Gunug Agung has had impressive eruptions in the past week. So far, in spite of the publicity, the event has remained relatively minor. We do not know whether these are the initial rumblings and in the next weeks and months there will be far larger explosions, or that this suffices to relieve the pressure inside and allows the volcano to go back to sleep. For the local population the second option would be preferable by far! If the eruption intensifies, we also do not know what level it may reach. Based on statistics, smaller eruptions are far more likely than big ones, and Agung is therefore expected to behave. On the other hand, larger eruptions do occur and can never be fully excluded. And if the size reaches VEI6 or higher, the impacts can become global. But even then, it is the local people who suffer most.
The figure (taken from the book ‘Global volcanic hazards and risks) shows why large eruptions are best avoided. It shows documented fatalities due to volcanic eruptions over the past 400 years. Obviously, the older the event, the less complete the record will be. The numbers are dominated by the largest events, and the largest events are dominated by two eruptions, both in Indonesia. These are also the two largest eruptions in Indonesia in recent centuries. It seems likely that past VEI6 or 7 events in Indonesia will also have had massive local impacts. Worldwide, every known VEI6 or 7 eruption since 1600 has resulted in fatalities. And Indonesia is particularly vulnerable: it is a densely populated country, with a high level of development going back millennia, and there is a tendency for people to live close to the most active volcanoes as that is where the land is most fertile. Nowadays, the volcano observatories in many countries do a marvellous job in saving lives. Most eruptions give warnings, through a variety of precursors. In the current decade, the most devastating eruption was from Merapi (also Indonesia!) in 2010, where over 380 people died. But almost half a million people were evacuated before the eruptions, and it as been estimated that this evacuation saved over 10,000 lives. The Indonesian authorities, guided by the volcanologists, avoided a catastrophe.
The risk of eruptions depends not only on the particular volcano, but also on the nearby population. Taken both into account, Indonesia stands out as the country with by far the highest level of danger. It has been estimated that over 60% of the global threat to people from volcanoes comes from Indonesia. This is calculated from the number of active volcanoes, and the number of people living within 30 km of one. The second highest country, the Philippines, accounts for only 10%.
Since 1950, 347 volcanoes are known to have erupted across the world. Before 1950, the records become rather less complete. Over the holocene, 886 volcanoes have had known eruptions. But even for the largest eruptions there are major gaps in our knowledge. It has been estimated that we do not know 85% of the largest holocene eruptions from before 1AD. This makes it difficult to derive eruption rates, and makes it tricky to estimate the likelihood of a VEI6 or 7 per century. The best current estimate is that a VEI6 eruptions happens once every 70 years. The number of VEI7+ eruptions is estimated in the same manner to be once every 3000 years, but that seems an underestimate. After all, there have been several such eruptions over the past 2000 years. And two of the largest three eruptions of the past millennium happened in Indonesia.
Let’s take a quick look at the major eruptions we know about in Indonesia. This is a whirlwind tour, and is very very far from complete. In fact, only a few biggies are listed.
1883 Krakatoa VEI6
This most famous of eruptions is also the smallest one considered here. The eruption volume was perhaps 20 km3, and it counts only as a VEI6. The death toll was still horrendous, because of the tsunamis which destroyed the nearby coasts. The pyroclastics flows added to the toll. The volcano has been busily rebuilding itself albeit not at exactly the same place. Anak Krakatau is trying to emulate its famous parent.
1815 Tambora VEI7
Tambora, in 1815, was the most recent major eruption the world has experienced. The volcano is located on Sumbawa island, east of Lombok. The explosion had worldwide impacts, including the ‘year without summer’. The eruption volume was in excess of 100 km3; it left a caldera 7 kilometer across. Before the eruption, the mountain was one of the tallest in Indonesia, estimated at 4300 meter. Afterwards, it had been reduced to less than 3000 meters.
1257 Rinjani VEI7
This mountain on Lombok is the location of the disastrous 1257 eruption. There used to be a mountain here, Samalas, but it disappeared in this explosion leaving a caldera of 7 km diameter. It is claimed to be the largest explosion of the last 2000 years. 25 km away, the pyroclastic deposits are still over 30 meter thick. The volume is not well known but it likely to be around or close to 100 km3. The eruption had worldwide effects, including dry fogs in Europe, and the death of 20,000 people in distant London, caused by crop failures and famine. It may have been to blame for the decline of the Mongol empire. And in Indonesia, it destroyed a kingdom.
