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:

 Krakatoa, Wikipedia

Anak Krakatoa, Wikipedia

Raw earthquake data downloaded from IRIS

Earthquake plots are the author’s own.

Quick Update on the Earthquake Swarm on the Reykjanes Peninsula

A large earthquake swarm started on the morning of 19 July at around 1:30 am at Fagradalsfjall on the Reykjanes Peninsula.  The largest earthquake had a magnitude of 5.1M.  At the time of writing, there had been 1,635 earthquakes in the last 48 hours recorded on IMO’s website (note that not all of these have been confirmed). IMO’s map and breakdown of the swarm are shown below:

Fig 1: Map of earthquakes in Iceland over the past 48 hours.  Source: IMO

Close up of the Reykjanes Peninsula:

Fig 2: Map of earthquakes in the Reykjanes Peninsula over the past 48 hours.  Source: IMO
< 1.0720
2.0 -3.0162
Fig 3: Breakdown of earthquakes by magnitude

This swarm is occurring on the east side of the swarms on the Reykjanes Peninsula which started late last year.  IMO have reported that these swarms (still ongoing) are associated with multiple magma intrusions.  The aviation code for the area is still green (IMO). IMO are in the process of evaluating the Fagradalsfjall swarm. 

The swarm at the Tjörnes Fracture Zone is still ongoing.

The eagle-eyed amongst you will note that there is some seismic activity at Katla.  Whether this will result in anything is anyone’s guess at the moment. 

We have not yet updated our earthquake data-set for the current swarm.  We will wait until IMO has had a chance to confirm more earthquakes

Update 24 July 2020

The swarm at Reykjanes is now less intense. In the meantime, Katla produced a shallow 3.0 M. IMO have remarked that earthquakes in the summer at Katla are not uncommon.

From memory, Katla was seismically active before the intense swarms started in August 2014 at Barðarbunga in the run up to the eruption at Holuhraun. This may have been a coincidence.

Fig 4: Map of earthquakes in Iceland over the past 48 hours.  Source: IMO

For further updates, please consult IMO.

The Armchair Volcanologist

21 July 2020

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

Source and Further Reading

“An earthquake swarm in Fagradalsfjall”, 20.07.2020

The Barðarbunga Volcanic System

Good Afternoon!

Fig 1: Barðarbunga: cropped image from photo 3 of 11 by Erik Sturkell, retrieved from Icelandic Volcanoes

In this post we continue our journey round Iceland’s many volcanoes.  We have reached the mighty Barðarbunga at the northwest corner of the Vatnajökull icecap.

Barðarbunga volcanic system lies in the Eastern Volcanic Zone, Iceland, near where the head of the mantle plume is thought to be.  The system comprises a 2,000 m high central stratovolcano with a 65 km2, 700 m deep caldera, the Veiðivötn fissure swarm running NE to SW, and the Tröllagigar and the Dyngjuháls – Holuhraun fissure swarms running NE; the entire system is c. 190 km long and 25 km wide.  

There is second central volcano in the system, Hamarinn, 20 km south west of the Barðarbunga central volcano. Hamarinn may be younger, indicated by the absence of both an intrusive complex and a caldera. 

There are geothermal areas near the caldera rim of Barðarbunga and the east of Hamarinn, the latter is the source of jökulhlaups. 

Fig 2: Map of the Barðarbunga volcanic system: central volcanoes, fissure systems and lava flows. Green barred squares indicate the locations of various volcanoes; BAR is Barðarbunga.  Retrieved from Icelandic Volcanoes.  See Sources for full accreditation.

The area is tectonically very active: the Eastern Volcanic Belt accommodates much of the separation between the North American and Eurasian Plates. The area is close to the junction with the Northern Volcanic Zone, where Barðarbunga’s neighbours, Askja and Herðubreið, can be found.

Eruptive History

According to GVP, there are 55 identifiable Holocene eruptive periods for the Barðarbunga system.  Some of the eruptive history has been hidden by the ice-cap.  However, lavas and tephra deposits on the ice-free sections of the fissures are more accessible.

Barðarbunga Central Volcano

The central volcano has had around 22 eruptions in the last 1000 years, most occurring between 1200 -1500 and in the 18th century.  The last known subaerial eruption was in 1910.

Barðarbunga’s lavas are mainly basalt/ picro basalt.  Her eruption types are explosive, phreato-magmatic with jökulhlaups, reflecting the impact of the ice-cap.  The central volcano produces eruptions in the order of VEI 3 to 4, producing tephra – both airborne and waterborne.  There is a silicic tephra layer in the ice-cap dating to the early 18th century but it is not clear that this came from Barðarbunga.  If it did, any rhyolite would have come from partial melting of the basaltic crust.

Magma is sourced from a depth of 10 km or more below the caldera; above this source is an intrusive complex and a lower density region, probably of caldera in-fill.  Magma may also be sourced direct from the mantle in the fissure swarms.  

Fissure Swarms

Fissure swarm eruptions are basaltic in the order of VEI 1 to 2, with a maximum of VEI 6 on the Veiðivötn fissure. 

The last three eruptions on the south west fissure swarm were the VEI 4 at Vatnaöldur in 877, the VEI 6 at Veiðivötn in 1477 and the VEI 2 at Tröllagigar in 1862-1864.  The first two of these were explosive tephra eruptions, producing 5 km3 to 10 km3 of tephra and small lava flows.  Both the Vatnaöldur and Veiðivötn fissures cut into the Torfajökull volcano, causing it to erupt with silicic tephra and lava.  The largest known effusive eruption on the SW fissure swarm is the Great Þjórsá lava which covers 900 km2 and reached the south coast via the Tungnaá and Þjórsá river valleys.

The Gjálp eruption in 1996 occurred on a subglacial fissure that links the Barðarbunga and Grímsvötn volcanic systems.  While it is thought that the magma was sourced from beneath Barðarbunga, based on seismic and geodetic data, the magma erupted subaerially was characteristic of Grímsvötn’s lavas.

The frequency of eruptions on the northern fissure swarm is not known; the last eruption was at Holuhraun which started on 29 August 2014 and lasted until February 2015.  Precursors to this eruption were a build up of seismic activity at Barðarbunga over seven years, which stopped immediately after the Grímsvötn 2011 eruption but resumed soon afterwards. The largest known effusive fissure eruption north of Vatnajökull is the mid Holocene 15 km3 Trölladyngja lava shield.

