Mount Etna, Not One Volcano But Many

Part 1: Tectonic Setting and History

Good Afternoon!

Fig 1:  Cropped image of Mount Etna with Catania in the foreground from a photo by BenAveling in 2007, published under CC BY-SA 4.0.  Source: Mount Etna – Wikipedia

Today we are looking at Mount Etna, Europe’s largest and most active volcano, sited on the eastern side of Sicily. Sicily has been at the heart of many Mediterranean trading routes. As a result, the volcano has been widely studied.  Etna has been designated a Decade Volcano by the UN.  She is home to indigenous species, Etna Broom, which thrives on its lower slopes.  In 2013, she was designated a UNESCO World Heritage Site.

Apart from the widely reported recent activity, she caught our eye because of the interesting tectonic setting between the African Plate, Eurasian Plate, Adriatic Plate and additional micro-plates.  The area has some of the deepest subduction in the Mediterranean region.  We start with her tectonic setting.

Fig 2: Cropped from an image by Velela of Etna Broom, published under CC BY-SA 3.0.  Source: EtnaBroom8142 – Genista aetnensis – Wikipedia

Mount Etna is a c.3,320m high complex volcano with four summit craters: the North East, Voragine, Bocca Nuova and the South East Craters and many cinder cones.  The volcano is made up of an old shield volcano, topped by stratovolcanoes, Ellittico and  Mongibello. It is truncated by small nested calderas. 

The volcano has erupted basalts, viscous granitic lava, trachyte and again basalt.  The last is rich in iron with microscopic gold particles.  The basalt is used for building and sculpture.  Until health and safety concerns banned it, the fluid lava was collected to be made into souvenirs  for tourists. 

The source for Etna’s lavas is not clear.  You will see from our earthquake plots below that there is little evidence of subduction beneath Etna, herself, although there is clear evidence of subduction beneath the neighbouring Aeolian Arc.  Various theories have been proposed: rifting; a hot spot; and, the intersection of breaks in the crust associated with the subduction of the African Plate under the Eurasian Plate, providing a channel for magma.

Tectonic Setting

The collision of the Eurasian and African Plate has a complex history. In the vicinity of Sicily, the Tyrrhenian and Ionian Seas opened, the Aeolian – Calabrian Arc suffered subduction, bending and compression and the edge of the Eurasian Plate was thrust over the edge of the African Plate.  The Adriatic Plate broke off and the Ionian Plate was subducted under the Tyrrhenian Sea.


The main tectonic features of Sicily are the crystalline units of the Calabrian arc; the Neogene Maghrebian fold and thrust belt created during the opening of the Tyrrhenian Basin; the Catania – Gela foredeep; and the Hyblean Plateau. 

Fig 3:  Map by FieldsetJ, published under CC BY-SA 4.0.  Source: Geology of Sicily – Wikipedia

The Calabrian Arc unit is located in the north eastern part of Sicily.  It is the tectonic unit that connects the Apennines with the North African Maghrebide.  The Arc migrated to the south east, driven by the subduction of the Ionian slab east of Sicily.  In the early Miocene, Calabria was attached to Sardinia.  Trench roll back to the east and extension in the middle Miocene, caused Calabria to break off from Sardinia. By the early Pliocene, back arc volcanism relating to subduction of the Ionian slab had created new crust in the Tyrrhenian Sea.  From the Pleistocene, the eastern portion of the Arc formed the Apennine mountains in Italy, while the Calabrian block had migrated to Sicily.  The migration is ongoing.  Palaeozoic igneous and metamorphic basement rocks are exposed at this part of the island.  Triassic late Cretaceous carbonate is overlain by Paleogene – Middle Miocene turbidites, Middle Miocene to – Messinian rift deposits, Messinian evaporites and early Pliocene chalk.

The Maghrebian fold and thrust belt, part of a subduction regime, comprises frontal accretion and detached nappes of Mesozoic carbonate, which has been folded in to a syncline, underlying the anticline of the Caltanissetta region.

The Catania – Gela foredeep extends from the northern margin of the Hyblean Plateau to the offshore Gela basin of southwestern Sicily.  It was formed due to bending of the carbonate platform caused by loading at the front of the Gela nappe.  In the late Miocene – Pleistocene sediments were deposited, including limestones, evaporites and sandy clays.

The Hyblean Plateau is on the Nubian Plate. In the early Miocene, regional uplift caused by thrusting and volcanism resulting in the emergence of a carbonate platform in the eastern Hyblean Plateau.  In the late Miocene, sea level fall and uplift caused the deposition of evaporites in the Mediterranean basin.  This was followed by subsidence in the foreland and deposition of sediments.  In the late Miocene – early Pleistocene, basaltic volcanic activity occurred; alkaline basalts were erupted during the late Miocene.  In the late Pliocene, large volumes of tholeiitic basalt was erupted and the foredeep north of the Hyblean foreland collapsed.

Mastrolembo et al(1) found evidence for independent lithospheric blocks by studying GPS velocities relative to the Eurasian and Nubian Plates. The Sicilian domain rotates clockwise with respect to Eurasia and is fragmented into three distinct blocks: the south eastern, the central and the north eastern Sicily blocks, which move independently from the African Plate.  

