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Volcanic Eruptions in Bali

Why are Volcanoes Dangerous?

Volcanic eruptions can be very dangerous in many different ways. Hot magma, rocks ash and gasses are thrown out which can cause lava flows, very dangerous avalanches of hot gasses known as pyroclastic flows, floods of slurry known as Lahars, ash clouds that can rise kilometres into the air and blow large distances around the world, ash fall that can bury cars and acid rain that can take the paint off you car. These devastating results of an eruption can kill hundreds of people, displace many more and may create health problems and destroy property and livestock of many more. Here we look at the different risks from volcanic eruptions and precautions you can take to avoid or minimise the effects

See also:

Dangers From Volcanoes

Volcanoes in Bali can be dangerous

Indonesia has 126 volcanoes which is more than anywhere else in the world. Many of these are active to varying degrees.

Here in Bali we have 2 active volcanoes. In the East is Mount Agung which, at 3,000 metres, is the largest mountain on the island, while it has been dormant for many years and last erupted in 1963 there is an ever present danger of eruption. A smaller volcano, Mount Batur, is situated in the centre of the island, it is relatively quiet though it has had ongoing low level activity for many years.

The 1963 eruption of Mount Agung was a violent one. Minor eruptions started on 24th February and continued for over a month releasing magma which flowed up to 7 kilometres from the crater. A violent eruption took place on March 17th which threw debris 8 to 10 kilometres into the air. Deadly Pyroclastic flows devastated villages killing an estimated 1,100 people. Heavy rainfall formed cold lahars which killed a further 200 people. Further pyroclastic flows from a second violent eruption on May 16th killed a further 200 people. The mountain calmed down again though minor eruptions carried on for almost a year.

How Do Vocanoes Erupt?

Volcaoes erupt when there is a build up of pressure in magama (molten rock) beneath which forces its way to the surface. How volcanoes erupt can vary considerably depending on physical conditions such as how much pressure has built up before an eruption takes place, how tightly sealed are the lava vents, the chemical composition of magma and the presence of water. Some volcanoes can be quite docile and may erupt in a fairly benign way but very often when volcanoes erupt in a series of explosions of explosions that can throw rocks ash and magma high into the sky an causes magma to flow down the sides of the volcano.

During eruptions explosions can occur and may be caused by:
1. Water coming into contact with hot magma or rock and producing superheated steam, this is a known as a phreatic eruption or
2. A sudden release of gasses dissolved in the magma as the magma reaches the surface and pressure on the magma is released, this is a magmatic eruption.

This rapid expansion of steam or gasses is the driving force behind the explosive nature of violent volcanic eruptions which can throw rock and debris great distances from the volcano and can pulverise rock to form volcanic ash which can be driven very high into the sky.

Incomprehensibly large forces are involved, the largest observed explosion in recorded history was the eruption of Mount Tambora on Sumbawa Island in April 1815, an explosion so huge that it blew the top third off the top of the 4,300 metre mountain and throwing out 160 cubic kilometres of rock and debris forming a plume more than 25 kilometres high. The explosion was heard 2,600 kilometres away and the ash fell as far as 1,300 kilometres away. The ash cloud produced circled the globe and caused crop failures in North America and Europe that lead to the worst famine of the 19th century. 11,000 to 12,000 people were killed by the eruption and a further 60,000 were killed by starvation and disease.

A much larger eruption occurred around 75,000 years ago when a volcano exploded in Northern Sumatra throwing out 2,800 cubic kilometres of magma and rock, of which 800 cubic kilometres was volcanic ash, and formed the large caldera in which Lake Toba now sits.

There are a number of dangerous results that can be caused by an eruption and each as its own peculiar risks attached to it.

Falling Rocks and Magma

The most obvious immediate danger from a sudden violent eruption is from falling rocks and magma, when we are talking about such enormous forces at play we can understand that very large boulders and molten rock can be thrown many kilometres away from the crater. Falling eruption debris can bring down buildings and other structures and can also set fire to anything combustible. Clearly such debris will very likely be fatal if it hits a person directly.


  • Generally speaking as long as we keep away from designated danger zones it is unlikely that falling rocks and magma will be a danger although we should keep in mind that it is impossible to predict how large and violent an eruption will be.

Earth Tremors

As a volcano becomes unstable pressure may build up in the surrounding area which causes earth tremors. In the build up to an eruption hundreds of earth tremors may be recorded each day, these ground movements can be measured and give vulcanologists much of the information they need to predict when an eruption is likely to occur.

Earth tremors are, of course, earthquakes and can result in damage, injury and death to anyone in the area. Building and wall collapse are the main causes of death from earthquakes.


  • Once again keep away from danger zones.
  • If you have to go into the area (perhaps to help people evacuate) try and keep away from buildings and other structures that could fall.

Pyroclastic Flows

Dangers from volcanoes include a pyroclastic flowPyroclastic Flows

Probably the most deadly danger from an erupting volcano in terms of the number of people they kill are pyroclastic flows. Pyroclastic flows occur after violent volcanic eruptions cause avalanches - very powerful surges of hot gasses carrying rocks, ash, pumice and other debris that race down the sides of the volcano.

