Category Archives: Famous Eruptions

Descriptions of famous eruptions

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).

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

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

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

The Smithsonian Institution’s Global Volcanism Program (GVP):

Earthquake raw data: IMO:

For updates on La Soufriere St Vincent:

News 784:

Nation News Barbados:

For updates on the new volcano at Geldingadalur:

Icelandic Met Office: (English site)

Icelandic Met Office: https:// (Icelandic site)

Reykjavik Grapevine:

Department of Civil Protection and Emergency Management | Almannavarnir

Mt. Pelée, La Soufrière St. Vincent And A Quick Tour Of The Plates Surrounding The Caribbean Plate

Good Morning!

On 4 December 2020 the alert level for Mt. Pelée was raised to yellow due to increasing seismicity above background levels; and, on 29 December 2020 the alert level for La Soufrière St. Vincent was raised to orange following an increase in seismic activity, changes seen in the lake and fumaroles and a new growing lava dome emerging in the summit crater.  

This led us to look into what drives volcanism in the area, notably the interaction of the Caribbean Plate with its surrounding plates.

Fig 1: Mt. Pelée on the left with St. Pierre, photo by Lee Siebert, 2002 (Smithsonian Institution); Soufrière St. Vincent on the right, photo by William Melson, 1972 (Smithsonian Institution).

Mt. Pelée is famous for destroying the town of Saint- Pierre and its inhabitants plus visitors – a total of 29,000 people – in a matter of minutes on the morning of 8 May 1902 in a pyroclastic flow during a VEI 4 eruption.  La Soufrière St. Vincent also erupted around the same time.  Both volcanoes are located in the Lesser Antilles on the Caribbean Plate. 

Mt. Pelée

Mt. Pelée is located on Martinique.  She is a 1,400m high stratovolcano located in the caldera of an earlier volcano; edifice failures have breached the south west section of the caldera. Her lava types are andesite, basaltic andesite, dacite and basalt, picro basalt.

54 Holocene eruptions are recorded by GVP. Her historic eruptions include 1792 (VEI 1), 1851 (VEI 2), 1902 (VEI 4) and 1929 (VEI 3).  Two lava domes were emplaced in the summit crater, l’Etang Sec, during the 1902 and 1929 eruptions. 

1902 Eruption

Prior to the 1902 eruption, Mt Pelée’s known eruptions had been mild.  Activity at the volcano started to ramp up gradually with fumaroles in the summit crater in 1889 to March 1902.  From 23 April 1902 phreatic activity cleared out old rock, starting with minor explosive activity. By 4 May 1902 ash was raining down on Saint- Pierre. On 5 May, 23 people were killed when near boiling water from the crater heated by rising magma overran a distillery in a lahar in the Rivière Blanche valley.

On 6 May 1902, new lava emerged creating a lava dome.  During 7 May 1902, small parts of the dome collapsed.  At 07:50 on 8 May 1902, explosions were heard and a large black cloud seen to emerge and flow down the volcano, engulfing Saint-Pierre and some of the ships in the harbour.  Most of the casualties were killed by hot gases and dust from the blast.  Several pyroclastic flows followed: on 20 May 1902 a second pyroclastic current swept over Saint-Pierre destroying several of the remaining buildings; the Rivière Blanche valley saw several PDCs over the ensuing months; and, Morne Rouge was destroyed and 2,000 people killed by a pyroclastic current on 30 August 1902.  During this activity a 300m lava spine emerged. After this eruptive activity continued until 1903. The lava spine has since been eroded.

Saint-Pierre had not been evacuated prior to 8 May 1902 for a couple of reasons: it was not known at the time that the volcano produced pyroclastic flows so the danger was not understood; and, an election was due on 11 May 1902, which politicians were keen should go ahead.  No evacuation order was given. 

When activity ramped up again prior to the 1929 eruption, people were evacuated in time.

La Soufrière St. Vincent

La Soufière St. Vincent can be found on St Vincent Island.  She is a 1,234m high stratovolcano with crater lake and lava domes.  The 1.6km wide summit crater is located on the south west edge of a 2.2km wide Somma crater; slope failure caused a breach in the Somma crater.  Her lava types are andesite, basaltic andesite and basalt, picro basalt.

