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).

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.

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 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)

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 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 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)

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.

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)

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).

The Blingkang Ignimbrite inside Batur I caldera, overlying extensive pyroclastic surge deposits. (Sutawidjaja)

 

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)

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)

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

166 thoughts on “Romantic Paradise Destination – The New Decade Volcano Program #6, Bali

  1. Interesting article 🙂

    Can’t argue with including the Lesser Sunda Arc being on the list, but why Bali over other contenders, or are there more volcanoes from the Lesser Sunda Arc on the list?

    • Bali is an easy choice for a few reasons.

      1. The volcanoes in the Bali area all have had large ingimbrite / caldera forming eruptions. So there is documented proof that they have and will create large eruptions. Additionally, every volcano on the island has had large eruptions – thus a 100% success rate of achieving a large eruption. Two volcanoes have done it twice, so it would seem likely that a third, which has constructed a large somma edifice and has had large historical eruptions in recent times could be a candidate for a secondary caldera eruption just like it’s brethren.

      2. There are candidates that could potentially realize a large eruption within the next 100 or so years as there has been enough time for the volcanoes here to rebuild their magma chambers / recharge from past large eruptions.

      3. Bali is fairly high in population, whereas the other lesser sunda islands are much more sparse.

      4. Islands complicate evacuations, since they require either air-travel or boat to leave. Most islands do not have the capacity to carry out extremely large-scale evacuations via boat or airplane.

    • Cbus gives a nice summary of the whys. As I point out in the article, both G. Rinjani on Lombok and G. Tambora on Sumbawa have had major eruptions very recently, geologically speaking, which almost certainly rule them out in the near future. Also, G. Sangenes on Sumbava is “extinct” as are several others. Sangeang Api? Possibly, if it suffered a complete collapse which could cause a region-wide tsunami as not that many people live there.

      With three active or potentially active volcanic systems, and a tourism-boosted population of about 4½ million, Bali stands out like a sore thumb. Take a look at the geological map and the extent of the Ubud ignimbrite and consider what a similar eruption today from any of these three systems would cause.

      • Both Tambora and Rinjani had a long period of quiescence before their large eruptions. Agung has had two VEI 5’s in less than 200 years, so some of the pressure / magma build up would have been released.

        I would argue that there is no harm in increasing monitoring of Agung but I would make sure that there were enough resources in fhe region to monitor other volcanoes quickly. The Lesser Sunda Arc is an area where volcanoes considered dormant or extinct can present nasty surprises.

        • Oh I agree that there are other volcanoes in the area that most certainly do need monitoring and none so more than the unstudied and unmonitored volcanoes of the Buyan-Bratan Volcanic Complex; Batukaru, Adeng, Pohen, Sengayang, Lesung and Tapak to name the most obvious candidates for a nasty surprise!

          But how you regard the danger posed by Agung depends on how you view the process of generation of evolved magmas. If they are indeed generated by remelting of a sedimentary layer as I hypothesised in the previous article, then the amount generated is a straight function of surface area – provided of course that enough energy in the form of hot, juvenile magma is infused into the system:

          The yellow line, a linear progression, can be made to represent a wide range of phenomenae such as the increase in size of eruptions or, in other words, the system’s ability to release pressure. The gap between magma and thus pressure generated and the system’s pressure release capability just widens until there will come a point when “all hell breaks loose”. This is what I fear may happen if the evolved magmas observed indeed are the result of an interaction between mantle-derived basalts and sedimentary deposits.

        • Tambora did not have a long repose period. And as far as I know there is no detailed stratigraphic study proving Rinjani to have had a long repose period either.
          In my opinion the “long repose meme” is just wrong. Instead it seems that it is equaly, or even more so, dangerous for a volcano to have frequent eruptions. The reason for this is that repeated influx of magma tend to build ever larger reservoirs as magma is influxed and disgorged.

          • But crystalizing magma becomes more gas-rich and explosive, so the chances of an explosive eruption is higher after a dormancy of ~1000 years. It is not the only way to get explosive behaviour though. Pressure changes can lead to changes in the magma reservoir, and these can be caused by expulsion of some magma. If your favourite volcano does this, an eruption can out it over the edge. I assume something like this happened in Krakatoa.

