A history of plates
Indonesia lies at the intersection of two major tectonic plates, the Eurasian Plate and the Indo-Australian Plate, resulting in an abundance of volcanoes. For example, Bali lies on the path of the famous Pacific Ring of Fire, where some 450 volcanoes have emerged, representing 75% of the Earth's volcanoes. This chain stretches almost linearly across the Pacific Ocean for some 40,000 km, bordering the coasts of numerous countries and islands located at the boundaries of tectonic plates and major oceanic faults, stretching from Patagonia to the Tonga Islands, via Alaska, Japan and Indonesia.
Indonesia's geographical position on the Pacific Ring of Fire is strategic. This zone is where the Pacific tectonic plate collides with several other continental plates, frequently generating earthquakes. This region is also home to many of the world's most important volcanic eruptions.
The long Sunda volcanic arc, stretching over 3,000 km from Sumatra to the Banda Sea, is home to the majority of Indonesia's volcanoes. This volcanic chain is the result of subduction (when one plate sinks beneath another advancing in the opposite direction) of the Indian plate beneath the Asian plate. Further north, tectonic complexity intensifies with several subduction zones, notably in Sulawesi-Sangihe and Halmahera, due to the convergence of various micro-plates.
The archipelago of 17,000 islands and 500 volcanoes
Indonesia is world-renowned for its high concentration of active volcanoes and for the considerable number of eruptions it has experienced over the course of history. These eruptions have often been disastrous, resulting in loss of life, damage to farmland, volcanic mudflows (lahars), tsunamis and glowing clouds. Among the most significant volcanic disasters of modern times, Indonesia has been the scene of two major events: the eruption of Tambora in 1815, which caused global climatic disruption and a "year without summer" in Europe in 1816, and the eruption of Krakatoa in 1883, the repercussions of which have remained deeply rooted in the collective memory.
Indonesia's volcanoes are characterized by their unique features, taking various forms, such as stratovolcanoes, shield volcanoes or calderas. These natural structures captivate scientific interest, while playing an essential role in daily life in Indonesia, affecting agriculture, climate and even the tourism sector.
The study of the country's volcanoes offers a fascinating insight into the earth's geological processes, and underlines the importance of understanding and monitoring these phenomena to reduce risks and protect populations. Although often destructive, volcanic eruptions in Indonesia are also a powerful reminder of the strength and majesty of nature.
Some 500 volcanoes dot Indonesia, of which almost 135 are active. Of these, 43 have experienced 184 significant eruptions in the last 1700 years, claiming the lives of over 57,200 people. The most catastrophic, in terms of casualties and damage, was that of Tambora on April 10, 1815, which alone claimed 11,000 lives. There are also 5 active underwater volcanoes along the coast.
The Kelud and Merapi volcanoes on Java are among the most active, responsible for thousands of deaths on the island. Since the year 1000, Kelud has erupted more than 30 times, with one major eruption reaching level 5 on the volcanic explosivity index, while Merapi has erupted more than 80 times.
Sumatra's volcanoes
Sumatra is characterized by the presence of the Bukit Barisan, an imposing mountain range crossing the island from north to south for some 1,700 km. This chain is the result of the displacement of the Australian plate, which is advancing at a speed of 5.5 cm per year. This tectonic movement is the source of frequent earthquakes, including the memorable one at the end of December 2004, and contributes to the formation of magma reservoirs beneath the island.
The region is home to 36 active volcanoes, all located on the island of Sumatra, with the exception of Weh, isolated on a neighbouring island to the north-west of Sumatra. This isolation is the result of a major Pleistocene eruption that submerged the plains between Weh and Sumatra. Among these volcanoes, Lake Toba stands out as the largest in the area, formed following the collapse of its caldera some 74,000 years ago. The highest point in the mountain range, and in the whole of Indonesia, is Kerinci, reaching an altitude of 3,805 metres.
Volcanoes of Java and the Sunda Strait
The Sunda Strait, which acts as a natural border between Sumatra and Java, is home to the volcanic island of Krakatau, positioned between these two large islands. Java, although smaller than Sumatra, boasts a higher density of active volcanoes. The island boasts 45 volcanoes, in addition to the 20 craters and cones that make up the Dieng volcanic complex.
On Java, Merapi stands out as one of the world's most active and dangerous volcanoes. In 2010, its intense activity led to the evacuation of 280,000 people and the death of 300 others. Merapi experienced a remarkable eruption in early 2023. Towards the end of 2023 and the beginning of 2024, Merapi once again entered an active phase. The volcano rises to an altitude of 2,885 metres, and its eruptions, characterized by lava flows, ash and gas, have a considerable impact on local air traffic and neighbouring regions, such as Bukittinggi, a town of 99,000 inhabitants located just 14 km from Merapi.
