Hotspot volcanoes

Earth’s mantle provides new clues to explain Indonesia’s explosive volcanoes

Agung, a volcano in Bali, had an explosive eruption in 2018. Credit: OL Andersen

Indonesian volcanoes are among the most dangerous in the world. Why? Through chemical analyzes of tiny minerals in lava from Bali and Java, researchers at Uppsala University and elsewhere have found new clues. They now understand better how the Earth’s mantle is composed in this particular region and how the magma changes before an eruption. The study is published in Nature Communication.

Frances Deegan, first author of the study and a researcher in the Department of Earth Sciences at Uppsala University, summarizes the findings.

“Magma forms in the mantle, and the composition of the mantle beneath Indonesia was only partially known. Better knowledge of the Earth’s mantle in this region allows us to more accurately model the chemical changes of magma as it breaks through it. the crust, which is 20 to 30 kilometers thick, before an eruption.

The composition of magma varies greatly from one geological environment to another and affects the type of volcanic eruption that occurs. The Indonesian archipelago was created by volcanism, caused by the collision of two of Earth’s continental tectonic plates. In this collision, the Indo-Australian plate is sliding under the Eurasian plate at a rate of about 7 cm per year. This process, known as subduction, can cause powerful earthquakes. The 2004 tsunami disaster, for example, was caused by movements along this particular plate boundary.

SIMS Instrument

The SIMS instrument at the Swedish Museum of Natural History in Stockholm, which the researchers used to study the oxygen isotopic composition of minerals in various lavas from Indonesia. Credit: Frances Deegan

Volcanism also appears in subduction zones. As the sinking tectonic plate descends into the mantle, it heats up and the water it contains is released, causing the surrounding rock to melt. This results in often explosive volcanoes which, over time, form arc-shaped groups of islands. Along the Sunda Arc, comprising the southern Indonesian archipelago, several cataclysmic volcanic eruptions have taken place. Examples are Krakatoa in 1883, Mount Tambora in 1815, and Toba, which had a massive super-eruption around 72,000 years ago.

The magma reacts chemically with the surrounding rock as it penetrates the earth’s crust before erupting to the surface. It can therefore vary considerably from one volcano to another. To better understand the origin of volcanism in Indonesia, the researchers wanted to know the composition of the “primary” magma, from the mantle itself. Since samples cannot be taken directly from the mantle, geologists studied minerals in recently ejected lava from four volcanoes: Merapi and Kelut in Java, and Agung and Batur in Bali.

Frances Degan

Frances Deegan, researcher in the Department of Earth Sciences, Uppsala University. Credit: Frances Deegan

Using the powerful beams of ions from a Secondary Ion Mass Spectrometry (SIMS) instrument, a state-of-the-art form of mass spectrometer, the researchers examined crystals of pyroxene. This mineral is one of the first to crystallize from a magma. What they wanted to determine was the ratio of the oxygen isotopes 16O and 18O, which tells a lot about the source and evolution of magma.

“Lava consists of about 50% oxygen, and the Earth’s crust and mantle differ greatly in their composition of oxygen isotopes. Thus, to trace the amount of material that magma has assimilated from the crust after having left the mantle, oxygen isotopes are very useful,” says Frances Deegan.

The researchers found that the oxygen composition of Bali’s pyroxene minerals was virtually unaffected during their journey through the earth’s crust. Their composition was quite close to their original state, indicating that a minimum of sediments had been driven into the mantle during subduction. An entirely different pattern has been found in the minerals of Java.

“We could see that the Merapi in Java had a very different isotopic signature from those of the volcanoes in Bali. This is partly because Merapi’s magma interacts intensely with the earth’s crust before erupting. This is very important because when the magma reacts with, for example, the limestone which is in central Java right under the volcano, the magma becomes full to the bursting point with carbon dioxide and water , and eruptions become more explosive. Maybe that’s why Merapi is so dangerous. It is in fact one of the deadliest volcanoes in Indonesia: it has killed nearly 2,000 people over the past 100 years and the last eruption claimed 400 lives,” says Professor Valentin Troll of the Department of Science of Earth from Uppsala University.

The study is a collaboration between researchers from Uppsala University, the Swedish Museum of Natural History in Stockholm, the University of Cape Town in South Africa, the University of Freiburg in Germany and the Vrije University (VU) Amsterdam in the Netherlands. The results of the study improve our understanding of how volcanism works in the Indonesian archipelago.

“Indonesia is densely populated, and anything that gives us a better understanding of how these volcanoes work is invaluable and helps us be better prepared in the event of a volcanic eruption,” says Frances Deegan.

Reference: “Source of the mantle of the Probe δ arc18O value revealed by intracrystalline isotope analysis” by Frances M. Deegan, Martin J. Whitehouse, Valentin R. Troll, Harri Geiger, Heejin Jeon, Petrus le Roux, Chris Harris, Marcel van Helden and Osvaldo González-Maurel, June 24, 2021, Nature Communication.
DOI: 10.1038/s41467-021-24143-3