Gas pulse, rock fracturing and sloshing. These are the processes that influence the presence of Radon gas detected by the monitoring station located near the top of Etna and that help to understand how the volcano works. The study, signed by INGV, was published in Geochemistry, Geophysics, Geosystems of the American Geophysical Union
Radon gas works as a tracer of eruptive activity and in some cases also tectonics. This is demonstrated by a study conducted by the National Institute of Geophysics and Volcanology (INGV) section of Catania-Osservatorio Etneo, just published in Geochemistry, Geophysics, Geosystems of the American Geophysical Union (http://onlinelibrary.wiley.com/doi/10.1002/2017GC006825/abstract).
Etna is one of the most active volcanoes in the world. It erupts with high frequency, especially in recent decades, and changes its appearance rapidly. Being a largely man-made volcano, its monitoring has a high social value. The dense network of roads, easily passable up to the highest altitudes, allows access to the summit in a short time. It is also for this reason that Etna represents a formidable natural open-air laboratory, where scientists can install and test increasingly dense, sophisticated and efficient networks of monitoring and surveillance instruments.
In recent years, Radon gas has also been analyzed at Etna. A natural radioactive gas that comes from the subsoil, considered by some to be a precursor to earthquakes, albeit with many distinctions, doubts and skepticism from the scientific community.
"Radon on Etna functions as a tracer of eruptive activity and, in some cases, also of tectonic activity" explains Marco Neri, principal researcher of the INGV-Osservatorio Etneo (INGV-OE).
But, to really understand the tectonic phenomena, it is necessary to compare Radon with the many other data that are produced daily by the instrumental networks of INGV-OE, strengthened in about forty years of monitoring and surveillance activities.
“A period of lively and varied volcanic activity of Etna was analysed, between January 2008 and July 2009. Nineteen months in which the volcano produced some seismic swarms, superficial soil fractures, a vigorous lava fountain and, finally, a long eruption that lasted 419 days”, continues Susanna Falsaperla, first researcher at INGV-OE and first author of the publication.
Enough to test the significance of the Radon detected by a station located near the top of Etna, about 3000 meters above sea level, in a place once known as the "Tower of the Philosopher" and now buried under meters and meters of lava flows that from 2013 to today have completely changed the physiognomy of those places.
“It has been discovered that the Radon of that monitoring station is essentially influenced by two processes. The first is related to the rising of magma in the central conduit of the volcano. This process takes place through gas "pulsations", that is, short and intense increases in Radon, which scholars define, in English, as gas pulse. The second is induced by rock fracturing, when the same rock breaks due to an earthquake or a seismic swarm”, continues Neri.
The results of the study have also shown that the Radon probe is "sensitive" even to earthquakes of relatively small magnitude and which occur several kilometers away from it. This can be explained through a phenomenon that the English call sloshing, and which literally means "slapping".
"The shaking of the rock, induced by an earthquake swarm", says Falsaperla, "can cause an oscillatory movement in the water table and in the magmatic fluids contained within the volcano, the effects of which can therefore radiate to a much greater distance than commonly imagined".
Etna is perpetually in a precarious balance: even a small phenomenon that occurs, for example, on the north side of Etna, can make its effects felt on the opposite side. Almost like a "butterfly effect".
Extended
We analyze short- to long-term changes (from days to months) in Radon (Rn) activity measured nearby (<2 km) the eruptive fractures that fed a lava effusion at Mt. Etna, Italy, between 13 May 2008 and 6 July 2009. The N120-140°E eruptive fractures opened between 3050 and 2620 m above sea level before a dike-forming intrusion fed the ∼14 month-long lava emission. Our high-rate data streams include: Rn, ambient parameters (barometric pressure and soil temperature), and seismic data (earthquakes and volcanic tremor) recorded from January 2008 to July 2009. The analysis highlights repeated episodes of rock-fracturing related to seismic swarms , and vigorous gas pulses and peak values in Rn emissions (maximum ∼4.1×105 Bq/m3 on 16 November 2008), which we interpreted in a conceptual model as the response to inputs from the magmatic system during the eruption. This multidisciplinary study: (i) provides evidence of a close relationship between Rn emission at a fumarole near the summit active craters and local earthquakes, and (ii) enables exploring the important role of the volcanic source on the temporal development of the Rn flux, which may account for the much higher (≫94 m/d) ascent speed of the Rn carrier (vapor) than diffusion. The close location of Rn probes to the active conduits, along with the application of our multidisciplinary approach, may shed new light on the internal dynamics of other active volcanoes worldwide.
Bibliographic citation:
Falsaperla, S., M. Neri, G. Di Grazia, H. Langer, and S. Spampinato (2017), What happens to in-soil Radon activity during a long-lasting eruption? Insights from Etna by multidisciplinary data analysis, Geochem. Geophys. Geosyst., 18, 5, 1-15, doi:10.1002/2017GC006825.
Photo 1 - Helicopter view of the upper eastern flank of Etna, taken from the southeast. The Radon probe used in the article is located about 1 km away from the summit crater area and about 2 km from the 2008-2009 eruptive fissure. Photo by M. Neri.
Photo 2 - Radon data measurement station located in the Torre del Filosofo site (South flank of Etna, about 3000 m altitude). In the background, the simultaneous eruptive activity can be seen about 1 km away from the monitoring station. Photo by M. Neri.
Rome, 6 July 2017