22 Holocene eruptions are recorded by GVP. Her historic eruptions include 1718 (VEI 3), 1812 (VEI 4), 1902 (VEI 4), 1971 (VEI 0) and 1979 (VEI 3).  The 1902 eruption occurred on 6 May 1902, killing 1,680 people. The 1812 eruption produced a new crater, cutting through the summit crater.  1971 eruption extruded a lava dome in the summit crater, which erupted explosively in 1979 to be replaced by another dome. 

Tectonic Setting

As noted above, both Mt. Pelée and La Soufrière St. Vincent are located on the Caribbean Plate in the Lesser Antilles. The Caribbean Plate is thick oceanic crust located between the North American and South American Plates.  The northern boundary of the Caribbean Plate is a transform boundary with the North American plate, running from Central America to the Virgin Islands. The Gonâve microplate and Puerto Rico Trench form part of the northern boundary. At the eastern boundary, the South American Plate subducts under the Caribbean Plate in the Lesser Antilles. At the western boundary, the Cocos Plate subducts under the Caribbean Plate, forming the Central American Volcanic Arc.  The southern boundary with the South American plate is a complex, comprising a convergent margin with the Panama Plate, a subduction zone with the North Andes Plate and a transform boundary with the South American Plate.  The main plates velocities relative to the African Plate are noted below.

North American PlateWest25 mm per year
Cocos PlateNorth east67 mm per year
Caribbean PlateNorth west10 mm per year
Panama PlateNorth west19 mm per year
Coiba and Malpelo PlatesEast 
North Andes PlateNorth west23 mm per year
Nazca PlateNorth east40 -53 mm per year
South American PlateWest27 – 34 mm per year

The origins of the Caribbean Plate are debated.  There are two main theories which attempt to explain why the less dense but thicker crust of the plate overrides the Cocos and South American Plates. It may have evolved millions of years ago from the Caribbean large igneous province, formed at the Galapagos hotspot, drifting to its current location as the plates moved to accommodate the widening of the Atlantic Ocean.  Alternatively, it may have formed from an old hotspot in the Atlantic.  These theories are based on the relative motions of the plates.  The first theory works on the basis that the Caribbean Plate is moving eastward compared to the North and South American Plates, whereas the latter uses the actual westward motion of the Caribbean Plate.

Recent Seismicity

Yes, we’ve downloaded earthquakes for the region from USGS’s earthquake search, taking a larger area than the Caribbean Plate in order to pick up the subduction zones.  In this case, it was not really necessary as most subduction is beneath the Caribbean Plate, but it was fun to find several microplates in the process: the Gonâve, Panama, Coiba and Malpelo Plates.  The Malpelo Plate was first identified as late as 2017 by Tuo Zhang, Richard G Gordon et al of Rice University.

The coordinates selected were: 3.760°S, 107.051°W to 26.838°N, 48.867°W for earthquakes with magnitudes of 2.5 or more between 1 January 1975 and 5 January 2021.  This picked up 80,751 earthquakes.

Fig 2: Earthquakes plotted by the author using data downloaded from USGS (see sources below).  Green dots denote earthquakes with magnitude between 2.5 M and 4.5 M, yellow dots earthquakes between 4.5 M and 6.0 M, orange stars, earthquakes between 6.0 M and 7.0 M and red stars, earthquakes over 7.0 M.  Some volcanoes are shown, these are denoted by blue triangles. Mt. Pelée and Soufrière St.Vincent are shown as yellow triangles. © Copyright remains with the author; all rights reserved, 2021

From Figs 2, 3 and 4 below, we can see the plate boundaries and the subduction zones on the western and eastern margins of the Caribbean plate are well marked by earthquakes and volcanoes; the subduction of the Caribbean Plate under the North Andes Plate is also visible (lower centre of the depth plot); and, the Puerto Rico Trench is also tectonically very active.  The Puerto Rico Trench has produced some large earthquakes and tsunamis.