Fig 4: Plot of earthquakes from 01/10/2004 to 13/02/2022 in eastern Sicily by the author.  Colour denotes earthquake depth: magenta 0 – 33km, green 33km – 70km, yellow 70km – 150km, orange 150km-300km, red 300km – 500km.  Blue triangles denote Holocene volcanoes. The orange triangle denotes Mount Etna.  Earthquake epicentres roughly delineate the boundaries of the tectonic blocks identified by Mastrolembo et al(1): STYR is the South Tyrrhenian block, NESI the North Eastern Sicily block, SC the central Sicily block, SECI the South Eastern Sicily block and CALA Calabria.  The yellow, orange and red dots are the Ionian Plate subducting under STYR.  © Copyright remains with the author; all rights reserved, 2022.

Deep faults transect the island.  Currently, the Tyrrhenian side of Sicily is being compressed and the Ionian side extended.  Mount Etna, itself, is located near to the junction of three major lithospheric faults which may reach the mantle: the east to west Mt Kumeta -Alcantara fault; the north -northeast to south-southwest Messina – Giardini fault which delimits the north coast of Etna; and, the Aeolian-Maltese fault which extends from Vulcano to Malta.

Mount Etna’s History

Mount Etna is a complicated volcanic system; she is thought to have evolved in four stages: 1) tholeiitic basalt; 2) Timpe shield-building; 3) Valle del Bove Central Volcanoes; and 4) Stratovolcanoes.

Tholeiitic Basalt Stage

Earliest activity at Etna has been identified as being the sporadic eruption of tholeiitic pillow lavas from fissures more than 500,000 years ago in the submarine environment of the Gela-Catania foredeep basin at Aci Castello.   300,000 years ago, Na affinity lavas were erupted in fissure eruptions on the alluvial plain of the Simento  River, forming a lava plateau.

Timpe Shield Building Stage

220,000 years ago, extension tectonics of the NNW-trending Timpe fault system drove activity. Sub alkaline – Na alkaline were lavas erupted in fissures along the Ionian coast built a 22 km NNW -SSE aligned shield volcano.  Volcanic activity also occurred to the south west and south east of the location of the now current edifice. 129,000 to 126,000 years ago, activity shifted to a location at the central portion of the location of the current volcano.

A 15km horseshoe depression in the east flank of the edifice was created during this period.  The edifice rested on basement rocks of Miocene argillaceous turbidites in the north and west, but on Pleistocene clay rich marine sediments to the south and the east.  The clays were unable to support the weight of the edifice, resulting in slippage and subsidence to the east. 

Valle Del Bove Central Volcanoes Stage

During this period magma ascent through the crust became more localised, allowing the building stratovolcanoes.  Several monogenetic volcanoes are visible in the Valle del Bove scars. Na-alkaline lavas were erupted.

121,000 years ago, effusive eruptions occurred on the southern edge of the volcano in the N – S aligned  Tarderia and Rocce volcanoes in the Valle Del Bove area.   Activity at Tarderia and Rocce  ended between 106,000 and 102,000 years ago, respectively. 

Trifoglietto volcano was also active at this time.  While the onset of activity at Trifoglietto is not known (there are no rocks outcropping to date), activity is believed to have ended c. 99,000 years ago with explosive benmoreitic Plinian eruptions. 

Activity then shifted to Monte Cerasa, Giannicola, Salifizio and Cuvigghiuni volcanoes.  Monte Cerasa volcano is a large composite stratocone in the central part of the present Valle del Bove with explosive eruptions and pyroclastic flows which transitioned to more effusive eruptions. Activity ceased here 93 ka ago. Giannicola volcano is a 300 m neck intruded in the northern flank of Trifoglietto volcano about 85 ka ago followed by alternating pyroclastic deposits and lava flows.  The main vent Salifizio volcano was located 1 km east of Trifoglietto.  She produced effusive lavas that covered the eastern sides of Trifoglietto and Monte Cerasa. Cuvigghiuni erupted intrusive bodies into the western side of Trifloglietto, followed by lava flows , spatter deposits and pyroclastic flows between c. 80,000 and 65,0000 years ago.  The final stage of her activity was effusive lava flows.

The Valle Del Bove, itself, was created during this period.  Subsidence of the Trifloglietto volcano to the east led to reduction in magma pressure, successive violent hydro-magmatic eruptions and debris avalanche flows which carved out the Valle. 

Stratovolcanoes Stage

In the fourth phase of growth , c. 60,000 years ago, Etna’s magmatic plumbing system had stabilised at its current position.  Two overlapping stratovolcanoes were formed, which form the current upper edfice: Ellittico between 60,000 and 15,000 years ago and Mongibello from 15,000 years ago to present.  Ellittico erupted trachybasalt, followed by mugearites, benmoreites and trachytes, reaching a height of 3,700m before 4 caldera-forming Plinian eruptions signalled the end of activity at Ellittico.

