Today’s post will describe the Buyan – Batan volcanic complex on Bali, Indonesia.  There are two caldera system on Bali, Buyan – Batan and neighboring Batur to the E.  All in all, Volcano Discovery lists six volcanic systems on Bali.  Three of them are recently active, Buyan – Batan, Batur and Agung.  Buyan – Batan and Batur both went caldera with significant resurgent activity afterwards.  Agung has not made it to a caldera eruption yet.  The other three volcanoes aren’t recently active, Seraja, Patas and Merbuk.  Now, on to Buyan – Batan. 

The Buyan – Bratan volcanic complex is centered around the 11 x 6 km Bratan (Catur or Tjatur) Caldera.  Its highest point is 2,244 m.  There are three shallow caldera lakes in the northern half of the caldera.  Several post-caldera, resurgent stratovolcanoes are found in the S half of the caldera and farther S outside the caldera.  The largest of these is Batukau (Batukaru), 10 km SW.  The cones are well formed, covered with thick vegetation, and thought to be inactive for hundreds to thousands of years. 

There are no known historic eruptions from Buyan – Bratan, which has an active geothermal field within the caldera.  There are hot springs found in over a dozen locations. 

The region is heavily populated with over 333,000 within 10 km of the caldera, many of them living inside the caldera itself.  There are over 4.3 million within 100 km. Most of these in Denpasar, some 46 km S. 

Climate in this part of the world is a tropical savanna, with hot, humid weather year-round.  There is little temperature change throughout the year, averaging 28° C.  There are two seasons, wet and dry.  Wet is Nov – May.  Dry, Jun – Oct.  Average precipitation is just under 175 cm/year. 

Bali is the only Hindu-majority province in Indonesia.  Up until the 1970s, the Balinese economy was largely agricultural.  Over the last half century, that has shifted to tourism as the single largest industry in terms of income.  This has made Bali one of Indonesia’s wealthiest regions.  As of 2003, around 80% of the Bali economy was tourism related.  Despite the growth of tourism, agriculture is still the island’s largest employer.  Fishing also provides a significant number of jobs.  There is a thriving artisan community creating an array of handicrafts.  Individual villages adopt a single product line (wooden furniture). 

Earthquakes and volcanoes in this part of the world are monitored by MAGMA Indonesia.  Only Batur and Agung are actively monitored.  There is an active seismic network for the entire island.  There is no dedicated webcam for Buyan – Bratan. 

Region

The three islands involved in this region are Java (W), Bali and Lombok (E).  As Java is some distance W and Lombok is a similar distance E from Buyan – Bratan, we will only touch on volcanoes on those islands and concentrate our efforts on the Volcano Discovery list of Indonesian volcanoes on Bali.  These volcanoes are referred to as Quaternary volcanoes, though that doesn’t mean they were all active in the last 10 ka.  All distances will be measured from Buyan – Bratan. 

The three closest volcanoes to Buyan – Bratan on far E Java are Baluran (98 km WNW), Ijen (101 km WNW) and Raung (119 km km W).  As of this writing, MAGMA carries activity at Ijen as Level I (normal) and Ruang as Level II (Alert).  Ruang is a satellite vent on the W flank of the larger Ijen caldera system. 

The two closest volcanoes to Buyan – Bratan on neighboring Lombok Island to the E are Rinjani (148 km E) and Punikan (120 km E).  Rinjani is the largest of these, with a 9 x 6 km caldera.  The active Barujari vent is on the far E part of this caldera.  As of this writing, activity at Rinjani is carried by MAGMA as Level II (Alert). 

Volcanic Activity on Bali

There have been three active periods of volcanism in Bali, 1.6 – 1.5 Ma, 0.7 – 0.5 Ma and 0.2 Ma -present.  Volcanic rocks W of the Bratan caldera were formed 1.6 – 1.5 Ma.  Volcanoes on the N aprons of caldera sommas were formed by the 0.7 – 0.5 Ma.  Tapis, the small volcano covered by Agung and Seroja (Seraya) on the E part of Bali were formed during this period.

