Category Archives: Decade Volcano

Today in Geological History; June 15th – Mount Pinatubo 1991

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Figure .1 Mount Pinatubo 1991

Before the eruption in the early 90’s, Pinatubo was a rather unassuming mountain on the island of Luzon, Philippines. Standing only 2000 ft above surrounding peaks, it was almost obscured from vision. It’s flanks covered in lush green forest, without an eruption in memorable history, people never saw it as a threat.  This change in June 1991 when it produced the second largest eruption of the 20th century (after Novarupta 1912).

Figure 2. Damage from the earthquake in 1990.

In 1990, on July 16th a magnitude 7.8 rocked the island of Luzon. A strike-slip along the Philippine Fault System it caused a surface rupture oer 125 km long. Killing  over a thousand people it became the deadliest earthquake in Philippine history but may also have been the start of something much greater, geologists have long been convinced that it is linked to Pinatubo’s activity the following year. However it has never been proven if this earthquake stirred the sleeping volcano or if the reawakening caused the quake. For a few weeks after locals reported steam coming from Pinatubo, but when it was visited by PHILVOLC’s scientists there was only landslide evidence and not emissions.

Seismicity kicked off activity again on March 15th 1991. The north-west side of the volcano felt a swarm of tremors increasing in intensity over the next two weeks. On April 2nd a 1.5 km fissure opened along the summit with phreatic explosions dusting the local area in ash. seismicity continued to increase causing volcanologist to rush to its flanks to place monitoring equipment they had never thought to place while the mountain lay in slumber. The volcanoes eruptive history had very been studied before and they were surprise to learn it had large eruptions as recent as roughly 500, 3500 and 5500 years ago. On April 7th the first formal evacuations took place. With the Clark Air base just 14 km from Pinatubo the USGS aided PHILVOLCS in setting up 3 zones the first, a 10 km radius from the summit was the initial area to be designated unsafe and people were quickly evacuated to safety. Further zones from 10-20 km and 20-40 km were deemed safe for now but people were told to be alert to the possibility of evacuation if the mountain showed any sign of getting worse.

Figure 3.

Activity stepped up in May with sulfur dioxide emissions rocketing from roughly 500 t p/d at the beginning of the month to over 5000 t p/d by May 28th. At this point emission slightly decreased and inflation began to increase rapidly leading many to believe pressure was building with the magma chamber.

On June 3rd the first lava was noticed signalling that a magmatic phase of the eruption had begun which eased some people’s mind as activity seemed relatively effusive. The first large explosion cam four days later on June 7th. An eruption column towered 7 km above the summit prompting the second wave of evacuations with people in the 10-20 km zone being prompted to leave their homes. A lava dome began to grow dramatically in the next few days reaching in excess of 600 ft wide. Activity seemed pretty constant at a low-level until 03:41 on June 12th when a new, more violent phase of eruptions began. As explosions intensified over the next few hours the eruption column grew to over 19 km. Pyroclastic flows surged as 4km from the summit in some valleys. Ash and tephra rained down on the surrounding area as the intense explosions lasted over  . The final wave of evacuations was called for on the morning of June 13th as a small but intense earthquake swarm saw in a third phase in the eruption.

Figure 4. On of the most iconic images from the 1991 eruption.

People as far as 40 km away, and even further if possible, were urged to leave the area as quickly and calmly as possibly as Pinatubo showed no signs of slowly down its activity. The column peaked again, this time over 24 km high. Several more large explosions were recorded for the next 24 hours including one at lunch time on the 14th with saw another 21 km eruption column and more pyroclastic flows obscuring the view of the flanks.

From midday on June 15th the eruption reached its most climatic point. By 2.30 pm on the June 15th readings stopped being received from seismometers and other remote censoring equipment which the USGS had placed at Clark Air base indicating the area had been over some by the pyroclastic material still being ejected at a terrifying rate. An ash cloud covered an area greater than 125,000 km2 bringing near total darkness to much of the island of Luzon and ashfall was recorded as far as neighbouring countries of Cambodia, Malaysia and Vietnam. By 10.30pm that night all fell quiet and Pinatubo’s fury seemed to be over.

Figure 5. Mapping the spread of the SO2 released by Pinatubo.

The VEI 6 eruption spat out over 10,000,000,000 tonnes of material and a whopping 17,000,000 tonnes of sulphur dioxide. It was the later which signed Pinatubo’s fate in people’s minds as the SO2 emitted quickly covered the globe causing the mean global temperatures to drop by 0.5°C for the following two years. Sulphur dioxide in the atmosphere reflects the Sun’s radiation back in to space meaning the Earth’s surface received up to 10% less sunlight in the following year. It also meant an increase in ozone damage, with the hole above the Antarctic being at the largest it had ever been.

An estimated 847 people lost their lives (many from collapsing buildings under the weight of the ashfall),but problems continued past initial fatalities in the aftermath. Over 2.1 million people have believed to have been affected by the disaster. Agriculture was severely effected both my ash fall and then following effects of the climate. Lahars plagued the region for years after with each heavy rain fall. It is often put by the end of 1992 the eruption and resulting lahars caused the country losses in excess of 400 million US dollars.

Despite this, the events of 1991 are often hailed a volcanological triumph with quick responses, prediction and evacuation believed to have saved the lives of thousands. It enabled us to gain an insight to volcanic impacts on climate and how we monitor the risks.

Figure 1. http://www.slate.com/articles/life/welltraveled/features/2012/tempting_fate_in_the_philippines_/dennis_rodman_s_dad_and_the_eruption_of_mount_pinatubo_.html

Figure 2. http://xuwenewegu.webatu.com/earthquake-in-philippines-july-25-2011.php

Figure 3. http://www.coolgeography.co.uk/GCSE/AQA/Restless%20Earth/Volcanoes/Mount%20Pinatubo.htm

Figure 4. https://www.uclm.es/profesorado/egcardenas/pa.htm

Figure 5. http://earthobservatory.nasa.gov/IOTD/view.php?id=1510

Today in Geological History; June 3rd – The 25th Anniversary of Unzen

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Today marks the 25th anniversary of the pyroclastic flow from Mount Unzen which claimed the lives of 43 people.

Mount Unzen

Mount Unzen is actually several over lapping volcanoes on Japan’s island of Kyushu. It was the cause of Japan’s largest ever volcanic disaster in 1792 when a lava domed collapsed and caused a mega tsunami which killed nearly 15,000 people. After this even the volcano lay silent until beginning to stir in 1989.

Seismic swarms began in the November of 89 about 10 km west of the summit and gradually migrated eastward until the first phreatic eruption a full year later in November 1990. By May 20th 1991 fresh lava began to flow from the highly inflated summit area prompting the evacuation of almost 12,000 locals.

volcano-unzenThe threat of another eruption to the scale of 1792 brought journalists and scientists alike flocking to the surrounding area to monitor the activity of Unzen and its potential threat. Sadly this curiosity resulted in the deaths of 43 when on June 3rd activity peaked due to a possible lava dome collapse. This sent a huge pyroclastic flow surging down its flanks and funnelled in to a valley point in the direction where volcanologists and journalists had set up a base at what was thought to be a safe distance, over 4.5 km, from the summit.

Activity continued well in to 1995 and over 10,000 pyroclastic flows were recorded over this period. By the end of the eruption a new lava dome was in place 1.2 by 8 km wide. Its volume was approximated at 0.1 cubic km.  In total, about 0.21 cubic km of plagioclase-phyric dacite magma was erupted over the course of the eruption at peak effusive rates of 7 cubic metres per second in 1991. Over 2000 buildings were destroyed by these flows in Shimabara City alone. Matters were further complicated between August 1992 and July 1993 when heavy rains caused multiple lahars destroying a further 1300 homes along the Mizunashi and Nakao Rivers, requiring the sudden evacuation of several thousand residents.

Mount Unzen has been placed on the official decade volcano list and is one of Japan’s most highly monitored areas.

Maurice and Katia Krafft

The Krafft’s were French volcanologists and soul mates who met at the University of Strasbourg. Their love for volcanology almost reviled their love for each other. The specialized in documenting eruptions as best and often as close as possible, their end was almost inevitable.

Their most famed contribution was the documentation of Nevado del Ruiz which when shown to the Phillipine president Corazon Aquino who was then convinced to evacuate the area surrounding Mount Pinatubo before its catastrophic 1991 eruption almost certainly saving hundreds if not thousands of lives. 

Over a 20 year period, when volcanology was still a relativity young science, the married couple documented hundred of eruptions. They fillmed over 300 hours of footage, took thousands of photos and published multiple books.

While in the Philippines during Pinatubo’s early stages, Maurice was interviewed by a local news agency where he told the journalist  “I am never afraid, because I have seen so much eruptions in 23 years that even if I die tomorrow I don’t care.” From here they flew out to Japan where activity was picking up at Unzen. The pair perished together when they were overcome by the pyroclastic flow on June 3rd.