Fig 3 Scatter plot of earthquakes round the Caribbean plate: latitude v longitude on the left and a depth plat on the right.  Earthquake with magnitude less than 4.5 are not shown in the depth plot to reduce noise.  Colour key as before. © Copyright remains with the author; all rights reserved, 2021
Fig 4: Scatter plot of the earthquakes at the Puerto Rico Trench: latitude v longitude on the left and a depth plat on the right. Colour key as before. © Copyright remains with the author; all rights reserved, 2021

Fig 4 shows subduction of the North American Plate on the right of the depth plot.  But there is also a line of earthquakes on the left of the plot which appears to indicate another subduction zone.  Neither subduction zone here, despite being seismically active, has active volcanoes associated with it.  It’s possible that there is another microplate here, but this is conjecture on our part until we can find an explanation or confirmation.

If you are interested or concerned by the alert statuses of Mt Pelée and La Soufrière St Vincent, you can find more information at L’Observatoire Volcanologique et Seismologique de Martinique, the National Emergency Management Organisation (NEMO) or GVP.

Hope you enjoyed our little tour.  We will be looking in more detail at points of interest in the future.

The Armchair Volcanologist

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

Sources & Further Reading

Raw earthquake data from USGS Earthquake Catalogue Search:

L’Observatoire Volcanologique et Seismologique de Martinique:

National Emergency Management Organisation (NEMO):

The Smithsonian Institution’s Global Volcanism Program (GVP):

Caribbean Plate – Wikipedia:

Zhang, Tuo; Richard G. Gordon; Jay K. Mishra, and Chengzu Wang. 2017. The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos-Nazca-Pacific plate motion circuit, 1. AGU Fall Meeting, New Orleans. Accessed 2018-06-06.

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

La Soufrière (volcano) – Wikipedia:ère_(volcano)

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.

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.

Mount St Helens : 18 May 1980 Eruption

Good Afternoon!

The 40th anniversary of the catastrophic eruption of Mount St Helens was on 18 May 2020.  On 18 May 1980, 57 people are known to have been killed, most probably by the initial lateral blast, not taking into account anyone unwise enough to have ignored the exclusion zone.  In addition, significant damage was done to the surrounding area.

Geologists Keith and Dorothy Stoffel, on a photography expedition, witnessed the catastrophic failure of the northern flank of the volcano: the north side of the volcano slid down then disintegrated in a massive debris avalanche. This was followed by the horizontal blast and the vertical ash column which reached a height of 20 km within 10 minutes. The threat of being engulfed in the ash cloud forced them to return to base.

Fig 1: Mount St Helens and Spirit Lake before the May 1980 eruption. Image cropped from one by Jim Nieland – US Forest Service, Public Domain

Geological Setting

Mount St Helens lies at the front of the Cascades volcanic arc in Washington. She is the most active volcano in the arc. Her lavas are dacite, andesite / basaltic andesite, basalt / picro basalt, trachybasalt / tephrite basanite and trachyandesite / basaltic trachyandesite(1). The lavas are sourced from depths of 7 km – 14 km beneath the base of the edifice. Eruptions occurring after 1980 were fed from magma at a depth of less than 4 km. 

Volcanism here is driven by the subduction of the Juan de Fuca plate under the North American plate.  The arc starts in northern California and reaches up to British Columbia, including composite volcanoes and volcanic centres, calderas and back arc basalt shield volcanoes.  Lava compositions range from intermediate to high silica lava domes to low silica lave flows in the southern end of the arc.  The presence of low silica lavas at the southern end of the arc is attributed to extensional tectonics permitting the rise of more fluid magmas.

Seismicity in the Cascades

We downloaded the earthquakes between 39.470°N, 130.808°W and 50.870°N, 118.696°W from 1975 to 31 May 2020 to see what the subduction zone below the Cascades looks like.  There were 18,173 quakes over 2.5, of which 48 were between 6.0 and 7.0 and 4 had magnitudes in excess of 7.0. We found two areas where there was some evidence of subduction, one at the northern end of the region (north of latitude 45°N) and the other in the south (south of latitude 42°N), otherwise the area was relatively quiet in this period. 

Fig 2: Seismicity and some volcanoes of the Cascades plotted by the author. © Copyright remains with the author; all rights reserved, 2020.

