Volcanoes – Indonesia Ring of Fire

Sangeang Volcano
Active Volcano Sangeang

Indonesia is one of the most volcanically active regions on Earth. Making up the SW region of the “Pacific Ring of Fire”, there are more than 130 active or dormant volcanoes in the country. With recent active in the Canary Islands, Iceland, and Hawaii it’s a good time to look more in depth into the volcanoes of Indonesia.

Where are Indonesia Volcanoes?

The vast majority of the volcanoes are located along the Sunda Arc. Imagine a line that connects the northern part of Sumatra and runs south then east across Java, Bali, Lombok, and all the way to Flores and beyond. This is the most active area of volcanism in Indonesia and boasts around 100 active and dormant areas. The other areas with a lot of volcanoes are the Halmahera Islands and North Sulawesi/Sangihe Island chain.

Gunung Ibu volcano
Active volcano Gunung Ibu in Halmahera

How are Volcanoes Formed?

To fully understand how volcanoes form it requires knowledge of the Earths plate tectonics. The surface of the Earth is made up of dozens of large “plates” that move around the molten core of the planet. These plates move slowly but are always in contact with one another. When the edge of one plate suddenly slips or slides across the edge of another plate it creates an earthquake. Plates also move under and over each other in an action called subduction. It’s the act of subduction that has created many of the islands of Indonesia and their volcanoes.


When one plate moves under another plate, the thinner oceanic plates typically slide underneath larger, more buoyant continental plates. As the smaller plate continues to slide under the larger one it begins to melt as it moves further into the core. This sinking and melting of the plate creates hot water and magma which then floats toward the surface and melts through the plate to create volcanos on the surface. The Sunda Arc along the southern and western coasts of Indonesia is the perfect example of subduction. The volcanoes on the islands of Sumatra, Java, Bali, and Nusa Tenggara are the result of molten activity as the Indo Australian plate moves under the Eurasia plate. The following diagram shows the process of subduction at work.

Subduction forming a volcano, Public Domain from Wikicommons, user Fredrik https://tinyurl.com/2e2wjruf

North Sulawesi and Halmahera are also areas of subduction and home to similar volcanoes to the Sunda Arc ones. However, in many areas of Indonesia there are no volcanoes at all. The island of Borneo has had volcanism in the ancient past but only at the far northern end. Other large islands such as Ceram and the Indonesian half of Papua also don’t host any volcanoes. The island of Papua is an interesting case as the western (Indonesian) half of the island is free of volcanism whereas the Papua New Guinea side is home to dozens of them.

Historic Volcanoes

With such a large number of volcanoes, the frequency of eruptions is only matched by Japan. The most active volcanoes are spread throughout the country and include ongoing eruptions in Java, Sumatra, and Halmahera. Sadly, with the rich soil from ash being good for farming, many people live near active volcanoes. This has resulted in a large loss of life throughout the years as eruptions have occurred. Java, Sumatra, Bali, Lombok, and Sumbawa have all experienced catastrophic eruptions that have had large effects on the population. Two of the largest eruptions in the last 10,000 years have happened in Indonesia. In 1883, the famous island volcano Krakatau erupted violently and created a large tsunami that killed over 30,000 people.

Mt Agung in Bali, major eruption in 1963

However, the Krakatau eruption pales in comparison to Mt Tambora. Tambora is located on the western end of Sumbawa island and is the site of the largest eruption in recorded human history. In 1815, this large 4300m high mountain exploded with more than 10x the power of Krakatau. The eruption deposited more than 100 cubic kilometers of rock and blew 1500 meters off the top of the mountain. More than 50,000 people were killed either directly or indirectly by this eruption. The amount of ash it spread into the atmosphere also affected weather worldwide for the following three years. The famine created around the world by the cold weather was the worst of the 19th century.

Lake Toba – Super Volcano

Yellowstone in the United States is well known as a “super volcano”. However, it’s not the only super volcano on Earth. Lake Toba in northern Sumatra is another super volcano with the potential to create a large amount of damage. In fact, its most recent eruption 74,000 years ago was larger than any Yellowstone eruptions. Thankfully there are no recent eruptive activity at Toba and no signs of any upcoming events!

Volcanoes are interesting geological features. Unlike earthquakes, scientists can and do monitor them for activity. They are able to understand when activity rises and announce ahead of time when local populations should evacuate an area. The volcanologists of Indonesia are some of the best in the world and actively monitor many of the most active cones in the country. Due to their efforts people have been able to keep safe and there has been relatively little loss of life over the last 30 years of activity.

Crater Lake of Mt Rinjani, Lombok

Observing Volcanoes

The Samata cruises throughout the Nusa Tenggara area which is home to dozens of volcanoes. They are a dominant part of any landscape and their prominent peaks are beautiful. Along the coast of Flores it’s possible to see five or six volcanoes at a time. In the Komodo area we often anchor the boat in ancient volcanic calderas. Sangeang, Batu Tara, and Pulau Raja are all small volcanic islands where we can observe old lava flows from the comfort of the ship. The opportunity to see how Earth’s geology works in real time is an eye opening experience not to be missed.

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