Harry Glicken

Man wearing a coat and hat and holding a pad of paper sits on a rock , with a lake and several mountains visible in the backgroundHarry was an American volcanologist who although was based at USGS was funded by outside organisations. He specialised in volcanic debris flows and was closely involved with research on St Helens with his doctoral thesis ‘Rockslide-debris Avalanche of May 18, 1980, Mount St. Helens Volcano, Washington‘ being recognised as a leading paper on the event.

Glicken cheated death on St Helens as he was meant to be the volcanologist on duty May 18th however swapped with his then mentor David Johnston who was killed by the blast.

Figure 1; http://raredelights.com/top-28-worlds-important-volcanoes/mount-unzen-in-japan/

Figure 2; https://curiousmatters.wordpress.com/2014/05/23/curious-facts-31-of-the-strongest-volcanoes-known-to-man/

Figure 3; https://en.wikipedia.org/wiki/Mount_Unzen

Figure 4; https://volcanogeek.wordpress.com/2011/09/20/maurice-and-katia-a-love-story/

Figure 5; https://en.wikipedia.org/wiki/Harry_Glicken

Figure 6; https://www.youtube.com/watch?v=Cvjwt9nnwXY

 

 

 

 

Kyushu Earthquake, Mt Aso and the Relationship between Volcanoes and Earthquakes.

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In the past week the Japanese Island of Kysushu has be ravaged by earthquakes.

2016-04-16Japan is a highly seismic area with noticeable quakes in some areas occurring nearly daily.  But things began to escalate for the Kyushu region on Thursday night when a magnitude 6.5 quake brought several buildings down. As rescue efforts began the region had two more huge after shocks during the night, one over Mg 6 and the other > Mg 5. By midday Friday the death toll stood at 9 with over 800 injured and although the aftershocks kept coming many >Mg 4 people were still being pulled from the rubble. Sadly these events were quite possibly a precursor to something larger.

At 01.25 local time (15.25 GMT) a Mg 7.3 struck just north of Kumamoto just kilometers from the large earthquakes which had already occurred. Much of the seismicity in the Kyushu region is related to the subduction of the Philippine Sea plate at great depth. However this series of earthquakes have occurred at very shallow depths several hundred kilometers northwest of the Ryukyu Trench. They have been cause by strike-slip faulting within the Eurasian plate.

Quake damaged houses in Kumamoto, Japan (16 April 2016)So far 22 more people have been reported dead but this is expected to rise in the coming days with at least 80 people known to betrapped in rubble. 11 of which are trapped in a Tokai university apartment in the town of Minami Aso.

 

The shallowness of the earthquakes means damage to the surface is high and it is not just collapsing building which are a hazard. People have fled the area down stream of a dam which collapsed soon after the earthquake. Landslides in the area have taken out roads and power lines and with heavy rain anticipated over the coming days JMA have advised mudslides will be a huge problem for rescuers.

 

 

 

 

 

 

The seismic problems of Kyushu may have also set in motion another geohazard in the form of Mt Aso. Yesterday one of my favorite volcanology bloggers Eric Klemetti tweeted “Quite a few volcanoes on Kyushu and these earthquakes have been centered near Unzen, Aso, Kirishima. This is NOT to say these earthquakes will trigger any eruptions, but could be worth watching over the next year.” Several hours late JMA reported a small scale eruption at Aso. Smoke plumes have migrated 100 meters above the summit and it is not yet clear if the activity is magmatic (caused by movement of magma towards the surface) or phreatic (steam explosion caused by heating of groundwater).

Eruptions and earthquakes do not always come hand in hand but each one can contribute to the other or not at all depending on the circumstances. One indication a volcano is about to erupt is volcanic tremors; these low frequency earthquakes are usually caused by the migration of magma or changes to magma chamber. Although they are rarely higher than a magnitude 4. On the other side large earth quakes can cause faulting in bed rock which allows magma to exploit a new weakness and find a path to the surface it previously could not intrude on. The same can happen for ground water with faulting caused by a quake allowing it to seep in to geothermal areas it previously did not have access to due to the impermeability of the rock. When earthquakes hit volcanic regions volcano observatories always keep a closer eye on vulnerable or highly active volcanoes as a precaution but it is not always needed.

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The Aso Caldera complex has one of the world’s largest calderas. It is comprised of a 25 km north-south by 18 km east-west Caldera and a central cone group comprised of Mt. Neko, Mt. Taka, Mt. Naka, Mt. Eboshi, and Mt. Kishima. Mt Naka where the eruption has just taken place is the most active with its most recent eruption taking place last October. Although much of Aso’s activity in the past century has been relatively small it has had a violent history with at least 4 VEI 7 events in the past 300,000 years.

It’s is not clear whether the earthquakes in the past few days did trigger the current current eruption but JMA are keeping a close eye on the situation and I will update this page as I know more.

 

 

Figure 1. http://earthquake.usgs.gov/earthquakes/map/

Figure 2; http://www.bbc.co.uk/news/world-asia-36061657

Figure 3; http://www.independent.co.uk/news/world/asia/japan-earthquakes-dozens-reported-dead-injured-second-quake-two-days-a6986931.html

Figure 4; http://mashable.com/2016/04/15/japan-earthquake-landslide-photos/

The New Decade Volcano List; #5 Trans Mexico Volcanic Belt

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The guys at Volcano Cafe have picked up where they left off with a rather interesting choice at number 5. Mexico is one of the more volcanically active countries in the world, with the likes of Popocatépetl and Colima frequently showing at least some signs of unrest.  This has been one of my favourites which they have put forward as it highlights the complexity of the region and how several systems can affect a region meaning threat can come from varying or even all sources!

Mexico City and the Trans Mexico Volcanic Belt – NDVP #5

The extinct volcano Sierra de Guadalupe rises 750 metres above Mexico City, it’s highest peak within 15 km of the centre of the city. In spite of conservation attemps, illegal buildings continue to sprout and at present the crater and debris avalanche have been completely covered by urban development. (Hotu Matua)

It is inevitable that the higher we get in this series, the more speculative our choices may seem. If everything was known about every volcano, identifying and motivating the choice of the ten most dangerous ones would be a relatively simple matter. As it is, our selections have to be based on what meagre information is available and educated guesswork as to what the full story might or could be. In our choice of number five, this is highlighted as we cannot even identify a single volcanic system as the main threat, but then the area occupied by the cities Mexico City, Toluca and Puebla is highly unusual.

Throughout almost its entire length, the Ring of Fire produces volcanoes aligned on and along the subduction zone forming a great arc of stratovolcanoes which has given rise to the term “Arc Volcanism”. But running across Mexico from Colima in the west to Pico de Orizaba in the east, the subduction zone makes an almost 90-degree turn and the volcanoes seem to align on a N-S line, perpendicular to the subducting plate. Three main such alignments are identified in recent scientific papers; Cántaro–Nevado de Colima–Colima de Fuego in the west, Tláloc–Telapón–Iztaccíhuatl–Popocatépetl (Sierra de Nevada) just east of Mexico City, and Cofre de Perote–Las Cumbres–Pico de Orizaba–Sierra Negra at the eastern end of the Trans Mexico Volcanic Belt, alternately known as the Trans Mexico Volcanic Zone. For the entire TMVB, volcanism has trended from acidic (dacite and rhyolite) to intermediate magmas (andesitic) as well as from north to south although there are numerous and noticeable exceptions to these identified trends.

The geological setting of the Trans Mexico Volcanic Belt. The numbers next to the arrows showing the direction are the annual subduction rates. The numbers along the isolines display the depth of the subducting plate as inferred from earthquakes. The TVMB is outlined in grey and the alignment of volcanoes mentioned are in yellow. Note how volcanoes (north-)west and (south-)east of the TMVB seem to align along the 300 and 100 km subduction isolines as opposed to transversing them as is the case in the TMVB. (Adapted from Macías 2007)

In addition to these three main lines of active volcanism, there are further lines of dormant or extinct volcanoes, one bordering the Mexico City plain to the west and the Toluca plain to the east with another one bordering the latter plain to the west. To complicate the matter even further, both north and south of these plains run lines of ancient, heavily eroded and extinct(?) volcanic edifices that seem to follow the subduction zone. If we also include the Puebla plain to the east of the Sierra de Nevado, there are more than 1.6 million inhabitants of Greater Toluca, 22.5 million of Greater Mexico City and 2.1 million of Greater Puebla, in all well in excess of 25 million.

Landsat image of the Toluca, Mexico City and Publa plains. The names of active to potentially active volcanoes in yellow, possible volcanic alignments are marked in blue and the 90-km-long Chichinautzin volcanic field immediately south of Mexico City, centred on the Aztec temple El Tepozteco, is circled. (Author)

Not only is the north-south alignment perpendicular to the subduction zone of the most recent volcanoes highly unusual. There is as well a dearth of large, explosive calderas in the TMVB. The semi-official blog GeoMexico laments: “There is still lots of work needed to fully unravel the geological secrets of Mexico’s Volcanic Axis which crosses the country between latitudes 19 and 21 degrees North. Unlike most volcanic belts elsewhere in the world, this one does not appear at first sight to correspond to any plate boundary. Another of the mysteries of this volcanic region, where igneous upheavals have shaped the landscape for several million years, is the relative dearth of calderas, the “super craters” formed either by collapse or by giant explosions.”