Holuhraun Eruption 2014 – 2015

Fig 3: Holuhraun eruption.  Cropped image from Barðarbunga: photo 8 of 11 by Alessandro La Spina, 4 September 2014. Retrieved from Icelandic Volcanoes. Note the fire fountains, spatter cones and volcanic gases.

The subaerial eruption started on 29 August 2014 at the Holuhruan vent 45 km NE of the Barðarbunga caldera; the eruption ended on 28 February 2015, having left an 85 km2 lava field and a 65 m deep depression in the Barðarbunga caldera’s ice cover.  The eruption was a large SO2 and other volcanic gas producer, however there was little ash or tephra.

The central volcano, Barðarbunga had inflated prior to the eruption then deflated during the eruption as evidenced by subsidence in the ice covering.  The volume of the subsidence was consistent with the dyke intrusion and the lava erupted at Holuraun, although there is seismic and geochemical evidence that some of the lava erupted at Holuhraun was fed direct from the mantle. It is estimated that 1.6 km3 lava was erupted. 

Since September 2015, seismic and GPS data show that the volcano has started to refill at a depth of 10 to 15 km.

Recent Seismic Activity

We looked at earthquakes in the Barðarbunga, Askja, Herðubreið and Holuhraun area (64.56°N, 17.65°W to 65.3°N, 16.1°W) for the period 1 January 2009 to 28 June 2020.  Not much activity had been noted in the area to the west and south west of Barðarbunga in our earlier plots; however, we had noted that heightened activity at Askja and Herðubreið had preceded the 2014 eruption at Holuhraun which lies between the three volcanoes, hence we included them in our plots.  The link between the centres is rifting in the crust to accommodate the separation of the North American and Eurasian Plates.

Fig 4: Earthquake plots for the period 1 January 2009 to 28 June2020: lat. v lon. geodensity and scatter plots and a depth plot; all by the author.  Green dots denote earthquakes ≤ 2 M; yellow circles, earthquakes between 2.0 M and 3.0 M; and, red stars, over 3 M.  Note in addition to the intense activity around Barðarbunga and Holuhraun, activity near Askja and Herðubreið. © Copyright remains with the author; all rights reserved, 2020.

There were 70,128 earthquakes in the period, of which 16,573 occurred before the 2014 -2015 eruption, 19,061 during the eruption and 34,494 post eruption; the average per calendar month was 247 pre eruption, 2,723 during the eruption and 539 post eruption; the maximum magnitude earthquake pre eruption was 3.9 M, during the eruption 5.5 M and 4.9 M post eruption; and, the deepest quakes had respective depths of 33.9 km, 31.0 km and 33.9 km.  These numbers include activity at Barðarbunga, itself, the Holuhraun fissure, Askja and Herðubreið.  The larger magnitude earthquakes occurred near the north and south caldera rim during the eruption.  Since the eruption all four centres have had elevated seismic activity.

Seismicity during the 2014 to 2015 Holuhraun eruption

Fig 5: Earthquake swarms: Herðubreið three months prior to the Holuhraun eruption, the swarms at Barðarbunga and Holuhraun in the first month of the eruption and then a sample three months later. The Barðarbunga and Holuhraun swarms started in August 2014 and continued with decreasing intensity to June 2015. © Copyright remains with the author; all rights reserved, 2020.

Three months prior to the eruption, there was an earthquake swarm at Herðubreið, noted here because the rifting event that preceded the Holuhraun eruption occurred on the same plate boundary.  Seismic activity at Herðubreið or Askja may be precursors to activity at Vatnajökull, if they, themselves, are not the main event or brewing something.  Something to watch out for bearing in mind the recent large swarms in the Tjörnes Fracture Zone and on the Reykjanes Peninsula.

The earthquake plots for August 2014 and November 2014 show the intense swarms from caldera collapse and also the rifting event.

We will look at Askja and Herðubreið in future posts.

The Armchair Volcanolgist

17 July 2020.

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

Sources and Further Reading

“Barðarbunga”, Guðrún Larsen and Magnús T. Guðmundsson (Institute of Earth Sciences – Nordvulk, University of Iceland).  In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes:

Fig 2: Map. After Björnsson (1988), Gudmundsson and Högnadöttir (2007), Jóhanneson and Saemundsson (1998a & b), Sigurgeirsson et al (2015). Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO.  In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes:

Smithsonian Institution Natural History Museum Global Volcanism Program (GVP):

Raw earthquake data: Icelandic Meteorology Office: IMO

Plots are the author’s own work.

Famous Eruptions – Introduction

All good books on volcanoes describe well-known eruptions, including Vesuvius 79 AD, Tambora 1815, Krakatau 1883, Mount Peleé 1902, Katla 1918, Mount St. Helens 1980, Pinatubo 1991.  Who are we to be any different?

There is a good reason for this. Most famous eruptions are large and explosive, causing loss of life and considerable property damage – in other words, headline-grabbing.   Later eruptions have impacted aviation – volcanic ash and jet engines are not a good mix; airports near erupting volcanoes are closed and flights re-routed to avoid the ash clouds. 

Catastrophic events are the reason volcanology is such an important subject. Understanding these and other eruptions is important to find out how to minimise the risk; i.e. reduce loss of life.

Researchers have shown that volcanoes usually very kindly give us some warning in the lead up to an eruption.  Magma is viscous, rarely moves fast and, when moving, causes earthquakes, usually small, but some are felt without equipment.  As magma ascends, degassing starts to occur, some of which is detectable at the surface.  Rising magma also causes ground deformation, e.g.  it causes the volcano to inflate, measurable by tilt-meters and GPS, and some of which may even be visible to the naked eye (Mount St Helens). 

I read somewhere that the first piece of equipment required to monitor a volcano is a seismometer, the second is another seismometer, as is the third; earthquakes are often the first sign of impending trouble (if I can remember / find the source, I will accredit it properly). Scientists observing volcanoes will have a raft of equipment in place: seismometers, GPS stations, tilt-meters and gas monitoring; satellite monitoring may also detect ground deformation.  Drones may be used to inspect craters which are not readily accessible or where it would be unsafe to visit. Our maps of Iceland include some GPS stations as markers to indicate where the earthquakes are occurring but still evidence that Iceland has several in place at its active volcanoes.

