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.
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.
Sadly I awoke this morning to the news Mount Sinabung in Northern Sumatra, Indonesia had claimed the lives of three farmers working in the fields by the Gembar Village. This figure has since risen to 7 and is feared to continue to rise with several more critically injured and the Red Cross and army looking for further victims.
Mount Sinabung has been in a near constant state of eruption since late 2013. Pyroclastic flows sweep down its flanks on a regular basis which has lead to 4 km exclusion zone being enforced around the summit. On February 1st 2014 people were killed by one such pyroclastic flows.
About 10,000 people have been displaced by activity at the volcano which has been on the highest state of alert for well over a year. Sadly the volcano is positioned in a relatively poor and over populated area of the world, many people have little choice but to continue to farm on the volcanoes fertile flanks. Officials have struggled to keep the people to stick the ‘red’ exclusion zones and it is unclear how many people were on the mountain at the time of the recent activity.
Head of the National Disaster Mitigation Agency (BNPB) Willem Rampangilei has instructed Karo Regent to take quick measures to vacate the red zones (Gamber village, Simpang Empat district and Karo Regency) but they know that this is easier said then done. The pyroclastic flows caused by partial collapses of the growing lava dome occurred in a series at 14:28, 15:08 and 16:39 local time on Saturday. Rescue attempts went through the night and in to Sunday morning. An ash column remained for hours, towering over the area darkening the skys and hampering the search operation.
The pyroclastic flow captured here to the left happened only a week ago on May 16th showing the power and regularity of such activity. On May 9th a lahar swept through Kutambaru near the Lau Barus River killing 1 and leaving one person still missing now also presumed dead.
Sinabung lay silently for nearly 400 year until springing back to life back in 2010. It has now killed at least 25 people since its rousing. Volcanism on the island of Sumatra is caused by the subduction of the Indo-Australian plate beneath the Eurasian plate along the Sunda Arc which creates the andesitic-dacitic composition magmas which are prone to such explosive activity. Sinabung sits just 25 miles north-east of the Toba Super Volcano caused by the same tectonic motion.
In the past week the Japanese Island of Kysushu has be ravaged by earthquakes.
Japan 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.
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.
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.
I am pretty sure I have covered this event before but seeing as today marks the 5 year anniversary of one of the worst natural disasters in the past decade I thought it deserves a much more in depth look. The events of March 11th destroyed the lives of hundreds of thousands of people and claimed the lives of nearly 20,000. For me, it opened my eyes to a world of geohazards and mad me realized this was something I wanted to study and understand so such loss of life would not happen again.
There are three elements to the events of March 11th that I am going to look at here; the earthquake, the tsunami and the Fukushima power plant. Each aspect a huge disaster in on there own but interlinked as they were caused devastation for Japan.
Japan is a volcanic island which stretches along where the North American, Pacific, Eurasian and Philippine plates all collide at different points. It is a part of the Pacific Ring of Fire, the world’s most tectonically active area. Practically all of our planets largest and most destructive earthquakes occur along the ring, one of which rocked the east coast of Japan at 2.46 pm JST (5.46am UTC) on March 11th 2011.
The magnitude 9 quake struck at the shallow depth of just 32 km roughly 70 km off the Oshika Peninsular. The area was already alert to seismic activity as several large foreshocks had occurred in the run up including a Mg 7.2 on March 9th and followed by three more above a Mg 6. Of course no one knew these were precursors to something much larger…
Initial reports from JMA and USGS put the March 11th quake at a 7.9 but this had risen first to a 8.8 and then to a 9 before most of the seismic waves had even hit Tokyo 373 km away. Luckily thanks to Japans intense seismic network the countries capital had at least 80 seconds warning before they felt the strong shaking.
The megathrust earthquake occurred where the Pacific Plate subducts beneath the North America Plate. The Pacific Plate moves at a relative speed of roughly 9 mm per year but it is not a smooth decent, tension can build and release in a large snap causing an earthquake. On March 11th this happened in epic style causing over ~50 meters of displacement near the Japan Trench which caused the tsunami which swept across the Sendai planes. The earthquake was so powerful that up to 1.69 meters of co-seismic deformation has permanently altered our planet and affected the Earths tilt shaving 1.8 microseconds of the day (not that we would ever notice!)