Pyroclastic flows tend to sweep down steep sided gulleys and valleys, they can travel at speeds of up to 700 kilometres an hour and the hot gasses can be in excess of 1,000 degrees centigrade. Typically they can have a volume of gas and volcanic materials of 1 to 10 cubic kilometres and they travel for several kilometres. They can destroy anything in their path, you cannot outrun them and the extreme temperatures mean instant death. As they reach more level ground on the lower slopes of a mountain pyroclastic flows decelerate and lose energy leaving piles of rocks, boulders and ash.


  • Keep out of the danger zones when a volcano is erupting. Do not linger in valleys.


Lahars are also dangerous in terms of fatalities. A lahar is formed when water mixes with ash, and other volcanic debris to form a slurry which flows down from the mountain carrying rocks and debris with it. The slurry looks and behaves like very wet cement which acts as a lubricant and is capable of carrying very large boulders.

Dangers from volcanoes include a laharA Lahar

Lahars can be hot or cold. They have a tendency to pick up speed as they descend and become larger as they collect rocks, soil, vegetation, trees, buildings and anything else in their path particularly on steeply sloping ground. It is typical for lahars to become many times larger than their original size and to travel at speeds of over 200 kilometres an hour. They tend to follow established water courses flowing down gulleys and valleys on a mountainside. As they reach flatter ground they slow down but, depending on how steeply sloping the ground is, lahars can flow for many kilometres from the foot of a mountain and are capable of wiping out whole villages in their path which is why they are often responsible for large numbers of fatalities in volcanic eruptions.

It should be noted that lahars can occur at any time, they don?t necessarily occur when there is an eruption. Heavy or protracted rainfall can accumulate and mix with volcanic debris lying on the unstable upper slopes of a volcano, eventually something gives, perhaps a small landslide or an earth tremor which can then trigger the start of a lahar.


  • Do not linger in valleys or watercourses that lead from volcanoes even if you are many kilometres away.

Lava Flows

The driving force behind volcanoes is magma, the molten rock beneath the earth?s crust. Many volcanoes exude magma when they erupt, once above ground magma is known as lava which may pool in the volcano?s crater or run down the sides of the mountain in a lava flow. Lava is hot, very hot, usually between 700 and 1,300 degrees centigrade. As it cools it forms igneous rock such as basalt or granite. The chemical composition of the lava can vary considerably and can affect the way the eruption occurs, how it cools and how hard it is once cool. The composition of the lava, how much heat it has retained, the rate of supply and the slope of the ground will determine how quickly it flows.


  • Obviously we need to keep away from hot, flowing lava. It may appear to be predictable but it can be very dangerous.
  • Don?t walk on it.

Toxic gasses and Acid Rain

Volcanic eruptions release huge volumes of toxic gas which can provide serious health hazards and produce acid rain.

Carbon Dioxide may be released which, when it is cold, is heavier than air and may collect in low lying areas or depressions in the ground. It is colourless and odourless and, with a concentration of only 3% in the air, you start to feel symptoms of headaches, dizziness, increased heart rate and difficulty breathing, a concentration of 15% leads to unconsciousness and death.

Sulfur Dioxide is colourless, it has strong smell and irritates the skin, eyes and respiratory system. It can dissolve in rainwater to cause acid rain.

Hydrogen Sulphide is known for its bad egg smell. In concentrations of more than 0.01% it becomes odourless and highly toxic leading to irritation of the respiratory system and the risk of pulmonary edema. Concentration of over 500 parts per million can result in unconsciousness in 5 minutes and death in an hour.

Hydrogen Halides can also dissolve in water to produce acid rain. They can also coat ash particles which can then poison water supplies, land and crops.


  • Do not breath in gasses from volcanic eruptions.
  • Protect water supplies and swimming pools from falling ash and acid rain.
  • Protect your car, motor cycle or other metallic objects from falling ash and acid rain.

Volcanic Ash

Volcanic ash from a volcanic eruption will affect more people than any other factor.

The eruption of Tambora in April 1815 threw enormous amounts of ash high into the atmosphere, held aloft and blown around the world by the jetstream the ash made the skies dark around the world resulting in lowering of earth?s temperatures, extreme weather events, worldwide crop failures and famine. The ash affected millions of people.

What is Volcanic Ash?

Under normal circumstances when a volcano erupts there are a series of ongoing explosions caused by both the rapid production of steam and the rapid release of gas in the magma as it comes to the surface. These explosions shatter the rock and magma into tiny particles, technically speaking volcanic ash refers to particles smaller than 2mm though most people include much large particles. Particles can be very small and can be carried on the wind for great distance.

These are particles of volcanic rock which can be very hard. Being created in explosions they are also usually very angular with sharp edges and corners. As a result Volcanic ash can be very abrasive.

To read more about volcanic ash see:

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