Scientists have identified a crystal in volcanic rocks that could help estimate the size of future eruptions. 

Volcanologists from the University of Geneva, in partnership with the University of Heidelberg, have devised a new approach based on a small crystal called zircon in volcanic rocks. This crystal contains uranium and thorium; radioactive elements whose crystallisation can be dated. 

Furthermore, zircon crystallises only within a specific temperature range which allows scientists to calculate the cooling rate of the magma beneath the volcano. The larger the volume of magma, the longer it takes to cool it down. With the aid of thermal modelling, volcanologists can use the estimated cooling rate of magma to determine the volume of magma under a dormant volcano. This magma volume is not directly observable because it is located at a depth of between six to 10 kilometres. 

“Knowing the size of a volcanic reservoir is important to identify volcanoes that are most likely to produce a large magnitude eruption in the future. Our method is a new way to evaluate the candidates for such eruptions,” said researcher Gregor Weber on Thursday.  

This method - published in the scientific journal Nature Communication - was used on the dormant Mexican volcano of Nevado de Toluca, which is located in a densely populated area near Mexico City. Calculations have determined that the maximum size of a future eruption will be 350 km3, about four times the volume of Lake Geneva. 

According to the University of Geneva, the most devastating volcanic eruptions in the last 100 years were those of volcanoes that rarely erupt. These dormant volcanoes - which often have large populations in the vicinity - have often gone under the radar of scientists. Around 800 million people live around volcanoes worldwide.