From 15,000 years ago to present, activity at Mongibello has been a mix of lava flows and a few Plinian to sub-Plinian eruptions of basaltic / picritic magma.  Silicic eruptions occurred in 8,460, 7100, 6100, 5000 and 4280 years ago.  In 122BC a Plinian eruption formed the Cratere del Piano, the summit caldera.  Other eruptions were basaltic, filling the caldera and building the cone.

GVP lists 241 Holocene eruptions for Etna, of which 196 have been confirmed.  These include 2 VEI 5, 1 VEI 4, 26 VEI 3, and 167 VEI 2 or less.  The VEI 5’s are the 122BC Cratere del Piano and another in 1500 BC.

Subsidence is continuing on the eastern flanks of the edifice, creating a series of rift faults and fault scarps.

Recent Seismicity

We have plotted earthquakes downloaded from EMSC for the period 1st October 2004 to 13th February 2022 for the area 37°N, 14°E to 39°N, 16°E. 

There is little evidence of a Wadati-Benioff zone under Etna, although the area is seismically active. Our plots clearly show the subducted Ionian slab under the South Tyrrhenian Sea.

Fig 5: Earthquake plots by the author for Southern Italy from 1st October 2004 to 13th February 2022 for the area, 37°N 14°E to 39°N,16°E.  In the left plot green dots denote earthquakes less than 3M, cyan stars over 3M.  In the right plot, colour denotes year.  Blue triangle are Holocene volcanoes, red Pleistocene, orange (left) and black (right), Etna. © Copyright remains with the author; all rights reserved, 2022.
Fig 6: Video of earthquake plots by the author for Southern Italy from 1st October 2004 to 13th February 2022 for the area, 37°N 14°E to 39°N,16°E.  © Copyright remains with the author; all rights reserved, 2022.

An Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2022

Sources & Further Reading

There is no shortage of reading matter about Mount Etna.  A quick internet search will probably fund you what you want.  Here are the resources we used:

  1. 1.       Brunella Mastrolembo Ventura, Enrico Serpelloni, Andrea Argnani, Alessandro Bonforte, Roland Bürgmann, Marco Anzidei, Paolo Baldi, Giuseppe Pulisi, “Fast geodetic strain-rates in eastern Sicily (southern Italy): New insights into block tectonics and seismic potential in the area of the great 1693 earthquake – ScienceDirect”, Earth and Planetary Science Letters 404 (20 14) 77-88.
  2. Stefano Branca, Mauro Coltelli, Emanuela De Beni & Jan Wijbrans , Geological evolution of Mount Etna volcano (Italy) from earliest products until the first central volcanism (between 500 and 100 ka ago) inferred from geochronological and stratigraphic data | SpringerLink
  3. Alwyn Scarth, Jean-Claude Tanguy, “Volcanoes of Europe”, Terra Publishing, 2001
  4. Geology of Sicily – Wikipedia
  5. Mount Etna – Wikipedia
  6. Raw earthquake data:

Is it Ok? An earthquake swarm west of Ok, Iceland

Good Morning!

At the end of 2021 a large earthquake swarm started to the west of the Ok shield volcano in Iceland. At the time of writing the swarm is ongoing.  The Icelandic Meteorological Office (IMO) reported on their Icelandic site on 1st February 2022 that there is no evidence of magma intrusion. 

Fig 1: Ok volcano looking northwards, with a small tuya in the foreground.  Image from Google Earth. The remnant of the Okjökull glacier is visible with a small lake in its centre at the summit of the volcano.  The current swarm is to the west of the volcano.

The swarm is occurring west of Ok in Borgarfjörður in a low temperature region outside the main volcanic and rift area.  The earthquakes are thought to be rifting caused by horizontal tension in the crust, here of the North American Plate.  The swarm is the largest to date in the region and is continuing at the time of writing. So off we trotted to plot the earthquakes.

What’s in the Area?

We looked at the area 64.2°N, 19.2°W to 65.3°N, 21.8°W which contains the Langjökull ice cap, two Holocene volcanic systems and several Pleistocene volcanic systems.

The Langjökull ice cap is the second largest ice cap in Iceland located at the northern end of the Western Volcanic Zone (WVZ).  The WVZ is a slow spreading rift that forms the western boundary of the south Iceland microplate. Sinton et al.’s study in 2005 showed that there is no evidence for the decline in the WVZ in the Holocene; the shield eruptions are long duration with low effusion rates fed by magma from the mantle. c. 40 eruptive units have been located in the WVZ; we are focussing on those in the Langjökull area in the northern part of the zone.

Holocene Volcanoes

VolcanoLatitudeLongitudeLast Known EruptionDominant Rock Type
Prestahnúkur64.583-20.6663350 BCEBasalt / Picro-Basalt
Langjökull64.85-19.7950 CEBasalt / Picro-Basalt

Prestahnúkur is west of the Geitlandsjökull glacier in the south west part of Langjökull.  It is a 90 km ling 15 km wide  fissure system with a rhyolitic peak, on a Pleistocene basaltic plateau.  The central volcano is a hyaloclastite  massif; it has shield forming olivine tholeiite basaltic effusive eruptions, with some rhyolite.   The fissures extend southwest-north east, reaching under the Þórisjökull and Geitlandsjökull glaciers.  There is a parallel sub swarm  of tuyas 4–6 km west of the main swarm.  The last significant eruption of the central volcano was in the Pleistocene and on the fissures c.900 CE.  The Holocene eruptions occurred on rift zones to the north and south west of the volcano. 

