Management | Almannavarnir

Krakatau, Sunda Strait, Indonesia: 1883 Eruption & 2018 Tsunami

Good Afternoon!

Krakatau’s VEI 6 eruption of 1883 is the next in our series of famous eruptions.  We have also included a summary of birth of Anak Krakatau from the caldera, and the December 2018 cone collapse and resulting tsunami.

The 1883 eruption is not only famous for the catastrophic destruction of Krakatau Island, pyroclastic flows, global cooling, devastating tsunamis and a death toll of between 36,164 to 120,000 people, but it is also the first major eruption to have been reported globally by telegraph. A typo in the telegraph led to the west calling the volcano Krakatoa.

The 2018 eruption, cone collapse and tsunami are well-documented by various sources. Our go-to resource here was primarily GVP.

Fig 1: Lithograph: Parker & Coward, Britain. 1888. Public Domain

Geological Setting

Katakatau lies to the west of the Sunda Strait between Sumatra and Java.  It is the site of a much larger 7 km wide caldera which may have been formed during eruptions in 416 AD or 535 AD.  The edges of the caldera are marked by Verlaten and Lang Islands.

Before the 1883 eruption, Krakatau was a 9 km long verdant, wooded island formed from three volcanoes, Rakata, Danan and Perbuwatan in the caldera.  Another small island in the group was Polish Hat.  The islands were uninhabited but used by local fishermen, woodcutters and the Dutch and British navies.

Fig 2: Map by ChrisDHDR showing Krakatau Island before the eruption and the site of Anak Krakatau, Public Domain

Eruptive History

GVP records 56 known Holocene eruptive periods for Krakatau, of which only 10 precede the 1883 eruption.  The evidence for the 10 is historical observations from 250 AD to 1684, so perhaps earlier activity has been lost under the debris from more recent events. Activity after 1883 relates to building of Anak Krakatau.

The 535 AD eruption of Krakatau may have caused the volcanic aerosol veil that dimmed the Sun (filtered out sunlight) for eighteen months, causing crop failures, cooling and hiding Canopus, a bright star used by Chinese astronomers to mark the seasons – more likely if the eruption was a large caldera forming event. The other contender (preferred by some) is the 408 AD – 536 AD Tierra Blanca Joven eruption of Ilopango, El Salvador. On the other hand, why exclude one? Both may have contributed in some way.

Krakatau’s lavas are typical of a subduction zone: andesite, basaltic andesite, dacite, trachyte and trachydacite; and, also basalt and picro basalt.  The last mentioned indicates that there may be more rapid magma ascent through extensional faulting in the area.

The 1883 Eruption: 100 Days of Activity

The Intro

The only known precursors to the 1883 eruptions are a large earthquake on 1 September 1880 followed by a period of increasing seismicity.  Unfortunately, the area is seismically very active, being near the convergent margin between the Sunda Plate and the descending Indo-Australian Plate so, without modern instrumentation, there was insufficient information to interpret escalating events.

On the 20 May 1883 eruptive activity started at the Perbuwatan crater with series of loud explosions audible 150 km away.  Light ashfall covered the area and a column of steam was visible.  Activity continued for a few days then calmed down enough for a party to charter a boat to the island on 27 May 1883; they were the only witnesses to the Perbuwatan crater, then about 1 km in diameter, 50 m deep, with a small pit generating a steam column and small explosions every 5 to 10 minutes.  By the end of June 1883, the summit of Perbuwatan had been destroyed and a second eruption column was visible at the centre of the island.

Captain Ferzenaar, a surveyor for the Dutch government, collecting ash samples on 11 August 1883, found a thick covering of tephra, all vegetation stripped bar a few tree trunks; three active eruption columns (one at Perbuwatan and the other two near the centre of the island); and, at least eleven other sites with some activity.  However, upwind of the eruption, he was unable to see more beyond the steam and ash.

The Cataclysmic Eruption

The main event occurred over 26 and 27 August 1883. There were very few survivors so it took while for Dutch investigators, led by Rogier D.M. Verbeek, a mining engineer and geologist, and British investigators from the Royal Society to reconstruct the events.  Their information came from various sources, including ships caught in the Sunda Strait, pressure gauges at the Batavia gasworks on Java, Dutch officials living in Batavia and Buitenzorg, and requests for information from the Royal Society for data further afield, including one printed in The Times.

Three days before 26 August, there had been a marked increase in activity on the island.  By 13: 00 on 26 August explosions were loud enough to be heard 150 km away.  By 14:00 a 25 km high black eruption column was visible.  By 17:00 activity was audible throughout Java and pumice was raining down on vessels in the Strait.  By 19:00 a Plinian eruption column with intense volcanic lightening was witnessed. Several ships were among those caught up in the eruption, one of which, the Charles Bal, trapped by poor visibility had to sail within sight of  the volcano to keep its bearings amid hot ash fall, volcanic gases, lightening and St Elmo’s Fires (static electricity which lit up the mastheads).

The eruption escalated on 27 August with large explosions at 05:30, 06:44, 10:02 and 10:52 in the morning (local time).  The noise from these woke people 3,224 km away in Australia; and, further away at a distance of 4,811 km, it was confused with gunfire.  Atmospheric pressure changes were detected globally.  These explosions generated a 40 km high Plinian eruption column that cut out the sun for up to two days in the vicinity; further away in Batavia, full loss of light lasted for just over an hour and a half.  There is some debate on what caused the large explosions, including the possibility of sea water reaching either the magma chamber or ascending hot magma.  

