Indonesia on the Ring of Fire Harmonious Coexistence of Man and Volcano

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Mount Semeru
Bird’s eye view of Curah Kobokan hamlet in Pronojiwo village, Lumajang, East Java, Wednesday (8/12). The hamlet was one of the worst affected by hot cloud avalanche caused by Mount Semeru eruption. It is also one of the closest settlements to the volcano. Photo: ZABUR KARURU/ANTARA

IO – Geology of Indonesia
The geological situation of Indonesia is unique. It is the meeting place of three major active tectonic plates in the world: the Indo-Australian Plate, which is moving northward, the Pacific Plate, which is moving westward, and the relatively stable Eurasian Plate, which is moving southward (stretching from Sumatra, Kalimantan, Java, Bali to Nusa Tenggara and further). As a result, rich resources of oil, gas, coal, metals, and non-metallic minerals are accessible. In addition, Indonesia contains 127 active volcanoes, accounting for 13% of the world’s total. Indonesia’s outstanding geographic features are due to its geology, which includes majestic mountains, deep valleys, and active faults on both the land and seafloor. The same restless, unavoidable fault movement, however, is also responsible for earthquakes, which, if the epicenter is on the seabed and has a magnitude greater than or equal to 6 on the Richter scale, may generate a tsunami, endangering roughly 46% of Indonesia’s coastal regions. Landslides are a constant danger in many locations, especially during the monsoon season, because many mountains have moderate to severe slopes.

Volcanoes in Indonesia
Magma is formed by the slow-motion collision of the Eurasian Plate and the Pacific Plate off the west coast of Sumatra, south of Java, Bali, and Nusa Tenggara, as well as the interaction between the Indo-Australian Plate (oceanic plate) and the Eurasian Plate (continental plate) in northern Sulawesi and Halmahera. A subduction zone pressures molten rock upward to the earth’s surface when one plate is forced beneath another. This is how volcanoes are formed. They are cracks or fractures in the earth’s surface that allow magma, gas, or other fluids to escape. The material ejected by the volcano determines the type of volcanic eruption. First, magmatic explosions can be explosive or effusive, depending on the viscosity and gas content of the magma ejected from the volcano’s belly. Second, phreatic (steam-blast) eruptions occur when groundwater meets magma and produces steam. In Indonesia, many volcano eruptions are magmatic, phreatic, or a combination of the two.

The materials thrown up in the eruption gradually form a stratovolcano, which is a cone-shaped physical structure. This type of volcano makes up the majority of Indonesia’s volcanoes. ( Figure 1)

active volcanoes map
Figure 1. Indonesia’s active volcanoes are distributed across the country

The chemical composition of magma determines its classifcation. The majority in Indonesia are composed of SiO2 55-65 wt% (basaltic-acidic) with a temperature of 800-100°C, intermediate gas content, and a tendency toward explosive eruptions with hot phreatic eruption clouds. Several Indonesian volcanoes exhibit Merapi, Plinian, Strombolian, and other characteristics based on the form and behavior of eruptions. There are also effusive molten rock eruptions that build a lava dome that can trigger avalanches of lethal hot ash clouds (which occurred on Mount Semeru on December 4, 2021). With a Volcanic Explosivity Index (VEI) of 7 — the highest indexed level — the 1815 eruption of Mount Tambora on the island of Sumbawa was recorded as the most violent volcanic explosion in modern human history. Its volcanic ash blanketed the skies over the world, causing “the Year Without a Summer” in Europe in 1816, destroying crops, causing starvation, and resulting in the world’s highest death toll.

A curse and a blessing
Volcanoes can pose major risks on land and in the sky when they erupt, but they also give many benefits to those who live in the surrounding communities.

Of course, the immediate threat is catastrophic loss of life and property, as well as environmental damage in the “risk zones,” which are often densely populated in Indonesia. Volcanic eruptions provide both direct (primary) and indirect (secondary) risks in general.

Hot clouds spewed by explosive eruptions, in the form of an avalanche of gas-rich suspension of volcanic rock, gravel, ash, sand, and hot gas from the magma chamber into the valleys/rivers, which can travel at speeds of more than 200 kilometers per hour over distances of tens of kilometers, with temperatures up to 4,000 degrees Celsius, are direct (primary) hazards. The hot clouds are extremely hazardous to human life and pollute the environment. (Figure 2).

