Deciphering magma plumbing system at Fogo volcano using geodetic (InSAR, GNSS and gravimetric) data

Abstract

Fogo volcano is one of the most active volcanoes in the Atlantic, with 27 eruptions since the Portuguese settlement in the early 15th century. Located nearby the center of a 9 km-wide truncated caldera, Chã das Caldeiras, Fogo is a stratovolcano capped by a 500 m-wide summit crater that rises ~1 km above the caldera floor. It has been interpreted as a recent structure formed at the top of an older volcanic edifice that would have collapsed about ~117 or at ~73 ka. Evidences of this collapse can be seen on land as illustrated by the horseshoe shape of Chã das Caldeiras and the tsunami deposit recognized on the neighboring Santiago island and offshore, with landslide debris deposit; even though its origin is still debated. Eruptive activity at Fogo includes both explosive and effusive phases with alkaline products ranging from basic to intermediate compositions. Eruptive fissures that fed lava flows are mostly distributed along three rift zones that are north, southwest and southeast oriented. The recurrence time for eruption is ranging from 1 to 94 years, with a 20 year interval for the three last eruptions. The last eruption started on November 23, 2014, ending February 7, 2015. The 2014-2015 fissure was 700 m long and included up to seven vents where lava was emitted and where strombolian and vulcanian explosions took also place. This eruption as well as the 1951 and 1995 ones have produced extensive lava flows that threatened the local populations resulting in 2014-2015, in the total destruction of two villages, Portela and Bangaeira. Interferometric analysis of the 1995 and 2014-2015 eruptions at Fogo volcano using C-band SAR data have allowed to characterize the feeder dykes of the two eruptions without any evidence of shallow magma reservoir. Recent petro-geochemical analyses suggested the existence of a transient magma chamber, located <1.5 km below sea level, where magma would have been stored ~50 days prior to the onset of the eruption. To unravel the existence of this transient reservoir and the related volcanic plumbing system at Fogo and also investigate the processes leading to the eastward flank movement, we have considered X-band SAR satellite and field-based GNSS data. We used COSMO-SkyMed SAR data acquired from July 2013 to the end of the eruption in both ascending and descending configurations to investigate surface deformation during the pre and co-eruptive period. The tropical atmospheric conditions, the highly non coherent material composing Fogo volcano and the extensive lava field make challenging the interferometric analysis in this volcanic environment. Therefore, InSAR analysis was completed by GNSS measurements. The surface deformation was analyzed for the 20 years using a compilation of GNSS campaign data performed in 1998, 1999, 2000, 2014-2015 (eruptive period) and 2017, suggesting a horizontal displacement rate up to 2 mm/yr with respect to Nubia plate. The acquired gravimetric data (~90 points) since 2014 combined with the new computed Digital Terrain Model is used here to provide complementary observation on the magma plumbing system of the volcano. This is a contribution to Project FIRE (PTDC/GEOGEO/1123/2014) funded by FCT (Portugal). The GNSS solutions were computed using resources provided by C4G – Collaboratory for Geosciences (POCI-01-0145-FEDER-022151).