Volcanic Eruptions Driven By Bubble Growth
October 17, 2012

What Factors Decide The Size Of A Volcanic Eruption

Alan McStravick for redOrbit.com - Your Universe Online

From Pompeii to Mount St. Helens, we humans have watched in awe, and sometimes horror, the magnificence of volcanic eruptions.

As detailed in a great article earlier this month by redOrbit´s own Lee Rannals, monogenetic volcanoes, volcanoes erupting due to the combination of water and magma, are driven by a rapid expansion of gas bubbles that form as the water, previously trapped in molten rock, rises beneath the volcano. Researchers explain that the mechanism is not dissimilar to shaking a bottle of a carbonated beverage and then opening the lid. Or if shaking the bottle isn´t your style, maybe Mentos will more accurately convey the idea. Whether the volcano or the drink erupts in a style that is like the rare, large-to-gigantic eruptions that threaten entire communities, or as the more common, small eruptions that have a minimal impact on humans and the environment depends on the interplay between bubble growth and gas loss. For this reason, this study focused on investigating the formation and growth of bubbles and their effects on magma properties. The researchers believe their findings can provide a key to understanding volcanic eruptions, leading to better predictive models for future eruptions.

The international research team led by Professor Don R. Baker of McGill University´s Department of Earth and Planetary Sciences has published a new study in Nature Communications that suggests the difference between a small and large eruption depends on the first 10 seconds of bubble growth in molten rocks. Though the timing is so brief, the researchers believe their study findings point to a need to develop volcanic monitoring systems that can measure rapid changes in gas flux and composition during those crucial moments.

Utilizing technology from the Swiss Light Source facility in Villigen, Switzerland, researchers were able to examine the growth of volcanic bubbles in real time by heating water-bearing molten rock with a recently developed laser heating system. This new technology allowed the researchers to perform three-dimensional X-ray microtomography (CAT scans) of the samples during the first 18 seconds of bubble growth and foaming. Information gleaned from these observations allowed the researchers to measure the number and size of the bubbles, to investigate the geometry of the connections between bubbles, and to calculate how quickly gas was able to flow out of the sample allowing the foam strength to drop.

In the earliest part of the measurement it was noted that thousands of small bubbles were able to form in each cubic centimeter. Each of these bubbles trapped gas inside them, but they swiftly coalesced into a foam of larger bubbles, rapidly decreasing the overall strength due to a significant increase in gas loss. This entire process occurred in the first 15 seconds of bubble growth. From here, researchers then were able to determine which conditions of  bubble formation and growth eventually led to a failure in the molten rock.

Baker and his team were able to draw an hypothesis from their results, claiming that even molten rocks with small amounts of water have the potential to create devastating, large eruptions. In most cases, gas is able to escape rapidly enough to outpace bubble growth. This is the method by which smaller eruptions occur. The team states, however, under exceptional rates of bubble expansion, or conditions where the bubbles cannot coalesce, the larger, more dramatic eruptions may result.

For centuries, and even millennia, we humans have been at the whim of nature where volcanoes are concerned. The findings in this current study take us a small, though important step closer to the goal of being able to predict the types and intensities of eruptions that will occur in various volcanic regions on Earth. “Future work will need to concentrate on the first few seconds of bubble growth and the effect of crystals on the bubble growth,” Baker said.