Langjökull comprises the Hveravellir central volcano and a 100 km long and 20 km wide fissure system.  A 600m thick ice cap partly covers the system.  The central volcano is another hyaloclastic massif with a silicic component. It has had six recorded  Holocene eruptions: 2 VEI 2, Hallmundahraun 950 CE and Kjalhraun 5850 BCE; and, 4 VEI 0, Lambahraun 2050 BCE, Krákshraun 2550 BCE, Strytuhraun 3550 BCE and Leggjarbrjótur 8600 BCE. The Hallmundahraun lava flow covers 240 km2 .  Kjalhraun is a shield which erupted 11km3 lava 7,800 years ago.  The dominant magmas are olivine tholeiite basalt.

Skjaldbreiður is a shield volcano that lies in the southern part of the Langjökull system.  It erupted 13km3 of basaltic lava in the early Holocene.  The lava flows formed the basin of Þingvallavatn and Þingvellir, where Iceland’s parliament, the Alþing, was founded in 930.

Pleistocene Volcanoes

VolcanoLatitudeLongitudePrimary Volcano Type
Hreppar64.42-19.5Fissure vent(s)

The Geysir geothermal area lies in the Haukadalur basin near the southern end of the Langjökull system.  Earthquake activity in June 2000 temporarily activated the normally somnolent Grand Geysir

Eiríksjökull is Iceland’s largest tuya.

Ok erupted during interglacials in the Pleistocene.  It used to have a summit glacier, the Okjökull, whose disappearance has been attributed to climate change.  Local volcanologists have reminded us that the ash from the 2010 Eyjafjalljökull eruption  would have contributed to ice loss.  “Not Ok” was a documentary about the lost glacier.  Ok overlies some of the tuyas of the sub swarm to the west of Prestahnúkur.

Hreppar is two NE-SW trending ridges which extend from the rhyolitic Kerlingarfjöll volcano located SW of the Hofsjökull ice cap.

Recent Seismic Activity

As we have extended our database of Icelandic earthquakes back to 1995, we have plotted the earthquakes for the period from 1995 to 7th February 2022 for the area 64.2°N 19.2°W to 65.3°N 21.8°W.

There is a relatively low level of background activity compared to the other volcanic regions of Iceland.  However, the following months each saw more than 200 earthquakes: 55 (July 1999) near Þórisjökull and Prestahnúkur  , 66 (June 2000) near Geysir, 67 (July 2000) near Geysir, 69 (September 2000) near Geysir, 155 (November 2007) near Hveravellir and Geysir, 240 (December 2014) near Geysir, 325 (January 2022) near Ok and 326 (February 2022) near Ok.  Note: that February 2022 is only one week.

We noted six warms to the west of Ok, including the current swarm.  They appear to be aligned along a rift / fissure.  The current swarm near Ok is one of the largest in the database to date.

Fig 2: Earthquakes by month for the Langjökjull region for the period 1995 to 7th February 2022 by the author. © Copyright remains with the author; all rights reserved, 2022.

Our geoscatter and scatter plots (including a rotating 3D scatter plot of the earthquakes to the west of Ok) are contained in the following video.

Fig 3: Video of earthquake activity in the Langjökjull region for the period 1995 to 7th February 2022 by the author. © Copyright remains with the author; all rights reserved, 2022.

It’s not clear from the plots, themselves, whether or not there is any magma movement associated with the current swarm or, indeed, any of the other activity identified.  The size of the current swarm indicates that something may be going on near Ok.  Only time will tell if volcanic activity will ensue.  In the mean time, the plots do shed some light on the activity along rifts associated with plate seperation.

Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2022.

Sources and Further Reading

Icelandic Meteorological Office: for raw earthquake data

Kristján Sæmundsson (Iceland GeoSurvey) (2019). Prestahnúkur. In: Oladottir, B., Larsen, G. & Guðmundsson, M. T. Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from

Guðrún Larsen and Magnús T. Guðmundsson (Institute of Earth Sciences – Nordvulk, University of Iceland) (2016): Langjökull, Hveravellir. n: Oladottir, B., Larsen, G. & Guðmundsson, M. T. Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from

John Sinton, Karl Grönvold, Kristján Sæmundsson, “Postglacial eruptive history of the Western Volcanic Zone, Iceland”, Geochemistry, Geophysics, Geosystems, AGU, Volume 6, Issue 12, December 2005,

For the Pleistocene volcanoes we consulted Wikipedia and the Smithsonian Institution – Global Volcanism (

Hunga Tonga-Hunga Ha’Apai, Major Eruption and Tsunami, 15th January 2022

Good Morning

A major eruption occurred on the island of Hunga Tonga-Hunga Ha’Apai on 15th January 2022 at 04:14:45 am (UTC).   As with Krakatau, news of the eruption arrived before the pressure wave from the blast due to modern technology (now the internet, then the telegraph).  The eruption was preceded by a smaller one at the end of 2021 which ended 11th January 2022.  Activity picked up again on 14th January 2022.  Our thoughts are with all those affected.

