Beyond Pressure: The Role of Shear in Volcanic Eruptions

Traditionally, the explosiveness of a volcanic eruption was likened to a shaken bottle of champagne. As pressure decreases during ascent, dissolved gases rapidly form bubbles, propelling the liquid upwards in a dramatic spray. This model explains many explosive events, but it fails to account for instances where gas-rich magma flows calmly, such as the 1980 eruption of Mount St. Helens in Washington state, or the Chilean volcano Quizapu. Why didn’t these volcanoes explode despite possessing ample gas content?

Researchers at ETH Zurich have now demonstrated that gas bubble formation isn’t solely dependent on pressure drops. Shear forces – the friction and internal movement within the magma – can independently trigger the creation of bubbles, even at constant pressure. Imagine stirring honey; the faster you stir, the more air gets incorporated. Similarly, as magma flows through the narrow conduits of a volcano, the varying speeds create shear, “kneading” the molten rock and generating gas bubbles.

How Shear Forces Create Degassing Channels

These bubbles don’t simply appear randomly. The research indicates they form primarily near the edges of the volcanic conduit, where shear forces are strongest. Crucially, these bubbles can coalesce, forming channels that allow gas to escape gradually. This process effectively “degasses” the magma, reducing the potential for a violent explosion. The more gas present, the less shear is required to initiate this degassing process.

Conversely, even magma with relatively low gas content can become explosive if shear forces generate a large number of bubbles rapidly, causing a swift upward surge. This highlights the complex interplay between gas content and mechanical forces within a volcano.

To visualize these processes, the team conducted experiments using a viscous liquid saturated with carbon dioxide, mimicking magma. They observed that applying shear forces consistently led to bubble formation, even without pressure changes. These findings were then corroborated by computer simulations of volcanic eruptions, confirming the importance of shear forces in real-world scenarios.

What implications does this have for understanding volcanic hazards? Could we be underestimating the potential for eruptions in volcanoes where shear forces are particularly pronounced? And how can we better incorporate these findings into predictive models?

This research builds upon previous work examining pressure dynamics within volcanoes. For a deeper understanding of how pressure influences eruptions, explore resources from the U.S. Geological Survey.

Further research into magma composition and conduit geometry is essential. The Woods Hole Oceanographic Institution provides valuable insights into the relationship between volcanic activity and the marine environment.

Frequently Asked Questions About Volcanic Eruptions and Shear Forces

• What are shear forces in the context of volcanic eruptions? Shear forces are the frictional forces created by the movement of magma within a volcanic conduit, similar to stirring a thick liquid.
• How do shear forces influence the explosiveness of a volcano? Shear forces can create gas bubbles, which can either lead to a gradual release of pressure or, in some cases, a rapid and explosive eruption.
• Can a volcano with low gas content still erupt explosively? Yes, if shear forces are strong enough to generate a large number of bubbles quickly, even magma with low gas content can erupt explosively.
• What is degassing, and why is it important for volcanic activity? Degassing is the release of dissolved gases from magma. It’s crucial because a gradual release of gas reduces the potential for a violent explosion.
• How does this new research change our understanding of volcanic eruptions? This research highlights the importance of shear forces, alongside pressure, in determining the style and intensity of volcanic eruptions.