Why a Mount Agung eruption is so dangerous

DW: The volcano on Bali has erupted. What kind of volcano is Mount Agung?

Jacqueline Salzer: It's a composite volcano, also known as a stratovolcano. Such volcanos can often be found at tectonic subduction zones. In Bali, the Indo-Australian Plate is pushed below the Sunda Plate. It’s part of the Pacific Ring of Fire. In those areas, one can often find lava of high viscosity - unlike shield volcanos, for example, where the lava's more liquid. 

This means that, in the eruption zones, the volcanic lava, ash and other eruption materials pile up steeply. They form the typical conical volcanos, which can also be very high mountains. Mount Agung, after all, is the tallest mountain on Bali, reaching more than 3,140 meters (10,300 feet) high. 

How dangerous are such volcanos?

If the magma's highly viscous, the volcano can be particularly explosive. The reason is that gases are caught inside the magma, under pressure. The pressure then increases as the magma rises to the surface. But when the volcano erupts, it suddenly discharges those gases - they expand and escape. In doing so they literally tear the magma to pieces, which results in huge ash clouds. These eruptions are repeated and can continue for many days, because there's a constant source of fresh magma and gases rising to the surface and releasing energy. 

What kind of dangers are there for the inhabitants of the island?

During the last eruption, most victims were killed by pyroclastic flows. Those are hot clouds of gases, ash and rock debris that race down the flanks of the mountain. They pose the biggest threat.

It's an unbelievably fast event: The pyroclastic streams reach speeds of several hundred kilometers per hour. Nobody can escape them. You have to be far enough away from the volcano - and early enough - to be safe.

Another big danger are the "lahars." It's an Indonesian word, and it means a stream of mud that develops when the fluffy materials from the volcanic eruption get mixed with rainwater. There's a lot of rain in Indonesia. These lahars flow down through riverbeds and valleys and carry huge pieces of rock and debris with them. They can tear down bridges and houses and can even cover entire villages. That's the second big danger from the volcano.

In addition, there are the clouds of ash that can rise more than 20 kilometers high. The heavy ash can then fall down on the settlements. Another threat are the hot streams of liquid lava.

How far should one move away from such a volcano? Is it better to leave the island?

There are simulations for many dangerous volcanos, for the impact of an eruption. The civil defense authorities perform such calculations, but unfortunately they can never be 100 percent certain.

In the case of Mount Agung, they've evacuated an area of twelve kilometers around the volcano. More than 80,000 people have had to leave their homes so far.

Lava flows can be expected in an area within several kilometers of the volcano. Pyroclastic streams can reach much further - sometimes dozens of kilometers. In Mount Augung's last eruption, the lava flows traveled seven kilometers and the pyroclastic flows up to ten kilometers. 

The ash cloud can reach considerably further. This will then affect air travel, and it also makes breathing difficult. Here in Germany, for example, we were able to detect traces of volcanic eruptions that occurred in Iceland.

There are also big pieces of rock flying around - the so-called "bombs." How big can they get?

It depends on the eruption. During the last eruption of Mount Agung, they were "head-sized."  Now, the authorities have also warned of rocks in the range of tens of centimeters. But there have been cases in which the bombs were several cubic meters in size and weighed several tons. It all depends on the intensity of the eruption and on the distance to the crater. In principle we can say: The farther away from the crater, the smaller the bombs.

The last great eruption of the volcano was in 1963/64. That was roughly half a century ago. Is there a specific rhythm in which such eruptions occur?

You can't really define a rule for it. We know that there was a big eruption of Mount Agung in 1843 as well, but we know very little about what exactly happened.

There are some volcanos that seem to follow certain cycles, but they are more the exception than the rule. Under no circumstances should one rely on a prognosis based on such factors, or try to predict the time of the next eruption based on such models.

Also, one should keep in mind that, in the case of volcanos that erupt more frequently, people are more used to dealing with the situation. This is the case with Mount Etna in Sicily. People there have experienced it before. In Bali, most people can not remember the last eruption.

Mount Manaro on Vanuatu ist another volcano located along the Pacific Ring of Fire. It has erupted at the same time when Mount Agung showed its first signs of activity in late September. Authorities there evacuated 7,000 of a total island population of 10,000. Do the two eruptions have anything to do with one another? 

Vanuatu is 5,000 kilometers away from Bali. It's a different tectonic plate boundary, and even the composition of the rock is different. The lava on Vanuatu is considerably more liquid, forming literal lava lakes. One can hardly compare the two volcanos. That this is happening at the same time is a coincidence.

But that does not mean that the volcano in Vanuatu is any less dangerous. There's a fresh water lake in the crater of Mount Manaro. The interaction of large amounts of water with hot lava poses additional risks. It could lead to explosions, and the water from the lake could also result in Lahars. That eruption will develop in a different way than on Bali. But regardless, the dangers there also include volcanic ashes, gases, bombs, lava and pyroclastic flows. 

Dr. Jacqueline Salzer works at the German Research Centre for Geosciences in the field of earthquake and volcanic physics. In particular, the geophysicist monitors active volcanos via cameras and radar data from the Earth observation satellite TerraSAR-X. The main focus of her work is learning about changes in the geometry of volcanos before, during and after an eruption.

This interview was conducted by Fabian Schmidt.