The most recent active period can be divided into 100 ka chunks.  200 – 100 ka, extensive volcanism took place in Batur and Bratan regions.  Batukau volcano, SW of Bratan, EL 706 m volcano near Pasek (located between Batur and Bratan) and Cemara (S of Agung) were formed.  The shield volcanoes that went caldera creating sommas of Batur and Bratan started erupting in the same period. Materials from these eruptions covered that of volcanoes older than 0.5 Ma.

Activity since 100 ka continued at the Batur somma and built Abang peak.  Agung started erupting 50 ka, constructing the current cone that partly covers older Tapis and Cemara volcanoes.  Batur produced caldera forming eruptions multiple times over the last 30 ka.  These calderas formed between the aprons of volcanoes formed 0.5 – 0.2 Ma.  Post caldera resurgent activity continued in both calderas along with new, pre-caldera activity at Agung.

The most widespread surface rocks on Bali are products of Buyan – Bratan caldera formation, Seraya, and recently active Batur and Agung volcanoes.  It is difficult to distinguish volcanic rocks from Buyan – Bratan from those of the more recent Batur.  It is also difficult to find volcanic rocks from the Buyan – Bratan caldera forming eruption as most of it is covered by younger ignimbrites.  The most recent significant volcanic activity on Bali took place in 1963 when Agung erupted producing a VEI 5.  This was followed by a VEI 3 2017 – 2019. 

Batur 

The Batur volcanic field (BVF) is located some 27 km E from Buyan – Bratan.  It is classified as a Quaternary field.  Batur covers some 2,300 km2.  The field is located some 150 km above the Benioff zone.  Crust beneath Bali is about 20 km thick with an oceanic velocity structure.  Our former home discussed Batur along with the three main volcanoes on Bali in a 2017 post.  Batur appears to have two magma chambers – 4 and 18 km beneath the volcano.

The BVF is formed by two nested caldera and an active cone within the smaller caldera.  The older, larger caldera (Caldera I) measures 14 x 10 km.  The younger (Caldera II) is circular at 7.5 km.  The rim varies 1,267 – 2,152 m.  The highest peak of Batur volcano tops out at 1,717 m, 686 m above the surface of Lake Batur.  The ancestral stratovolcano was likely higher than current 3,142 m Mount Agung. 

The Bunbulan lava dome complex is located in the NE sector of Caldera I and forms the NE wall of Caldera II.  It consists of at least six lava flows with minor pyroclastic flow and fall deposits.  The base are up to 30 m thick massive dacite lava flows.  They grade to thinner andesitic flows 5 – 6 m thick with brecciated bases. 

The young resurgent Batur stratovolcano erupted at least 22 times since the 1800s, mostly producing Strombolian eruptions and lava flows.  A recent eruption VEI 2 in 1974 produced basaltic andesitic lapilli and lava flow.  A VEI 2 eruption 1963 – 1964 produced mostly lava flows.  The most recent eruption was a VEI 1 in 1999. 

Batur Caldera Formation

Caldera formation is thought to proceed in seven stages.  Pre caldera activity (Stage 1) built the pre-caldera stratovolcano, about 3,000 m above sea level of basalt and basaltic andesites.  Stage 2 erupted the Ubud Ignimbrite 29.3 ka, creating the larger Caldera I.  The depression was deeper in the NW and leaked andesite from the evolving magma reservoir.  It erupted high silica andesite lavas as Stage 4.  Stage 5 erupted the Gunnungkawi and Batur Ignimbrites 20.1 ka and andesitic magma through cracks or ring fracture to produce domes and dikes.  The second caldera collapse (Stage 6) formed the smaller, nested Caldera II and a second collapse of Caldera I.  This formed the lake, maars and cinder cones in the caldera moat.  The terminal stage (Stage 7) is postcaldera volcanism, with the young basalt to basaltic andesite stratovolcano in the center of Caldera II.

Caldera I formation took place 29.3 ka.  It deposited the 84 km3 dacite Ubud ignimbrite, via a VEI 6.9 eruption.  Ubud covers at least 1,200 km2 of Bali around the caldera.  The caldera subsided as a block, forming a distinct basin structure.  There is no significant destruction of the previous stratovolcano.  The collapse is thought to be a Valles-type collapse. 