The Krakatau volcano, often mistakenly called Krakatoa in English, is located 40 km off the coast of Java, in the Sunda Strait that separates Java from Sumatra. It is one of the world's most active volcanoes. Its eruption on August 27, 1883 is considered one of the most devastating of the modern era, with a power estimated at around 13,000 times that of the Hiroshima atomic bomb. The eruption resulted in a colossal disaster: volcanic ash was propelled over 80 km into the air, reaching as far as Singapore and plunging the Strait into darkness for almost two days. A total of 20 km3 of rock was ejected from the earth. The explosion was felt at extreme distances, as far away as Perth in Australia and Rodrigues Island near Mauritius, i.e. almost 5,000 km. The ashes dispersed in the atmosphere so affected the sky that for three years, the sunsets were exceptionally spectacular, to the point of causing fire alarms in the USA, where residents mistook them for large fires. Tsunamis generated by the eruption destroyed around 200 villages with waves up to 30 metres high, circling the globe up to seven times. It is estimated that over 36,000 people perished as a result of the waves, with some experts putting the death toll at over 120,000. As a result of this catastrophe, the Earth's global temperature dropped by 1.2°C over the following five years.
Following its explosion, Krakatau was reduced to a submarine volcano. In 1927, a new volcano rose from the sea, named Anak Krakatau, meaning "the son of Krakatau". This volcano, reaching a height of almost 350 metres, continues to grow. It began an eruptive phase in 1994 and emitted large clouds of smoke in May 2009. On October 3, 2011, Anak Krakatoa released an ash cloud exceeding two kilometers in height, heralding an intensification of its activity. Towards the end of that week, the number of seismic tremors associated with the volcano reached nearly 6,000 per day. On December 22, 2018, an increase in activity at Anak Krakatoa, which began during the summer, led to an underwater landslide and triggered a tsunami in the Sunda Strait. As of December 25, 2018, the authorities had counted 429 victims and many missing on the islands of Java and Sumatra. The volcano's height has been reduced from 338 meters to 110 meters, and a two km² area of the volcanic island has collapsed into the sea. According to the Indonesian Vulcanology Agency, between 150 and 180 million m³ of rock and ash were precipitated into the ocean. Another eruptive phase began on April 10, 2020. On April 24, 2022, an eruption sent an ash cloud to an altitude of 3,000 meters.
The volcanoes of the Lesser Sunda Islands
The archipelago of the Lesser Sunda Islands comprises a series of islands, of which Bali, Lombok, Sumbawa, Flores, Sumba and Timor are the most notable. These islands lie to the north of the Australian continental shelf and together are home to 31 volcanoes.
Among these volcanoes, Lewotobi attracted attention when it erupted on December 23, 2023, and this activity continued into early 2024. The summit of Lewotobi reaches a height of 1,703 meters. The eruption released lava, ash and gas, with significant repercussions for surrounding areas and air traffic. Clouds of ash spewed from the volcano reached an altitude of 3,400 meters, underlining the scale of the eruption.
The volcanoes of the Banda Sea
Located in the south of the Moluccan archipelago, the Banda Sea encompasses a group of minor islands. This region is geologically distinctive, being the point of convergence of three major tectonic plates (Eurasian, Pacific and Indo-Australian), a phenomenon that began as early as the Mesozoic era. This particular geological zone is home to nine volcanoes.
The volcanoes of Sulawesi and the Sangihe Islands
Sulawesi is a K-shaped island made up of four distinct peninsulas. Its central region is mountainous and mainly non-volcanic in the southern part. By contrast, the island's volcanic activity is concentrated on the northern peninsula and extends northwards to the Sangihe Islands. In this area, there are 14 active volcanoes, including an as yet unnamed submarine volcano. In April 2024, Mount Ruang in the north erupted, forcing thousands of people to evacuate.
The volcanoes of Halmahera
Halmahera, an island in the north of the Moluccan archipelago, developed as a result of the interaction of three tectonic plates. This geological phenomenon gave rise to two intersecting mountain ranges, forming four distinct peninsulas separated by three deep bays. On the western side of the island, an arc of 16 volcanoes stretches from north to south, and also extends to some of the surrounding volcanic islands such as Ternate and Tidore.
Among these volcanoes, Dukono, located in the Molucca region, is particularly active. Rising to an altitude of 1,229 meters, the volcano continually emits lava, ash and gas, greatly affecting surrounding areas and disrupting air traffic. The town of Tobelo, with a population of 10,000, lies just 13 km from Dukono. During eruptions, clouds of ash can rise to an altitude of 4,000 metres.