Mount Merapi in 2010
Figure 2. Environment damage caused by fows from Mount Merapi in 2010 (before the eruption)
Mount Merapi in 2010
Figure 2. Environment damage caused by fows from Mount Merapi in 2010 (the aftermath)


Lahar is a mudflow composed of pyroclastic material, stony debris, and water that forms on volcano slopes as a result of volcanic eruptions. Deep crater lakes can be found on several volcanoes, including Mount Kelut and Mount Ijen in East Java and Mount Dempo in South Sumatra. The pH of the water is generally near 0, making it acidic.

A gas-rich lava dome will form when molten, viscous lava collects inside or on the crater’s exterior. Volcanic gases, particularly CO2, are colorless and odorless and can cause asphyxiation, organ damage, and even death if inhaled.

Volcanic eruptions release bursts of incandescent materials and fine volcanic ash, which can suffocate and stunt crops and the environment, as well as cause significant respiratory infection when inhaled. The abrasive nature of volcanic ash can cause damage to airplanes, fouling jet engines and empeding thrust when it is blasted high into the sky. In 1982, a British Airways Boeing 747 flying from London to Auckland few into a cloud of fine volcanic ash thrown up by Mount Galunggung’s eruption, causing all four engines to fail and nearly resulting in a fatal crash. Following this terrifying event, volcanologists established “Volcanic Ash Advisory Centers” (VAAC) in a number of nations throughout the world. They’re in charge of coordinating and sharing information regarding the emergence of potentially dangerous volcanic ash clouds in the atmosphere, which could endanger aircrafts. (Figure 3).

The nine VAACs
Figure 3. The nine VAACs across the world, as well as the territories they monitor.

Lahar flow is an example of an indirect (secondary) danger. “Lahar” is an Indonesian term that has gained international recognition. Rainwater mixed with volcanic deposits such as ash, sand, gravel, and rock will rapidly flow down rivers/valleys on volcanic slopes after rains. Because the majority of Indonesia’s volcanoes are stratovolcanoes, lahar flows can reach speeds of more than fifty kilometers per hour. If it forms from hot cloud deposits, it can be extremely hot, posing a threat to human life while wreaking havoc on infrastructure such as bridges, settlements, and the natural environment.

A lava dome-generated hot cloud avalanche can travel at speeds of several hundred kilometers per hour. Although it is not as hot as a hot cloud eruption, it is nevertheless exceedingly hazardous to people’s lives and the environment. The temperature can reach up to 3,000 degrees Celsius.

Volcanoes are a blessing in disguise in the long run. They provide abundant mineral, fertilizer, sand, and rock deposits for infrastructure development on the area. They may help to drive local economies, particularly in type C mining (sand, andesite, soil, gravel, and pebble, for example), by giving jobs for a large number of people. The ash and fine sand in the area make the ground exceedingly productive, making it suitable for agriculture. It’s no surprise that agricultural products from locations around active volcanoes in Indonesia are outstanding, such as Salak Pondoh in Magelang and Muntilan (around Mount Merapi), Medan oranges from Karo regency (around Mount Sinabung), and Malang apples (around Mount Kelut, Mount Semeru and Mount Bromo).

Volcanoes also create spectacular landscapes, such as in Puncak, West Java (Mount Gede and Mount Salak), Berastagi and Sibolangit (Mount Sinabung), Kaliurang (Mount Merapi), and Bali (Mount Merapi) (Mount Agung and Mount Batur). Some cultures also revere Volcanoes as sacred sites and the home of gods.

Natural materials spouted from the volcanoes are also known to have excellent water absorbing property, giving rise too many hot springs around them. Active volcanoes are also a crucial source of water for several major rivers in Indonesia. As a result, the relationship between volcanoes and the Indonesian people is a very harmonious one. Most of us recall drawing twin mountains, the sun, a paddy field, and the grandeur of nature around it when we were children. Natural beauty, fertile land, sufficient water, and means of livelihood all contribute to the appeal of places near volcanoes for settlement. Around four million Indonesians are thought to reside in the vicinity of volcanoes. Of course, this increases the likelihood of calamity in the event of an eruption.

Volcanoes are also a source of renewable energy. Indonesia has roughly 23,965 possible geothermal sites for a 5 Mwe power facility, with 127 active volcanoes. The country has the world’s greatest geothermal reservoir, estimated to be approximately 40%.