            To quote Tolstoy: “Happy volcanoes are all alike; every unhappy volcano is unhappy in its own way.” There is more than one ways to turn a crisis into a catastrophe.

          • I tend to think of this as more of “exception, but not the rule” type of thing. I don’t think a volcano NEEDS a long repose time to have a large explosive eruption, but I believe most of the large eruptions we’ve seen even in more recent time have come from systems that have been closed open and not frequently active.

            Santa Maria, Novarupta, Huaynaputina, Chicon, and Pinatubo all come to mind as volcanoes that had a significant repose period prior to their large events.

            If you get enough magmatic input into a system, it *can* create large eruptions while “leaking” smaller eruptions all the same. I tend to be of the personal belief that this is more often a product of volcanoes with stacked or multiple magma chambers as well as a prolific feeder systems. But that doesn’t mean this is the normal case scenario.

            As is always said, I don’t think there is a “rule” for how volcanoes act. But there definitely tendencies, and most volcanoes that awaken from a long slumber don’t do so effusively.

  2. Can’t argue with the selection unlike last one. It hits on a variety of high risk factors for a future large eruption along with having the requisite nearby population to have things get bad.

    Also, There looks to be a fourth caldera on the island as well further to the east, although it’s likely extinct based on appearance.

    As for the past eruption sizes, I would personally adjust the eruption sizes upwards a bit. I realize it’s good to make conservative estimates, but we have two gleaming recent examples in the same arc of volcanoes that had VEI-7 eruptions to compare the Bali volcanoes to.

    Both of these volcanoes (Tambora and Samalas/Rinjani) created calderas significantly smaller than the calderas in Bali. Considering relative caldera size, I would assume that the initial caldera forming eruptions of Batur and Bratan were decently larger than Tambora / Samalas, and the subsequent eruptions were likely similar in size to Tambora’s 1815 eruption. That being said, it’s just (educated) guesswork.

    • With the Ubud ignimbrite being calculated at 84 km^3, it is almost certain that the total DRE of that eruption of Batur was well in excess of 100 km^3 or 2½-3 x the size of Tambora, 1½-2 x the size of Rinjani.

  3. Sorry, newbie question here… What is the difference between a Nuees Ardentes and a Pyroclastic flow? I thought they are one & the same?

    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.

    • They are two different names for the same phenomenon, Jory. The name originates from the 1902 eruption of Mt Pelée in Martinique and it being a (former) French colony, they were first reported as “Nuees Ardentes” and only later did the English version take precedence, much like the Paris Meridian is nowadays known as the Greenwich meridian (as if an English village had automatic precedence over the capital of France…). 😉

      From the Wiki entry (which for once is correct):

      “The eruption also lent its name to the “Peléan eruption style”. Among those who studied Mount Pelée were Angelo Heilprin and Antoine Lacroix. Lacroix was the first to describe the nuée ardente phenomenon.”

  4. … and today, a horrible realization that all the old people that I saw sitting around the restaurant eating fish… were about my age.

  5. there are always volcanoes erupting in Indonesia, quiet often residue of ash comes across Australia, I have in some years a lot of it destroying plants and making them look not good and less fruit, a big one would possible make a difference to a wide area of the globe over a period of time, therefore loss of life accumulating, my take on the next big one would be further to the east, like New Britain going by the big ones previously moving across the arc, I am no expert just thinking

  6. So far I think this is your best sell as a new Decade volcano – not in terms of destruction (though it seems potentially to good at that) but more in terms of persuading the vulcanologists of the world that this is a destination they would ‘reluctantly’ be forced to study, and that the old Decade volcanoes should be switched to this new list. I could consider overlooking the ‘probably not quite yet imminent destruction’ and manning a volcano monitoring station somewhere there if there is enough funding to support flights and a hut with internet (so I can keep track of volcanocafe)

  7. I might somewhat disagree with your idea of renaming meridians – the reality was a bit more complex. Thanks for the articles. I returned from travels to find two very interesting posts waiting to be read. Agung seems a good choice. It seems that research for deciphering its past could help judge how much of a danger it currently poses. There is a 50/50 chance that the next ‘colossal’ eruption will be from somewhere in Indonesia, so you could have picked half you decadal list here!

    • Welcome back! Of course it is, it was said very much tongue-in-cheek and if asked point-blank, I would emphatically state that I disagree with my own presentation on the course of events that led to the Greenwich Meridian taking precedence.

