Under the coordination of the geochemist Don R. Baker, an international research team composed of members from McGill University in Montréal, Elettra – Sincrotrone Trieste, University of Trieste, Swiss Light Source and University of Chicago has investigated the growth of magmatic bubbles at very high temperature in hydrated basaltic rock from Stromboli volcano. The researchers, for the first time, could follow the evolution of the rock microstructure and correlate it with the potential destructiveness of the system. The study that has been published today in Nature Communications demonstrated that the first ten seconds of life of magmatic bubbles have a crucial role in the determination of the final character of an eruption.
“Although we cannot see inside a volcano during its eruption” - explains Don Baker – “we know that eruptions are driven by the formation of bubbles in molten rock under the volcano and their rapid expansion, much the same mechanism as can be observed by shaking a bottle of a carbonated drink and then opening the lid. Whether the volcano or the drink erupts dramatically or slowly loses its gas depends upon the interplay of bubble growth and gas loss. With this work we would better understand the formation and growth of bubbles and their effects on magma properties”.
The experiment was carried out on the TOMCAT beamline of the Swiss Light Source using synchrotron radiation. “By increasing the temperature of a water-bearing molten rock up to 1200°C, through a heating laser system – explains Lucia Mancini, researcher at Elettra and expert in X-ray imaging techniques – we obtained a sequence of high-resolution three-dimensional images of the sample by using an ultra-fast X-ray microtomography technique, during the first 18 seconds of formation and growth of the bubbles in the magmatic melt, and we visualized them and variations of their number and size”.
“Analysing the geometry and connections between bubbles” - adds Francesco Brun, engineer and expert in image analyis at the Univesity of Trieste and Elettra - “we were able to measure the number and size of bubbles and we could investigate the geometry of the connections between bubbles, calculate how quickly gas flowed out of the sample, and how rapidly the foam strength dropped. We discovered that at the beginning thousands of bubbles per cubic centimeter were created trapping gas inside them, but that they rapidly coalesced into a foam of larger bubbles whose strength rapidly decreased while the rate of gas loss rapidly increased. All these modifications were observed in the first 10 seconds of bubble growth”.
“From these results – explains Don Baker – we could infer that even molten rocks with small amounts of water have the potential to create devastating, large eruptions, but that in most cases the rate of gas loss increases more rapidly than the strength falls, resulting rapid gas loss and smaller eruptions. However, under exceptional rates of bubble growth, or conditions where the bubbles can not coalesce, the strength may drop faster than the gas loss increases, resulting in large eruptions, explaining more violent phenomena as the abrupt explosion of Stromboli volcano in April 2003”.
“Even if today we cannot fully understand the different mechanisms determining the intensity of a volcanic eruption – concludes Don Baker – this work shows that the difference between a small or large eruption appears to depend upon the first 10 seconds of bubble growth and indicates that volcanic monitoring systems need to be able to measure rapid changes in gas flux and composition in order to provide information on impending eruptions. Future work in this area will concentrate on the first few seconds of bubble growth and the effect of crystals on the bubble growth”.
References: Nature Communications, 16 ottobre 2012DOI: 10.1038/ncomms2134.
“A four-dimensional x-ray tomographic microscopy study of bubble growth in basaltic foam”. Authors: Don R. Baker, Francesco Brun, Cedrick O'Shaughnessy, Lucia Mancini, Julie L. Fife, Mark Rivers