A volcano erupting in Indonesia is not something that would normally cause too great a flurry, so what is the hype with Gunung Agung? Naturally, people remember well its last eruption which lasted almost a year, was very explosive and, sadly, claimed ~2000 lives. Additionally, this time after official reports about unrest began, there were also a number of brush fires on the slopes of the volcano, expelling huge clouds of gray smoke – it’s no wonder that people began panicking. Moreover, newspapers all over the world reported that Mt. Gunung was erupting; a German report even “knew” that the plume was “over 10 km high”. That’s what the internet does for you.
After I saw the thermal anomalies on satellite images I did a bit of fact-checking. Wild fires at this time of year, I was told, are common in Indonesia as farmers burn dry rice stalks after the harvest. (Unfortunately, the fires have also destroyed a large area of “protected” forest, seems the protection is not being much enforced by authorities). Also – as of today – the volcano has NOT erupted.
BALI, about the ISLAND
Bali, one of the 13,000 Indonesian islands, is located directly east of Java. About 4.2 Million people live there permanently, but about double that number may be visiting Bali each year. Its size is 145 km long and 95 km wide, big enough to find a safe place during a “normal”-sized eruption.
It is a part of the Sunda-Banda volcanic islands arc. Bali is dominantly covered by volcanic rocks, overlying the Tertiary carbonate rocks that outcrop in the southern and western part. The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, Bali and Lombok, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate. The latter underlies the islands and neighbouring Sumatra, along the Sunda Trench. It passes under the islands, where friction between the two plates can cause large Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes on the islands.
A string of volcanoes lines the northern part of the island, in West-East axis, along which the western part is oldest, and the eastern part is newest. Stratovolcano Mount Agung is the highest with 3014 m a.s.l. (the generally given hight of 3142 m or so is out of date, as the difference has been blasted off in the 1963 eruption). Bali sits also at the edge of the continental Sunda shelf, just west of the Wallace line, and was at times connected to the neighbouring island of Java, particularly when the sea level was low in the Ice Ages. Today, the two islands are separated by the 2.4 km Bali Strait.
G. Agung is on the NE side of Bali. It is part of one of three volcanic centres on the island: the Agung/Seraya volcanic complexes. G. Agung volcano is surrounded by the major calderas of Bratan and Batur to the NW, and the Rinjani-Samalas complex on the island of Lombok to the east. Because of its basaltic andesite lavas Agung can produce highly explosive eruptions – with two VEI 5 eruptions in 120 years this is a volcano to be reckoned with. Although, one should think that it has spent its power now for a longer period of rest.
A study of the Late Holocene tephrostratigraphic record showed that Agung has an average eruptive frequency of one (VEI>2-3) eruption per century. About 25 % of all its eruptions are of similar or larger magnitude than the 1963 event, and this includes the previous eruption of 1843 (estimated VEI 5, contrary to previous estimations of VEI 2). The latter represents one of the chemically more evolved products (andesite) erupted at Agung. In general it is thought that Agungs magmas are the product of repeated basaltic intrusions into basaltic-andesitic to andesitic reservoirs, with little compositional variations.
The abundance of sulfur in a magma is inversely proportional to silica content. Thus, the basaltic-andesite magma of Agung contained more sulfur than the higher silica magma of the historic Krakatau eruption. This high amount of SO2 gas emitted during Agung’s 1963 eruption formed 11-12 million metric tons of H2SO4 aerosols in the stratosphere – which might be linked to a subsequent drop of 0.3°C in average temperatures in the northern hemisphere. The graph (above left) shows how Agung caused the third biggest impact on world climate between 1958 and 2008.The patterns and distribution of earlier eruptions (1808, 1821, 1843, and 1963) show similar types. There is explosive activity (throwing sparks, lava fragments, pyroclastic rain and ash), as well as effusive pyroclastic flows (PFs) and lava flows. PFs from G. Agung tend to go down on the northern and southern slopes, as, to the E and W, the crater is flanked by two sharp ridges, like a pair of shoulders. PFs that are thrown out in E or W direction may even be divided by the “shoulders” to flow down on both sides.
Scientists modelled the main deep magma source of Agung to be at 18-22 km depth. Also, shallow level magma reservoirs exist at depths between 3 and 7 km (after the 1963 eruption). This trend is found to be characteristic of Sunda arc magma storage systems.
On Sept. 14, the alert for G. Agung volcano has been raised from Normal to “Waspada” (Attention), the second level of four. It had shown a significant increase in seismicity since the first volcanic earthquakes (EQ) had been recorded in early August. Also a new fumarole emitted steam up to 300 m.
When the numbers of volcanic EQ began to climb into the hundreds per day, the alert level was raised again to “Siaga” (Standby) on Sept. 18. At this time, apparently a small thermal anomaly was also detected within the crater. When an EQ of M 3.1 occured on 18 Sept. the volcanologist in charge reported stronger fumarolic activity and “visible sparks” from the crater.
The latest report by PVMBG (Sept. 22) states that seismicity of G. Agung continues to increase significantly, but no “crater smoke” has been observed.
On Sept. 19 there were 427 VA and 20 VB earthquakes.