2021 Eruption

Hunga Tonga-Hunga Ha’Apai has been erupting since 20th December 2021.  The initial eruption produced a large plume (height unknown) and was heard up to 2,000 km away in New Zealand.  Activity died down and the eruption was declared over on 11th January 2022.

This eruption added to the land created during the 2009 and 2014/15 eruptions which links the islands of Hunga Tonga and Hunga Ha’Apai.

Fig 1: Hunga Tonga-Hunga Ha’Apai pre the December 2021 eruption (image from Google Earth) on the left and Hunga Tonga-Hunga Ha’Apai on January 7, 2022 on the right (image from Planet (@planet) / Twitter)

January 2022 Eruption

Activity resumed on 14th January 2022 with a 20 km high ash cloud.  Geologists from Tonga reported large explosions and a 5 km wide ash column.  Their report can be found here Tongan geologists observe stunning eruptions at Hunga | Matangitonga .  At the time of writing this is the last update from the news source in relation to the eruption.

An image from @Planet taken, it is believed, two hours before the 15th January 2022 shows that the 14th January 2022 eruption had removed a lot of the subaerial land bridge between the two islands.

Fig 2: Hunga Tonga-Hunga Ha’Apai 15 January pre-eruption from Planet (@planet) / Twitter

On 15th January 2022, a much larger eruption occurred which was heard as far away as Alaska and Canada. USGS estimate the surface wave magnitude as 5.8. The pressure wave has travelled round the globe: it was measured as 7hPa in New Zealand, 2.5hPa in Switzerland and 2hPa in the UK.

Fig 3: Hunga Tonga eruption early on January 15, 2022, taken from the GOES-17 weather satellite.  Source: ( Public Domain).

 The eruption covered the surrounding islands in ash, blocking out the sun in Tongatapu, the main island in Tonga. Tonga lost its internet connection, probably due to damage to underwater cables so an assessment of the damage has not been possible.  The Australian and New Zealand Governments have offered assistance, including over-flights to assess the damage; the flights are hindered by the ash cloud.

Known Impact

A large amount of material is thought to have been ejected into the stratosphere.  This may cause temporary climate cooling in the region and possibly globally.  New land linked the islands of Hunga Tonga and Hunga Ha’Apai was cut through during the activity on 14th January 2022; how much remains has not been confirmed at the time of writing, but it is expected that more of the subaerial edifice has been lost.

A Pacific-wide  tsunami was generated which reached heights of 0.3m (Mexico), 1.3m (California), 2m (Chile), 2.5m (Vanuatu) and 1.5m (Tonga).  Low lying islands such as Atata Island were submerged by the tsunami.   Tsunami warning systems worked; the known fatalities are low.  Although closer to the volcano where residents had only minutes to flee to higher ground, the toll is not known. 

The cause of the tsunami is not known.  It may have been due to one or more of the following: edifice failure allowing to sea water to reach the magma chamber causing a large explosion; a large volume of pyroclastic material dumped into the ocean; or, a submarine landslip. We will have to wait until the experts have had a chance to assess.


Tonga is an archipelago of 169 flat low-lying coral limestone islands, some with volcanic bases, of which 36 are inhabited. It is home to 105,000 people, of which 70% live on Tongatapu.  Tongatapu has a wealth of archaeological remains, including traces of Lapita pottery from the earliest known settlers, the Lapita. Tonga was also the seat of the Tongan Empire which emerged in 950 AD and declined after 1500 AD; it is now a kingdom with a constitutional monarchy. Hunga Tonga-Hunga Ha’Apai lies 65 km to the north of Tongatapu.

Hunga Tonga-Hunga Ha’Apai is two volcanic islands joined by a land bridge created during the 2009 eruption.  Hunga Tonga is the island on the right of the images and Hunga Ha’Apai the one on the left.  The islands are the subaerial manifestation of the caldera of a submarine volcano.  Its lavas are andesitic.  Five Holocene eruptions are recorded by GVP: 1912, 1937, 1988, 2009, and 2014; all were VEI 2, except for the VEI 0 1988 eruption.  Other eruptions evidenced by rock and ash deposits, including evidence of pyroclastic flows, occurred in 1040 – 1180 CE and 1108 CE; the latter is believed to have formed the caldera and caused 1°C global cooling.

The volcano is situated 100km above the subducting Pacific Plate on the Tonga Plate in the Tonga Kermadec Volcanic Arc.  The Tonga Plate is a microplate which lies between the Pacific Plate, the Niuafo’ou Plate and the Kermadec Plate. 