As of 1999, there were seven calderas known in the belt, one of which is in fact no more than a crater lake, Lake Alchichica, with a diameter of 1888 meters. The largest of these seven calderas is the 15 by 21 km Los Humeros caldera in the state of Puebla, close to its border with Veracruz. It lies 55 km west-north-west of the city of Veracruz (Xalapa), relatively close to Puebla (Teziutlán). The main caldera is about 400 m deep and roughly oval in shape. Prior to its formation 460,000 years ago, lava emitted from this vent covered 3500 square km with ignimbrites. Later, two smaller calderas formed nearby, with ages of about 100,000 years (Los Potreros caldera) and 30,000 years (El Xalapazco) respectively.

The 11 km wide and 400 m deep, heavily eroded Amealco caldera is located at Garabato (= unintelligible scribbles), midway between the towns of San Juan del Río and Maravatio, about 125 km NW of mexico City. Caldera-related activity started in the Pliocene ca. 4.7 Ma ago and ended around ca. 2.2 Ma. The total volume of pyroclastic flow deposits and ignimbrites is in the region of 500 cubic km. The Huichapan Caldera in the central sector of the TMVB, also referred to as the Donguinyó-Huichapan caldera complex is 10 km in diameter and appears to be the result of two overlapping calderas that date to 5 and 4.2 million years ago respectively. The rocks from the older caldera are intermediate to basic in composition, while those from the more recent caldera are acidic (high silica content) rhyolites, another relatively unusual feature.

Since then, one very interesting albeit ancient feature has been discovered in the Coxcatlán-Tilzapotla region, about 100 km south of Mexico City, just south of the TMVB. The elliptical NW-SE oriented dome structure, approximately 30 x 52 km, encompasses the Tilzapotla collapse caldera, rhyolitic domes, large volumes of ignimbrites, as well as the Buenavista intrusive body, and the Coxcatlán and Chautle plutons located west and east of the structural margin of the caldera, respectively. Previous geochronological studies carried out on the silicic and intermediate magmatic rocks places the uplift in the dome area in the late Eocene (~38-34 Ma). This suggests that doming was related to emplacement of magmas into the crust prior to collapse of the Tilzapotla caldera at 34.3 Ma.

The approximately 11 x 13 km Tilzapotla caldera is located on top of this large, rhyolitic dome feature. “The caldera is defined by a 33 x 24 km semi-elliptical structure that encircles the largest exposures of the Tilzapotla ignimbrite and corresponds to the structural margin rather than the topographic rim. A central uplifted block limited by NW-trending faults is the main indication of a resurgent stage. The caldera structural margin is surrounded by extensive exposures of Cretaceous marine sequences that structurally define a broad elliptical dome (45×35 km) originated in the first stage of the caldera evolution. There is evidence showing that the 34 Ma Tilzapotla ignimbrite represents the climatic event of the caldera collapse.” (Morán-Zenteno et al 1998) This begs the question of how the very large dome feature itself was formed. It covers some 1500 square kilometres to a height more than 1,000 m above the surrounding plains with a total thickness in excess of 800 m. If we make allowances for surface depression and 34 My of erosion, the total volume emitted is in excess of 1,500 cubic kilometres of silicic magma.

The observed absence in the TMVB of the elsewhere omnipresent large explosive calderas is a conundrum. Either they have been masked by the products of subsequent volcanic eruptions and rapid, tropical erosion and still await discovery, or, volcanism in the TMVB is sufficiently different to almost preclude these eruptions. However, the presence of the >500 km3 Amealco caldera, the 15 by 21 km Los Humeros caldera and the 10 km Huichapan Caldera rather points to the former being the case. In order to gain an insight into how very complex Mexican volcanism can be to unravel, at this point I recommend a look at the reconstruction by Diaz & McDowell (page 11); “Figure 7. Volcanic evolution of the Amealco caldera and peripheral volcanoes”. It is unfortunately too large to reproduce here, so please, take a look!

http://www.geociencias.unam.mx/~ger/2000_GSASP_334_Ame.pdf

If we turn our attention away from the very largest types of eruptions, there are several large and highly dangerous volcanoes in the Toluca – Mexico City – Puebla area. To the SW of Toluca lies the giant stratovolcano Nevado de Toluca and 50-70 km east and southeast runs the Sierra Nevada mountain range comprised of four major volcanoes:

The 4,680 m a.s.l. high Nevado de Toluca volcano as seen from the city of Toluca, 24 km away. (Wikimedia Commons)

Nevado de Toluca

In the Nahuatl language, “Xinantécatl” means “naked man”. Alternately, the name has been interpretated as “Chicnauhtécatl”, “nine hills” which given the volcano’s appearance seems the likelier. Nevado de Toluca is a composite volcano of late Pleistocene-Holocene age with a calc-alkaline andesitic to dacitic composition. The northern flank of Nevado de Toluca has a relative elevation (prominence) of 2015 m with respect to the Lerma river basin, and its southern flank has a relative elevation of 2900 m with respect to the Ixtapan de la Sal village. The elliptical 1.5 by 2 km wide crater of Nevado de Toluca is breached to the east. The interior holds a dacitic central dome and the remains of two ancient scars, located on the SE and NE flanks of the volcano which are related to the partial collapse of the edifice. Unusually for volcanic lakes, the two crater lakes are alkaline, not acidic.

El Refugio quarry located 15 km northeast of Nevado de Toluca crater showing an exposure of the 37,000 yr B.P. block-and-ash flow deposits (Macías 2007)

Nevado de Toluca was built upon the intersection of three fault systems with NW-SE, NE-SW, and E-W orientations. This structural geometry favoured the formation of coalescent pyroclastic fans that reach all the way to the cities of Toluca and Metepec, 25 km to the NE of the volcano. During the late Pleistocene, the southern flank of Nevado de Toluca collapsed twice, originating debris avalanche deposits that were transformed into debris flows with distance. The scars produced by these collapses have disappeared due to subsequent volcanic activity and glacial erosion. The older flow can be traced to distances up to 35 km from the summit while the younger event near the end of the Pleistocene ( > 40 kA) generated a debris avalanche, the “Pilcaya Debris Flow”, that travelled more than 55 km from the summit. Activity then continued with three very large explosive eruptions – the Lower Toluca Pumice ca. 21,700 yr B.P., the Middle Toluca Pumice ca. 12,100 yr B.P. and the Upper Toluca Pumice ca. 10,500 yr B.P. The pyroclastic deposits of these eruptions are mostly covered by subsequent and “smaller” Plinian eruptions.

The Sierra Nevada Volcanic Range

From north to south, the Sierra Nevada Volcanic Range comprises the volcanoes Tláloc, Telapón, Iztaccíhuatl, and Popocatépetl. Previously, it was considered that volcanic activity began to the north and migrated south but new evidence obtained from previous studies, field reconnaissance and radiometric dating paints a slightly different picture.

During the past 10,000 years, there have been repetitive Plinian eruptions of Popocatépetl including some historic events and the 1994–present eruption, but Holocene activity has not been limited to Popocatepetl alone. 9,000 years ago, Iztaccíhuatl produced the Buenavista dacitic lava flow. As is obvious, magmatism of the Sierra Nevada Volcanic Range has not kept a continuous north to south migrating path as had been previously surmised. Rather, it has shifted back and forth chaotically throughout its evolution.

Tláloc

Volcanism at the Sierra Nevada Volcanic Range likely started 1.8–1.4 Ma years ago with the construction of Paleo-Tláloc volcano, today buried by younger deposits. The activity continued between 1.07 and 0.89 Ma with the emplacement of dacitic domes, lavas and associated pyroclastic flows (“San Francisco” 1 Ma, “Chicoloapan” 0.9 Ma). Then between 0.94–0.84 Ma, the main edifice of modern Tláloc was built up through the emission of dacitic lava flows. Although Popocatépetl took over as the centre of eruptive activity about 320 kA, Tlaloc reawakened with the emission of rhyolitic magma at 129 kA followed by the emplacement of the El Papayo dacite (118 kA) to the south and Téyotl summit lavas (80 kA).