29,300 BP Batur VEI7
Behind Agung, on Bali, lies a much less imposing mountain. And behind this mountain hides an elliptical caldera measuring 14 by 10 km in size. This is the site of another monstrous eruption. The ejecta, called the “Ubud Ignimbrite” covers an area of about 1200 km2. A later eruption, about 20,000 years ago, was four times smaller, and formed the current lake. Both large eruptions were dacitic. The Ubud ignmbrite, which comes from the earlier, larger eruption, has a volume of 84 km3. The full ejecta must have been more. Over time, a new dome has formed in the centre: it still erupts regularly.
33,600 BP Ranau VEI7+
This is another remnant of a very large eruption. Ranau is a 14 km by 8km caldera, rectangular in shape, which is filled by Ranau lake. The caldera formed in an eruption that left its ejecta over a region at least 100 km long. The tuff in one area 50 km from Ranau is still several meters thick! Seminung Volcano has now grown up in the caldera, and is perhaps the reason why the remainder is rectangular. Prior, the caldera could have been as large as (perhaps) 20 by 20 km. However, it is also possible the caldera has been distorted over time by movement along the Sumatran fault and was a bit smaller than that. The eruption was rhyolitic. The total volume is not well known, but is certainly in excess of 100 km3 – possibly by a lot.
52,000 BP Maninjau VEI7+
This beautiful lake is located in the Padang Highlands, Sumatra, about 300 km to the south of Toba. The lake is 16 km by 7 km, and has a volume of 100 km3. The caldera is bit larger, 20 km by 8 km. There have been two eruptions here, where the first one formed the main caldera, and the second a few thousand years later enlarged the southern part of the caldera. The first eruption is the major one: it left its deposits over an area of 8500 km². The eruption produced a volume of 220–250 km³.
The caldera formed over a fairly short period of time. There were a series of closely spaced pyroclastic flows and surge deposits, followed by three high volume lava flows going both east and west.
74,000 BP Toba VEI8
It is too large to comprehend. The lake is 100 by 30 km in size, with a surface area of 1100 km2. The volume of water is a staggering 240 km3 which makes it larger than the Dead Sea. The eruption that caused this lake was equally staggering, the largest known on Earth in the last 2 million years. The volcano that was responsible had had several big eruptions before, but the final one blew everything away. It erupted 2800 km3 although much of this was not explosive. The lava flows reached both oceans! The flows and ejecta cover an area of more than 20,000 km2 with a typical thickness of 50 meter. The explosive phase, producing ash, lasted around one week and produced 800 km3. That is 100 km3 per day! The effusive phase may have lasted longer. It is hard to believe that anything in this part of Indonesia was left alive.
Eruptions like Toba are very rare, and they may only occur once per million year or longer. It is perhaps a midway point between a major eruption and a flood basalt. No worries.
Huge eruptions, of the size of Maninjau or similar, seem to happen here perhaps every 20 thousand years in Indonesia. The ones we know about are all on Sumatra: that island seems particularly prone to caldera formation. Smaller eruptions, say the common-garden VEI7 variety, happen here as often as once every 500 to 1000 years. They tend to occur further east, towards Lombok. VEI6’s, like Krakatoa, can probably occur anywhere and anytime. They happen in Indonesia once every few hundred years at least and perhaps more often.
The lakes left by the largest caldera forming eruptions can last a very long time. These eruptions destroy the magma chambers that feed them and no new volcano will form: there may be rebound which fills in the central part of the caldera, but this is not volcanic in itself and new volcanoes will more likely form further along. The longevity means that we may know most of these calderas. No other Toba-sized eruption has happened on Sumatra over the past million years: we would have known.
But smaller eruptions, say Tambora-sized, appear not to leave lasting scars, or we would see them everywhere. The magma chambers survive these blasts, and over time a new volcano grows and fill the caldera. This may happen within a few thousand years. The new mountain may not erupt as massively as its progenitor: there is no guarantee that VEI7’s put in repeat performance. But in Indonesia, anything is possible.
Albert, Nov 2017