Nobody knows if the present-day volcanic activity at Agung Volcano, Indonesia, will abruptly turn into a catastrophic explosion, one that would sweep away the Island of Bali and change the world’s climate patterns.
The mandatory evacuation of tens of thousands of residents in the “danger zone” was a much-needed precaution, after this gigantic, 3148-m-high stratovolcano erupted at the end of November, sending plumes of volcanic ash thousands of metres into the air and triggering lethal mudflows known, in the Indonesian language, as lahars.
Why is Indonesia so prone to volcanic hazard? Well, as a matter of fact this stunning archipelago – 17500 islands, from the major ones such as Sumatra, Java, Celebes, Kalimantan, to the thousands of tiny, pristine islets that are a heaven for tourists – is threatened by all kinds of geological-related hazards, from volcanoes to earthquakes, from tsunamis to landslides and lahars.
Yes, few places in the world are as dangerous as Indonesia when it comes to natural hazards: It may be claimed that this is the place that reveals, better than any other, the inner workings of Earth, that relentlessly shape the appearance of our Planet. Here, two plates converge against each other, and, as the Indo-Australian plate descends underneath the Euroasian plate, it reaches tens of kilometers down within Earth’s mantle: Here, the rock that makes up much of the mantle, known as peridotite, undergoes melting, thus gives birth to magma, the gasoline that fuels all volcanic eruptions.
Once formed, it takes time for magma to reach the surface, and when it does, it produces volcanic eruptions that can be as quiet and beautiful to watch as Kilauea’s (Hawaii), or as violent and destructive as Agung’s latest eruption in 1963, which claimed 1148 lives.
Indonesian volcanoes are the ones that have produced, so far the most cataclismatic explosive eruptions ever recorded on Earth. Four out of 147 active volcanoes belonging to the archipelago are absolute leaders in terms of dmaage caused at the worldwide level: Toba, Samalas, Tambora, Krakatoa.
Toba is a gigantic caldera, a supervolcano that rivals Yellowstone for the sheer power of its eruptions: The blast that rocked the world 75000 years ago, sending 3000 cubic kilometers of ash and pumice into the stratosphere, changed Earth’s climate so much that, according to a much-debated hypothesis, may have reduced the world’s population to the brink of extinction.
Samalas volcano (Lombok Island, East of Bali) has been recently recognized as triggering the first “year without a summer” in human history. In 1258, there was no summer in most locations of the Northern Emisphere. Middle Age cronicles tell apocaliptic stories of chilly and rainiy summer months, that wrought havoc to agriculture and caused misery and loss of life all over Europe. The culprit was Samalas “ultra-plinian” explosion that projected 40 cubic kilometers of fragmented material into the air and affected atmospheric patterns for years.
Almost 600 years later it was the turn of Tambora, another major stratovolcano on the Island of Sumbawa, East of Lombok: In the Spring of 1815, the violence of the parossistic explosion literally decapitated the once majestic, 4300-m-high stratovolcano, to the present-day, 2800-m-high volcanic edifice. The huge amount (150 cubic kilometers) of pumice and ash spewed into the atmosphere caused the most severe climate disturbance ever produced by a volcano and leading to the infamous “Year without a Summer”.
The latest – and by no means the last – cataclismatic eruption in the Indonesian archipelago was the one that, in August 1883, caused the almost total destruction of the volcano that made up the beautiful Island of Krakatoa, in the Sunda Strait, between Sumatra and Java. When the island volcano exploded, it uttered the loudest sound ever recorded in human history: The immense blast was heard as far as Perth, Australia, 3000 km away. Two thirds of the original island were completely obliterated.
The void created in the magma chamber located into the ocean crust underneath the island, as a consequence of the emission of 18 cubic kilometers of pumice and ash, caused the whole island to implode into Sunda ocean waters. The following tsunami wiped out coastal communities along the shores of Sumatra and Java, with waves as high as 30 m that rushed onshore, reaching as far inland as 3 km and causing the death of at least 40000 people.
As is the case in all eruptive episodes, nobody is able to tell if activity at Agung will slowly come to a close, or if it will escalate in a mammoth-sized eruption similar to the worst ones that preceded it. In the latter case, we will all have to brace for the eruption’s impact on worldwide climate for months or years to come.
Federico Pasquaré Mariotto (Associate Professor, University of Insubria)
Fabio Luca Bonali (University of Milan-Bicocca)