Volcanic disaster mitigation
A sequence of actions aimed at lowering the risk posed by volcanic eruptions is known as volcanic catastrophe mitigation. The Center for Volcanology and Geological Hazard Mitigation (PVMBG), which is part
of the Energy and Mineral Resources (ESDM) Ministry, is in charge of
this initiative in Indonesia.

The first stage of mitigation entails extensive research on the features of volcanoes based on magma composition, which can throw light on the behavior, morphology, distribution, and types of historical eruption materials, as well as the creation of a disaster-prone zone map (KRB). This gives you a graphic representation of which areas are safe and which are risky for human settlement and activity. (Figure 4).

The map of disaster-prone zone (KRB)
Figure 4. The map of disaster-prone zone (KRB) around Mount Semeru (Directorate of Vulcanology, 1996)

KRB for volcanoes are divided into 3 zones: KRB III (zones potentially vulnerable to hot clouds, expelled incandescent materials, lava bombs, toxic gases, volcanic ash fall), KRB II (zones vulnerable to the expansion of a hot cloud avalanche, expelled incandescent material, ash fall, and toxic gases), and KRB I (zones vulnerable to lahar foods).

The results of the study are also used to determine the strategies for continuous observation of volcanic activity. These can be carried out visually and instrumentally. Instrumental observations record changes in the energy of volcanic activity. Seismic observations record data on changes in kinetic energy associated with the movement of fluids (magma, steam or gas or all three combined) in the volcano’s chamber as volcanic tremors. If there is an increase in the frequency and seismic energy, it indicates an increase in volcanic activity that can potentially lead to an eruption.

The elastic potential energy of a volcanic belly is studied by surface deformation, as evidenced by the distension (uplift) of the volcanic edifice generated by fluid movement. Temperature readings from a variety of volcanic heat sources represent variations in thermal energy in the volcano’s core. The chemical composition of gas and water can also be used to determine the rise in magma volume near the surface.

Visual observations are also made to see changes in the volcanic environment that could not be observed instrumentally, such as the changes in the height and color of fumes, vegetation around the peak, solfatara penetration in the crater and so forth.

Volcanic observations are used to provide an early warning of volcanic activity so that communities are aware of and can be more prepared in the event of imminent eruptions that can put lives in danger. Early warnings are not used to predict when exactly the eruption will happen or how big it will be – only when communities need to take precautionary measures. The volcanoes will run their natural course.

Dr. Ir. Surono DEA IPU

Volcanic early warning systems consist of 4 levels: Level 1 (Normal), Level 2 (Advisory), Level 3 (Watch), Level 4 (Warning). Level 4 can be followed by an eruption, or no eruption, depending on the observed volcanic activities.

Volcanic hazard mitigation also involves educating the communities living around active volcanoes to raise their safety awareness and preparedness in learning about the characteristics of the volcanoes, potential hazards and procedures for evacuation. Emergency response, cooperation between the communities and all stakeholders are key to success in volcanic disaster management.

Volcanic disaster mitigation is currently focused on pre-eruption through “pentahelix” collaboration between experts, media, community, central and regional government, and the private sector to reduce the negative impact of eruptions. Harmonious coexistence Indonesia, which is part of the “Ring of Fire,” boasts the world’s the largest number of volcanoes. This provides it with an abundant supply of energy (geothermal and micro-hydro), food (fertile soil), and water, not to mention volcanic tourism. Therefore, it’s only natural that Indonesian people “make peace” with the volcanoes and live in harmony with them. Let’s compare this to a host-guest relation. We are the guests, and the volcanoes are the hosts. We must respect each other’s space in order to live in peace. People should avoid living near KRB III zones because hot clouds traveling at high speeds can represent a major threat to lives in the case of eruptions. This area should be clear of settlements and activities around it must ceased if there are signs of eruption. KRB II zones are habitable but careful and comprehensive disaster risk assessment must be carried out. To coexist with volcanoes, spatial planning based on holistic community protection from the risk analysis of volcanic eruptions based on KRB zones is the key.

Volcanoes have existed since the beginning of time. As such, we must respect them as guests. KRB is similar to the rules that guests must observe in order to stay out of harm’s way. This must be stated clearly and enforced consistently. Otherwise, it’s a disaster in the making, and we have only ourselves to blame.