Fingerprinting interaction of magma and carbonate rocks in Java: an explosive volcanic brew
Why are some volcanic eruptions more explosive than others? The violence of an eruption depends on the type of magma involved, as well as the way that magma interacts with the rocks that it encounters on its way from the deep Earth to the surface. This project will explore how and when carbonate rocks can cause a volcanic eruption to become more explosive than we would otherwise expect.
Carbonate rocks contain large amounts of mineralised carbon dioxide; for example, limestone is made entirely of the mineral calcium-carbonate. When subjected to magmatic heat carbonate minerals can break down releasing CO2 gas, which then forms bubbles in the magma. These bubbles reduce the magma’s density and accelerate it upwards through the crust. As it ascends the pressure on the magma decreases causing even more bubbles to form, just as removing the top from a bottle of carbonated drink allows CO2 bubbles to grow and expand. If enough CO2 is added then the magma ascent and bubble expansion can lead to an explosive (pyroclastic) eruption rather than gently oozing (liquid) lava. These different eruption styles represent completely different hazards to local populations.
Previous work by Durham students has shown that pyroclastic and lava eruptions from Sumbing volcano, on the Indonesian island of Java, began with the same type of magma. However, crystals in pyroclastic rocks contain excess calcium acquired from breakdown of carbonate rock through which the magma passed. This also added CO2 which contributed to the pyroclastic nature of the eruptions.
This project will track the development of magma as recorded by crystals in rocks produced by Sumbing’s pyroclastic and lava eruptions. Crystals grow in magma by adding new material to their surfaces while the magma travels through the crust. This generates a forensic record of compositional changes from the core to rim of individual crystals, much like growth rings in a tree. Colleagues in Durham and at the Universities of Leeds and Uppsala will work to explore how the chemical records in carbonate-affected, pyroclastic eruptions diverge from those in the “control group” of carbonate-free, lava eruptions. This will highlight the full range of chemical effects produced by interaction of magma with carbonate rock, allowing estimates of how much CO2 was added to the magma, and determination of whether CO2 was all added at one time or over a period of time while the crystals grew.
Many volcanoes around the world, including well-known peaks such as Popocatepetl and Mt Etna, emit large quantities of CO2 gas. But measurements of total CO2 emission do not reveal how much of this gas might be added from carbonate rocks in the crust. This means that we don’t fully understand the potential for explosive eruption hazards at many volcanoes. By developing the crystal record as a tool to track how magma changes when it encounters carbonate in the crust, this work will provide a new way to constrain our understanding of this potentially lethal, volcanic brew.