Given that volcanoes give some warning, why are people killed?  Unfortunately, magma ascent and the build up to an eruption is a slow process with many stops and starts.  A major eruption may be preceded by a few small throat clearing events before the volcano unleashes the main eruption. An evacuation may take place, but if it is quiet between the precursors and the main event, people may think that the volcano has finished and return home to be at ground zero at the wrong time, irrespective of whether or not any alert is in place. People living close to volcanoes tend to be farmers with strong bonds to both their land and livestock. On the other hand, the volcano may not behave as expected – e.g. a phreatic eruption or edifice failure.  Volcano hazard assessment is an interesting topic for future posts.

We will also look at some of the more famous eruptions over the coming weeks.

We’ve already touched a bit on Katla (The Katla Volcanic System, Myrdaksjökull – the not so cuddly Katla) and Mount St Helens (Mount St Helens – 18 May 1980 Eruption) so we’ll carry on by looking at Tambora’s 1815 eruption (Tambora 1815).

The Armchair Volcanologist

7 July 2020

Tambora 1815

This is the first of out famous eruptions series.  Why not start with one of the largest?

Fig 1: Tambora’s caldera by Tisquesusa, 2017; published under CC BY 4.0

Tambora produced one of the largest known eruptions in recorded history with a climate impacting VEI 7 and possibly the largest Holocene eruption (other contenders for a VEI 7 being Kurile Lake, 6440 BC, Mazama, 5700 BC, Kikai Caldera, 4300 BC, Cerro Blanco, 2300 BC, Thera (Santorini), 1620 BC, Taupo, 180 AD, Baekdu, 946 AD, and Samalas (Rinjani), 1257 ).

 Tambora’s once proud 4,300 m stratovolcano lost around a third of its height and acquired a 1 km deep, 6 km wide caldera over the space of a few days in April 1815.  Sumbawa and the surrounding islands were devastated. Climate abnormalities (cooling and severe storms) were noted round the northern hemisphere along with crop failure and famine.  Tambora is accredited as the cause of the 1816 “year without a summer”. The eruption released 50 km3 of magma, 150 km3 of tephra, 80 million tonnes of sulphur dioxide and 18 mega tonnes of fluorine, along with water vapour and other aerosols.

The eruption was chronicled by local eye witnesses.  However, the then lieutenant governor of Java, Sir Stamford Raffles, keen to develop trade in the area, was not so keen to broadcast it further afield to potential investors; outside the region, the eruption went largely unnoticed by the West, already distracted by the Napoleonic wars.

Geological Setting

Before the eruption, Sumbawa Island was a pleasant prosperous island, trading mung beans, corn, rice, coffee, pepper, cotton, wood and horses.  Despite a wealth of natural resources, the people to farm them were in short supply so there was also a large slave trade and piracy.

Tambora occupies the entire Sanggar Peninsula on Sumbawa Island in the Sunda Arc of the Indonesian Archipelago.  Here the Australian Plate subducts beneath the Sunda Plate at a convergence rate of 7.8 cm per year.  Plotting the earthquakes in the region for 1972 to date clearly shows the Wadati-Benioff zone, with volcanoes sitting around 70km above the descending plate. 

Fig 2: Scatter plot of the earthquakes showing the subduction zone (Wadati-Benioff zone) where the Australian Plate meets the Sunda Plate by the author.  Green dots represent earthquakes < 5M, yellow circles, 5M to 6M, red stars, >6M, blue triangles, some volcanoes. © All rights reserved, 2020

Tambora is a 60 km wide stratovolcano with trachybasalt and trachyandesite lavas.  She formed a caldera c.43,000 years ago which was in-filled by Pleistocene lava flows.  During the Holocene her eruptions have been explosive: three eruptions in the Holocene occurred before the 1815 event, identified by radiocarbon dating, 740 AD, 3050 BC, and 3910 BC; three further smaller eruptions have been observed since the 1815 event, 1819 VEI 2, 1880 VEI 2 and 1967 VEI 0, which extruded lava domes and small lava flows on the caldera floor.

1815 eruption

Tambora had been dormant for over a thousand years; magma cooling and fractional crystallisation had been occurring along with the exsolving of high-pressure magma; over pressurisation was happening. Tambora awoke in 1812 with minor activity during the period 1812 to 1815 while magma ascended from the reservoir c.4 km below the edifice. 

The 1815 eruption proper started with a short Plinian eruption on 5 April 1815 of trachyandesite, lasting two hours, producing a 33 km eruption column; the explosions were heard as far away as Sumatra and were confused with gun fire. Following this, between 5 April and 10 April, there was a relatively low level of activity. 

On the evening of 10 April, a second short Plinian eruption occurred, lasting three hours, producing a 44 km eruption column.  Pumice rained down on local villages for 2 hours.  Volcanic winds destroyed trees and property.  The eruption column collapsed as the vent was eroded.  Pyroclastic density currents (PDCs) raged over the next three to four days, creating phoenix ash clouds, covering the area in ash and destroying villages, along with inhabitants.  During this phase the volcano, no longer supported by magma, subsided, creating the current caldera.

Tsunamis were generated when the PDCs reached the sea.  Sanggar was engulfed in a four-metre high tsunami at around 10 pm on 10 April; this tsunami reached Java a couple of hours later with a height of two metres. 

Explosions were heard during the night of 10 to 11 April up to 2,600 km away.  Locations within a 600km radius suffered darkness for a couple of days and a chilling of the atmosphere while sunlight was blocked. 

Seven or more ignimbrite layers were deposited during the second phase of the eruption.  Only 2.6 km3 of deposits remain on dry land; the heavier ejecta ended up in the sea and lighter aerosols were scattered round the globe.  Pumice rafts up to 5 km wide and trees trunks littered the Flores Sea, providing a hazard for shipping for several months.

Eruptive activity continued intermittently up to August 1819. 

Local and global impact of the eruption

Sumbawa was stripped of its vegetation. The death toll in Sumbawa and neighbouring Lombok was in the order of 60,000, 10,000 from the eruption itself with the remainder from disease and famine from polluted water and loss of crops and livestock.  Children were sold into slavery in order for them to survive or apparently killed to avoid a slow death from starvation or worse. 

The effects were also global.  The volcano had lobbed 50 km3 of matter plus gases into the stratosphere.  Larger particles fell back to Earth but smaller aerosols hung around for three years causing adverse weather phenomena (e.g. storms), global cooling, failed harvests, famine and disease in much of the northern hemisphere.

