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expert reaction to the eruption of Mount Merapi in Indonesia

Many people died when an Indonesian volcano, Mount Merapi, had a series of eruptions.


Dr Dave McGarvie, Volcanologist, Open University, said:

“”The Merapi eruption is unlikely to have an effect on Krakatau. This is because there is no close physical connection between the two volcanoes. Were they side-by-side then there might be, but they are not. So separate plumbing systems feed the magmas that erupts at each of these volcanoes, and therefore what happens at one is completely independent of what happens at the other.

“These volcanoes are part of a volcanic arc, which is the large-scale plate tectonic structure produced in this region (fuelled by escape of the Earth’s heat). These volcanoes (and all the other c.130 volcanoes on this arc) are the surface channels for the escape of this heat, but because this heat is channelled into discrete and isolated areas – we get individual volcanoes instead of a continuous long crack in the crust gently weeping lava.

“On a separate note Krakatau is very unlikely to do another ‘big’ Krakatau (as per 1883), for the same reason that Mount St Helens is unlikely to do another ‘big’ 1980 eruption – during these big eruptions both volcanoes lost the material that was essential to making both of these eruptions so big.”

Is Merapi a volcano that regularly produces medium-sized eruptions or can it generate a ‘big’ one?
“In its current stage of evolution Merapi is going through a cycle of dome growth and dome collapse, which generates small to medium sized eruptions. When domes collapse on steep slopes the hot lava within dome interiors becomes exposed and it fragments as it tumbles, producing pyroclastic flows. Collapse of eruption columns also generates pyroclastic flows. Pyroclastic flows are the killers. (What we are seeing at Merapi is broadly similar to what happened at Montserrat.) For a ‘big’ one to happen there would have to be either a substantial build-up of magma within the volcano and its plumbing system that could be released quickly. This could happen if the amount of material ‘buffering’ the eruption in the high level conduits was removed. Another way of doing this is for part of the volcano to collapse, which is what happened at Mount St Helens in 1980. There are indications (from the Indonesians monitoring Merapi) that there is a larger build-up of pressure within the volcano, leading to fears that there could be a big eruption.”

Can Merapi ‘do a Krakatoa’?
“Merapi cannot do a Krakatau simply because there is a lack of water around. (Krakatau is/was a volcanic island surrounded by seawater.)”

Are volcanic eruptions on the increase? And if so, why?
“There is clear evidence that Merapi’s eruptions have tended to become more explosive with time (and by this we mean thousands of years). Some would argue that with global warming putting more water into the oceans (through melting of land-based ice) that this will suppress volcanic eruptions. The converse of this is that if you remove ice from volcanic areas such as Iceland you may encourage more volcanic eruptions!”

Are we expecting more eruptions?
“At Merapi we can expect more eruptions whilst magma is still moving from depth into the volcano itself. This magma movement can (and will be) detected by monitoring – for example seismic energy associated with magma movement, ground deformation as magma pushed its way into the plumbing system beneath the volcano and into the volcano itself.””


Dr Colin Macpherson, Department of Earth Sciences, Durham University, said:

Could Krakatoa do another Krakatoa?
“Krakatau has been active during the past week, mainly with small explosions. The geologic record of Krakatau suggests that it will generate several thousand years of these small eruptions before another cataclysmic event like 1883.

“It is unlikely that the current activity at Krakatau is directly linked with what is happening at Merapi (other than that they both sit on the same plate margin between the Indian and Eurasian plates). There are dozens of other volcanoes between Merapi and Krakatau (and between Krakatau and the site of the recent tsunami) that are not erupting at the moment. The activity at both is a coincidence. There are frequently more than two Indonesian volcanoes erupting at any given time.”

Is Merapi a volcano that regularly produces medium-sized eruptions or can it generate a ‘big’ one?
“The recorded history is of frequent (by geological standards) but moderate to small eruptions. However, its presence in such densely populated part of the Indonesian island of Java means that Merapi could have a devastating effect on life and property. But there was no culture of keeping written records before the arrival of westerner explorers so it is not possible to say with certainty that this is how the volcano always behaves.”

Can Merapi ‘do a Krakatoa’?
“Not as such because the edifice of Merapi is entirely above water. In 1883, seawater was able to percolate down into the crust beneath Krakatau and get heated by the rising magma, before and during the eruption. The result was an extremely explosive eruption. Merapi is on land so will not experience this. However, the magma does carry its own supply of gases (including water) which is what makes it erupt ash and pyroclastic flows in such an explosive manner.”

Are volcanic eruptions on the increase? And if so, why?
“At any one time there are several volcanoes erupting at various locations around the globe. It is not clear that there is any increase in frequency at present. But there have been a couple of eruptions this year that have had a major impact on society, so it may seem that way. By volcanic standards both Eyjafjalljokull and Merapi have (so far) been fairly modest eruptions.”

Are we expecting more eruptions?
“Can’t answer that.””


Prof Dougal Jerram, Dept of Earth Sciences, Durham University, said:

Is Merapi a volcano that regularly produces medium-sized eruptions or can it generate a ‘big’ one?
“Merapi commonly has small eruptions, it also has large eruptive phases like the present one every 10- 15 years, and even larger ones on a longer timescale. Its largest recorded eruption was in 1006 but limited information is available about the actual size.”

Can Merapi ‘do a Krakatoa’?
“Not exactly as Krakatoa’s violent eruption was partly fuelled by seawater, as the island collapsed during eruption; Merapi is on the mainland so this is less likely.”

Are volcanic eruptions on the increase? And if so, why?
“Not specifically but we are increasingly able to record and monitor them and man’s increasing presence on the planet means we are becoming closer to the environments affected by them.”

Are we expecting more eruptions?
“From Merapi yes, it is a very active volcano.””


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