The second collapse (Caldera II) took place around 20.1 ka erupting the 24 km3 dacite Gunungkawi and Batur Ignimbrites.  This eruption was a VEI 6.4.  The Gunungkawi ignimbrite in the caldera is interbedded lapilli ash surge, ash fall, pumice lapilli fall and thin pyroclastic flow deposits all covered by a thick massive pyroclastic flow deposit.  This covers central, S and N Bali with a single flow unit.  The deposits are not welded.

A third collapse was a second collapse of Caldera I.  This is based on a displacement of andesites and the Batur Ignimbrite less than 500 m forming the Kintamani Terrace.  The Batur caldera is similar to the Platoro Caldera in Colorado, in that it existed as a depression before completion of the caldera forming eruption.  There is some dispute over the collapse being caused by withdrawal of magma rather than eruption of the magma, meaning this was likely a cold collapse.

There is a third ignimbrite, Blingkang, from a VEI 4.9 Plinian eruption at the Batur Caldera dating 5.5 ka.  It erupted 0.09 km3 of non-welded sheet 1 – 4 m thick with pumice in a pumice ash matrix.  Peneloken and Penulisan fall deposits are more recent and cover the rims of both calderas and the SW flank of the volcanic field. 

Agung 

3,142 m Gunung Agung is the highest peak on Bali.  It is 42 km E from Buyan – Bratan, 17 km E from Batur.  The volcano is considered sacred to the Balinese people.  They hold ceremonies and make offerings to the volcano on a regular basis.  Our former home discussed Agung in three 2017 posts.  The link is to the most recent of them.

Agung is also active, with many historic eruptions.  Over 52 tephra fall eruptions are recognized over the last 5.2 ka.  Eruption frequency over the last 500 years (one every 100 years or so) is consistent with eruptions described in historical texts.  One particularly deadly event took place in 1711, killing hundreds and causing destruction on all sides of the volcano.  Multiple VEI 2 events took place in the 19^th Century.  An 1843 eruption is listed as a VEI 5.  Agung erupted in 1963 with one of the largest 20^th Century eruptions worldwide, a VEI 5.  The most recent eruption was a VEI 3 2017 – 2019. 

Agung’s recent stratigraphy is dominated by basaltic to basaltic andesite scoria falls separated by thin paleosol layers.  There are the occasional pumice lapilli deposits.  There is no evidence for large Plinian fall or ignimbrite deposits.  Fall deposits are widespread on all sides of the volcano.  Analysis of deposits from 1843 and 1963 show these are similarly sized eruptions.  Triggering mechanism for both seems to be injection of basalt into an existing andesite reservoir.  Agung appears to have two magma chambers – 3-7 km and 12 – 19 km beneath the volcano.

Cemara is a smooth peak and a steep cliff on the E apron of Agung.  Small hills distributed to the SE of Cemara may have formed by a sector collapse.  This requires further study.

Seraja 

Seraja (Seraya, Mount Lempuyang) is located on the E coast of Bali, next to currently active Agung, 59 km E from Buyan – Bratan.  Its heavily eroded summit tops out 1,142 m.  While listed as Quaternary, it is very likely extinct, though hot springs are found on its flanks.  It has a visible crater on Google Maps measuring 3.7 km across.

Patas 

Gunung Patas is a 1,412 m tall volcano located some 34 km W from Buyan – Bratan.  It is one of three mountains located inside the West Bali National Park in the western region of Bali.  The volcano is topped with a 4.9 km diameter crater / caldera that does not appear to be breached.  While there is not much published on the volcano in technical literature, it is quite popular with the hiking community.  

Merbuk 

Gunung Merbuk is a heavily eroded, inactive 1,388 m stratovolcano volcano some 53 km W from Buyan – Bratan.  Its slopes are covered with greenery and considered an opportunity for off-the-beaten-track exploration and hikes.  It is also listed as Quaternary, though no activity is obvious over the last 10 ka.  I have found no mention of hot springs or other hydrothermal features on its slopes.  An inspection on Google Maps shows an elliptical feature 5.8 x 4.4 km.  The feature is heavily eroded and may be more the result of the eye wanting to see something than an actual crater.  It is also a target for hikers. 