Tlaloc has always been considered the oldest volcano of the Sierra Nevada Volcanic Range (and extinct), but recent field data have revealed that Tlaloc was very active during late Pleistocene with a series of five explosive eruptions at 44, 38, 33, 31, and 25 kA and the growth of the summit dome. One of these eruptions produced the 1.58 km3 (DRE) Multilayered White Pumice (MWP), a rhyolitic pyroclastic sequence that consist of abundant white pumice (up to 96 vol.%), rare gray pumice, cognate lithics, accidental altered lithics, xenocrysts. The pumice clasts contain phenocrysts of quartz, plagioclase, sanidine, biotite, rare Fe–Ti oxides, monazite, zircon and apatite. Xenocrysts are represented by plagioclase, microcline, orthoclase and quartz likely coming from a deeper plutonic body. Both pumices have a rhyolitic composition (74.98 ± 1 wt.% SiO2 in water free basis) which represents one of the most acidic products of Tlaloc and the entire Sierra Nevada Volcanic Range. (Macías 2011)

Telapón

The inauspicious 260 m high (elevation 3,600 m) steep-sided Cerro Papayo dacitic lava dome marks the vent of the Telapón volcano on the north flank of Iztaccíhuatl formed approximately between 0.38 Ma and 0.34 Ma ago with the emplacement of lava flows and a dome. The 21 cu km compound lava field covers 84 sq km and includes flows that travelled long distances in opposite directions – into the Valley of Mexico and towards the Puebla basin. In addition, the Papayo lavas overlie glacial moraines about 12,000 years old, thus Telapón has been active until the very end of the Pleistocene. The lithology of Telapón shows two periods of activity. First, an andesitic-dacitic Lower Volcanic Event that was emplaced between 1.03 MA and 65 kA, and second, a dacitic-rhyolitic Upper Volcanic Event emplaced between 65 to 35 kA. (Macías 2007).

Photograph of Iztaccihuatl which clearly shows the resemblance to a sleeping woman. (Uncredited photograph, labels added by author)

Iztaccihuatl

The name “Iztaccíhuatl” means “White woman” in the Nahuatl language. Linked to the Popocatepetl volcano to the south by the high saddle known as the Paso de Cortés, it is a 5,230 m (1,560 m prominence) dormant volcanic mountain. Despite its relatively modest prominence, the volume is a staggering 450 km3, which is 100 km3 greater than that of Mount Shasta, Oregon. Iztaccíhuatl began its activity ca. 1.1 Ma ago. From then until 0.45 Ma several volcanic edifices were formed. At that date, the Los Pies Recientes cone was devastated by a Mount St. Helens–type event which destroyed the southeastern flank and produced a massive debris avalanche accompanied by large pyroclastic flows.

The summit ridge consists of a series of overlapping cones constructed along a NNW-SSE line to the south of the Pleistocene Llano Grande caldera. Andesitic and dacitic Pleistocene and Holocene volcanism has taken place from vents at or near the summit. Areas near the El Pecho summit vent are covered in flows and tuff beds younger than glaciation approximately 11 kA, yet GVP states that “The Global Volcanism Program is not aware of any Holocene eruptions from Iztaccihuatl.”

The once glacier-covered peak of Popocatépetl stratovolcano rises above Tlamacas to its north in this photograph from 1968. The sharp peak at right is Ventorrillo, the summit of a predecessor to Popocatépetl, the eroded Nexpayantla volcano. (William Melson)

Popocatépetl

Popocatépetl is the most active volcano in Mexico, having had more than 15 major eruptions since the arrival of the Spanish in 1519 with the most recent in 1947. In Nahuatl, the name means “Smoking Mountain”. Popocatépetl reaches 5,426 m a.s.l. with a prominence of 3,020 m with a base diameter of about 25 km. The crater is elliptical with an orientation northeast-southwest. The walls of the crater vary in height from 600 to 840 m. It lies 70 km southeast of Mexico City and more than one million people live within a radius of 40 km from the summit. According to paleomagnetic studies, the volcano is about 730,000 years old.

Popocatépetl used to be covered by glaciers, but due to increased volcanic activity in the 1990s, the glaciers covering Popocatépetl greatly decreased in size and by 2001 they were gone. Historically, Popocatépetl has erupted predominantly andesitic magma but it has also erupted large volumes of dacite. Magma produced in the current cycle of activity tends to be a mixture of the two.

There are at least four debris avalanche deposits around Popocatépetl volcano. The oldest comes from the failure of the SE flank of Iztaccíhuatl volcano, and the other three come from the flank collapse of paleo-Popocatépetl, the youngest being the 23,000 yr B.P. deposit. The modern volcano was constructed to the south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 AD, have occurred from Popocatépetl since the mid Holocene, accompanied by pyroclastic flows and voluminous lahars that swept through the basins below the volcano.

Some 23,000 years ago a lateral eruption, greater than the 1980 Mount St. Helens eruption, resulted in the lateral collapse of the ancient Popocatépetl cone. The explosion generated a debris avalanche deposit that reached up to 70 km to the South from the summit. The decompression of the magmatic system caused a lateral blast that emplaced a pyroclastic surge deposit accompanied by a Plinian eruption column which deposited a thick pumice-fall layer on the southern flanks of the volcano. The column then collapsed and formed an ash flow that charred everything in its path. The deposit reached up to 70 km from the summit, covers an area of 900 km2, and if we assign an average thickness of 15 m, a volume of 9 km3 is obtained. This deposit overlies paleosoil that contains charred logs radiocarbon dated at 23,445 ± 210 yr. Disseminated charcoal found in the ash flow deposit yielded an age of 22,875 +915/−820 yr. (Macías)

During the past 20,000 yr the explosive activity of Popocatépetl has been characterized by four major events (14,000, 5000, 2150, and 1100 BP) and four minor events (11,000, 9000, 7000 and 1800 BP) The events that occurred at 5000 and 1100 BP had a similar evolution. They began with hydromagmatic explosions that dispersed wet pyroclastic surges up to 20 km from the summit. These explosions opened the magmatic conduit, decompressed the magmatic system, and formed >25-km-high Plinian column.

From our perspective, it is of interest to note that the last three Plinian eruptions of Popocatépetl coincide with three important events in Mesoamerican history: The 3195–2830 B.C. eruption coincides with the 3114 BC beginning of the Mesoamerican Calendar. The 215 BC eruption coincides with the transition from the Preclassic to the Classic period. The last major eruption, which probably occurred in 823 AD, coincides with the Classic-Postclassic periods transition.

The Parque Nacional El Tepozteco is at the centre of the Chichinautzin volcanic field. It consists of a small temple to the Aztec god Tepoztecatl, a god of the alcoholic pulque beverage. (unearthingarchaeoblog)

The Chichinautzin Volcanic Field

The Chichinautzin volcanic field contains more than 220 Pleistocene to Holocene monogenetic vents and covers a 90-km-long, E-W-trending area immediately south of Mexico City. It is formed primarily of overlapping small cinder cones and shield volcanoes with a mainly basaltic-andesitic to andesitic composition with a thrachytic component as well as some dacite evident. The highest peak of the Sierra Chichinautzin is the Volcán Ajusco lava-dome complex at 3930 m a.s.l. There have been at least eight eruptions within the past 10,000 years with the most recent about 1670 radiocarbon years ago (~340 AD) from the Xitle scoria cone. These eruptions have typically been VEI 3 with one registered as a VEI 4. A very modest estimate based on an oval 60 x 90 km with an average emplaced height of 250 m yields a figure of 1,050 cubic km for the volume of the dome but the true figure could be more than double that. From the list of sources in the GVP entry for the Chichinautzin volcanic field, it would seem that some individual cones, vents and flows have been studied, but not the feature as a whole. What is it? What is its true age? Why is it so large, far larger than the initial shield deposited during the first development stage before volcanism shifts to construct (a series of) stratovolcanic edifices? Is there a significance to its position on the same isoline above the subduction zone as Pico de Orizaba, Popocatépetl and Nevado de Toluca?

Summing up

The geological setting of the Mexico City basin is unusual in that the subduction zone makes an almost 90-degree angle and that the major volcanoes do not follow the subduction zone but rather form lines at right angles to it. Instead of showing a neat progression, volcanic activity has been shown to jump “chaotically” (Macías 2011) both geographically as well as petrologically. There is a marked absence of identified caldera structures in the area, yet in the middle of it, right at the southern edge of the city limits, lies a more than 1,000 km3 large Pleistocene to Holocene dome structure that has been active until recently, one that is not well studied.

In addition to this, the Nevado de Toluca volcano has already produced eruptions sufficiently large to deposit ignimbrites at distances greater than 25 km from its summit and Popocatépetl clearly has the potential to do so. Both these volcanoes (and Iztaccihuatl) have suffered several major edifice collapses where deposits have been traced to distances greater than 55 and 70 km respectively.

With almost 30 million people living within 100 km, Mexico City will remain on our list until the mysteries of why the “currently and recently active” volcanoes of the TMVB align perpendicular to the subduction zone as well as where and why the very large, caldera-forming eruptions (VEI 6 to 7) have disappeared to have been unravelled. It will remain on our list until we have a thorough investigation of the past and likely future evolution of the gigantic Chichinautzin volcanic field as well as a better understanding of the risks posed by the large stratovolcanoes in the vicinity.

The more I delved into this subject, the more intricate it became and the more I realised just how little I understood. The TMVB as it passes the Toluca – Mexico City – Puebla area once fully investigated may well deserve a place higher up on the list (or possibly even be struck from it), but with the material and understanding at present, we will leave it at a provisional fifth place on our list.