  8. Good morning. I was surfing the web for the Colima eruption and found this interesting, informative site. Maybe some of you already know about it. This link is about Colima volcano and the Colima Volcanic Complex. The eruption video is from January, 2015. If you click the box at the top marked “Geo Events” a large list of articles about earthquakes and volcanoes comes up. Lots of great reading.

    http://www.earthscope.org/science/geo-events/colima-mexico-volcanic-eruption

    rescued from akismet /Hobbes

    • http://www.earthscope.org/information/about – an interesting read.

      The EarthScope scientific community conducts multidisciplinary research across the Earth sciences utilizing freely available data from instruments that measure motions of the Earth’s surface, record seismic waves, and recover rock samples from depths at which earthquakes originate.

    • Hey, I know that guy! Well, I know who he is.. Brenner Phillips is quite often on board the R/V Nautilus working with and piloting the rovs (remotely operated vehicles) at depths of up to 4,000 meters.

        • cool. I noticed the whole crew was “glowing” as the women say, guessing the water temp there is somewhere north of 26C, seeing that glass surface just reminds me of brain baking in Galveston bay complex when the wind lays down

    • That is interesting. Whether the creatures would know to get out of there sensing danger should the worse happen or not survive at all.

  9. Thank you Henrik. So much information. Great Post and a good choice……..As we progress up the list the scenarios get more ….. Worrying? Scary? I am not into doom mongering at all but the hazards from these often quiescent volcanoes seems to certainly be underestimated. How well is Bali monitored?

  10. Nice article on this acid ‘ocean lake’. Note to Carl; decadal volcanoes could be under water.. But it is a pity about their inferences about ocean acidification being harmless: I wonder which part of chemistry they don’t believe in! Sharks survive anything, but how many shelled organisms did they find?

    • “Acidification” is a misnomer. The oceans are having episodes of less alkalinity.

      GL Edit: In this instance, the nomenclature is a directional notation. The ocean pH is still well above 7.0… except in situations like around Bob when it was erupting when some reports stated the pH as low as 5.0, which is in fact quite acid. Which, if you note, is how the water on the inside of the volcanic caldera was described. Back when Bob was doing it’s thing, fish were dieing swimming through that muck. The very same junk that certain politicians were proposing sending amateur dive photographers into just to down play the volcanic threat and save scaring off investors.

      No sock puppet intended. I was on my phone at a restaurant. I am the guy with a torch and have not hidden that fact.

      • Please correct me if I am wrong, but are not corals susceptible to very small changes in pH as well as temperature?

        • There is a cut-off point, where carbonate begins to dissolve into the water. If the pH falls below this, corals and any outside shell will dissolve which obviously is a bit of a problem if you are a mollusc. At the moment this already happens below about 4km depth: the depth varies a bit with oceanic conditions. We are now putting CO2 in the top of the oceans and are in danger of creating a second ‘forbidden zone’ in the top few hundred meters. Eventually a balance will be reached where enough dissolves to keep the pH stable again, but at a lower level.

  11. Or how the French failed to take the opportunity when the Greenwich Observatory closed to get their meridian back!

  12. don’t worry about it, just logged out of WP and put the VC data in and the post above is in

  13. And a quick infomercial… of sorts.

    Many of you have heard of “Tungsten”… it’s an element. It is a very tough metal that has a pretty high melting point… about 3422 °C, ​(6192 °F). One of the more common ways of working with it, is to cast it using “Sintering” methods. This way you do not have to achieve and maintain the ultra-high melt temperature, so high that it could actually deform/melt the cast that it is being poured into. All you have to do is to compact a powdered form of it into the mold, and then heat it to a point where the molecules in the particles start to drift into adjacent particles. This effectively welds the mass into one cohesive unit.

    This is also how Ignimbrite forms. You may have seen the term used here on VC to denote areas where volcanoes have deposited material. This is quite true, but it is also characteristic as to how the material got there. Ignimbrite comes from a term coined by Zealand geologist Patrick Marshall, which essentially means “fiery rain”. In other words, from a pyroclastic flow. Pyroclastic flows are the result of an eruptive plume that no longer has the energy to loft skyward and then collapses under it’s own weight. The material can easily be still in excess of 800°C, when it hits the ground, it rockets across the landscape at phenomenal speed… pretty much incinerating everything in it’s path. Once the mass settles, the individual particles weld together into a pretty solid mass. Welded tuff is similar, but probably achieved it’s solid mass characteristic due to lower temperature particle activity, such as a slower compaction while still quite hot.

