An international study headed by IRD researchers (IRD/CNRS/Université Blaise Pascal volcano and magma laboratory), the University at Buffalo and the United States Geological Survey sheds new light on the physical mechanisms that cause pyroclastic flows during volcano super eruptions. Until now, only the diluted mixture turbulent model was used to explain the considerable distances covered by these flows. For the first time, researchers have been able to demonstrate the existence of dense flows caused by a very high eruptive output and sustained interstitial gas pressure. These results, which can be used to better assess volcanic hazards, were published on 7 March 2016 in Nature Communications.
Explosive volcanic super eruptions where the volume exceeds 500 km3 of magma, are rare but extremely devastating cataclysmic phenomena. They generate pyroclastic flows that are a mixture of gas and extremely hot rock fragments, denser than the atmosphere, and that run down the slopes of volcanoes, destroying everything in their wake. The deposits from these flows, known as ignimbrites, can be found at distances of over 100 km from the eruptive centre.
Volcanologists have long endeavoured to understand the processes at work during the transportation and depositing of pyroclastic flows, in order to assess the natural hazards related to these phenomena.
Two fundamentally different physical mechanisms are likely to occur: a rapid, diluted flow (containing less than 1% particles in volume terms), the turbulence of which holds the particles in suspension, or a mixture with close to maximum concentration of particles, within which the interstitial gas pressure reduces internal friction. Until recently, only the diluted flow model had been proved in quantitative terms, requiring propagation velocities of over 200 m/s.
The objective of this research is to understand the behaviour of pyroclastic flows using a clearly characterised example to define a model applicable to all super eruptions occurring on Earth.
In this study, researchers studied the ignimbrite at Peach Spring (Arizona, United States), formed by flows of more than 170 km during an eruption that occurred 18.8 million years ago, giving off over 1,300 km3 of magma and leading to the formation of a hug volcanic crater (or caldera).
They looked closely at large blocks of rock (> 0,5-1 m) found in the ignimbrite studied, initially present in the substrata but carried along by the pyroclastic flows. An initial analysis revealed that the movement of these blocks could not have been triggered by diluted flows at realistic speeds.
To understand this entrainment process, the researchers then worked in the laboratory to simulate the propagation of pyroclastic flow on a particulate substratum. The procedure, developed with partners from the University of Chile, means generating, on a small scale, a gravity flow comprising a dense mixture of solid particles and air. Thanks to these experiments, the researchers demonstrated, for the first time, that a pressure gradient generated at the base of the flow can lift particles from the substratum; they are then borne in the flow and taken downstream.
Applying the experimental principle that links the size of the substratum particles carried to the speed of the flow, the authors were able to calculate the speed of the pyroclastic flows that formed the Peach Spring ignimbrite: between 5 and 20 m/s. This speed over a minimum distance of 170km, was then used to determine the duration of the eruption (between 2.5 and 10 hours), and its flow rate (107-108 m3/s), higher than any known to date.
Using this combination of laboratory experiments and data from the field, the researchers concluded that during a super eruption, a high eruptive flow over several hours and sustained interstitial gas pressure in pyroclastic flows can be more effective than an extremely rapid diluted suspension when it comes to covering very long distances.
This new take on the propagation mechanisms of pyroclastic flows forces us to reconsider the interpretations of many of the ignimbrites generated by super eruptions in the Earth’s history. It opens new possibilities for better assessing volcanic hazards, especially along the Andes, which encompass some of the planet’s most active volcanoes. Chimborazo, Cotopaxi, Tungurahua (Ecuador), Ubinas, Misti (Peru), Lascar, Villarrica, Calbuco (Chili), etc.
Managing volcanic risks and hazards is a priority for IRD and is part of the long-standing scientific cooperation with partners in countries in the Global South, in the Andes (Chile, Ecuador and Peru) and in the Indian and Pacific oceans (Indonesia, Vanuatu).
Since 2015, a consortium has been formed in Latin America and serves as a regional cooperation instrument. This is the VIMESEA project. Coordinated by IRD and the national commission for scientific and technological research in Chile (CONICYT) and funded by the European Commission, this project aims to deepen knowledge into the mechanisms of volcanic eruptions in the Andes and their impacts on the environment and societies. It involves several European research institutes (CNRS magma and volcano institute, Université Blaise Pascal and IRD, Bristol University, University of Munich, Pisa volcanology institute) and others from South America (University of Chile, Peru’s geological, mining and metallurgy institute, Ecuador’s geophysical institute).