Merapi is the most active volcano in Indonesia and its unpredictable, explosive eruptions are a constant risk to the communities living along its flanks. Some scientists have proposed that the cause of the explosive eruptions might be carbon dioxide, released when magma eats away at the underlying limestone crust and transforms carbonate minerals in the limestone. Until recently, however, no one had been able to show how efficient the digestion of limestone was as a source of gas emissions at Merapi; a process that globally would represent an underappreciated piece of the deep carbon cycle.
Now in a new paper  in Scientific Reports, researchers show that chunks of limestone swept up by the rising melted rock, degas carbon dioxide, and it’s a surprisingly rapid and efficient process. Sean Whitley, Ralf Gertisser, Ralf Halama (all at Keele University, UK), Katie Preece (Swansea University, UK) and DCO Reservoirs and Fluxes Community members Frances Deegan and Valentin Troll (both at Uppsala University, Sweden), used advanced analytical techniques to examine bits of transformed limestone, called skarns, to estimate how much carbon the limestone lost. Their findings suggest that Merapi and other volcanoes sitting on limestone can move significant amounts of carbon dioxide into the atmosphere at rates that impact global climate. This process may even help explain “hothouse” periods in Earth’s past.
“It’s a bit like one of those experiments where people drop Mentos into a bottle of Coke,” said Deegan. “If you drop a bit of limestone into your magma, it’s going to start to degas and fizz up very quickly.”
More than a decade ago, scientists became interested in volcanoes like Merapi because of abnormal carbon isotope concentrations in their gas emissions, which hinted at unusual sources of the carbon. Isotopes are atoms of an element with a different number of neutrons in the nucleus, and researchers can measure their concentrations to track the movement of carbon through the environment. “People only recently have started to notice that quite a lot of carbon degassing from volcanoes might be coming from crustal sources,” said Sean Whitley who analyzed the Merapi samples as part of his PhD work. “Previously they thought it was coming just from the mantle or from recycling of marine carbonates into the mantle. There’s possibly a whole new recently recognized reservoir of carbon dioxide that people are starting to consider.”
To investigate the interactions between magma and limestone, Whitley analyzed several skarns collected from Merapi. He applied a technique called secondary ion mass spectrometry (SIMS), which can measure oxygen and carbon isotopes from a tiny location, only a few micrometers in diameter, within a sample. Based on the isotopic composition of carbonate minerals in the skarn, Whitley used modeling to recreate what happened to each piece of limestone as it moved through the volcano.
“Merapi is pretty hungry for limestone and it seems to be dissolving the limestone very rapidly,” said Deegan. “We have evidence now that this process is really efficient.”
The speed of this interaction does not bode well for a better warning system for the people living in Merapi’s shadow. “Some scientists believe that this relatively quick release can actually affect the explosiveness of a volcanic eruption, because gases are the major driver of explosive eruptions,” said Gertisser. Vesuvius in Italy and Popocatépetl in Mexico are similarly explosive volcanoes that sit on limestone. “Geologically this process would be very rapid,” said Troll. “If you have fresh magma and new fresh limestone coming together, this reaction would take hours to days and probably up to a week until we see all the results translating to surface phenomena.”
The carbon dioxide released by the interaction between limestone and magma has implications for global climate. Warmer times in Earth’s history supported more extensive growth of coral reefs and other marine animals whose remains accumulated to form limestone crust. Some of these periods also happened to have more volcanic activity. Together, these factors can create a positive feedback loop where more limestone formation causes volcanoes to release more carbon dioxide, which warms the planet and encourages even more reef growth. For example, during the Cretaceous hothouse period about 90 million years ago, this phenomenon may have accounted for some of the previously unexplained carbon dioxide in the atmosphere, which led to an ice-free planet.
Currently, the contribution of limestone to the flux of carbon through volcanoes is not adequately estimated, but should be taken into account for climate models and predictions of global warming. “I would hope that in the long run, we as a community will be able to quantify the extra, ‘piggyback’ carbon dioxide parcels from the crust that really have not been accounted for very well,” said Troll.
Now that the researchers have uncovered what happened to the carbon from Merapi’s skarns, they are investigating traces of metals that some of the rocks carry as well. They suspect that the rush of alkaline limestone as it dissolves into the magma could create just the right conditions to cause iron and copper ore to crystallize. Of course, it would be impossible to mine an active volcano, but what the researchers learn about the processes occurring beneath Merapi could point prospectors to older, eroded volcanic sites that would be good spots for mining valuable copper ore.
Main image: Merapi is the most active volcano in Indonesia. It may be especially explosive because rising magma dissolves limestone in the crust to release carbon dioxide. The process is much like the classic volcano demonstration that mixes vinegar and baking soda to create a bubbly eruption. Credit: GFZ Potsdam, Germany