The global effects were exacerbated, according to ice-core sampling, by another large catastrophic eruption in 1809.  The source for the 1809 has yet to be identified; the volcano and eye witnesses may not have survived the eruption.  However, the 1809 and 1815 eruptions together caused a little ice age.

Can this happen again?

VEI 7 eruptions are rare.  As mentioned earlier, there are currently only eight other contenders in the Holocene. That is not to say they won’t happen again in the future. 

My guess is that to get another large event, not only do you need a large magma source and a build-up of pressure but also an element of edifice failure.  The largest part of the Tambora eruption in terms of magma output was while the PDCs were in full swing; these occurred as or after the vent eroded.  We will see in the accounts of other eruptions, there was some edifice failure.

In the meantime, Mount Tambora and her Indonesian sisters are well-monitored.

The Armchair Volcanologist

6 July 2020 (updated 23/08/2020 & 25/08/2020 to include more potential VEI 7s (the list is getting longer!)

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

Sources and Further Reading

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

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

“Tambora: The Eruption That Changed the World”, Gillen D’Arcy Wood, Princeton University Press, 2014

Mount Tambora – Wikipedia,

Earthquake data from Incorporated Research Institutions for Seismology (IRIS) Earthquake Browser:

Plot by the author.

The Magic of Magma

Good Afternoon!

Magma is the molten source of many of the rocks at the Earth’s surface; the others come from meteorites or other terrestrial processes.  Here, we look at how magma is generated and how it affects volcanic eruptions.   This is a long one; you may want to dip into it as a reference rather than read it all in one go, but bear with me.

Fig 1: Holuhraun on 4 September 2014 by peterhartree.  Published under CC BY-SA 2.0. Note the fire fountains, effusive lava flows and gas emissions.

There are several different types of volcanic eruption, varying from effusive to explosive.  The type is determined by the magma composition- both its silica content and its volatile (gas) content.

How is magma generated?

From our earlier post, we know that the heat required to generate magma comes from nuclear reactions at the Earth’s core.  This heat makes its way to the surface via conduction, convection and radiation.  The surface rocks are cold enough to be solid.  However, we saw that the heat from the core drives plate tectonics; it is the variations in temperature and pressure caused by plate motion, combined with changes in rock composition, that create the conditions for rocks to melt and create magma.

Effusive Continental Rifts / Ocean Ridges

Effusive eruptions (think Hawaiian eruptions such as Kilauea) tend to occur at rifts or fissures, cracks in the Earth’s crust that let magma reach the surface relatively quickly and allow gases to escape gently.  Such fissures are common at constructive plate boundaries (continental rifts or mid ocean ridges), caused by plate separation.  Magma is generated in a process called decompression melting; as the plates move apart the overlying pressure is reduced, which in turn reduces the melting temperature of the mantle.  Because magma ascends relatively rapidly there is less time for it to mix with other magma or the crust.  This magma tends to be basic basalt. 

Magma ascent at ocean ridge
Fig 2:  Magma ascending at a Mid Ocean Ridge by the author after the many examples available.  Arrows denote plate motion.  Not to scale. © All rights reserved, 2020.

More Explosive Subduction Zones

Magma may also be generated at what is called subduction zones.  Subduction zones occur where plates meet: a denser oceanic lithosphere descends beneath either a less dense continental lithosphere or other oceanic lithosphere. These zones were discovered by two scientists independently researching earthquakes, Hugo Benioff and Kiyoo Wadati. Earthquake foci delineate the descending slab; this zone may be referred to as the Wadati-Benioff zone.   Magma is created from the descending slab by a process called hydration melting: as the slab descends water is squeezed out which mixes with the overlying asthenosphere, lowering its melting temperature.  Hydration melting occurs between 50 km to 200 km, with volcanoes accumulating at around 70 km above the descending slab.  

Magma mixes with the asthenosphere and the crust which increases its silica content. Partial melting of the mantle results in basalts (<52% silica); partial melting of the descending ocean crust provides andesites (52% to 65% silica); and, partial melting of the continental crust gives rhyolites (> 65% silica).  Increasing the silica content of the magma increases its viscosity and the explosivity of eruptions.

These plate boundaries are called destructive boundaries; it is believed that the descending plate is destroyed in the process. 

Fig 3:  Magma generation at a subduction zone by the author, after the many sources available.  Large arrow denotes plate motion; stars, the site of hydration melting; and, the smaller arrow, magma ascent.  The accretionary wedge is rock scraped off as the plates meet. Earthquake foci, not shown, would indicate the path of the descending plate. Not to scale. © All rights reserved, 2020.


There is another process where plates meet called obduction; one plate rides over the other.  It is mentioned because obduction revealed the origins of oceanic crust.  Ophiolites, the remnants of old oceanic crust, show that this crust is made up of peridotites from the mantle overlain by old magma chambers, sheet dyke complexes, pillow lavas and sediments.  The magma was sourced from partial melting of the peridotite.  Sheet dyke complexes are complexes of vertical magma intrusions where magma has filled gaps caused by extension from plate separation.  Pillow lavas occur when basalt is erupted underwater.

Magma Evolution

We know that not all magma is basic basalt: there are others, including andesite and rhyolite.  These are formed from basalt by a process called magma evolution.

Magma rises slowly under its natural buoyancy, being around 10% less dense than the surrounding rock.  It tends to accumulate in magma chambers / reservoirs at depths of around 5 km to 20 km below the volcano.  Magma evolves by one or more of the following:

  • Magma mixing: one batch of magma mixes with another in the magma reservoir;
  • Assimilation: the rising magma collects surrounding rock during its ascent;
  • Fractional crystallisation: as magma rises, it cools, which causes the components with higher melting points to crystallise out.

Evolution tends to increase the silicate content of the magma and increase its viscosity; the higher the viscosity, the slower magma moves.

The table below shows a comparison of rock and magma compositions for some its key elements.  The convention for magma composition is based on the oxide equivalents of the elements, presumably on the basis that oxygen does not occur on its own in rock. 

Fig 4:  Comparison of key elements in the composition of different magmas by the author after various sources.  © All rights reserved, 2020.

So how does magma get to the surface?

The short answer is plate tectonics. Distortions in the crust from plate motion either squeeze magma out via existing or new fractures, or provide a wide enough pathway for the magma to ascend.  This is why we see so many volcanoes around plate boundaries.

