Indonesia’s volcanoes are among the worlds most dangerous. To explain why, researchers from Uppsala University have used chemical analysis of tiny minerals in lava from Bali and Java. They now understand better how the Earth’s mantle is composed in that region and how magma changes before an eruption.
The study’s author Frances Deegan says, “Magma is formed in the mantle, and the composition of the mantle under Indonesia used to be only partly known. Having better knowledge of Earth’s mantle in this region enables us to make more reliable models for the chemical changes in magma when it breaks through the crust there, which is 20 to 30 kilometers thick, before an eruption.”
Magma composition varies greatly from one geological environment to another, and has a bearing on the kind of volcanic eruption that occurs.
Volcanism often arises in subduction zones. When the sinking tectonic plate descends into the mantle, it heats up and the water it contains is released, causing the surrounding rock to melt. The result is volcanoes that are often explosive.
Magma reacts chemically with surrounding rock when it penetrates Earth’s crust before breaking through the surface. It can therefore vary widely between volcanoes. Since samples cannot be taken directly from the mantle, geologists studied minerals in lava recently ejected for different volcanoes.
They examined crystals of pyroxene which is a mineral that is one of the first to crystalize from a magma. They wanted to determine the ratio of the oxygen isotopes O16 and O18 which reveals a great deal about the source and evolution of magma.
“Lava consists of roughly 50 percent oxygen, and Earth’s crust and mantle differ hugely in their oxygen isotope composition. To trace how much material the magma has assimilated from the crust after leaving the mantle, oxygen isotopes are very useful,” Deegan says.
https://phys.org/news/2021-06-knowledge-earth-mantle-indonesia-explosive.html