The New Decade Volcano Program; #6, Bali

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A week later than posted on Volcano Cafe, here is number 6 on the guys proposed list! Introducing the dangers of Bali…

Romantic Paradise Destination – The New Decade Volcano Program #6, Bali

Sunset from the 3,148 m high summit of Gunung Agung. The peak in the distance is G. Abang, a remnant of a far loftier peak, Ancestral Batur. (WikimediaCommons, photo by Mrllmrll).

It has often been pointed out that the deadliest volcano is the one you did not know about. This is our dilemma. When you try to identify the potentially most dangerous ones, by necessity you have to go out on a limb to find those that are not well known nor well studied and there is always the chance to end up with egg on your face. But in this we are not alone. As an example, it was long thought that a particularly heavy layer of volcanic dust in ice core samples dated to c. 3650 BP belonged to Thera. Only recently has most of this been identified as belonging to the far larger, contemporaneous, 100km3 DRE Aniakchak eruption in the Aleutians, Alaska.

When it comes to large volcanic eruptions, one of the more striking features is the Sunda Arc that runs from Sumatra via Java and the Sunda Strait through the Lesser Sunda Islands. Sumatra is home to the Toba caldera, source and result of the largest volcanic eruption in the past 100 kA. Recently, a vast body of magma underlying Java was discovered, one that feeds that islands prodigious volcanic activity. But of the southern part of this arc; the Sunda Strait and the Lesser Sunda Islands, little is known. Yet this part of the Sunda Arc is home to two of the largest volcanic eruptions of the past 1,000 years; Rinjani (~1257 AD, <80 km3 DRE) and Tambora (1815, 33 – 41 km3 DRE). Sufficient to say, was there a repeat of either of those eruptions today, the islands hosting these giants are home to some 4½ million people each and neither such VEI 7 blast would be survivable. As both had “mega colossal” eruptions recently geologically speaking, neither is a good candidate for another one in the foreseeable future. But on the premise that a similar magmatic feed into a similar geological setting will most likely result in similar volcanic activity, let’s take a closer look! Lightning did after all strike twice here within the past millennium!

The Sunda Arc from Eastern Java through the Sunda Strait and the Lesser Sund Islands where the Australian plate subducts under the Sunda Plate at a rate given as 6-7 cm per year, relatively high. Note that for Sumatra and Western Java, the subducting plate is the Indian plate. The names of the Islands is green, active volcanic complexes, calderas and the larger stratovolcanoes are denoted in red. Sangenes (yellow) is thought to be extinct.

From a birds-eye view, this area is characterised by the formation of very large stratovolcanic cones with a prominence in excess of 3 km (eg Raung, ancestral Catur, Ancestral Batur. Agung, Rinjani, Tambora and the partly submarine Sangeang Api), volcanic complexes (eg. Biau, Buyan-Bratan and Batur) and 10-15 km calderas (eg. Biau, Bedegul, Batur). It all comes together on Bali, tropical island paradise and the place to go for a romantic holiday. Apart from the 1963 VEI 5 (5.3) eruption of Gunung Agung, little is known about the volcanism of Bali.

Bali

With a population of 4,225,000 as of January 2014, Bali is home to most of Indonesia’s Hindu minority which according to the 2010 Census constituted 84.5% of the island’s population. Just over a quarter of a century ago, the economy was mainly based on agriculture. Before the 2003 terrorist bombings, over 80% of the economy was tourism-related and Bali had become the richest of all Indonesian territories. Annual tourism is in excess of eight million with five being Indonesian and the remaining three international. To crown it all, Bali was host to the 2013 Miss World pageant.

The main tourist locations are concentrated to the South; Sanur on the east coast which once was the only tourist location, Kuta with its beach and close to the Ngurah Rai International Airport, Ubud in the centre of the island and the newer development Nusa Dua and Pecatu. Kuta, the main tourist location, lies 55 km from the centre of the Buyan-Bratan volcanic complex, 61 km from the centre of the Batur Caldera and 60 km from the peak of the 3,031 m high Gunung Agung, The town of Ubud is basically at half that distance while Dempasar, the capital with over 800,000 inhabitants, is within 50 km of all three.

The crust beneath Bali Island is about 18 km thick and has seismic velocities similar to those of oceanic crust (Curray et al, 1977). The depth of the Benioff Zone beneath the Batur Volcano is 165 km, which has been computed by multiple linear regression analyses (Hutchison, 1976). The depth of the seismic zone beneath the arc reaches to approximately 650 km depth between Java and Flores. The oldest widely exposed rocks are lower Tertiary shallow marine sediments, which are intruded and overlain by plutonic and related volcanic rocks in a zone only slightly south of the present-day volcanic arc (Bemmelen, 1949). The rocks of the Sumatra to Bali sector range from tholeiitic through calc-alkaline to high-K calc-alkaline series.

Geologic map of Bali Island. Note the extent of the “Buyan-Bratan and Batur Tuffs and Lahar deposits” and compare with the previous map of Bali showing the main settlements. (After Purbo-Hadiwidjojo, 1971)

Volcanism in Bali is concentrated to three areas, the Buyan-Bratan volcanic complex which formed roughly 100,000 years ago but holds several young stratovolcanic cones to the SSW, the Batur Caldera which formed <100,000 to 25,000 years ago and has the highly active stratovolcanic cone of Batur. Both areas contain large lakes within the caldera perimeters. Finally, there is Gunung Agung which had a powerful VEI 5 eruption as recently as 1963. However, the eruptive record of Agung extends no further back than to the 1808 VEI 2 eruption and that of Batur to a VEI 2 eruption in 1804. Being located just south of the Equator, the tropical climate and vegetation quickly covers whatever volcanics that have been deposited. This may create a false sense of security.

Buyan-Bratan Volcanic Complex

The Buyan-Bratan volcanic complex is also known as the Bedegul caldera, Bratan caldera, Catur or Tjatur caldera. The southern caldera wall has disappeared beneath a superimposed field of young, heavily vegetated stratovolcanoes including Gunung Batukaru (2,276 m), Adeng (1,826 m), Pohen (2,063 m), Sengayang (2,087 m), Lesung (1,865 m), Tapak (1909 m). Although the ancestral volcano is known as Mt Catur, the location of today’s Catur (2,096m) on the NE calera rim argues that it may not be a volcano even if it is sometimes referred to as being one.

The age of the 6 x 11 km Bedegul caldera which formed when ancestral Mount Catur collapsed is unknown although it must be substantially older than ~30,000 years and possibly even hundreds of thousands of years. The field of young stratovolcanoes to the SW, the Byan-Bratan Volcanic Complex, is heavily vegetated, thus the latest period of activity remains unknown but has been tentatively placed hundreds or thousands of years ago (Wheller, 1986). Two of those stratovolcanoes, Tapak and Lesung must have formed after the last large eruption of the nearby Batur Caldera as they not covered by deposits of its youngest dacitic pumice eruptions. As this has been dated to 20,150 years ago, these stratovolcanoes with prominences of 625 and 669 m respectively as measured from the surface of Lake Beretan must therefore be less than this age. Inside the caldera, geothermal activity is exploited at the Buyan-Bratan geothermal power plant and there are at least a dozen hot springs in the area.

The municipality of Beretan is a major Hindu enclave and contains a Shiivaite temple, the Bratan Bali. (Indonesia Tourism)

The outline of the remaining caldera walls suggest that there may have been two events; the first forming the 9 to 10 km diameter Western part with the stratovolcanic cone of Tapak forming subsequently near the centre, the second forming the smaller 5.5 to 6 km diameter Eastern part. Very tentatively and assuming that the calderas were formed by the subsequent collapse of those edifices following a major eruption, also assuming that the ancestral volcanoes were similarly steep to the nearby Mount Agung, we can make an educated guess at the size of those eruptions. Ancestral Catur (Catur A) would have been about 3,300 m high (a.s.l.) and the caldera bottom, allowing for subsequent infill, would have been about 600 to 800 m deep as measured from the remaining walls. This yields a figure on the order of 52 + 16 = 78 km3 or borderline VEI 7 for the larger caldera, Catur A. Catur B would have been about 2,400 m a.s.l. and the caldera ~500-600m deep as measured from the remaining walls prior to infill. This results in figures of 11.3 + 4.7 = 16 km3 or a small to medium-sized VEI 6 eruption. Please note that this is speculation on my part! No doubt better-informed readers will hasten to correct my assumptions from a position of superior knowledge!