Fig 4: Earthquake plots by the author showing the subduction zone underlying Hunga Tonga-Hunga Ha’Apai and Tongatapu.  Green dots denote earthquakes less than 6 magnitude, yellow stars greater than or equal to 6 and less than 7 magnitude and red stars greater than or equal to 7 magnitude. Blue triangles denote active volcanoes and the orange triangle Hunga Tonga-Hunga Ha’Apai.  The orange circle denotes Tongatapu.  ©copyright remains with the author; all rights reserved, 2022

The Tonga Kermadec Volcanic Arc lies on the 3,000 km long 100 Ma old Tonga Kermadec Ridge.  The ridge has two segments: the Tonga Ridge in the north and the Kermadec Ridge in the south.  There are two back arc basins to the west of the ridge: the 6 Ma old Lau Basin and 2Ma old Havre Trough, that began opening at 6 Ma and 2 Ma respectively. The Tonga Kermadec Ridge moves independently of the Indo-Australian and Pacific Plates to its west and east, resp.

Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2022.


Raw earthquake data: USGS Earthquakes | U.S. Geological Survey (

Hunga Tonga – Wikipedia

Tongatapu – Wikipedia

Tongatapu – WikipediaTonga-Kermadec Ridge – Wikipedia

Fagradalsfjall: Aviation Code Downgraded to Yellow

Good afternoon!

Seismic activity at Fagradalsfjall, Iceland, has remained low. The recent swarm is now considered over. The aviation code has been lowered to yellow as a result.

Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2022.


Aviation colour code map:

Icelandic Met Office:

Fagradalsfjall: Update 02/01/2022

Good afternoon!

Updated Earthquake Plots, 2nd January 2022 13:50

Good afternoon!

We have updated the earthquake plots for Fagradalsfjall, Iceland, for the swarm which started on 21 December 2021. 

The swarm had an intense initial period while magma moved along a lateral dike, followed by a less intense period, accompanied by magma ascent.  The Icelandic Met Office, IMO, say that the swarm is following the same pattern as that which preceded the eruption in March 2021. 

Magma is ascending under Fagradalsfjall, itself.  The outlying earthquakes, e.g., at Svartsengi and Krýsuvík, are what Iceland calls triggered earthquakes.  Triggered earthquakes are tectonic in nature, arising as local faults respond to magma movement. 

Fig 4: Geodensity plot by the author of earthquakes at Fagradalsfjall the period 21.12.2021 to 02.01.2022.  © Copyright remains with the author; all rights reserved, 2021.
Fig 5: Geoscatter plot by the author of earthquakes at Fagradalsfjall the period 21.12.2021 to 02.01.2022.  Colour denotes age: red is the oldest and yellow the youngest. © Copyright remains with the author; all rights reserved, 2021.
Fig 6: Scatter plot by the author of earthquakes at Fagradalsfjall the period 21.12.2021 to 02.01.2022.  Colour denotes age: red is the oldest and yellow the youngest. © Copyright remains with the author; all rights reserved, 2021.
Fig 7: 2D Depth v Earthquake scatter plot by the author of earthquakes at Fagradalsfjall the period 21.12.2021 to 02.01.2022.  Colour denotes age: red is the oldest and yellow the youngest. © Copyright remains with the author; all rights reserved, 2021.

Armchair Volcanologist

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


Raw earthquake data:

Update 31/12/2021

Following a drop in seismicity over the past couple of days, visitors are now advised to avoid the area.

A similar drop in seismicity was observed immediately before the March 2021 eruption.

If the volcano does erupt again, this would be considered a new eruption in the same place; the earlier eruption was declared over three months after the cessation of activity on 19 September 2021.

Update 28/12/2021

The earthquake swarm at Fagradalsfjall, Iceland, is continuing.  While the swarm has slowed down a bit, 19,000 earthquakes have been recorded by the automatic SIL system since the swarm started.  14 earthquakes over 4.0 magnitude have occurred.  An alert for the risk of rockfall and landslides in the area has been raised; visitors are advised to stay away from the area.

We have plotted the confirmed earthquakes from 20 December 2021 to 28 December 12:50.  This shows the dike propagation south west of Geldingadalur, with additional activity near Kýsuvík and Svartsengi.

Fig 1: Geoscatter plot of the earthquake swarm between 20.12.2021 and 28.12.2021 12:50 by the author.  Red denotes oldest quakes and yellow the newest.  © Copyright remains with the author; all rights reserved, 2021
Fig 2: Scatter plot of the earthquake swarm between 20.12.2021 and 28.12.2021 12:50 by the author.  Red denotes oldest quakes and yellow the newest.  © Copyright remains with the author; all rights reserved, 2021
Fig 3: Geodensity plot of the earthquake swarm between 20.12.2021 and 28.12.2021 12:50 by the author.  © Copyright remains with the author; all rights reserved, 2021.

Only time will tell where and when lava will emerge.  In the meantime, if you are lucky enough to be in the area, be aware of the risk of rock fall and landslip.

Armchair Volcanologist

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


Raw earthquake data:

Icelandic Met Office:

A Christmas Present for La Palma; Eruption Declared Over 25th  December 2021

Good evening!

There was some good news for La Palma over the week end; the eruption was declared over on 25 December 2021.  The eruption, itself, ended on 13 December 2021 at 22:21.  There was a precautionary period of waiting before announcing the end of the eruption.