Eruption Style

Magma contains dissolved gases (e.g. water, carbon dioxide, sulphur, and halogens: chlorine and fluorine).  When the constraining pressure decreases, the gases are released from solution: e.g. carbon dioxide exsolves at depths of several kilometres and sulphur at around 1 km.   This is a process called degassing (think of carbon dioxide released from a fizzy drink when the cap is loosened.) These gases may be held in the magma as bubbles or it may escape via fractures in the crust to the surface.  Measuring gas emissions around volcanoes indicates whether or not new magma is close to the surface.  We have seen with Grímsvötn that Icelandic scientists have reported the presence of sulphur dioxide near the caldera rim, indicating that Grímsvötn may be heading for a new eruption.

Effusive eruptions tend to occur with the less viscous basaltic magmas from which gases can escape gently.  More explosive eruptions are caused when the gases are trapped in more viscous magma and accumulate either as foam or as bubbles; it is their rapid expansion causes an explosive eruption. If a degassing event is large enough, less viscous magmas may also produce explosive eruptions.

When gas bubbles coalesce and expand, large bubbles form which rise with the magma.  If the magma reaches the vent, the bubbles explode with the reduction in constraining pressure, throwing out chunks of lava and ash; some of the chunks can be large boulders.  If gas bubbles get trapped in the magma chamber, when the accumulated pressure is high enough, the gas is forced out as a jet strong enough to cause fire fountains from less viscous basaltic magmas or ash / shards from fractured more viscous magma.  Fire fountains are / were visible in some of Etna’s eruption, in the 2014 eruption at Holuhraun or in the 1783 eruption of Laki. Famous explosive eruptions include Pinatubo, 1991, Krakatau, 1883, and Vesuvius, 79AD,

Impact of ground water or ice

We have seen explosive eruptions from some of Iceland’s basaltic volcanoes.  Another source of volatiles is ground water (aquifers) or glaciers.  When magma gets close enough to these, the heat from the magma flashes the water to steam; the rapid expansion of the water / steam results in an explosive eruption.  If only old rock is involved, the eruption is referred to as phreatic; if new magma is involved, the eruption is referred to as phreatomagmatic.  Phreatic eruptions may be a vehicle to clear the vent / provide a pathway for new magma to reach the surface. Recent phreatic eruptions include White Island, 2019, Mount Ontake, 2014; both caused loss of life because of the unpredictable nature of this eruption type.

Future Topics

There is another important process for magma that we have not touched here: mantle plumes.  We will look at this later.

We will also look in more detail the various locations for volcanoes, including the earthquake foci plots that indicate activity (usually tectonic).  Our Icelandic posts show the influence of the Mid Atlantic Ridge; others will look at subduction zones.

We may get diverted / distracted on the way to look at current events of interest.  (Probably, no “may” about it 😉 ).

Thank you for staying with me,

The Armchair Volcanologist

1 July 2020

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

Sources and Further Reading

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

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

“Volcanoes Global Perspectives”, John P. Lockwood and Richard W. Hazlett, Wiley-Blackwell, 2010

“Wadati-Benioff zone”, Wikipedia:

“Igneous rock”, Wikipedia:

“Magma”, Wikipedia:

More Trembles in the Tjörnes Fracture Zone

Good Evening!

A large earthquake swarm started in Iceland in the Tjörnes Fracture Zone (TFZ) on 19 June 2020, still ongoing at the time of writing.  The Civil Protection Authority, Iceland, has declared a state of uncertainty; the TFZ is capable of producing large destructive earthquakes. 

The Icelandic Meteorological Office (IMO) has reported that over 9,000 earthquakes have been detected by their SIL earthquake monitoring system.  The swarm includes three earthquakes over 5.0: on 20/06/2020 a 5.6 and a 5.4, both 20 km north east of Siglufjörður; and, on 21/06/2020, a 5.8 30 km north, north east of Siglufjörður.  The day before the swarm started there was a small earthquake with a depth of 92.3 km, which is unusually deep for the area.

 IMO is in the process of manually confirming the earthquakes – a mammoth task!  We have updated our earthquake data set up to 28 June 2020 based on the earthquakes confirmed so far in order to take a look at the swarm.  In the area, 65.25°N, 21.5°W to 67.25°N, 15.0°W, 2,010 earthquakes from 1 June 2020 to 28 June 2020 were available to download.

Statistics so far

Fig 1: Summary of earthquakes for 1 June 2020 to 28 June 2020 by the author; © All rights reserved 2020.  A later download may give a more comprehensive picture.

Updated earthquake plots

Our plot for month 54 (1 June 2020 to 28 June 2020) shows that the swarm is occurring on the western end of the TFZ, near the junction of the Eyjafjarðaráll Rift and the Húsavík-Flatey Fault.

Fig 2.1:   Earthquakes 1 June 2020 to date in the Tjörnes Fracture Zone by the author. Green dots denote earthquakes less than 2.0M; yellow circles, quakes between 2.0M and 3.0M; red stars, quakes over 3M; orange triangles, volcanoes or volcanic features. Image on the right is a zoom in on the current earthquake swarm. © All rights reserved, 2020.
Fig 2.2:  Earthquakes 1 January 2016 to 28 June 2020 in the Tjörnes Fracture Zone by the author. Green dots denote earthquakes less than 2.0M; yellow circles, quakes between 2.0M and 3.0M; red stars, quakes over 3M; orange triangles, volcanoes or volcanic features. Red ellipse indicates the location of the current swarm. © All rights reserved, 2020.

So what is going on?

Prior to this swarm, most activity in our data set from 1 January 2016 had been on the Grímsey Oblique Rift and Húsavík-Flatey Fault, with a large swarm occurring to the north west of Grímsey in February 2018 (see our earlier post).  The current swarm appears to be the western end of the system catching up. 

This swarm is, however, relatively deep for oceanic crust.  Whether or not there is any associated volcanic or geothermal activity remains to be seen. 

The swarm on the Reykjanes Peninsular that started in late 2019 is still ongoing, albeit at a reduced rate. Are the two swarms linked? The obvious answer is yes – they are both on transform fault systems which accommodate the spreading from the Mid Atlantic Ridge as it crosses Iceland. But is there more to it? Possibly, the crust could be fracturing to accommodate other activity in Iceland – e.g. uplift from the mantle plume. If yes, we may see some more activity e.g. in the vicinity of the Vatnajökull Icecap in the next few months. But we’ll have to wait until the real experts opine.