It was the forth largest earthquake ever recorded and the largest ever to strike Japan.
The displacement on the sea bed in turn caused a huge displacement of water in the Pacific ocean its self. Across a 180 km stretch there was recorded up thrust of 6-8 meters. Above the rupture the tsunami waves would have looked like no more than ripples on the surface radiating out across the ocean. It is as the waves reach the continental shelf and the water is forced upwards that they begin to take on their characteristic ‘wall of water’ appearance.
At its maximum height (recorded at Miyako, Iwate) the waves hit 40.5 m high (133 ft). The Pacific has the most comprehensive tsunami warning systems in the world but even this gave only about 15 minutes warning from the earthquake to waves hitting the coast line. Travelling at speeds up to 500 mph the water surged up to 6 miles (10 km) inland.
It was not just Japan which felt the repercussions of the event. Tsunami waves propagated out through out the entire Pacific. 11,000 miles away the coast of Chile experienced waves in excess of 2 meters along with most of the America west coast right up through to the Aleutian Islands and as far south as Antarctica where it broke chunks off the Sulzberger Ice Shelf
An estimated 5 million tonnes of debris began washing up on shore lines across the Pacific in the months and even years after the initial Earthquake. In April 2013 a 20 ft boat ran aground in California and was later identified as belonging to the marine sciences program at Takata High School, Japan. NOAA have kept tracks and aimed to clear as much of the debris as possible to minimize risk to ships and wild life but the operation can take more than a decade.
The melt down at the Fukushima was the worst nuclear disaster the world has seen since Chernobyl in 1986.
The plant ran by TEPCO had 3 of its 6 units shut down for inspection when the earthquake struck. Units 1, 2 and 3 then under went automatic shutdown cutting off power. 50 minutes later the waves up to 15 meters high breached the measly 5.7 meters seawalls and flooded the basements of the turbine buildings and disabling the emergency generators. The lack of power meant the cooling systems of the 3 active reactors failed and eventually the heat caused by decay caused the containers to burst leaking radioactive material.
It was classified a Level 7 on the International Nuclear Event Scale (INES) and its was the way the event was handled from the very beginning my TEPCO which saw the escalation in the threat. Approximately 15 PBq of caesium-137 was released along with some 500 PBq of iodine-131, luckily all the failed reactors were in concrete containment vessels, which limited the release of strontium-90, americium-241 and plutonium.
No deaths were caused by the events or short term radiation exposure but it is thought people in the area worst hit will have a slightly higher risk of developing certain cancers in the future. Now 5 years on there is still a 12.5 km is still in place with thousands of people still exiled from their homes. The wild has reclaimed the land making it look like a scene from an apocalyptic film.
It could be centuries before the area is truly deemed safe to live on again.
Pre-Warning; This has happened before
Japan is no stranger to tsunamis; the 1896 and 1933 Sanriku earthquakes (Mg 8.5 and 8.4 respectively) also brought deadly waves. For this reason tsunami barriers have been constructed both on and off shore, trees were planted along the coastline, vertical evacuation buildings were built to the highest standards and regular evacuation training was introduced. But none of these were built to with stand the sheer force of a tsunami of this magnitude.
In 2001 a team from Tohoku University published an article in Journal of Natural Disaster predicting such an event occurring every 800-1100 years. Within the Sendai Plain there is evidence of at least 3 major tsunami deposits all left within the past 3000 years. On July 9th 869 BC what is believed to be a magnitude 8-9 earthquake occurred off the coast of Sanriku causing a major tsunami which left deposits up to 4 km inland. So given that we knew an event like this had occurred before, why was Japan not better prepared for March 11th?
Sadly human nature does not always listen to the reason of science. It is often easier to believe ‘it won’t happen in my life time’ and then brush the threat under the carpet for future generations. The problem is it does not matter how much we study the mechanics of our planet we are still no where near being able to predict these disasters with any degree of accuracy meaning preparation is our best defence.
A report issued by the Japanese government in May 2015 claimed the events of March 11th 2011 caused $300 billion dollars. A confirmed 15,894 people lost their lives, 2,562 people are still unaccounted for.