On Sept. 20 there were 563 VA and 8 VB earthquakes.
On Sept. 21 there were 592 VA and 82 VB earthquakes.
(VA = Volcanic Earthquake, VB = Shallow Volcanic Earthquake; according to PVMBG)
There is an access restriction order in place for a radius of 9 km around the volcano, extending to 12 km in areas N, SE and SSW of it. Over 11 000 people of the ~ 50 000 who live in the restriction zones have already evacuated to temporary shelters.
Local sources say, though, that farmers still go back daily to tend to their farms. (Hazard Map) (Updated 26/09/2017)
THE 1963-64 ERUPTION
In 1963 Gunung Agung erupted simultaneously with its neighbour Batur. This was to become one of the most important volcanic events of the 20th century, and the largest and most devastating eruption in Indonesia since the 1883 eruption of Krakatau. On February 18, local residents heard loud explosions and saw clouds rising from the crater of Mount Agung. On February 24, lava began flowing down the northern slope of the mountain, eventually traveling 7 km in the next 20 days. On March 17, the volcano erupted (eventually producing a VEI 5), sending debris up to 10 km into the air and generating massive PFs. On this occasion also the top 300 ft (or more) of the mountain was blown off. According to observations, PFs travelled at an average speed of 60 km per hour and reached up to 13 km to the south and 14 km to the north. The flows devastated numerous villages, killing approximately 1500 people.
Lahars caused by heavy rainfall after the eruption killed an additional 200. A second eruption on May 16 led to pyroclastic flows that killed another 200 inhabitants. Much of the agricultural land at the foot of the volcano was destroyed. The final output of volcanic materials was calculated as 0.95 km³ DRE (dense rock equivalent).
“Ancient Balinese texts demand that the most important of Balinese Hindu rituals, the Eka Dasa Rudra, is performed every 100 years to purify our world. Such a ceremony was scheduled at Besakih temple in 1963. In February of that year Mount Agung started to rumble, and high priests argued that this was a bad omen from the Gods and that the wrong date had been scheduled for the ceremony. By this time though President Sukarno had arranged to attend with a group of foreign dignitaries and he ordered that the event must go ahead.
On March 8th the mountain was in a full pre-eruption phase and on March 18th it blew with such force that the top 100 metres or more was blasted away. Lava spewed over much of eastern Bali, a devastating gas cloud swept across villages and ash destroyed all crops. Up to 2,000 people are thought to have died and tens of thousands of homes were destroyed. Quite remarkably, the mother temple of Besakih perched high on the slopes of the mountain was relatively untouched by this event, fuelling the local spiritual reasoning for the eruption. Further, many Balinese argued that this eruption was a portent of dreadful things to come and felt vindicated when the whole nation was plunged into civil crisis in 1965. – The cleansing rituals from the 1963 ceremonies were finally finished at Besakih in 1979.“
The eruption ended in January 1964, leaving a newly shaped crater of 500m diameter and 200m deep.
Video: When G. Agung erupted in 1963, not only the caretaker of Agung refused to evacuate, even almost all adult males from some villages “welcomed” the spill of Agung’s lava kneeling and praying:
The Balinese “compass”
Here, people do not talk of north, south, east and west. They refer instead to kaja, kelod, kangin, and kauh. Wherever you are in Bali the direction facing Mount Agung is kaja and this is the most sacred and pure point on the compass. Generally speaking, if you live in the south, kaja is northeast: if you live in the north, kaja is southeast. The opposite and most impure direction is kelod; that is facing away from Mount Agung and towards the sea. Kangin means facing the rising sun that evokes birth and beginning. Kauh is the direction looking west towards the setting sun, a symbol of ending and death. (from gaiadiscovery.com)
A living model of sustainability
Another interesting thing is the Balinese irrigation system: “A thousand years ago, rice farmers in Bali created a complex, ritual-based farming system in which “subaks,” self-governing, democratic organizations of farmers, managed the shared use of irrigation water among terraced rice paddies. Subak heads met in sacred temples, stationed along irrigation pathways, to coordinate planting and watering schedules in ways that would ensure an ecologically responsible sharing of resources. Although it has faced challenges, Bali’s ancient water temple system remains largely in place today. Hundreds of water temples are sites of festivals, rituals and religious offerings, thanking the gods for the gift of irrigation water, which flows through canals and tunnels from Bali’s volcanic Lake Batur into numerous rivers and streams and to the jewel-like terraced fields. This system is considered a living model of sustainability.” (from Earth Observatory Singapore, see more images with this post: http://www.earthobservatory.sg/project/bali)
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Agung on GVP
– G. Agung (on Badan Geologi’s website)
– Reports (on Badan Geologi’s website)
– A 5000-year record of […] Gunung Agung (2015, paywalled)
– Inflation of Shallow Magma Reservoirs […] by InSAR (2012)
– Geothermal systems […] Bali (PDF, 2015)
– G. Agung on MIROVA (look up “Batur”)
– New Decade Volcano Program #6 (VolcanoCafe, Blog)
– […] Magma Supply System of Agung and […] (PDF, 2014)
– […] Bali’s Mountains and Lakes (Blog)
– Mount Agung, Wikitravel