For updates, please consult IMO.

The Armchair Volcanologist

29 June 2020.

Source for earthquake data: Icelandic Meteorological Office (IMO):

Plots are the author’s own work.

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

What is a Volcano?

Good Morning!

As this blog is about volcanic and seismic activity, a word or two on what a volcano is might be helpful.

A volcano is defined by the Oxford English Dictionary as mountain or hill with a crater or vent through which rocks, rock fragments, lava, hot vapour and / or gases are or have been erupted through the Earth’s crust.  However, said mountain or hill may be quite small, even just a depression or a rupture in the Earth’s surface.

The island of Vulcano is the source of the term, volcano, itself.  Vulcano Island is located in the Tyrrhenian Sea, north of Sicily, made up of several active volcanoes, including calderas. 

Fig 1: Vulcano Island by Brisk g. in the public domain. Source

So what causes the lava and other matter to be erupted?  What is the Earth’s crust?  These are questions some of the questions we will look at in this blog. 

Heat generated at the Earth’s core drives the geological processes which result in volcanic activity. For starters, we will look at the basics of the earth’s composition, magma and the source of the energy to enable eruptions to occur.

Basics of the Earth’s Composition

This is a very basic description of the Earth’s composition.  The Earth is made up of three main parts: the core, the mantle and the crust. The core is very hot and temperatures decrease towards the Earth’s surface.  Most of the Earth is solid, only the outer core is liquid.  Evidence for this structure has been gleaned from seismic studies, notably how the different wave types generated by an earthquake pass through the Earth, geophysics and the study of rocks.

Fig 2:  Earth’s structure and composition, not to scale, by the author. © Copyright remains with the author; all rights reserved. 2020.

The Core

The radius of the Earth is around 6,378 km, in other words the centre of the Earth’s core can be found 6,738 km down.  The core makes up around one third of the Earth’s mass.  It is made up of an outer core which starts at around 2,900 km down and an inner core which starts at around 5,100 km down.

Material in the core is too dense to make its way to the surface, so there is some uncertainty over its composition.  What we do know is inferred from geophysical studies of the Earth and the chemical analysis of meteorites.  During the Earth’s formation, as rocks and fragments combined to form the planet, denser matter sunk towards the core under gravitational and other forces.  Iron is the chief component of the core, with nickel at the inner core and a lighter element in the outer core (possibly, oxygen, sulphur, carbon, hydrogen or potassium).  The iron in the core and the electrical currents in the molten outer core are the source of the Earth’s magnetic field. 

Seismic studies have shown that the outer core is impermeable to earthquake shear waves (S waves) so acts like a liquid.  Whether or not a layer is liquid or solid is down to the balance between temperature, pressure and chemical composition: while the inner core is around 4,700°C, immense pressure keeps the rock solid.

The Mantle

The mantle is composed of solid rocky materials that are less dense that the outer core; it makes up two thirds of the Earth’s mass.  Density differences mean that the mantle is a distinct layer from the outer core.   The most abundant elements in the mantle are silicon and oxygen, that form silicates.  The mantle is made up of around 45% silica. Magnesium and iron are the third and fourth most abundant elements.  Many other elements are to be found in the mantle, but these tend to be depleted near the boundary with the crust.

The composition of the mantle is inferred from xenoliths (small fragments of rock) contained in some basalt magmas and kimberlites.  Whether or not these are representative of the mantle as a whole or just the fragments that have been erupted is open for debate.

The upper mantle is joined to the crust; the combined layer is referred to as the lithosphere. Below the lithosphere, also in the upper mantle, is the asthenosphere.  The asthenosphere, being weaker than the lithosphere, enables lithospheric slabs to move around (plate tectonics).  The asthenosphere moves at the rate of a few centimetres a year from a process called solid-state convection; hot mantle rises, transfers heat to the lithosphere and the resulting cooled mantle sinks.  The heat in the lithosphere is dissipated through conduction or via rising magma.

The lithosphere is around 120 km thick.  It’s boundary with the asthenosphere is defined by the temperature at which rocks become ductile, around 1,350°C.

The Crust

The crust is a silicate rich brittle layer covering the mantle; it comprises less than 0.5% of the Earth’s mass.  There are two types of crust: oceanic crust, c. 6 km to 11 km thick, mostly basalt, which makes up ocean floors; and, continental crust, c. 25 km to 90 km thick, composed of igneous rocks (granite and andesite), sedimentary rocks and metamorphic rocks, which, as the name suggests, make up the continents and the continental shelves.  Igneous rocks are those resulting from volcanic processes. Sedimentary rocks are those made up of fragments produced by erosion or decay of rocks on the surface.  Metamorphic rocks are sedimentary or igneous rocks altered by changes in temperature and / or pressure. 


Magma is the molten rock from either the mantle or the crust, itself, that makes its way through the crust to where it may be erupted as lava at a volcano or volcanic fissure.  Magma and lava are the same rock: it is magma until it is erupted; and, lava is the erupted matter.

The composition of the magma and how it is generated determine the eruptive style of the volcano: e.g. effusive or explosive.


The energy required for matter to be erupted is heat from the Earth’s core.  The Earth’s core is made up of radioactive materials; their radioactive decay generates heat.  Most heat today is generated from four long-lived radioactive isotopes: two uranium isotopes,235U and 238U; one thorium, 232Th; and, one potassium, 40K.  Additional heat came from the decay of the shorter-lived aluminium isotope, 26Al earlier in the planet’s formation.  Asteroid bombardment has also added kinetic energy. 

So we know have the Earth’s crust, magma and heat.  What happens next?  Watch this space. 

The Armchair Volcanologist

24 June 2020

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

Grímsvötn – Grumbling Quietly

Good Afternoon!

While browsing IMO’s website  a few days ago, I saw that signs have been detected that Grímsvötn is getting ready for another eruption, IMO ; a team of scientists noted large sulphur dioxide emissions near the south west caldera rim, indicating that magma is close to the surface. At the time of writing, the alert level for Grímsvötn remains at green.

Grímsvötn is Iceland’s most active volcano, erupting every 10 years and last erupting in 2011 with a VEI 4. 

