Monitor the Kīlauea volcano on the island of Hawaii to recognize possible precursors of eruptive events and make operational decisions to minimize their negative effects. These are the results of the study, the result of the INGV-USGS scientific collaboration, published in EOS Research Spotlight
Monitoring volcanoes in "real time" allows obtaining information on the state of activity which can then be used by the authorities in charge to make operational decisions, in order to minimize the negative effects in the event of paroxysmal events. A collaboration between the National Institute of Geophysics and Volcanology (INGV) and the US Geological Survey (USGS) has been underway for some years, focused on the study of geophysical signals acquired at Kīlauea (volcano in Hawaii). One of the objectives of this collaboration consists in the recognition of precursors capable of providing indications on possible changes in the state of activity before the eruptive phases. The results of the research Insights into shallow magmatic processes at Kīlauea Volcano, Hawaii, from a multiyear continuous gravity time series were published in the Journal of Geophysical Research and highlighted in the EOS Research Spotlight (Exciting new research from AGU journals: https://eos.org/research-spotlights/the-gravity-of-volcanic-eruptions).
“The scientific collaboration between INGV and USGS, explains Daniele Carbone, INGV researcher, “began in 2010, when the Hawaiian Volcano Observatory (HVO) of the USGS decided to install gravimeters (instruments for measuring small variations in acceleration of gravity) in continuous acquisition at Kīlauea. To then continue with the analysis and interpretation of the acquired data”.
Kīlauea is a volcano located on the island of Hawaii, the largest of the archipelago of the same name. Its origin is linked to the presence under the earth's crust of a hot spot, i.e. a hot point of ascent of molten rock. The summit crater of Kīlauea (Halema'uma'u) hosts a lava lake that floats between 70 and 150 m below the crater rim.
“The Kīlauea has characteristics that make it ideal for applying the gravimetric method, as it is possible to install instruments close to active structures, thus allowing to increase the signal-to-noise ratio. Furthermore, the level of the lava lake is continuously monitored via a thermal camera and therefore it is always possible to have indirect information on the state of activity”, states the researcher.
The variations in gravity that are observed with gravimeters are the reflection of variations in mass around the point of observation. In the case of Kīlauea, lowering the level of the lava lake causes a decrease in gravity. In fact, there is a decrease in mass, given that a certain volume of lava is missing in a position close to the observation point.
“Knowing the shape of the crater and the level variation, we calculated this volume and, through gravimetric measurement, we estimated the density of the material that occupied this volume. With the result of a density equal to 1-1,5 g/cm^3. But, since the density of magma is usually much higher (about 3 g/cm^3), it has been deduced that the observed value is due to the presence of a large quantity of gas bubbles (it is in fact a foam), which lower the average density of the material inside the crater".
In many active volcanoes, monitoring is essentially based on data relating to ground deformation, seismic activity and gaseous emissions. Gravimetric observations represent a valid alternative. In fact, the variations in time of the acceleration due to gravity reflect changes in the distribution of masses in the subsoil and can indicate conditions of disequilibrium that precede paroxysmal events.
“Using four years (2011-2015) of data acquired at a distance of only 150m from the rim of the summit crater of Kīlauea, which hosts a lava lake, gravimetric variations strongly correlated with the level of the lake itself have been highlighted. These data have made it possible to estimate that the density of magma in the most superficial part of the plumbing system below the crater is very low, slightly higher than the density of water. A substantial part of the volume in question is therefore occupied by the gaseous phase”, continues Carbone.
Some transients, observed in the temporal sequence, indicate phases of disequilibrium probably due to low-depth magmatic intrusions. These events cannot be highlighted using other techniques and emphasize the importance of the gravimetric method for recognizing possible precursors of eruptive events.
Extended
Continuous gravity data collected near the summit eruptive vent at Kīlauea Volcano, Hawaiʻi, during 2011–2015 show a strong correlation with summit-area surface deformation and the level of the lava lake within the vent over periods of days to weeks, suggesting that changes in gravity reflect variations in volcanic activity. Joint analysis of gravity and lava level time series data indicates that over the entire time period studied, the average density of the lava within the upper tens to hundreds of meters of the summit eruptive vent remained low—approximately 1000–1500 kg/m3. The ratio of gravity change (adjusted for Earth tides and instrumental drift) to lava level change measured over 15 day windows rose gradually over the course of 2011–2015, probably reflecting either (1) a small increase in the density of lava within the eruptive vent or (2) an increase in the volume of lava within the vent due to gradual vent enlargement. Superimposed on the overall time series were transient spikes of mass change associated with inflation and deflation of Kīlauea's summit and coincident changes in lava level. The unexpectedly strong mass variations during these episodes suggest magma flux to and from the shallow magmatic system without commensurate deformation, perhaps indicating magma accumulation within, and withdrawal from, void space—a process that might not otherwise be apparent from lava level and deformation data alone . Continuous gravity data thus provide unique insights into magmatic processes, arguing for continued application of the method at other frequently active volcanoes.
Lava lake inside Kīlauea's summit crater
(Credit: USGS http://hvo.wr.usgs.gov/multim
Lava lake inside Kīlauea's summit crater
(Credit: USGS - http://hvo.wr.usgs.gov/multime
Lava level in the lake (top), radial tilt (middle), and gravity (bottom), during periods of several weeks in 2011 (left) and 2012 (right). The three time series are well correlated; this indicates that the gravimetric variations are induced by volcanic processes.