This is an important milestone in the process to recover and rebuild.

Fig 1:  Seismic signal showing the end of the eruption. Source: IGN



The eruption started on 19 September 2021 as a flank eruption on the Cumbre Vieja volcano and ended on 13 December 2021.  The duration of the eruption was 85 days and 8 hours – the longest for which there are historic records.

The eruptive style is described as Strombolian fissural with phreatomagmatic pulses.

The average height of the plume was 3.5km and its the maximum height was 8.5 km on 13 December 2021.


The cone’s height is now 1,121m.

There are six craters, with diameters ranging between 106m and 172m.


200 million cubic meters erupted, covering an area of 1,219 hectares with an average thickness of 12m and maximum width of 3,350m.

The maximum temperature was 1,140°C.

Two lava deltas were created covering c. 48 hectares (one c. 43 hectares and the other 5 hectares).


2,988 buildings were destroyed and 138 damaged, according to satellite data. This is initially analysed by the local authorities as 1,345 residential properties, 180 agricultural, 75 industrial, 44 leisure, 16 public and 16 other; the shortfall is attributed to properties having more than one building within their boundaries.

Infrastructural damage includes: 73.8 km of road have been damaged along with streets and crossings; and, 130km power lines, 85 medium voltage towers, 1500 low voltage poles and 19 distribution centres.

We do not have an up to date analysis of the extent of the damage to crops and livestock.

Around 7,000 people were evacuated.

There was sadly one fatality which is under investigation.

Seismic Activity

9,135 earthquakes were recorded in the period from 11 September (the onset of the swarm) and today, 27 December 2021.  Due to the severity of the volcanic tremor, many smaller quakes were not recorded.

We have plotted the sequence.  The results are shown in the video below.

Fig 2: Earthquake plots by the author of the swarm from 11/09/2021 to 27/12/2021. © Copyright remains with the author; all rights reserved, 2021.

 Earthquake activity is continuing at the time of writing.

Armchair Volcanologist

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


El Time:

Raw earthquake data: Instituto Geográfico Nacional (IGN)

Fagradalsfjall, Iceland, Aviation Code Raised to Orange

Good afternoon!

A large earthquake swarm started at Fagradalsfjall late on 21 December 2021, 2-4 km NE of Geldingadalir, thought to be caused by a lateral dike intrusion.  Due to the increased seismic activity, the aviation code was raised to Orange yesterday, 22 December 2021.

Fig 1: Icelandic Met Office, IMO’s map of the swarm.  Source: IMO

The eruption at Fagradalsfjall had stalled on 18 September 2021, with no new lava flows to the time of writing.  Ground deformation indicated that magma was still flowing into the crust.

Fig 2: Ground deformation at Krýsuvík.  The red line denotes the start of the March 2021 eruption. Source: IMO

We have downloaded and plotted the earthquakes from 1 September 2021 to 23 September 2021 (source for raw data: IMO ). This includes the swarm which started near Mount Kelir in late September 2021, which may or may not have heralded the current reactivation of the dike.

Fig 3: Geoscatter plot of the swarms from 1 September 2021 to 23 December 2021 by the author.  Colour denotes age: red being the earliest and yellow the most recent. © Copyright remains with the author; all rights reserved, 2021.
Fig 4: Scatter plot of the swarms from 1 September 2021 to 23 December 2021 by the author.  Colour denotes age: red being the earliest, and yellow the most recent. © Copyright remains with the author; all rights reserved, 2021.

While it is considered likely that the activity will result in a renewed eruption, when and where is not certain.

Armchair Volcanologist

23 December 2021

© Copyright remains with the author; all rights reserved.

Sources included in the text.

Grímsvötn, Aviation Code Raised to Orange 6th December 2021 (Since Lowered to Yellow)

Good morning!

Update 8 December 2021

The alert level has been lowered to Yellow. To date no eruption has followed the jökulhlaup or increased seismicity. As Grímsvötn may erupt with little warning, the situation remains closely monitored.

Original Post 6 December 2021

Fig 1: Líndal. A. (1 June 2011). Grímsvötn: photo 3 of 14. Retrieved from

The alert level for Grímsvötn  has been raised to orange following the draining of the caldera lake over the past few days, the resulting jökulhlaup, and increased seismicity today.

Fig 2: Aviation codes for Icelandic Volcanoes.  Grímsvötn is orange.  Source: IMO

The ice-cap has subsided by c. 77 metres.  The water drained via the Gígjukvísl river; the discharge in Gígjukvísl river yesterday around noon was 2800 m3/s, reducing later in the day.

Fig 3:  Measurements of the jökulhlaup as it impacted the Gígjukvísl river.  Source: Línurit (

At the time of writing, there have been 25 earthquakes in the last 48 hours in the vicinity of Grímsfjall.  A magnitude 2.3 occurred at 06:15, followed shortly afterwards at 06:16 by a magnitude 3.6.  There have been a few aftershocks.  No volcanic tremor has been detected.

