How pore size distribution affects fracture-enhanced permeability in lava domes
Using X-ray micro-tomography and Lattice Boltzmann simulations on Mt Unzen dacite, this study demonstrates that while fracture number density primarily controls pore connectivity, fracture width is the dominant factor influencing permeability in lava domes, with both metrics being significantly modulated by the initial pore size distribution.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Volcanic domes are not the smooth, rounded hills they often appear to be from a distance. Built from thick, slow-moving lava that piles up over a vent, these structures are constantly under stress. As the hot rock cools and shrinks, or as pressure builds from trapped gases, the solid crust cracks. These cracks are not just cosmetic; they are the primary highways for gas to escape. If the gas cannot find a way out through a connected network of pores and fractures, pressure builds until the rock shatters, potentially triggering an explosive eruption. Understanding how these hidden pathways form and connect is essential for predicting whether a dome will grow quietly or collapse violently. The key lies in the rock's internal architecture: the size and arrangement of its tiny holes, and how fractures link them together.
A team of researchers from Germany, the United Kingdom, and Japan has taken a detailed look inside a piece of volcanic rock to map this hidden world. They studied a sample of dacite, a type of volcanic rock, collected from the 1990s eruption of Mount Unzen in Japan. This specific rock came from a deposit of blocks and ash, representing the dense, crystal-rich material that often makes up the core of a lava dome. Using high-resolution X-ray imaging, the scientists created a three-dimensional digital model of the rock's interior, revealing a complex landscape of pores. These pores range from tiny, microscopic voids to larger, millimeter-sized bubbles. The researchers then used a computer simulation to mimic the process of fracturing. Instead of physically breaking the rock, they digitally introduced cracks into the 3D model, allowing them to watch how the fractures spread and connected the existing holes. They tested two different starting conditions: one where the rock contained both tiny and large pores, and another where only the large pores remained, effectively removing the microscopic ones to see how that changed the outcome.
The results revealed that the way fractures connect the rock depends heavily on the size of the pores they are trying to link. When the rock contained both large and small pores, the fractures had to work harder to connect everything. The tiny pores acted as isolated islands that required a high density of cracks to bridge the gap. However, when the researchers simulated a rock with only large pores, the fractures connected the network much more efficiently. In these simulations, the number of cracks was the most critical factor for creating a connected path. Once the cracks were numerous enough to link the pores, the rock became permeable, meaning gas could flow through it. Yet, the width of those cracks mattered most for how fast the gas could actually move. A small increase in the width of the fractures caused a massive jump in permeability, allowing gas to escape much more rapidly.
The study also showed that the initial state of the rock's connectivity plays a surprisingly minor role once fracturing begins. Even if the rock started with very few connected paths, the introduction of fractures quickly created a highly connected network. This suggests that in a real volcanic dome, the act of cracking can rapidly transform a sealed, pressure-building rock into a permeable system capable of venting gas. The researchers found that the permeability of their digitally cracked samples fell within the range observed in natural rocks from Mount Unzen, though their simulations sometimes showed slightly higher flow rates. This difference likely stems from the idealized nature of the digital cracks, which lack the rough, jagged surfaces found in real fractures that can slow down gas flow. Despite this, the model successfully captured the fundamental behavior of how rock texture controls gas escape.
These findings offer a clearer picture of why some lava domes behave differently than others. The interplay between the size of the pores and the density of the fractures determines whether a dome will release gas gently or hold it until it explodes. If a dome is rich in crystals and has a specific distribution of pore sizes, it may require a certain amount of fracturing to become permeable. Once that threshold is crossed, the rock's ability to let gas escape changes dramatically. This understanding helps scientists assess the stability of volcanic domes. By knowing how the internal texture of the rock influences its ability to fracture and vent, researchers can better predict the conditions under which a dome might fail, leading to either a slow collapse or a sudden, dangerous eruption. The work underscores that the safety of a volcanic edifice is not just about the pressure inside, but about the intricate, microscopic geometry of the rock itself.
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