    The point? Anytime you see “Ignimbrite”, think pyroclastic flow deposit.

    If you’ve seen the bodies in photographs of the victims at Pompeii, they were killed by this type of event. The greatest error that “news” programs make, is claiming that these are intact bodies. They are far from it. They are casts of the cavities that were found in the ignimbrite deposits as they were being excavated. Still a very horrible way to die. Within three breaths or so their lungs were seared and they were entombed in superheated ash deposits where they fell.

    Caveat: Not a geologist, just an avid reader.

    • It’s also the same process that welds snow avalanche material into ice after it stops. The energy of motion sinters the snow into what has been described as concrete by those caught in it. Cheers –

      • Interesting! Made me think of the glassy Pele’s tears, although perhaps they form in just the opposite way!

  14. Excellent article.

    (First post here, so hello, old Bardarbuddies and Sharknado fans)

    /First time comments have to be manually approved. It is a part of the security settings we have had to set into place. Welcome back Eddie! Admin

  15. According to the BBC, Colima in Mexico is showing signs similar to that of the major 1913 eruption.

  16. Just had a thought regarding silicic volcanism in Iceland. I know it gets discussed how there may be a deep lying shard of continental terrain that may play a role the intermediate magmas seen at Hekla. But why is there not any consideration for the Hreppar microplate as the source for these magmas? Couldn’t the Hreppar microplate be that very source of intermediate magmas?

    • Ding! I did quite an extensive amount of discussion on it back before I gave up on understanding Hekla.

      • But alas… in the end I discovered I had been wrong all along due to the lack of an ample supply of xenoliths in the lavas ejected.
        In the end I cracked the enigma of Hekla due to a paper by Sturkell. I am just saving that one for an article later on when I need a subject at hand.

        Hint, the solution to Hekla is so simple that Henrik quite literaly winced with evil Mirth when I told him over a beer.

        (I always save Hekla for a rainy day, in more ways than people think)

  17. Logged on to the webcam – wasn’t expecting to see much at night, but that definitely isn’t the case.

    • Glad you got the pic on here. I’m watching it and wishing to put it on here. I can’t quite do that yet. 🙂

  18. Looking at Iceland drums for the past 24 hours, there are a lot of weird seismic signals across Iceland, some stronger, some weaker, but generally all mid to more low frequency.

  19. A qwuery to the current Dragon: why does the header to this page read “Because volcanoes are Ewesome”? Awesome, I can appreciate, but Ewesome sounds like a blog about Sheep. Baaaa.

    /Released from the Dungeons. Admin

    • The answer to your question is that it harketh back to the days of the Eyjafjallajökull. During the eruption one stormy night there was a sheep flying infront of the webcams. Oddly enough we do believe that the flying sheep later resurfased in the shape of a carcass that was eaten on television by Bear Grylls (and was vomited out profusely as usual).
      It started our infatuation with sheep around volcanoes (and our dislike of Bear Grylls).

        • Technically, if I remember correctly, the sheep that Carl mentions was not actually flying, more of a plop and roll, struggle to get up, plop and roll again as it battled against the wind. From what I understand, this lasted for it’s entire transit across the camera field of view. A cow would have just put it’s long axis in alignment with the wind and continued to chew it’s cud. Cows figured out a long time ago that its much more difficult to get knocked over along their long axis.

  20. My eye just caught this, hope it is not old, old news. Underwater volcanoes discovered off Australia…

  21. Thank you all for the kind remarks on my being a grandma for the first time. Nearly sent you a video of the 6 month old laughing out loud at her father scolding the dog. Must be Geolurking’s influence…

  22. Pardon!!! Reading backwards I now saw Edward’s posting onb the same ones. Time to tape my beak…

  23. What’s with being frequently rebuffed for a so-called “Database Error” when trying to so much as just read an article here lately? I am fairly certain I’m not making any errors…

    • Well, as explained earlier, we had to move house because of an attempted hostile take-over … Luckily we got this site up and running and it is located at a temporal home until we get things sorted out, so bear with us. It’s not your fault you get to see those pages, it’s the server being too busy…