5 years on the area is yet to recover. An estimated 174,000 are still displaced mainly due to the exclusion zone still heavily in place around the Fukushima plant. Soon as the initial rescue operation was completed the Tohoku Earthquake Tsunami Joint Survey Group was assembled. A team of natural scientists and engineers from 63 universities world wide set out to understand what made this tsunami so powerful and how we can protect our selves from further events. By the end of 2011 the Japanese government had passed laws to establish “tsunami-safe cities” and pledged billions of dollars to an intense 5 year clean up operation. It was clearly a bigger job than they originally thought….
Today there are still over 60,000 people living in temporary accommodation.For residents once living near the Fukushima power plant they will probably never return to there own homes. Sendai is still trying to recover from the tragic events but also now living in fear that this could occur again.
It is for this reason I choose to go in to studying geoscience. We all live at the mercy of our planet and most of us never even consider the risk the land beneath our feet poses. Prediction, preparation and knowledge can save lives and this is what I one day want to help with.
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!
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.
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 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.
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 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 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!
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.
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.
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.
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.
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)
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.
Acknowledgement: I am indebted to Shérine France for finding and bringing Watanabe et al 2010 and Geiger 2014 to my attention.
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;
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!
I sat in a bar last Thursday night discussing the rarity of large-scale natural disasters with several of my non-geologically minded friends. They were shocked to hear just how common certain hazards actually were. I explained that media coverage is scarce depending on fatalities or area of the occurrence, but we have actually been lucky not to have seen any particularly deadly events in a while. Sadly I spoke to soon…
Figure 1. Remains of the Bhaktapur temple, once a UNESCO world heritage site
At 11.56 am local time (06.11 UTC) Saturday, Nepal was rocked by its strongest earthquake in decades. The Mg 7.8 quake was intensified by its shallow depth, just 11 km below the surface, meaning waves had less time to dissipate and disperse leaving the region to feel its full force. This morning officials have raised the estimated death toll again to just under 4000 but it could be weeks before we have real figures. The rescue operation has been hindered by hundreds of aftershocks many over a Mg 5 and even several over a Mg 6 making them devastating events in their own right.
The effects where far-reaching with all neighbouring countries feeling the tremor. India has so far reported 67 deaths and China and Bangladesh 20 and 8 respectively. 19 people lost their lives on Mount Everest as the quake induced avalanches across the snowy region. It is the mountains highest fatality since explorers began ascending its flanks in 1953, scores of hikers are still believed to be stranded.
Figure 2. South Base camp was struck by an avalanche soon after the earthquake killing 12
Why has this earthquake been so devastating?
The answer is a combination of two factors;
Firstly the region is extremely tectonically active. The processes that caused this earthquake are the same that have given rise to the highest mountain range on the planet, the Himalayas. The Indian plate is moving north-northwest towards the Eurasia plate at a rate of 40-50 mm per year. The thick continental crust of the Indian plate instead of simply subducting under the Eurasian plate in to the mantle, like oceanic crust would, actually lifts the opposing plate creating the Himalayan mountain range. Large scale earthquake are relatively common along the subducting surface of the Indian plate as it sticks to the overriding plate, there have been 4 in the last century including Saturdays event. The largest was a Mg 8.0 back in 1934 roughly 240 km southwest of the recent quake; over 17,000 lost their lives. The last was back in 1988 claimed nearly 1500.
Secondly despite all this knowledge the country and many of its neighbours are grossly unprepared for such events. Nepal is one of Asia’s poorest countries, many of its residents live in extremely poorly constructed buildings and in many cases shacks. Building codes and regulations in place in richer at risk regions, say San Francisco, are simple non-existent in the Himalayan region. The epicenter was just 77 km north-west of the countries densely populated capital Kathmandu, were it’s believed at least 800 have lost their lives. Surrounding villages have seen complete devastation with no building and very few people surviving.
Figure 4. Huge displacement due to the quake.
Only last month at the UN World Conference on Disaster Risk Reduction identified the potential threat in the region but no further measures were put in place. Hopefully when the rebuilding process begins steps will be taken to limit the threat of future earthquakes.