The Indonesian archipelago is dotted with 145 active volcanoes, more than half of which have erupted since human records began. Most of these volcanoes belong to the Sunda arc, a volcanic island chain formed by the subduction of the Indo-Australian plate beneath the Eurasian plate which underlies Indonesia. In the heart of the Sunda arc, about 30 km north of the city of Yogyakarta, lies the majestic peak of Merapi volcano soaring to an altitude of nearly 3000m.
Merapi, literally meaning ‘Fire Mountain’ in the local language, is arguably Indonesia’s most dangerous volcano, with a history of deadly eruptions. The volcano is frequently active, with eruptive episodes occurring every few years posing a threat to more than 1 million people living on the slopes of volcano. Over the last century, eruptions at Merapi have typically consisted of the gradual and effusive growth of a sticky plug of lava called a ‘lava dome’ on the summit of the volcano, which eventually collapses gravitationally when it grows too large, generating pyroclastic flows. This type of activity has occurred frequently in past years, including recently in 1992, 1994, 1997-98, 2001-02 and 2006, usually lasting for a few weeks or months each time. In fact, this type of pyroclastic flow has become so synonymous with Merapi in recent decades, that they have been termed ‘Merapi-style’ pyroclastic flows. Most recent eruptions have been classed with a volcanic explosivity index (VEI) of 2 or 3, on an open-ended scale, with the largest ever volcanic eruptions in worldwide history assigned a VEI of 8.
On 26 October 2010, Merapi violently erupted spewing flows of hot rock and gas (pyroclastic surges and pyroclastic flows) kilometres away from the summit and devastating the surrounding area. Further explosions continued daily, for approximately two weeks, before activity started to decrease in the middle of November. At the peak of activity on 5 November, pyroclastic flows travelled 16 km (about 10 miles) from the summit, destroying everything in their path. In stark contrast to other recent eruptions at Merapi, no lava dome grew before the eruption and there was little warning time before the first high-energy explosion tore across the southern flanks.
During the 2010 eruptive episode (VEI 4) more than 300 people were killed, making this most recent eruption the greatest volcanic disaster at Merapi in 80 years. Over 300,000 people were evacuated from their homes within a 20 km radius of the volcano and moved to temporary shelters in safer areas away from the fiery reaches of the volcano. Thanks to the detailed geological monitoring and timely warnings by the Indonesian Centre of Volcanology and Geological Hazard Mitigation and the resulting rapid evacuations, it is estimated that 10,000 – 20,000 lives were saved.
Merapi is a complex volcanic edifice, with a long history of eruptions dating back at least 170,000 years. The currently active part of the complex is a younger cone referred to as New Merapi, which is only a few thousand years old. This has gradually grown on top of the remnants of an older volcanic edifice (Old Merapi), which was destroyed by one, or possibly several, large Mount St. Helens-style collapses, forming the horseshoe-shaped crater that can be seen today high on the eastern flanks of the volcano. In contrast to the mainly effusive dome-forming activity of the 20th Century, rocks preserved in the geological record shows that during the Holocene different types of volcanic activity were commonplace at Merapi. Larger explosive (sub-Plinian) eruptions with a VEI 3 or 4 were common, as well as pyroclastic flows formed by the collapse of eruptive columns or fountains of hot gas and rock debris.
So what does the future hold for Merapi and the people living on its hazardous slopes? Will future eruptions consist of relatively benign lava dome growth, will they be marked by sudden explosions like in 2010, or will they be on an even larger scale such as those evidenced in the geological record? Scientists face a challenge to unravel the driving forces behind Merapi’s activity. Past eruptions hold the key to future eruptive style, so unlocking the secrets of what lies behind Merapi’s activity will help volcanologists to prevent further catastrophes occurring.