The southernmost stratovolcano of the Buyan-Bratan volcanic complex is the 2,276 m high Batukaru, which means "coconut shell" in Balinese. It has a prominence of ~1,500 m as its edifice can be traced to just below the 800 m topographic isoline. (Bali Foto Galerija)

Apart from the already mentioned Gunung Tapak (1909 m), the volcanic field subsequent to the caldera forming event(-s) includes at least another five major stratovolcanoes – Batukaru (2,276 m), Adeng (1,826 m), Pohen (2,063 m), Sengayang (2,087 m), Lesung (1,865 m). There is no information on any eruptive activity but as previously stated, due to the tropical climate and vegetations, all we can definitely state is that there has been no activity in the past two to three hundred years as there is no historical record of any. With at least two of them being younger than ~20,000 years, the likelihood is that all have been active recently, geologically speaking. What their presence does suggest however, is that the original magmatic system of ancestral Catur (Catur A & B) has been well and truly destroyed and that if in the future, there is renewed volcanic activity in the Buyan-Bratan volcanic complex, this will be from one or more of these young stratovolcanoes and most likely not greater than VEI 3, possibly a very small VEI 4 eruption in the sense that the eruption of Eyjafjallajökull in 2010 counts as one. As an example, at Tapak there are at least five layers of scoria separated by four layers of paleosoil, indicative of at least five periods of extended eruptive activity separated by four periods of repose. (Watanabe et al:2010). Watanabe and his co-authors repeatedly lament the fact that while Batur Caldera nowadays is relatively well studied, almost no research whatsoever (apart from their own exploratory field study, author’s note) seems to have been undertaken of the less easily accessible Buyan-Bratan Caldera and volcanic complex.

Batur Caldera

Ancestral Batur was an approximately 4,000-meter high stratovolcano, nearly a kilometre higher than present-day Agung (3,148 m), which had an enormous eruption in prehistoric times to form the outer, 10×13.8 km caldera around 29,300 BP which today contains a caldera lake, Danau Batur. The inner 7½ km caldera was formed at about 20,150 BP.

Gunung Batur (1,717 m.a.s.l., prominence 700 m) is a small stratovolcano in north-central Bali and its most active. It has several craters and remains active to this day. The first historically documented eruption of Batur was in 1804 and it has erupted over 20 times in the last two centuries (VEI 1 – 2). Larger eruptions occurred in 1917, 1926 and 1963. Clinopyroxene from the 1963 eruption of Batur record crystallisation depths between 12 and 18 km, whereas clinopyroxene from the 1974 eruption show a main crystallisation level between 15 and 19 km. Furthermore, plagioclase melt thermobarometry indicates the existence of shallow level magma reservoirs with depths between 2 and 4 km for the 1963 eruption and between 3 and 5 km for the 1974 event (Geiger:2014). This suggests the existence of a very large and rather deeply lying primary or lower magma chamber as well as a moderately substantial upper magma chamber.

The term “Batur” often refers to the entire caldera, including Gunung Abang, Bali’s third-highest peak, which is situated along the rim. Batur is a popular trekking mountain among tourists, as its peak is free from forest cover, offers spectacular views and is easily accessible.

The substantial lava field from the 1968 eruption (Batur III, VEI 2) that began on Jan 23rd and ended on Feb 15th 1968. (Martin Moxter)

Batur has produced vents over much of the inner caldera, but a NE-SW fissure system has localized the Batur I, II, and III craters along the summit ridge. Historical eruptions have been characterized by mild-to-moderate explosive activity (Strombolian?) sometimes accompanied by effusive emissions of basaltic lava flows from both summit and flank vents which have reached the caldera floor and the shores of Lake Batur in historical time.

The Batur caldera formed in two stages. Through radiocarbon dating, we have a relatively good idea of when. The first and larger of these is associated with the 84 km3 dacitic ignimbrite known as the “Ubud Ignimbrite” which in locations is over 120 m thick. About 29,300 years BP, Ancestral Batur had a “mega-colossal” VEI 7 eruption which caused a steep-walled depression about 1 km deep and over ten km in diameter. The second ignimbrite, the 19 km3 dacitic “Gunungkawi“ Ignimbrite”, erupted about 20,150 years BP from a large crater in the area of the present-day lake. The second eruption triggered a second collapse, which created the central 7½ km diameter circular caldera, and formed a basin structure. Both the Ubud and Gunungkawi Ignimbrites are of a similar dacitic composition although the latter is more mafic, white to red in main with less than 10% dark grey to black dacitic pumice clasts. In the case of the second of these ignimbrite, two different cooling layers were identified. The lower, thus first ejected, is finely grained and welded, hence it was far hotter. In places, it is between 5 and 20 m thick. The upper, coarser, partially welded and hence “cooler” unit has suffered much erosion but is in places up to between 50 and 70 metres thick. The calculated volume of erupted material for the Ubud (84 km3) and Gunungkawi (19 km3) Ignimbrites coincide with and are proportional to the size of related collapses of Caldera I (80 km3) and Caldera II (18 km3).

After these eruptions, there were two further ignimbrite-producing eruptions, both mainly intra-caldera. The Batur Ignimbrite is a densely welded dacitic ignimbrite, typically 50 – 200 m thick, which at one point overflows the caldera rim to form 30 to 70 m thick layers of non-welded ignimbrite. The Blingkang Ignimbrite is a non-welded to moderately welded intra-caldera ignimbrite deposit between 5 to 15 metres thick. Sparse charcoal clasts scattered in this sheet give an age of 5,500 ± 200 years B.P. The thick phreatomagmatic and surge deposits which are found below the ignimbrite indicate that this was preceded by phreatomagmatic eruptions. In addition to these four sequences, basaltic to basaltic andesite lavas and pyroclastic deposits are inter-layered with and underlie the ignimbrite sequences, particularly in the southern slope of the caldera.

In spite of the frequently erupting modern Gunung Batur with its moderately sized eruptions, this caldera cannot yet be said to have shot its bolt due to the implied existence of a very large magma reservoir, one that was apparently not destroyed by the caldera-forming eruptions. Both the Batur and Buyan-Bratan calderas illustrate a recurring theme where first a very large stratovolcanic edifice is built after which there is a substantial VEI 7 ignimbrite-forming eruption followed by the formation of a dacitic to andecitic dome complex after which a large, ignimbrite-forming VEI 6 eruption follows. Even if one of these volcanic complexes almost certainly is no longer capable of such large eruptions and the other probably not in the foreseeable future, there remains one gigantic stratovolcano on Bali, one that has dimensions of 8 x 11 km as measured at the 1200-m isoline, 2,000 m above which its somewhat truncated summit towers.

Gunung Agung

Gunung Agung photographed from about 60 km to the south during an overflight of the main tourist areas of Bali. Except for the extreme bottom of the picture, the entire plain visible is covered in ignimbrite deposits from the Batur I eruption of about 29,000 BP (WikiMedia Commons)

Located in the eastern part of Bali, Mt Agung is a young basaltic to andesitic composite volcano. Bordered to the east by the inactive or extinct volcanic cone Seraja, to the south by an ancient volcanic complex and to the NW by a valley that separates it from the Batur volcanic complex, Agung goes all the way down to the Indian Ocean to the NE and through a long unimpeded decline over the Buyan-Bratan and Batur ignimbrites and lahar deposits to the SW and WSW, all the way to the capital Denpasar and beyond. South of Agung, there are older Tertiary volcanic deposits as well as remnants of coral reefs. The present-day volcano is surrounded by older Quarternary andesitic and basaltic-andesitic lavas and pyroclastic deposits, something that has prompted the conclusion that Agung overlies an older caldera formation (S. Self et al:1979).

The eruptive record of Agung goes back only to 1808 when the volcano had a VEI 2 eruption. Since that date, Agung erupted again in 1821 (uncertain) and 1843, both VEI 2 eruptions after which it remained dormant for 120 years until the great eruption of 1963. Prior to 1808 is a big unknown, although the relative symmetry of the mountain, the state of its upper slopes as well as a comparison with similar volcanoes suggests that Agung would have erupted relatively frequently.

On February 18th 1963, locals reported hearing a loud explosion after which a dark eruption cloud rose over Agung. The first explosions were probably phreatic or phreatomagmatic. On February 24th, highly viscous lava oozed over the northern slope, 0.5-0.8 km wide and 30-40 m in height. It was moving so slowly that it took 18 to 20 days to reach 500 m a.s.l after travelling some 7 km down from the peak. This works out at a speed of about 4 mm per second or 14 m per hour. The volume of lava erupted was estimated to be on the order of 0.05 km3. After this, the eruption continued with a combination of effusive and explosive events.

On March 17th came the main eruption. The eruption cloud reached 8-10 km above the volcano but the lower portions fell down the slopes as nuees ardentes that travelled with a speed of about 60 km/hour up to 12-15 km from the crater down the valleys to the south and east. From this description, it seems the eruption was peléean. The pyroclastic flows destroyed many villages around the volcano and caused the deaths of many people living near the river valleys. Estimates are that 820 people were killed by the pyroclastic flows, 163 people were killed by ashfall and volcanic bombs and a further 165 people were killed by lahars.

