What happened when Mount Agung last erupted in 1963? Eruption killed more than 1,100

Mount Agung had been dormant for 120 years prior to its eruption in 1963 – it was Indonesia’s largest and most devastating eruption in the twentieth century and is estimated to have killed between 1,100 and 1,900 people. 

Not much is known about the eruption prior to this in 1843, and according to the Smithsonian Institution, there have only been two further explosive eruptions in 1808 and 1821 – although the latter eruption remains uncertain.

The devastation of the 1963 eruption was evident not only in the number of deaths and injuries caused by the ongoing eruption, but the far-reaching effects across the whole of Indonesia and the short term climatic impact. 

The 1963 eruption was one of the first witnessed volcanic eruptions to have a short lived climatic impact due to vast amounts of sulphur being injected into the higher atmosphere. Estimates of the global temperature decrease vary from between 0.1C to 0.4C. 

Thousands of people were killed in lava flows, mudslides, lahars and a deadly gas cloud in the 1963 eruption which lasted almost a year. 

Lava flowed 7km (4.5 miles) from the summit crater and ash fell as far away as Bandung and Jakarta in western Java – a distance of about 1,000km from the volcano.  

The eruption began on February 18 1963 after two days of tremors with the ejection of incandescent material and ash flows and the fall of ash, sand and lapilli-size particles.

A viscous lava flow started on February 19 down the north slope of Agung and did not end for 26 days. 

However what followed was an explosive eruption on March 17 – the first of two events – that produced a huge eruption column estimated to have reached heights of 19km to 26km (12 to 16 miles).

This explosive eruption lasted seven hours and generated deadly pyroclastic density currents (PDCs) – a fast moving current of hot gas and volcanic matter – and lahars which devastated a huge area of Bali.

Mount Agung 1963 eruption Getty

Mount Agung 1963 eruption: More than 1,100 were killed in the eruption in 1963

People had just minutes to flee, but many were incinerated when the gas cloud – which can travel hundreds of kilometres an hour – engulfed the area and further villagers were buried under mud or burned by lava. 

Cold and hot lahars (debris flow composed of pyroclastic material) rapidly formed in the torrential rainfall that followed this eruption and destroyed villages and constructions on the southern slope of Agung and reached all the way to the coast. 

Professor of Planetary Geosciences at the Open University said: “They were not expecting it to devastate such a large area, people were killed who were too close to the volcano. People who thought they were far away enough to be safe weren’t safe.”

It is estimated the majority of casualties happened during this first explosive eruption in which vast amounts of basaltic andesite (volcanic rock) fell from the sky.

“The mountain was erupting. There was a huge cloud above the mountain and then that cloud fell to the earth in lumps,” eyewitness Nengah Nese told the BBC.

“It sounded like lots of bombs going off.

“I remember pointing to the sky and asking my father what it was. I thought it was a bear, My father said it was a storm.

“The wind blew strongly and the river filled with lava.”

This deadly eruptive phase was followed by small explosions, ash showers and pyroclastic flows which continued until a second more intense explosion on May 16.

The four hour explosion produced a reported 20km ash column above the volcano summit.

This phase produced a larger volume of pyroclastic flow material compared to the March explosion, probably caused by the collapse of the ash column and led to destruction in villages at the foot of the volcano.

Following this second explosive phase, the eruption continued until January 27 1964 – almost one year after seismic activity first began – generally decreasing in intensity after the end of May 1963.

Even so, this period was punctuated with larger explosive events and produced ash columns of 3km to 4km and an eruption column was reported to have reached as high as 9km on May 31. 

During the rainy season of November 1963, lahars transported pyroclastic flows which had been choking the valley and devastated river canyons and lowlands and led to even more deaths. 

Mount Agung eruption: Getty

Mount Agung eruption: Around 100,000 people have been evacuated since the recent eruption

It is estimated during the period of Mount Agung’s eruption, 950 million m3 of material was erupted and 650 million m3 of magma was ejected from the mountain. 

It remains unclear whether the eruption we are seeing at the moment will be as large or destructive as the 1963 eruption, around 100,000 people have been evacuated from villages around the mountain.

Diana Roman, a geologist at the Carnegie Institution of Washington, told the Washington Post it was impossible to predict what Agung would do.

Mount Agung eruption: Getty

Mount Agung eruption: The recent eruption has been ongoing since September last year

“The current activity at Agung is pretty small-scale, although it is having a significant impact on local air traffic and the local population,” Roman said.

“We do not have any basis for knowing whether this eruption will intensify, continue at its current level or stop.”

This article has been put together with help from scientific papers:

  • ‘A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards’ by Fontijn et al
  • ‘Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung, Bali, Indonesia’ by S. Self and A. J. King
  • ‘The 1963–1964 eruption of Agung volcano (Bali, Indonesia)’ by Stephen Self and Michael R. Rampino