Pyroclastic density currents (PDCs) made it to southern Sumatra, killing 2,000 people. The inhabited islands of Sebesi and Sebuku between Krakatau and Sumatra were devastated, with no survivors. Hot ash from the PDCs burned people as far away as Kalimbang, Sumatra.

The tsunamis caused the most of the remaining fatalities (estimates of the total number of fatalities vary from c. 36,000 to 120,000).  A series of tsunamis devasted the shores of the Sunda Strait; waves reached a height of 25m on the coast of Sumatra and 40m on Java. The town of Anjer was washed away. The largest tsunami wave at Batavia was detected at 12:36 on 27 August.  The waves reached as far as Auckland, New Zealand.  The tsunamis may have been caused by displacement of large amounts of seawater from the rapid deposit of ash in the caldera form discrete explosions, collapse of the eruption columns or edifice collapse.  As you will see later, the edifice failure of Anak Krakatau in December 2018 caused a catastrophic tsunami.

The eruption calmed down after the four large explosions, with some outbreaks of minor activity, to be quiet after 28 August 1883.

The Immediate Aftermath

Two thirds of Krakatau Island had disappeared – either blown apart by the eruption or sunk as part of the creation of a 300m deep caldera; only the southern section of Rakata remained.  Pumice and a small rock were all that remained of the northern part of the island.   

20 km3 of dacite pyroclastic material had been erupted as tephra, pyroclastic density currents and the rest deposited into the sea and on surrounding islands.  Deposits enlarged Verlaten and Lang Islands and two new islands were formed: Steers and Calmeyer.  Polish Hat, however, had disappeared.  Steers and Calmeyer were later eroded by sea water.

The 40 km high eruption column had reached into the stratosphere, where ash was spread round the globe, initially in the tropics but then migrating northwards and southwards, lingering for a couple of weeks.  Aerosols filtered sunlight resulting in vivid sunsets; the sun is reported as appearing as green or blue, depending on its angle in the sky.  Filtering of the sunlight caused global cooling probably in the order of 0.34°C in 1884.

The Birth, Collapse and Regrowth of Anak Krakatau

Krakatau has remained active with over 40 eruptive episodes since the 1883 eruption. Anak Krakatau (Child of Krakatau) emerged in 1927 from the caldera and had reached a height of 338m by 2018, only to lose a large part of the new cone in December 2018 when a relatively small eruptive episode (VEI 3), which started in June 2018, caused edifice failure.  The edifice collapse was preceded by an eruption at 21:03.  Øystein Lund Andersen, a photographer, recorded that by 21:05 a dark plume obscured the volcano and earlier incandescence.  At 21:27 the first tsunami wave hit the shore, travelling 15m inland; at 21:31 a second much larger wave followed.

Two thirds of Anak Krakatau had been destroyed. The tsunamis killed 437 people, injured 31,943, displaced a further 16,198 and damaged 186 miles of the shore line in Sumatra and Java.  Ash and gases cleared Kecil Island and a large part of Anak Krakatau, itself, of vegetation.  

This edifice collapse had been predicted.  Volcanologists from the University of Oregon had noted in January 2012 that the cone, formed on a steep slope of the 1883 caldera, was vulnerable to edifice collapse, especially on the western side.

The eruptive activity has continued, initially underwater, producing Surtseyan activity.  Cone rebuilding is continuing with both submarine and subaerial activity.  Recently, there was a small magmatic eruption in April 2020, producing two ash columns that reached 14 km and 11 km height and lava fountains.

 Recent Seismicity

Volcanism in the area is driven by the subduction of the Indo-Australian Plate under the Sunda Plate.  The Sunda Strait is seismically very active, possibly because it is accommodating the change in direction between the northern and eastern arms of the plate boundary.  Krakatau, itself, lies in the bend of the Arc above the Wadati-Benioff zone. 

We looked at the earthquakes in the region 8.67°S 101.00°E to 3.94°S 110.09°E for the period 1971 to 14 July 2020; this area includes the southern end of Sumatra, the Sunda Strait and the western end of Java.  We downloaded the earthquake data from IRIS’s earthquake browser. The download comprised mostly earthquakes with magnitude over 4.0; smaller volcanic / tectonic earthquakes were not included in the data set.

Fig 3: Density plot and depth plot of earthquakes between 1971 and 14 July 2020 by the author.  Green dots denote earthquakes with magnitude below 4.5, yellow circles, earthquakes between 4.5 and 6.0 and red stars, earthquakes over 6.0. © Copyright remains with the author; all rights reserved, 2020.

Our plots show the subduction zone in the curve of the Sunda Volcanic Arc, with more intense seismic activity in the northern arm of the arc. The intense areas of activity in the northern arm starts in 2000, preceding the 2004 Banda Aceh earthquake, which is north of the area in our plot, and continuing for a few years afterwards.

The Armchair Volcanologist

7 August 2020

© Copyright remains with the author; all rights reserved, 2020.

Sources and Further Research

“Volcanoes”, Second edition, Peter Francis and Clive Oppenheimer, Oxford University Press, 2004

“Volcanoes, Earthquakes and Tsunamis”, David Rothery, Teach Yourself, 2010

Smithsonian Institution Global Volcanism Program, Krakatau: https://volcano.si.edu/volcano.cfm?vn=262000

 Krakatoa, Wikipedia https://en.wikipedia.org/wiki/Krakatoa

Anak Krakatoa, Wikipedia https://en.wikipedia.org/wiki/Anak_Krakatoa

Raw earthquake data downloaded from IRIS http://ds.iris.edu/ieb/

Earthquake plots are the author’s own.