Changes in magma prior to eruptions make Indonesia's volcanoes among the most dangerous in the world, according to a study published Thursday by Nature Communications.

Using chemical analyses of tiny minerals in lava from volcanoes in Bali and Java, there is now increased understanding of the composition of the Earth's mantle in the region, the researchers said.

The oxygen composition of pyroxene minerals from Bali, for example, change little as they pass through Earth's crust, meaning they remain close to their original state prior to eruptions.

This suggests that a minimum of sediment is drawn down into the mantle during subduction, which occurs when two of Earth's continental tectonic plates collide.

However, an entirely different pattern was found in the minerals from Java, which appear to interact "intensively" with the Earth's crust prior to eruptions, the data showed.

"When magma reacts with, for instance, the limestone that's found in central Java right under the volcano, the magma becomes full to bursting point with carbon dioxide and water, and the eruptions get more explosive," study co-author Valentin Troll said in a press release.

"That may be why Merapi's so dangerous," said Troll, a professor of earth sciences at Uppsala University in Sweden.

Merapi is one of the deadliest volcanoes in Indonesia, as multiple eruptions have killed nearly 2,000 people over the past 100 years, according to Troll.

The composition of magma varies greatly from one volcano to another, and it helps determine what, if any, volcanic eruption that occurs, he and his colleagues said.

The Indonesian archipelago was created by volcanic activity caused by a collision of two of Earth's continental tectonic plates.

As part of this ongoing collision, the Indo-Australian plate slides beneath the Eurasian plate at a speed of roughly 7 cm annually, a process known as subduction, which contributes to volcanic activity.

During subduction, when the sinking tectonic plate descends into the mantle, it heats up and the water it contains is released, causing the surrounding rock to start melting.

The result is volcanoes that are often explosive, the researchers said.

For this study, they sought to determine the composition of the region's "primary" magma derived from the Earth's mantle.

Because samples cannot be collected directly from the mantle, they analyzed minerals in lava recently ejected from four volcanoes, Merapi and Kelut in Java, and Agung and Batur in Bali, they said.

Using powerful ion beams from a secondary ion mass spectrometry instrument, they examined crystals of pyroxene, one of the first minerals to crystallize from a magma, the researchers said.

While the analysis revealed that oxygen composition of pyroxene minerals from Bali was impacted little as it passed through the Earth's crust, it showed the opposite in the samples collected from the Java volcanoes.

"Indonesia is densely populated," study co-author Frances Deegan said in a press release.

"Everything that gives us a better grasp of how these volcanoes work is valuable, and helps us to be better prepared for when the volcanoes erupt," said Deegan, a researcher at Uppsala University.

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