A comparison between the lavas erupted by Batur and Agung reveals that while Batur tends to erupt more trachytic magmas, the magmatic system of Agung produces more evolved magmas. The comparison between the historic and modern lavas indicates that, at present and probably, the Batur caldera does not produce the types of more evolved magma required to cause ignimbrite-forming eruptions. Unfortunately, it seems the lavas of Agung have not been similarly analysed and the data is based on a single eruption, that of 1963. (H. Geiger:2014)

For the 1963 Agung eruption, results from clinopyroxene melt thermobarometry suggest dominant crystallisation levels between 18 and 22 km depth. Plagioclase melt thermobarometry indicates the existence of shallow level magma reservoirs, with depths between 3 and 7 km for the 1963 eruption, located around the boundary between the (upper) sedimentary and the oceanic type mid- to lower crust. The deep magma storage regions notably coincide with lithological boundaries in the crust and mantle beneath Bali, at the boundary between MOHO and crust, while the shallow reservoirs are consistent with recent geophysical studies that point to regional shallow level magma storage. An along-arc comparison reveals this trend to be characteristic of Sunda arc magma storage systems. According to Harri Geiger, the author, the result “highlights the utility of a thermobarometric approach to detect multi-level systems beneath recently active volcanic systems.” (Geiger: 2014)

Summary

As was remarked at the beginning; a similar magmatic feed into a similar geological setting will most likely result in similar volcanic activity. This premise is further substantiated by the conclusion presented by Geiger, that the deep magma storage regions notably coincide with lithological boundaries in the crust and mantle and that this is a characteristic of the Sunda Arc. The conclusions that can be inferred from these observations are:

  • Very large caldera-forming, ignimbrite depositing eruptions VEI 6 to 7 are a characteristic of Lower Sunda Arc volcanism
  • The location of the deep magma reservoirs is such that these are not likely to be destroyed by the caldera-forming eruptions unlike those at other locations (e.g. Roccamonfina, Mt Mazama, Aniakchak)
  • Bali contains no less than three such volcanic systems of which the currently inactive Buyan-Bratan Volcanic complex is in a phase of stratovolcanic dome construction, the Batur Caldera is in the process of rebuilding a main stratovolcanic edifice while the Agung system is meandering towards the end of that phase
  • All three volcanic systems pose potential hazards to the Balinese population and require further studies as well as systematic monitoring
  • Of the three, the greatest danger is posed by the Agung system and at present, there is insufficient data to rule out a very large, caldera-forming and or ignimbrite depositing eruption

For these reasons, Bali is our proposed number six on the New Decade Volcano program.

Henrik

Acknowledgement: I am indebted to Shérine France for finding and bringing Watanabe et al 2010 and Geiger 2014 to my attention.

Igan S. Sutawidjaja, “Ignimbrite Analyses of Batur Caldera, Bali, based on C14 Dating”, Jurnal Geologi Indonesia, Vol. 4 No. 3 September 2009: 189-202. http://oaji.net/articles/2014/1150-1408334776.pdf

K. Watanabe, T. Yamanaka, A. Harijoko, C. Saitra and I W. Warmada; ”Caldera Activities in North Bali, Indonesia”, Journal of S.E. Asian Applied Geology, 2010
http://geologic-risk.ft.ugm.ac.id/fresh/jsaag/vol-2/no-3/jsaag-v2n3p283.pdf

O. Reubi & I. A. Nicholls, “Structure and Dynamics of a Silicic Magmatic System Associated with Caldera-Forming Eruptions at Batur Volcanic Field, Bali, Indonesia”, Victorian Institute of Earth and Planetary Sciences, 2005. http://petrology.oxfordjournals.org/content/46/7/1367.full.pdf

H. Geiger, “Characterising the Magma Supply System of Agung and Batur Volcanoes on Bali, Indonesia”, Uppsala 2014
http://uu.diva-portal.org/smash/get/diva2:759515/FULLTEXT01.pdf

New Decade Volcano List; #7 Mountain of Greatness

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Sadly the other week the awesome VolcanoCafe sight came under attack by an old member of the admin and was decimated. Luckily for all of us avid readers it can now be found on http://www.volcanocafe.org Now it is back  up and runnin Carl and Henrick have managed to throw up an unexpectid number 7 for their new decade volcano list. Introducing Mount Cameroon…….

Mountain of Greatness – DVP # 7

Mount Fako, old lava flows. Wikimedia Commons.

Few volcanoes on the planet represent such an awesome sight as the majestic Mount Cameroon. It stretches from the edge of the Atlantic at Bakingili Beach and reaches an astounding height of 4040 meters. Due to its prominence it is regularly dusted with snow at the top.

Mount Cameroon, or as I am used to calling it, Mount Fako, is the only volcano to date that I have worked professionally with as a geophysicist. As volcanoes go it is somewhat of a “terra incognita”, and to be quite frank, most that has been written about the volcano is just not correct. So, there is an ample chance here to set a few things straight, do some real science, and also put the limelight on one of those volcanoes of the world that is both highly dangerous and completely unmonitored.

Geologic setting

To understand Mount Fako we first must start with the geologic setting, and also come to terms with the geologic timescale of West African Volcanism. There are 3 distinct geological features that we need to contend with as we speak about Mount Fako.

The Cameroon Volcanic Line

Mount Manengouba Caldera.

The first one is the Cameroon Volcanic Line, it consists of 4 volcanic Islands, 2 large seamounts, Mount Fako itself, Manengouba, Bambouto, The Western Highland with Mount Oku, Ngaoundere, Mandara and Biu. Volcanism in the Cameroon Volcanic Line spans a time period of 49 million years and contains two distinct periods.

The first period consists of magmatic domes and maars, most of them are heavily eroded today and requires specialized knowledge to find. This period ended about 33 million years ago and can be seen as a proto-volcanic phase.

View from inside the caldera of Mount Bamboutos.

The second period started 32 million years ago at Mandara and Mount Oku. The ensuing volcanism is highly programmatic and follows a pattern where the volcanoes are born through large scale basalt eruptions creating layers between 50 and 600 meters thick. After that comes a period of trachytic lava with minor rhyolitic ignimbrites, after that comes a large caldera event with subsequent dyke formations and phreatomagmatic eruptions of diminutive scale.

The eruptive phases of the volcanoes spans from millions of years to tens of millions of years. There is no good explanation to why the basaltic eruptions during a fairly short time switch to highly explosive volcanism. My suggestions is that the large basalt flows necessitate large volume magma reservoirs that over time fills with residue from earlier eruptions and also that the magma reservoirs becomes inundated with stale base rock low in volatiles.

Mount Oku with the caldera lake.

The formation of Cameroon Volcanic Line has erroneously been attributed to a hotspot or mantleplume. And to the naked eye there seems to be a telltale track of volcanic islands and volcanoes. There is just a problem, there is definitely no hotspot or mantleplume to be had. I will though get back to this later on.

Let us start at the Northeast and work our way down to Mount Fako. The first volcano we stumble upon is Biu, very little is known about the volcano except that it morphologically follows the normal composition for a CVL volcano and that is started its activity less than five million years ago.

Mount Ngaoundere with one of the for the Cameroon Volcanic Line so common phonolitic plugs.

To the southeast comes the 32 million year old volcano of Mandara with an unstudied volcano due south. Further southeast of that unstudied volcano is the massive caldera of Nagoundere.

The group above is a distinct group of its own, not due to being morphologically different; instead they sit on a different rift system than the rest of the volcanoes. This rift system is roughly horseshoe shaped and transects the Central African Shear Zone that is home to the volcanoes below.

Annobón Island, a real tropical paradise where you can get down and dirty with your phonolite plugs.

Now it is time to continue with the Western Highlands that consists of two main volcanoes. The northernmost of those is Mount Oku that was active 31 to 22 million years ago before it went caldera forming Lake Oku. Southwest of Mount Oku we find the massive caldera of Bambouto that was active between 21 and 14 million years ago.

Next in line is the 1 million year old active volcano of Manengouba that is situated northeast of Mount Fako. It is a part of the Fako volcanic zone but is a younger and distinctly separate volcano. What makes Manengouba so interesting is that it took less than 1 million years before it went caldera.

Now it is time to get really serious with the plugs. Pico Cão Grande on Sao Tomé.

If we for now skip Mount Fako itself and jump to the other end of the CVL we find the miniscule volcanic island of Annobón and its volcano Pagalu. This diminutive Island formed during an unusually short volcanic period that started 5 million years ago and lasted less than 1 million years.

Next in line is Sao Tomé that is one large shield volcano. It started to form 13 million years ago and the volcano is still believed to be active due to the young cinder cones situated on the southeast side of the island. It is also well known for the Pico Cão Grande volcanic monolith.

Beutifal shield crater lake with a shield in the background. Pico Basile volcano on Bioko Island.

To the northeast of Sao Tomé we find the island of Principe that erupted from 31 million years ago to 14.7 million years ago.

The next island is Bioko that is housing no less than 3 major shield volcanoes that have been active historically. Volcanism here started 1 million years ago and eruptions occurred last in the 19th century.

Central African Shear Zone

All of the volcanoes from Pagalu up to that peskily unstudied volcano is situated on the CASZ, through that unstudied volcano runs the previously mentioned horseshoe shaped fault zone.

The CASZ formed around 640 million years ago and was volcanically active around that period. Previously western scientists believed that the CASZ was tectonically inactive until an M5 earthquake occurred and was monitored on a temporary seismometer. Local sources have though always stated that large earthquakes happen frequently along the shear zone, especially during eruptive phases where houses commonly have been leveled by the intense seismic activity.

