The deadliest and most destructive phenomena associated with volcanoes are pyroclastic flows, fluidlike avalanches of hot gases, ash, and rock fragments that race down volcanic slopes, destroying everything in their paths. These phenomena typically consist of two parts: a dense, valley-hugging flow and a more diluted, gas-rich ash cloud known as a surge. Although scientists have successfully modeled the behavior of the dense flows, an incomplete understanding of the physics controlling the surges has made the full suite of hazards associated with pyroclastic flows extremely difficult to predict.
Now Kelfoun has overcome this limitation by developing a new numerical model capable of simulating both portions of pyroclastic flows as well as their interactions, during which the surges and flows can, by exchanging material, spawn one another. In contrast to earlier, more computationally demanding models, this tool employs a depth-averaged method in which all physical properties are integrated perpendicular to the ground: the same approach that is commonly used to simulate landslides. What’s more, the method allows for rapid calculations to assess hazards if an eruption is imminent.
The study’s results indicate that thick flows move more quickly than thin flows and that any factor that allows a dense flow to attain a speed of about 25 meters per second can create a surge. This finding helps to explain why a change in topography, such as the narrowing of a river valley or a break in slope, can generate a surge.
To test the new model’s ability to reproduce an actual eruption, the author joined with a team of researchers for a companion paper that simulated two phases of the intensely studied 2010 eruption of Indonesia’s Mount Merapi. Despite prompt and widespread evacuations, this tragic event, which produced more than 100 pyroclastic flows, still killed an estimated 367 people and caused at least $600 billion in economic losses.
Overall, report the researchers, the model was able to reproduce the general characteristics of both types of eruptive phases that occurred during the eruption, including the different deposits’ extent, volume, thickness, and paths, as seen in the video below. A concentrated block-and-ash flow is in red, a dilute ash cloud surge is in blue. Yellow to green colors and contour lines are the thickness of surge deposits. Time is sped up: 1 second in the video represents 50 seconds in reality.
Although the team also found some significant differences between the model output and the documented phenomena, this model, as the first to simulate both portions of pyroclastic flows, represents a major advance in the capability to predict ash cloud surges and their devastating consequences. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014013, 2017 and https://doi.org/10.1002/2017JB013981, 2017)
—Terri Cook, Freelance Writer