Buyan – Bratan

The Buyan – Bratan volcanic complex today consists of a main caldera and multiple resurgent cones in the S half of the caldera and farther outside the caldera to the S.  There has been no historic activity recorded at Buyan – Bratan, though there is a vigorous hydrothermal system that powers hot springs and a geothermal field within the caldera. 

While I have listed dates for various events at the system, estimates of these dates, particularly the date of caldera formation vary widely.  For example, caldera formation is estimated in some places before 500 ka, while in others 200 – 130 ka. 

Buyan – Bratan grew in multiple stages, though generally divided into pre-caldera, caldera, and post-caldera stages.  The initial Bratan volcano grew about 400 ka.  It was a large basalt volcano.  The next stage was caldera formation somewhere 200 – 130 ka.  This created the 6 x 11 km main caldera.  There are today, three intra-caldera lakes, Bratan, Buyan and Tamblingan. 

The 6 x 11 km Buyan – Bratan caldera is a circular depression open to the S.  There is a caldera wall on the NE side.  There are multiple younger volcanoes inside the caldera, Tapak, Lesung and Pohen.  To the S, likely just outside the S caldera wall are Sengayang and Batukau volcanoes.  These have craters on top indicating that each erupted at least once recently. 

Outcrops of erupted material from Buyan – Bratan pre-caldera to caldera formation are limited and unclear.  There is a major exposure at the Gitgit waterfall where several lava flows show columnar jointing.  These lava flows are most likely pre-caldera.  The lavas are covered by weathered tuff breccias, likely from pyroclastic flows, though emplacement mechanism is unknown. 

Most of the caldera floor is covered by ash to lapilli sized scoria pyroclastic falls.  The scoria can be found surrounding Tapak and Lesung.  There are at least five layers of scoria at Tapak separated by paleosols.  The deposit may have come from Strombolian eruptions that form the young cones of Lesung, Tapak and Pohen inside the caldera. 

Post-caldera Resurgent Activity

Post-caldera, resurgent activity quickly started, building at least six post-caldera stratovolcanoes from the center to the S of the caldera starting with the basaltic Catur, 125 ka.  This was followed by andesitic Buyan-Bratan 94 ka.  One publication lists Sengayang constructed between Buyan – Bratan and basaltic andesitic Tapak 66 ka.  Next was basaltic Pohen 55 ka, basaltic andesitic Lesong (Lesung) 31 ka, finishing up with basaltic Adeng 14 ka.  The system has been quiet for the last 14 ka. 

The 2,276 m Batukau (Batakaru) is located 3 km the S of Sengayang and may or may not have been a post-caldera, resurgent volcano, though Smithsonian GVP does describe it as one.  It is topped by a 2 x 3 km amphitheater open to the SE.  There is little published about this volcano outside the hiking and travel world. 

Mount Bratan (Gunung Catur) is located on the NE caldera rim.  It has been cut in half by the collapse. 

Tapak has three craters aligned N -S.  Lava from the central crater flowed to the NW, dividing the Tamblingan and Buyan lakes.  This lava covered the tip of a lava flow from Lesung volcano.  The central cone of Tapak covers part of the S crater.  The direction of the E lava flow is controlled by the shape of the N cone.  The central cone segment of Tapak is the latest eruption of the post-caldera volcanoes.  A lava flow S from Tapak reached the S foot of neighboring Adeng. 

Lesung has two craters with a gully cut on the pyroclastic cone from the N to the W slope.  Lava from the S crater flowed down the W flank beyond the caldera rim.  This flow is similar in age to the Penelekan tephra from the Batur caldera.  Debris avalanche deposits are distributed on the SE foot of Pohen, though there are no obvious landforms on Pohen.  This may indicate sector collapse and rebuild took place on Pohen before more recent activity at Tapak or Lesung both of which erupted in the last 10 ka. 

Adeng is a smallish 1,828 m volcano some 5 km ESE from Sanghyang.  It is topped with a smallish amphitheater open to the SE.  Once again, though a possible member of post-caldera volcanic activity, there is not much written about it outside the hiking and travel blogs. 