Fig 1 Grimsvotn 2011 eruption.  Photographer: Sigurjónsson,O.  Grímsvötn (GRV): photo 2 of 14.  Retrieved from Icelandic Volcanoes:

Geological Setting

Grímsvötn is one of six active volcanoes under the Vatnajökull ice cap: Bárðarbunga, Kverkfjöll, Grímsvötn, Esufjöll, Þórðarhyrna, Öræfajökull.  Apart from Þórðarhyrna (THO in the map below), the other volcanoes are different volcanic systems.

The Vatnajökull volcanoes are part of the Eastern Volcanic Zone in Iceland.  Volcanism here is caused by rifting and extension from the separation of the North American and Eurasian Plates.  As noted in an earlier post, the Eastern Volcanic Zone accommodates 40 to 100% of the separation.

Our description of the Grímsvötn volcanic system is largely based on Magnús T. Guðmundsson and Guðrún Larsen’s description in Icelandic Volcanoes (ref. Sources below for the full accreditation).

The Grímsvötn volcanic system

The Grímsvötn volcanic system, itself, is made up of two central volcanos and fissure swarms.  It is partly covered by ice.

Fig 2 The Grímsvötn volcanic system showing craters, central volcanoes and fissure swarms.  Retrieved from Icelandic Volcanoes (see Sources below for full accreditation).

The Central Volcanoes

The Grímsvötn central volcano is a 1722m high, 15-16km diameter caldera complex covered by the Vatnajökull ice-cap, with ice depths of 100m to 700m; she has an 8km by 10km ice-filled caldera.  Grímsfall (GFUM) is the highest point on the caldera rim. There is a subglacial lake in the caldera under a 200 – 300m ice shelf with an associated geothermal area. The lake has been the source of many jökulhlaups.

The Þórðarhyrna central volcano, also subglacial, is a 1650 high with a 15 km diameter, connected to Grímsvötn by a subglacial ridge.  The volcano, itself, has a small intrusive complex but does not appear to have a large magma reservoir. There is a geothermal area near Pálsfjall.

Ice cover has restricted study of the volcanoes.  However, Grímsvötn has been around for long enough to develop a caldera – possibly more than 100,000 years.

Grímsvötn’s lava types are tholeiitic basalt with basaltic andesite and dacite / rhyolitic outcrops in the Þórðarhyrna central volcano.  The presence of a shallow magma reservoir is inferred from the geothermal field in the caldera.  The 2011 eruption of Grímsvötn produced 0.8km3 basaltic tephra.

Þórðarhyrna is less active than her neighbour; the last eruption occurred in 1903 with a VEI 4.  It is possible that she had a second eruption in 1753, resulting in jökulhlaups.  Again, ice cover has limited geological study.  There is little seismic activity near Þórðarhyrna. 

The Fissure Swarms

The fissure swarm is about 100 km long and 18 km wide.  Rifting is believed to occur along the entire swarm.   The northern end of the fissure swarm is covered by the Vatnajökull ice-cap; the southern 80km is ice-free.  Subglacial ridges characterise the northern end of the fissure, but not the ice-free southern end where crater rows delineate the fissure, including the Laki.

Three known subglacial eruptions have occurred since 1867 at Gjálp 10km to 15km north of Grímsvötn, itself.  The eruptive products include subglacial ridges and some airborne tephra.  The 1996 eruption produced basaltic andesite. 

Four effusive eruptions have been identified in the ice-free section of the fissure swarm southwest of Grímsvötn in the last 8,000 years; lava volumes have been between 1 km3 to 14 km3 with up to 0.7km3 of tephra.  The largest fissure eruption was the Laki eruption in 1783 to 1784.  No eruptions have been identified for the ice-covered section of the fissure swarm.

The Laki Fissure Eruption 1783 -1784

This eruption was well documented at the time; the Reverend Jón Steingrímsson’s 1788 account in “A complete description of the Síða Fires” gives a detailed eye-witness account. 

The 1783 eruption occurred on 27km long fissure and lasted from 8 June 1783 to 7 February 1784.  The early phase consisted of a series of ten or more explosive tephra events, each followed by effusive lava flows.  Grímsvötn, itself, erupted in July 1783 to May 1785 causing ash fall and jökulhlaups.

The Laki eruption was pre-empted by earthquakes of increasing intensity from mid-May to 8 June 1783 when a large ash cloud and ash fall appeared, followed by lava columns over 1km high from new fissure to the north.  Volcanic gases filtered out sunlight, making the Sun appear red.  Accompanying rainfall was acidic, irritating people’s eyes and skin.  Lava flows filled river gorges, overflowing to cover surrounding farmland.  During the eruption, Mount Laki was destroyed; I am not sure how big she was and how much her destruction contributed to the vast tephra output.

The eruption is rated a VEI4, having produced 0.7km3 of tephra which covered more than 8,000 km2, and 14 km3 of lava. Volcanic gases, including fluorine, killed more than half of the livestock and the “Haze Famine” killed 20% of the Icelandic population.  Further afield, 100 million tonnes of sulphur dioxide, having reached the jet stream, spread acidic sulphate aerosols round the Northern Hemisphere, damaged vegetation and crops in Europe and Alaska, caused severe winters and annual cooling of around 1.3°C that lasted for two to three years.

Fig 3: Laki Crater Row: Photographer: Sigurðsson, O.  Grímsvötn (GRV): photo 1 of 14.  Retrieved from Retrieved from Icelandic Volcanoes:

According to GVP, the Grímsvötn volcanic system has had 86 Holocene eruptions ranging from VEI 0 to VEI 6.  The VEI 6 occurred around 10200 BP and is the thought to be the source of the Saksunarvatn Tephra, a basaltic tephra which covered an area of 2 million km2 around the North Atlantic.  The Saksunarvatn Tephra, like the Vedde Ash from Hekla, is a geological time marker, although radiocarbon dating of the Saksunarvatn Tephra shows that it may have come from seven eruptive events over a 500 year period from 10400 BP to 9900 BP

Grímsvötn’s most recent eruptions from 1996 to 2011 range from VEI 3 to VEI 4. They were preceded by a small increase in seismicity and small earthquake swarms, except for the 1996 Gjálp eruption.  The 1996 eruption was preceded by a 5.4 earthquake on Barðabunga’s northern caldera rim,  swarms over a two day period at Barðarbunga’s north and northwest caldera rims and at Grimsvotn’s southern caldera rim, followed by a swarm from the north Bardarbunga caldera rim that migrated to Gjálp.