The Cameroon Volcanic Line showing the CVL, the CASZ and the Benue Through. Also visible is the horseshaped fault zone. Image taken from

The CASZ was volcanically active both 640 million years ago and also 130 million years ago during the break up of Pangea.  One should note that the 3 active periods do not rule out smaller scale volcanism in between. As such the CASZ is the oldest volcanic feature on the planet that is still active.

The CASZ used to continue in the form of the Pernambuco Fault in Brazil, but as some people have noticed, the breakup of Pangea occurred and the Shear Zone ended up divided across two continents by a sizeable ocean.

Benue Through

Eruption of Cameroon in 2000. Private photograph taken from Buea.

At the same time as the single largest eruptive episode started at Paraná-Etendeka with both trap formations and the largest explosive eruptions on record the West African Craton and the Congo Craton started to separate at what is today the Benue Through.

Volcanism at Benue Through started prior to the Paraná-Etendeka event at 149 million years ago and continued for roughly 100 million years.

As the breakup of Pangea was completed the Benue Through separation of Cratons reversed and the Through started to close up, that created a heavily folded zone adjacent to the CASZ. I would seriously try to remember this feature in your mind as I get back to the hotspot and mantleplume issue.

The Hotspot

The reigning theory for the volcanism on the Cameroon Volcanic Line is that it is created by a hotspot that is travelling in an ENE direction. Only problem is that the time record does not support this at all. To be quite frank, the pattern of age of the volcanic centers is entirely random. Let us repeat the ages from north to south. 5, 32, unknown, 11, 31, 21, 1, 3, 1, 31, 14 and 5. Either I have grown dimwitted or there is just not any time sequence that is associated with a hotspot track 1 600 kilometers long.

Tomographic map of 25km depth. Do note the position of the African Superplume and keep track of it. No Visible signs of a hotspot here at CVL. Image made by DownUnder for Volcanocafé.

Some have tried to save this by surmising that there is another hotspot there and they also favor to put in influence from the Saint Helena Hotspot in the mix. It still does not blend very well with reality.

So, if the time does not indicate a hotspot, what does? Well, the temperature of the erupted magmas is quite enigmatic. The volcanoes have erupted varied temperature magmas with the heat record at 1 338C and the coldest at 1 106C with a medium temperature of 1 280. That would put it at 220C below the temperature of the Hawai’i hotspot and en par with the Icelandic Hotspot. As such that would be a fairly cold hotspot, but those exist as we know from Iceland.

Tomographic map of 25km depth. Do note the position of the African Superplume and keep track of it, also note how cold the two cratons are. No Visible signs of a hotspot here at CVL. Image made by DownUnder for Volcanocafé.

Only problem is that the hotspots of Iceland, Hawai’i and the African Plume are caused by upwelling from deep within earth and all 3 of those are clearly visible when you create tomographic charts of the mantle.

A tomographic chart shows anomalies in the speed at which sound travels after an earthquake. The most clearly visible such entities are the Icelandic Hotspot and plume upwelling and the African Plume residing under Eastern Africa. Those can be seen very deep indeed.

Tomographic map of 25km depth. Do note the position of the African Superplume and keep track of it. No Visible signs of a hotspot here at CVL. Image made by DownUnder for Volcanocafé.

Problem is just that if we go and look at the CVL we see nothing as such, actually we even find inverse anomalies at depth showing the area to be slightly cooler than expected.

The next theory is that the Benue Through is causing a localized upwelling of material from below the LAB (Lithosphere-Asthenosphere Boundary). Only problem is that this is not evident from the tomographic maps either.

Tomographic map of 200km depth. Do note the position of the African Superplume and keep track of it. No Visible signs of a hotspot here at CVL. Image made by DownUnder for Volcanocafé.

This leaves us with a conundrum. We only know that there is no hotspot causing the volcanism. We also know that the volcanism is extremely extended in time.

Volcanism is caused either by hotspots, spreading rifts like the MAR or subduction caused melt. We know that for about 50 million years there was spreading rift volcanism going on adjacent to the CVL at the Benue Through, we also know that this started after the CASZ volcanism. We also know that there historically has been no subduction going on there. Sooner or later subduction in the area will start, but we are not there quite yet geologically speaking.

Tomographic map of 300km depth. Do note the position of the African Superplume and keep track of it. No Visible signs of a hotspot here at CVL, instead the temperature is below average at the litosphere/adenosphere boundary, definitely not a Mantleplume nor a hotspot there. Image made by DownUnder for Volcanocafé.

We are here left with a 640 million year old riddle regarding volcanism. Either we are missing something, or we have a fourth form of volcanism going on at the CVL. Sadly the CVL and Mount Fako is highly understudied. This is the first reason that Mount Fako should be on the new Decade Volcano Program.

Mount Fako

Even though it is sited as being a stratovolcano Mount Fako is actually a fissure row of volcanic craters. In some respects it reminds of an effusive cousin of Iceland’s Hekla volcano in shape. Eruptions at the volcanic fissure line started 3 million years ago with large scale basalt flows that built up an elongated shield. As volcanism continued with shorter lava flows the sides have grown increasingly steeper until a steep sided elongated hull like shape formed.

As volcanism progressed the lava flows has grown increasingly volatile rich and eruptions often take place at 2 or more places. One of the sites will be high up on the volcano and will be explosive in nature and further down the fissure there will be an entirely effusive eruption causing lava flows that often reach down to the Atlantic Ocean.

The eruptions span between VEI-2 and VEI-4 with VEI-2 sized eruptions being the by far most common type.

During eruptions the volcano becomes highly seismic with extensive and intense earthquake activity that often affects the capital of the Southwest Region Buea heavily with raised houses and deaths occurring. Normally residents of Buea are forced to sleep outdoors during eruptions to not risk that their houses cave in on them.

The lavas erupted are bimodal with basalts as the main component, but the other component are trachytes and phonolites signifying a volcano containing more evolved lavas in an intermediary stage. The sheer size of the 1 400 cubic kilometer volcano, the unstable flanks and the evolving magmas, point to a volcano nearing its end stage.

If we compare Mount Fako to its post caldera brethren to the northeast we can see that they reached about the same size before they went caldera. The volcano does though not yet hold evolved enough magmas to form ignimbrite flows.

The main forms of hazard are through seismicity and flank collapses. For flank collapses the cities of Buea and Limbé are in the strike distance. The gravest danger of this volcano is though not through an explosive eruption.

Instead the gravest risk is that a large basalt flood event will occur like the one that was potentially witnessed by Hanno the Navigator 450 BC. Another large effusive eruption would not kill people directly, instead gas content and destruction of cities and farms would cause the death toll.

Mount Fako is today not monitored at all. There is no active Seismometer, no GPS, no Inclinometer. Instead the park rangers are tasked with observing what is going on visually and forward the information to anyone interested in knowing it.

Together with the risk to the large local population and the scientific conundrum that Mount Fako poses it clearly merits to be placed at place number 7 on our proposed new Decade Volcano Program.

CARL (text) & DownUnder (tomography)

http://www.volcanocafe.org/science/mountain-of-greatness-dvp-7/

The Decade Volcanoes

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As I reblogged my last post, a revision of the Decade volcano list by the authors of VolcanoCafe, I thought before I bring you the new list I should really explain what the original one actually was!

As mentioned in one of my earliest articles, the list was complied in 1990 by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) a nongovernmental society. The aim was to select the worlds most hazardous volcanoes and put measures in place to keep a closer eye on them and raise awareness across the globe on the threats they pose, for a decade (1991-2000 The UN’s International Decade of Natural Disaster Reduction). Based on varied criteria from historic eruptions to local populations, the following made the cut;

Figure 1. USGS map of the decade volcanoes.

15 Years on the list is still going all though monitoring in some areas may have slackened slightly. It has seen some success such as the diversion of a lava flow on Etna back in 1992 and has helped form a better understanding of phreatic eruptions on Taal. It has sadly also come at great loss on several occasions as well. Despite increased monitoring of Unzen in 1991 pyroclastic flows killed 43 including volcanologists Katia and Maurice Krafft and Harry Glicken.  And even closer to the project, in 1993 the Decade Volcano conference took place in Pasto, Columbia an expedition from the conference to the Galeras crater occurred on February 14th when the volcano suddenly erupted. 3 tourists and 6 volcanologists including Professor Geoff Brown, Head of Department of Earth Science at the Open University, all lost their lives.

Many volcanologist are sceptics and/or critics of the program, hence the call for a revamp. Personally I feel any thing which promotes volcanic awareness is great all though there are some which need much more than others. Volcanoes are ever evolving and unlike most geological features can change in minutes rather than millennia and therefore prehaps a decade is too long for reviews of such a program. I know which have made my list, it will be interesting to see what makes the cut for the guys at VolcanoCafe!

Figure 1. http://listas.20minutos.es/lista/volcanes-de-la-decada-decade-volcanoes-301649/