      • “This site … is located … at a temporal home”

        OK, but what some of us want to know is whether there will be volcanoes and the Sheepy Dalek bar at our *eternal* home 😉

    • We are sorry for the inconvenience.
      As Lughduniense said, we are planning to move again to a permanent server home. We are just exploring to different paths to see which gives the best result for the money. This time we will be on our own owned servers. Question is just if it will be a Spiffy dial-a-server-cloud-thingamabob or a couple of beefed up servers in two different cities.
      We are on the problem.
      /Admin

    • “I am fairly certain I’m not making any errors…”

      Quite correct. The issue when presented with a message such as this, is that there was an issue with your “session” trying to connect to a “pipe” (logical handle) on the database server. When the software making your request (the connection/refresh or what have you) tried to go look up what it needed to process your new page, it either got no data, or it got the wrong data… and then threw the error back at you to let you know it did not succeed in what you had “told” it to do.

    • Anyone’s guess! Ice volcanoes seem fairly common in the outer solar system. The contrast between dark and light regions could suggest depositions. (Looking at the maps my impression was that the light material is below and the dark is something lying on the surface.) But you would not expect to see active volcanoes.

  24. 1. The world is all that is Cheese.
    1.1 The world is the totality of all that is Cheese, not processed cheese.
    1.11 The world is determined by the Cheese, and by these all the Cheese.
    1.12 For the totality of all the Cheese determines both what is the case, and also that is not the case.
    1.2 The Cheese in logical space is the world.
    1.21 Any one can either be the Cheese or not be the Cheese, and everything else remain the same.

    • “To a worm in horseradish, the world is horseradish”

      — Malcom Gladwell → “What the Dog Saw: And Other Adventures”

      Specifically from part one; “Obsessives, Pioneers, and other varieties of Minor Genius”

      • I’m pretty sure that the “horseradish” term is a substitute for a more vile substance, but was used to make it more polite.

        • Yep that’s a point. Volcanoes do a good job of marking their territory and can be rather noisy. =^-^= 😀

  25. Saw this on our friend, Thorbjorg Agustdott’s twitter. 15m deep graben at Holuhraun.

      • And just as a note, I’d say that the sight of an active lava flow is a decent indication that things won’t likely “get messy” here. Lava flows occur when degassed magma is pushed out onto the surface. Unless there is a deep batch of gas-rich magma (which is unlikely since evolved magma is usually at the top of a magma reserve), I wouldn’t expect any larger explosive eruptions. At worst, this ends up similar to Sinabung with some dome collapses leading to small pyroclastic flows, although Colima’s magma isn’t viscous enough right now to build up the rocky domes Sinabung tends to grow.

        • Not sure that is always true; there have been a number of cases where lava flows of degassed magma from the top of a magma column have been seen at the start of an eruption which got quite lively later, -possibly because the sudden removal of a large quantity of material reduced pressure on the volatile-rich stuff Down Below. Examples? Agung in 1963. Vesuvius from the 17th century to 1944. Mayon, in several 20th century episodes

        • I think Michael Don is quite on the money here. The reduced pressure of the overlying already degassed magma can easily “cross the threshold” of non-degassed magma at lower depths and tip into a bubble nucleation frenzy of activity.

          Sparks made a comparison to a sigmoid curve in the way that some volcanoes behave.

          And the thing about a sigmoid, is that once you go far enough along the curve, the switch to a new set of circumstances can be quite dramatic.

          • Afterthought…

            This curve is why well “kick” can be such a vexing problem. Once formation fluids make it into the mud column of the drill string, bubbles that form can quickly decrease the density of the column, allowing more and more formation fluid to intrude into the bore. Left unchecked, it can quickly become a runaway process, leading to a full on blowout.

          • Just a small remark. There were postulates made and then disagreement by others. All this was done in a pleasant scholarly manner without sneering or arrogance. Congratulations to all for keeping this site civil and informative.

  26. well, at least they have a breeze to dissipate the gases, and a little rain to cool the rockfalls. 🙁

    do you have any insight into seismo instruments around there?

    • gremlins in the machine, that was to be a threaded reply to mikegrimsvotn 15/07/2015 at 02:28

      GL Edit: Well, it sort of is, it’s just not tagged to it. I can put it there and fake it, but it might wind up with my avatar.