Fig 4: Map of earthquakes at Vatnajökull.  Grímsfjall is roughly where the green star is; the green star is the location of the 3.6M earthquake.  Source: Vatnajökull (

Volcanic eruptions have occurred after draining of the caldera lake in the past, attributed to the sudden decrease in pressure destabilising the system; IMO cite the 2004, 1934 and 1922 eruptions.

Armchair Volcanologist

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


Icelandic Met Office: IMO

Icelandic Volcanoes: Icelandic Volcanoes (

Mount Semeru, Eruption 4th December 2021

Update 9/12/2021

Sadly the death toll from Mount Semeru now stands at c.40 as of noon today (local time). Over 100 have been injured.

The lava dome collapsed from 1st to 6th December 2021 generating both lava and pyroclastic flows. The largest dome collapse occurred around 14:47 on 4th December 2021 to be followed by pyroclastic flows from 15:10, causing the fatalities. Rescue efforts have been hampered by later eruptive activity.

Sources: and

Original Post, 5/12/2021

Fig 1: Image from 1985 eruption of Semeru, cropped from a photo by Tom Casadevall, 1985 (U.S. Geological Survey).  Source:  GVP

Sadly, 13 people have been killed, 7 missing and c. 100 injured, at the time of writing, following the collapse of the lava dome on Mount Semeru on 4 December 2021. Ash rose to a height of 50,000 feet, covering 11 villages.  Flows of hot gas and lava travelled 800m at least twice on Saturday, reaching a nearby river.  Two bridges were destroyed, including one connecting Lumajang to Malang. 900 people have been evacuated.  10 trapped in mines were rescued. Heavy rain and a thunderstorm had led to destabilisation of the lava dome. The volcano had been on alert level II (Waspada).

Mount Semeru is 3,657m high a stratovolcano with pyroclastic cones and fissure vents. Lake-filled maars follow a N-S trend cutting through the summit. She is located in the Lumajang district, East Java.  Java lies above the subduction zone where the Indo-Australian Plate subducts under Sunda Plate .  Her lavas are Andesite / Basaltic Andesite and Basalt / Picro-Basalt.  Over 1m people live within 30km of the volcano.

GVP records 65 Holocene eruptions since 1818, which range from VEI 1 to VEI 3.

Our thoughts are with those affected.

Armchair Volcanologist

© copyright remains with the author; all rights reserved, 2021

Source& Further Reading

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

Indonesia: Mount Semeru volcano eruption kills 13 as thousands flee | The IndependentMount Semeru volcano eruption death toll at 13 as ash covers vehicles up to their roofs – World News – Mirror Online

Cumbre Vieja Eruption, Update 19 November 2021

Good evening!

The eruption of Cumbre Vieja continues unabated with Strombolian activity, lava fountains from many vents, lava flows, and ash emissions. .  As of 16 November 2021, Copernicus reported that 1,042.1 hectares of land have been covered by lava.  Rainfall now adds to the hazards created by volcanic ash.  The cone reached a height of 1,130m by 10 November 2021. Sadly, one fatality has occurred; one person who had gone to assist with ash clearance was found dead in his home.

Fig 1: Screenshot on 19/11/2021 from RTVC’s webcam monitoring the eruption.  Source: DIRECTO | Erupción del volcán en La Palma – YouTube

Seismicity picked up again in the last few days along with a ground uplift detected at GPS stations LP03 and LP06.

Fig 2: Earthquake count by day by the author for earthquakes from 11/09/2021 to 19/11/2021 (part day), created from publicly available earthquake data provided by IGN.  © Copyright remains with the author; all rights reserved, 2021.
Fig 3: Ground deformation at GPS stations LP03 and LP06.  Source: IGN

 To date there have been 5 earthquakes of 5 Mag. or more.

Fig 4: List of earthquakes greater than or equal to 5 Mag. for earthquakes from 11/09/2021 to 19/11/2021 (part day), extracted from publicly available earthquake data provided by IGN.  © Copyright remains with the author; all rights reserved, 2021.

Here are updated earthquake plots.  All of the action is centred around two depths: 7 -16 km and 30 -39 km, with a very few earthquakes in between; there are a few earthquakes with depths of more than 39 km.

Fig 5: Earthquake count by depth by the author for earthquakes from 11/09/2021 to 19/11/2021 (part day), extracted from publicly available earthquake data provided by IGN.  © Copyright remains with the author; all rights reserved, 2021.

For the updated video, we have provided the scatter plots for only earthquakes greater than  or equal to 3.0 Mag. because the smaller earthquakes obscure the action; all earthquakes are included in the geoscatter plots.

Fig 6: Video by the author of geoscatter plots for earthquakes from 11/09/2021 to 19/11/2021 (part day) and scatter plots of earthquakes greater than or equal to 3.0 Mag. for the same period.  © Copyright remains with the author; all rights reserved, 2021.

Magma still appears to be stalling at the two depths: 7 -16 km and 30 -39 km prior to ascent (refer to La Palma: Earthquakes and Magma Plumbing for more information).  How much of it reaches the surface remains to be seen.

Armchair Volcanologist

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


Raw earthquake data: Instituto Geográfico Nacional (IGN)

Other links are provided in the text.

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