EXPERT REACTION: Mt Agung continues to erupt

As with other stratovolcanoes, the course of the current eruption of Mt Agung stratovolcano on Bali, Indonesia, is unpredictable.

The two initial eruptions last week were small, probably phreatic explosions, triggered by the rise of molten rock or magma high into the volcano where it interacted with groundwater and explosively opened the vent, ejecting steam and rock debris.

The eruption that started on 26th November, involved the explosive ejection of fragmented magma, rock debris and volcanic gas that drove a column of this material several kilometres into the atmosphere.

The volcanic ash is being dispersed by atmospheric winds southeastwards, posing a hazard for aircraft, resulting in closure of airports on Bali and Lombok. The fine ash can not only clog the jet engines of aircraft, but poses respiratory health problems to the population.

It can also contaminate water supplies and cause other machinery to malfunction, including power generators.

The volcanic column of ash, rock debris and gas can become overloaded with rock debris and ash, collapse and generate very hot, dynamic pyroclastic flows which are capable of flowing in excess of 10 kms from the vent area at speeds of tens to over 100 km/hour. These can be hot enough the ignite vegetation and man-made buildings.

With a lot of loose ash on the ground, when it rains, the ash can be mobilised as volcanic mudflows (called lahars in Indonesian), that can also flow tens of kilometres down major valleys. These can be very hazardous and fatal.

As the erupting magma loses gas, lava flows may begin to flow from the vent down valleys, posing another hazard.

The current eruption may last for hours or spasmodically for a few days. The eruption could then cease or be reactivated, in several days, weeks or months, even several times, as was the case during the 1963-64 eruption of Agung volcano, which produced all the hazards listed above, and killed 1,600 people. 

After more than two months of high seismic activity, the Balinese volcano Mt Agung is now erupting magma (molten rock).

Deep magma is a mixture of melt, crystals and dissolved gas. As magma rises to the surface, gas bubbles form and expand, pressurising the system and causing fragmentation of the melt into small particles – similar to opening a bottle of champagne!

This process has generated a 3 km-high cloud of hot ash and gas over Mt Agung. Volcanic ash can damage aircraft engines, so all flights to and from Bali have been cancelled on 27 November.

Further flight disruptions will depend on the course of the eruption and the direction of the wind. Authorities have increased the alert level, expanded the evacuation zone to 10 km around the crater and distributed masks to the population.

It is very difficult to predict if the eruption will continue, become bigger, or come to rest, however past eruptions of this volcano have been very explosive. The latest eruption took place more than 50 years ago, in 1963, and produced lava flows, dangerous pyroclastic density currents and lahars (destructive mudflows).

In addition, the injection of large quantities of sulphur-rich gas into the atmosphere decreased global temperatures by 0.1-0.4 degrees C in the months to year after the eruption.

The 1963 eruption had a death toll of more than 1,000 people.

Current volcano monitoring efforts have improved dramatically and currently guide hazard evaluation and emergency planning by authorities.

Monitoring activities include tracking ground movement and deformation as well as measuring the volume and composition of gas coming out of the crater. Since the magmatic eruption started, ash fall has also been collected for analysis.

All these data provide a way for volcanologists to ‘get inside’ the volcano and track the movement of magma and its eruption potential.

The Agung volcano commenced a sustained ash eruption on Saturday afternoon, with plumes reaching over 3000m high.

The volcanic eruption has now moved on to the next, more severe, magmatic eruption phase, where highly viscous lava can trap gasses under pressure, potentially leading to an explosion.

The alert status is at the maximum level (IV), and the airports in Denpasar and Lombok are closed.

From my own eye-witness experience, the situation on Bali is very calm, and life is mostly continuing as normal outside of the 8-10km exclusion zone. People have been evacuated from the exclusion zone for several months.

The local authorities are extremely experienced in managing volcanic eruptions, and have the situation extremely well in-hand.

The volcanic activity at Mt Agung has increased significantly in the past few days, and the volcano is now sporadically emitting an ash column several thousand meters high above the summit.

For the time being, the winds are taking this ash eastwards rather than towards the main airport on Bali.

The ash is fragmented magma, and is a clear indication that a fairly direct route from the underground magmatic plumbing system has opened up to the summit of Agung. The activity could continue at this level for several days.

The main concern however, for the Indonesian authorities and everyone living on the island, is that an increase in the amount of magma in motion will develop, leading to the possibility of a major failure of the volcanic edifice and a catastrophic eruption of the type that occurred at Agung in 1963-64.

After a lengthy period of earthquake activity in September and October 2017 caused by magma (molten rock) intruding at shallow levels beneath Mt Agung, the volcano had a small water-driven eruption (phreatic) on the 21 Nov. This was followed by a larger magma-driven eruption, which started on the 25 Nov and is continuing.
 
The current eruption has produced a high column of grey-black ash, which is composed of tiny, sharp, rock fragments and gas. The ash plume has reached 3,000 metres above the summit of the volcano. Exploding lava in the crater has been reported by the Indonesian National Disaster Management Authority.
 
The status of Mount Agung has been raised from standby (level 3) to alert (level 4) starting from 27/11/2017 at 06:00 WITA. The Indonesian Centre for Volcanology and Geological Hazard Mitigation has recommend a no-go zone with a radius of 8-10 km around the volcano. The situation is considered dynamic and could change at any time. People are encouraged to evacuate in an orderly and calm manner. 
 
A NASA Satellite has detected sulphur dioxide (SO2) emissions from the volcano spreading east-southeast of Agung over Lombok.
 
The most significant volcanic hazard at present is from falling volcanic ash. People are advised to wear masks and eye protection and stay indoors.
 
People traveling to/from Bail are advised to check with their airlines and tour operators as several airports in the vicinity have been closed. People are also recommended to follow the advice of local authorities and keep an eye on the Australian Government’s Smartraveller web site.

Mt Agung is part of the Pacific Ring of Fire.

These active or sometimes dormant (means they show no apparent activity for decades or centuries but can still be revived) volcanoes have a high volatile content (water, CO2, SO2), which when close to the surface are released from the melt - pretty much as bubbles out of a champagne bottle.

This will causes molten rock to burst into tiny fragments (so-called de-fragmentation), which will be carried at high velocity into the sky. We call this a volcanic ash plume.  

Dependent on weather conditions, this ash will remain in the atmosphere and can be carried with winds. If a plane flies through the ash, it will cause abrasive damage to the windscreen and more importantly, when heated up in turbine engines, can be melted again.

When melt droplets chill against ambient temperature, a fine layer of volcanic glass then stalls the engine. Hence the danger to aviation services. 

The initial activity at the volcano a few weeks ago already indicated its sub-surface activity and eminent danger.

It has to be understood that the energy of the eruption can not accurately be predicted, which obviously affects the nature of the ash plume. Hence airline precautions.

However, once the volcano has erupted, it is unlikely that a more violent eruption is following straight away, so that the local geological survey apparently sees no need to raise the danger level.