There was a lot going in the 1960’s. This was the time of the generation battle, where the children who had grown up in a time of austerity and a cold war, rebelled and looked for something different – exploring, ignoring the boundaries but never quite finding what they were looking for. Martin Luther King and John F. Kennedy were shaping the future, and paying dearly. East and West were fighting, from Cuba to Vietnam. Europe suffered through the terrible winter of 1963. Who cared about a volcano no one had ever heard about? Even if it was one of the largest eruptions of the 20th century?
Bali is different. This island of 4 million people has its own culture, and in fact was an independent kingdom until 1906, with a distinct culture. The island is not large: 150 km by 110 km. Most here is volcanic in origin, and those volcanoes, as elsewhere in Indonesia, are alive and active. Several are over 2 km tall. The highest is Gunung Agung, at just over 3 kilometer. The climate is monsoon, with a very wet December to April but little rain from May to September. Tourism is big business, and at times it seems most of Australia is here. It seems paradisiac. But when the fertile soil of paradise comes from volcanic ash, life is never totally secure. The eruption of 1963, little recognised in the rest of the world, brought devastation.
Three volcanoes are located in close proximity to each other. Furthest east is Seraya, deeply eroded with no intention of every erupting again. In the middle is sacred Agung, with Bali’s largest temple, Pura Besakih; the climb to the top takes about four hours from there. West of Agung is Gunung Batur, 2150 meter high, with its enormous, 25,000 year old caldera. Visible further to the east, beyond Bali, is Rinjani, site of the enormous 1257 eruption, showing that this area has history. Batur and Agung both are quite able to erupt. But while Batur does frequent but minor, Agung does dangerous. Its 1963 eruption was a particularly bad one.
Although there had been fumarole activity in the crater of Agung, the mountain had not erupted since 1843. On 16 Feb 1963, with Europe shivering in its worst winter for two centuries, earthquakes began to be felt in Bali, although not strong enough to do any damage. Two day later, 18 Feb 1963, explosive activity began, minor at first. Lava flows were first seen on 19 Feb, one day after the first explosion, flowing through a gap on the northern rim. The lava had probably first appeared together with the explosions the dat before, but this had not been seen. On 20 Feb, a pyroclastic flow which reached one village. These were the first casualties. Rain caused some lahars over the next days.
A video of the early days of the eruption is available on this icon of the 2010’s, youtube:
Over the next four weeks the lava kept flowing although the amounts were not huge. The lava flow reached a length of 7 kilometer. It was 1 kilometer wide and over 50 meter thick; the volume over the next three weeks is estimated at 0.05-0.1 km3. The explosions now began to strengthen, and ash was rising as high as 3 kilometer above the peak of the volcano. From March 8, explosions were becoming severe.
And so the main event began. At 5:30am on 18 March, major explosions began, lasting for 3.5 hours. The ash fell as far as 1000 kilometer away, in Jakarta. One of my secondary school teachers once told me that while he lived in Jakarta, he had experienced volcanic ash – and I think that may have been this eruption. Pyroclastic flows came over the rim of the caldera. Rain coming after the eruption caused lahars. These flows even reached the coast, destroyed villages and caused many casualties.
This event took some of the pressure off. The source of the lava flow was cut off after it, and the lava flow slowly came to a halt. Lava refilled part of the crater but it did not overtop the rim again. After 18 March the explosions, although continuing, lessened in intensity and by the middle of May, the eruption seemed to be subsiding. But this was deceptive. On 16 May, there was another major explosion, lasting 4 hours. Again the pyroclastic flows came, and again there was major destruction, now mainly around the foot of the mountain. Two days later there was a larger earthquake, and more explosive events although not of the same size. The lava now stopped completely. After the end of May, the explosions became less, and although the eruption continued until 1964, from here on it was a minor one. But when the rainy season started, in November, the ash soaked up the rain, became unstable and started to move. This again lead to devastating lahars, and even more casualties. And near the end, Batur joined in with its own, unrelated eruption.
By the end of the eruption, over 1100 people had died and the economy of Bali had been devastated.
During the first four weeks of the eruption, lava flows and explosive activity occurred at the same time from within the caldera. It seems problematic that these came from the same location; more likely is that there were several vents in the caldera, some giving phreatic explosions and one or more expelling lava. But no observations were made within the caldera. It was, after all, during the peak of the rainy season.
The ash of the 17 March eruption was blown to the west, covering all of Java. On the slopes of Agung, the layer was up to 50 cm thick, and it reached 0.1 cm in Jakarta. The ash of the 16 May was less widespread, and was blown mainly the the north where the layer reached 40 cm in depth. Much was blown out to sea, and some fell 175 km away on the island of Sempanjang. Pyroclastic deposits are found in the valleys on the slopes of the volcano but it appears they did not reach much into the low lands. Instead, the major damage was mostly done by the lahars, which was partly due to the fact that the main event occurred during the rainy season. In a way, this was a foreshadowing of what would happen later at Pinatubo, where the eruption coincided with a typhoon.
So how much did this 1963 eruption erupt? The ash layers are fairly well mapped, and summing everything up suggest that the 17 March event caused 0.4 km3 of ash. The 16 May event is less accurately mapped, but it is estimated at 0.3km3. This is fresh ash: the dense-rock-equivalent (the hole in the crater it left) is less, and this is estimated at 0.28 km3 in total. Adding the lava makes the total eruption 0.4 km3 DRE.
The precise cause of the event is not known, hampered by a lack of observations. The deposits show that two lava types mixed in the eruption. Self & Rampino suggest that older, andesite magma was already present, and that in Feburary 1963, new basaltic magma was injected into this reservoir. This resulted in lava filling the crater and causing the highly viscous lava flow. A second pulse of fresh magma high in volatiles caused the 17 March explosion. The magma became capped, but in May this cap broke because of the pressure from below, and a new explosion occurred.
Although the ash cloud was only reported to be 10 km high, it likely was much higher than this. The eruption rate makes it likely that it reached twice as high, 18-20 km. In fact, a year later a plane measured some ash from this eruption at 20 km altitude. Not only had the plume reached the stratosphere, some of the ash there stayed aloft for a long time.
After 1964, Agung went back to sleep, and the people returned to the local villages. The mountain would sleep for over 50 years. But now, a prince has come and kissed the sleeping beauty awake. A new injection of magma has taken place, and since a week there is explosive activity. I today’s event, like in 1963, the ash is reaching some 3 kilometer above the mountain, 6 kilometer above sea level. And again it has started close to the rainy season.
It is difficult to predict how this will evolve: If it remains similar to 1963, the next step will involve lava, which may be viscous and slow moving. However, eruptions rarely do copy each other. There are lessons, though, and they show that the biggest danger is not from the lava, and not even the pyroclastics. Once the rains come, the biggest danger is from lahars, coming down the river valleys. Following those valleys, they can reach far beyond the mountain.
Agung has history, and the history shows that this sacred mountain should not be taken for granted. The current eruption may end without much more activity: volcanoes do as they please and if we are lucky, the gods have now ben satisfied, the prince giving his marching orders, and the mountain allowed to fall asleep again. However, if it behaves as in 1963, this may just be the beginning and we could be in for a dangerous few months. Only time will tell.
Main reference source:
Self, S., & Rampino, M. R. (2012). The 1963-1964 eruption of Agung volcano (Bali, Indonesia). Bulletin of Volcanology, 74(6), 1521-1536.