Recent Seismicity

So, what does our earthquake data set tell us about the likelihood of an eruption at Grímsvötn?  The answer is a disappointing “not a lot”.   We can see that Grímsvötn has a fairly steady stream of earthquakes but no obvious swarms.  However, given the proximity of Grímsvötn to other volcanoes, we may have attributed some of Grímsvötn’s activity to another volcano in error.  Plots are shown below, including one for Vatnajökull which shows the problem.

Fig 4: Earthquake plots by the author of seismic activity in the Vatnajökull region, Iceland, from 1 January 2016 to 14 June 2020. Green dots denote earthquake below 2M, yellow circles earthquakes between 3M and 3M and red stars earthquakes over 3M. © Copyright remains with the author; all rights reserved, 2020.

The earthquake plots of the Vatnajökull region show the SW-NE trending fissure swarms and also a SE-NW trending line of earthquakes.  The head of the mantle plume is considered to be under the Vatnajökull ice-cap; perhaps we are seeing its influence on the plate junction?  We can also see the proximity of Grímsvötn to Bárðarbunga.

The Grímsvötn system, with 3,326 earthquakes, is not the most seismically active volcano; activity is overshadowed by seismic activity at Bárðarbunga (5,464 earthquakes), Askja and Herðubreið (a combined 15,645 earthquakes) and Öræfajökull (4,770 earthquakes).  The 2014 eruption of Holuhraun was both preceded and accompanied by intense seismic activity at Bárðarbunga, notably near the edges of the caldera, and deflation at Bárðarbunga.  Since the eruption, Bárðarbunga has started to re-inflate. Our data set starts a year or more after the end of that eruption.

Looking more closely at Grímsvötn we see that earthquake activity is focused on the south east of the caldera and at an E-W trending fissure to the north east of the volcano.  The E-W fissure is parallel to similar lines of activity further north at Bárðarbunga’s caldera.  We also picked up some activity at Þórðarhyrna.

Fig 5: Geo-scatter and scatter earthquake plots by the author of seismic activity in the Grímsvötn region, Iceland, from 1 January 2016 to 14 June 2020. Green dots denote earthquake below 2M, yellow circles earthquakes between 3M and 3M and red stars earthquakes over 3M; black triangles are GPS stations and orange triangles, volcanic centres. © Copyright remains with the author; all rights reserved, 2020.
Fig 6: Scatter earthquake plots by the author of seismic activity in the Grímsvötn region, Iceland, from 1 January 2016 to 14 June 2020. Green dots denote earthquake below 2M, yellow circles earthquakes between 3M and 3M and red stars earthquakes over 3M; black triangles are GPS stations and orange triangles, volcanic centres. © Copyright remains with the author; all rights reserved, 2020

The earthquakes are telling only part of the story.  Grímsvötn has had a steady stream of earthquake activity during the period, but without the SO2 measurements from scientists, we would not be certain that magma, itself, was near the surface.

For updates on Grímsvötn, please visit IMO’s website (details below).

The Armchair Volcanologist

22 June 2020

Sources and Further Reading

“Grímsvötn”, Magnús T. Guðmundsson and Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes:

“Þórðarhyrna”, Magnús T. Guðmundsson and Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes:

Fig 2: Map: After Guðmundsson and Miller (1997), Guðmundsson et al (2013a), Jóhannesson and Sæmundsson (1998a), Jóhannesson et al (1990). Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO.  In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes:

Smithsonian Institution Natural History Museum Global Volcanism Program (GVP):

Earthquake data: Icelandic Meteorology Office: IMO

Plots are the author’s own work.

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

A Quick Update on Activity on the Reykjanes Peninsula

Good Afternoon!

The earthquake swarm which started in December 2019 is continuing, let’s have a quick update on the stats.


There have been 19,675 earthquakes in the Reykjanes Peninsula area 64.4°N, 23.0°W to 63.7°N, 21.0°W for the period 1 Jan 2016 to 14 June 2020, of which 14,258 (72%) have occurred in the last six months, most associated with the swarm near Svartsengi.

Fig 1: Statistics for the earthquake swarm to date by the author.  Month from start refers to the start of our data extraction (January 2016). © Copyright remains with the author; all rights reserved, 2020.

Seismic Activity

Our updated scatter plots show that there is more shallow small earthquake activity above the lithosphere than in our earlier plots. 

Fig 2: Latitude v Longitude geoscatter plot and depth plot for earthquake activity in the vicinity of Svartsengi 1/01/2016 to 14/06/2020 by the author.  Green dots denote earthquakes <2M; yellow dots, earthquakes greater than or equal 2.0M and less than 3.0M; red stars, greater than or equal to 3M.  © Copyright remains with the author; all rights reserved, 2020

Geodensity Plots

The geodensity plots for months 48 (December 2019) onwards (Figs 3.1 and 3.2) show that the most intense action started to the east of Mt Thorbjörn and has migrated west to Svartsengi and beyond.

Fig 3.1 Geodensity plots: top row months 48 and 49 (December 2019 and January 2020); bottom row month 50 and 51 (February 2020 March 2020) by the author. Note that the colour intensity is calculated based on the data set for the specified month. © Copyright remains with the author; all rights reserved, 2020
Fig 3.2 Geodensity plots: top row months 52 and 53 (April 2020 and May 2020); bottom row month 54 (June 2020, to 14/06/2020) by the author. Note that the colour intensity is calculated based on the data set for the specified month.  © Copyright remains with the author; all rights reserved, 2020.


IMO has confirmed that uplift has resumed in the vicinity of Mount Þorbjörn.  Ground deformation is clearly visible on the GPS plots.

Fig 4: Uplift in the vicinity of Mt Þorbjörn as shown in recent GPS plots published by IMO: GPS Þorbjörn. THOB moved south eastwards, SENG moved north eastwards and ELDC moved westward; all showed uplift.


We are still looking at an unusually large swarm, accompanied by continued uplift in the vicinity of Mt Þorbjörn.

At the time of writing, there has been no change in the uncertainty phase declared by Icelandic Civil Protection .

The Armchair Volcanologist

15 June 2020


Raw earthquake data and GPS plots downloaded from the Icelandic Met Office:

Earthquake plots are the author’s own work.

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

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