      Yep. It did. Sorry, no work around at my level. 🙁

  27. Thank you for a fascinating article. Batur is the only active volcano that I have ever climbed. The structure of that one is laid out like a textbook but I could never get my head around the Bratan complex – this report helped a lot. Years ago, when I was last in Bali, my simplistic impression was that the caldera eruptions were moving West to East and that Agung was getting to the kind of size that Batur was when it last went caldera.
    One thing that I never found a definitive answer to was the origin of the Goa Lawah Bat Cave on the east coast of Bali. The sandy beach is tellingly black there so my suspicion is lava tube. Does that make sense?

    Welcome Robin! Your comment landed at the reception room as all first comments must be approved by an admin… Enjoy VC! /Lugh

  28. Is there an open vent near the base of the steep slope of Colima volcano in Mexico? I was looking earlier today at a persistent but small area that was smoking; after a while I decided it was probably just hot rockfall from above.
    About ten minutes ago I was watching again at the webcam and suddenly a BIG puff of smoke blew out but only hung around for a few minutes – I don’t think that was rockfall. I saved the image but I’m not sure how to post it.

    • Never mind – I just saw the timelapse for earlier in the day and it did it a few times – must be just big rockfall at the lower right edge of the pyroclastic flow zone down the front of the volcano.

  29. … and something for the lurkers.

    Seismic tomography is really cool, but one of the more vexing problems is coming up with an accurate model of how the crust of the earth is supposed to respond to passing seismic waves. Knowing this, and comparing how an actual measured wave responds in comparison to a theoretical wave, you can determine low velocity vs high velocity areas. The actual math behind it is beyond my skill set, but on the grand scheme of things, this is the basis of it.

    Many of you are already aware that the accepted source of magma along subduction zones is the 110 km depth dehydration melt off of the subducted slab. Minerals that have had water taken up into their structure tend to have a lower melting point, and the magma percolates off of this region and drives the more well known volcanic systems of the Kamchatka peninsula, The Cascade Volcanoes etc.

    While studying seismic travel times, geologists have discovered oddities in how the waves propagate through the earth. From many hours of analyzing this, they have found specific depths at which the speed of seismic waves change. As a general rule, the denser a material is, the faster the wave moves through it. One long standing model was/is AK135. A more recent one is IASP91. Here is a plot of a section of that model using IASP91 from IRIS.

    Notice the strange jumps in speeds at different depths. These are called discontinuities. More can be learned about what Geologists think is happening at these transitions.

    Not mentioned in the explanatory link, is Ringwoodite. It made the headlines in the last few months or so since it was discovered that it can take up water in the form of OH radicals into it’s mineral structure. It’s also quite possible that it undergoes a state change near the 660 km depth region, releasing it’s water. Whether this can generate an additional dehydration melt zone or not, I don’t know. But it is quite coincidental that the more active volcanic regions of subduction arcs, have the subducted plate angling almost straight down… placing the 660 km contour pretty much directly under the 110 km contour.

    Anyway, it’s just an idea. Feel free to mull that one over a bit.

    • This is a earth reference model from 1991.
      Have you tried any latest earth models with 3D and quite better resolution?

      • Or the updated version of the AK135, the AK135-F from 1995.

        • No, but thank you for cluing me into it’s existence. I’m really interested hearing more about how you are doing the tomography. Everytime I try to explore the subject, I wind up much like the dog in the photo. (“Jake”)


          This link is here just so I don’t loose it. It has no significance for the discussion.

  30. No volcanoes on either Pluto or Charon yet, but perhaps records of past activity. This (https://www.nasa.gov/image-feature/new-horizons-close-up-of-charon-s-mountain-in-a-moat) is a detail from Charon, near the day/night terminator. The mountain near the top seems to be sitting in a hole. That could have been caused by some melting below the surface – a caldera that didn’t quite make it. Ice is a bit different from rock, and ice magma (i.e. water) doesn’t behave like liquid rock. Magma is lighter than rock and rises, but water is denser than ice and stays put. So the surface sinks but doesn’t get drowned. Anyway, speculative, but this is what you might get if there is enough energy to melt some ice at depth but not enough to vaporize any.

  31. Pingback: The New Decade Volcano Program; #6, Bali | Geologically Speaking

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