Tectonics

As with the rest of Indonesia, volcanic activity is driven by the N collision of the Indo – Australian Plate into the Eurasian (Southeast Asia) Plate.  The subducting plate is an oceanic plate 130 – 80 Ma.  The subduction rate is 6 – 7 cm/yr. 

The Indonesian volcanic arc extends 6,000 km from N Sumatra to the Maluku Sea.  It consists of four volcanic belts.  The longest of these is the Sunda Belt from the N tip of Sumatra through Java, Bali, Lombok, Sumbawa and Flores.  The other three belts are Banda, Halmahera and N Sulawesi. 

There are over 500 eruptive centers along the Sunda Belt.  129 of these currently active.  These represent over 13% of active volcanoes worldwide.  In Sumatra and Java, volcanism may have been active since the Triassic.  Activity in the eastern part of the arc is much younger, mid to late Miocene. 

Continental crust in Sumatra – Java is 20 – 30 km thick, thinning to 18 km beneath Bali.  Seismic velocities are similar to those of oceanic crust.  The Benioff Zone is 165 km beneath Batur.  Magma chemistry slowly changes from continental to oceanic basement beneath Bali. 

The oldest exposed rocks are shallow marine sediments overlain by plutonic and volcanic rocks.  Igneous activity took place in three cycles, the first deposited old andesites.  The second cycle erupted are slightly more alkaline.  Distinction between second and third cycle volcanic is not particularly clear cut.

Conclusions

Volcanism on Bali is most impressive with two significant caldera systems and a young, active stratovolcano.  There is an ample magma supply for the entire region.  While there has been no recent activity at Buyan – Bratan, there is still an active hydrothermal system indicating magma beneath it.  Given the number of smallish stratovolcanoes in and around the caldera, the active hydrothermal system and its vigorously active neighbors, I would treat this system with a great deal of respect. 

Additional information

Geomorphological classification of post-caldera volcanoes in the Buyan – Bratan caldera, North Bali, Indonesia, Okuno, et al, Dec 2017

Genesis of dacitic magmatism at Batur, volcano, Bali, Indonesia:  Implications for the origins of stratovolcano calderas, Wheller & Varne, Jul 1986

Characterizing the magma supply system of Agung and Batur volcanoes on Bali, Indonesia, H Geiger, 2014

Bali Energy Limited, Geothermal Location, Bedugul Geothermal Field

Caldera activities in north Bali, Indonesia, Watanabe, et al, Sept 2015

Assessment of Bedugul geothermal prospect using a numerical reservoir modeling, Keintjem, et al, Jul 2023

Sustainable water management:  new perspectives, design and practice, Nakagami, et al, Jan 2016

Analysis of debris flow hazard in volcanic soil by the flood flows modeling (DFLOWZ) and Nakayasu Synthetic Unit Hydrograph, Sinarta, et al, Mar 2023

Debris flow hazard assessment based on resistivity value and geological analysis in Abang Mountain, Geopark, Batur, Bali, Sinarta, et al, Nov 2019

K – Ar analysis of the post-caldera lavas of Bratan volcano in Bali Island, Indonesia – Ar isotope mass fractionation to light isotope enrichment, Kitagawa, et al, Aug 2013

The geological hazards of Bali Island, IS Sutawidjaja, Sept 2014

Estimating the impact of inflow on the chemistry of two different caldera type lakes located on the Bali Island (Indonesia), Polkowska, et al, Apr 2015

Analysis of gravity anomaly and seismicity in the Bali region, Tobing, et al, Nov 2021

Ignimbrite analysis of Batur caldera, Bali, based on 14C dating, IS Sutawidjaja, June 2014

Interpretation and mapping of geologic structure in the Bali basin based on integrated gravity, magnetic and seismic data analysis, Kurnida, et al, Dec 2023

A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia):  implications for eruption frequency and volcanic hazards, Fontjin, et al, Jul 2015

Long-term volcanic history preceding caldera-formation in Bali, Sunda arc, Toshida, et al, May 2013