In order to see the subduction zone clearly we have had to removed the “noise” from the Blanco Fracture Zone.

Fig 3: The subduction zone north of latitude 45N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.
Fig 4: The subduction zone south of latitude 42N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.

Eruptive History

Mount St Helens is a relatively young volcano, formed around 40,000 to 50,000 years ago.  The current edifice was built in the last 2,200 years.   Before the eruption Mount St Helens was conical stratovolcano made up of lava, ash, pumice and other deposits with layers of basalt and andesite.  Dacite lava domes extruded from the summit and on the northern flank at Goat Rocks. 

Mount St Helens has had 44 Holocene eruptions according to GVP(1), of which one was VEI 6, and five were VEI 5, including the 1980 eruption.  Two years prior to the1980 eruption USGS volcanologists produced a hazard assessment, predicting an eruption with the next couple of decades. 

The 18 May 1980 Eruption

A 4.2 earthquake on 20 March 1980 and increased seismic activity showed that the volcano was reawakening.  Ash eruptions followed from 27 March 1980. A harmonic tremor, indicating magma ascent, set in.  Between the initial earthquake and to the earthquake preceding the eruption on 18 May 1980 there were 228 earthquakes over 2.5 and thousands more smaller quakes.

By the end of April, a 2 km wide cryptodome (bulge) had deformed the northern flank, swelling upwards at a rate of 1 metre per day to 150 metres above the existing topography by 12 May 1980.  Ground deformation was now presenting a serious hazard.  

Fig 5   The cryptodome on the north flank 27 April 1980, cropped from an image by Peter Lipman in  – CVO Photo Archives Mount St. Helens: A General Slide Set, Public Domain

On the morning of 18 May 1980 magma had reached a level of 2 km below the edifice. The slope failure was triggered by a 5.1 earthquake on the morning of 18 May 1980 (or vice versa).  A debris avalanche of rocks, glacier ice, soil and other debris ensued, reaching Spirit Lake and the Toutle River valley and ending up east of the Camp Baker logging base, 20 km further down the valley. 

Depressurisation of the system, resulted in the violent explosive lateral blast. The blast outran the initial avalanche, felling trees in an area of 600km2 and causing the most loss of life.  The blast included heated old lava from the volcano and left a relatively thin ash layer. 

After the blast, pumice was erupted in a vertical eruption column and pyroclastic currents covered the northern side of the volcano.  Pyroclastic deposits reached a depth of 40 metres in the Upper Toutle Valley.  It is estimated that at the time of the eruption the pyroclastic flows had temperatures in the region of 700°C.  Hot ash reaching Spirit Lake caused secondary eruptions as the water flashed to stream.

During the eruption, Mount St Helens lost its pristine cone and around 400 metres height; it was left with a 1.6 km wide crater, open on the northern side.

Fig 6 The day before the eruption and a few days later.  Both images are from Johnson’s Ridge (named after the volcanologist who lost his life in the eruption) and are by Harry Glicken – USGS Cascades Volcano Observatory, Public Domain

Post Eruption

From mid-June 1980, lava emerged into the crater forming a lava dome; subsequent eruptions have been crater-based lava dome.

Could Mount St Helens produce another catastrophic eruption? 

As she has a history of VEI 5s and a VEI 6, the answer is probably, yes.  Her lava types do produce explosive eruptions, including the dacite lava domes. If any of the remaining slopes are unstable, there could be future slope failure should ascending magma cause enough deformation but it’s possible that she may have to rebuild the cone first. 

In the meantime, a risk would be to the unwary straying too near to the crater should she have a smaller e.g. phreatic eruption, which are unpredictable. Phreatic eruptions occur when ground water in the edifice heated by hot rocks or magma flashes to steam.

GVP(1) have added a commemorative feature to its website giving a lot more information, including footage of the eruption.  We recommend that you take a look; it is well worth the visit(2).

The Armchair Volcanologist

2 June 2020

References & Further Reading

  1. Smithsonian Institution Natural History Museum Global Volcanism Program (GVP):
  2. Mount St Helens 40th Anniversary, GVP:
  3. Mount St Helens, Wikipedia:
  4. 1980 Eruption of Mount St Helens, Wikipedia,

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