The cryptic summit graben of Mt. Etna volcano

John B Murray

doi:10.1016/j.jvolgeores.2019.07.024

Abstract

Previous studies have inferred the likelihood of a summit graben at Mt. Etna, but until now there has been no direct observation or evidence of such a feature. Here, the results of 43 years of deformation measurements at the summit are reported, in which the presence of an actively forming north-south graben 1.5 km wide is evident, with a maximum subsidence of nearly 4 m in 43 years. Its bounding faults have in most places been hidden by the eruptive deposits of 190 summit eruptions, but horizontal measurements across these faults show a maximum value of nearly 9 m of east-west extension. Most of the subsidence and extension has taken place during the injection of eruption feeder dykes, with extreme values for single events of −1.7 m subsidence during the injection of the 2002–2003 dyke, and 4.4 m extension across the 1989 feeder dyke, but subsidence and extension continues to take place slowly during inter-eruptive periods. Analogue modelling indicates that its origin is probably a consequence of the gravitational spreading and downslope sliding of the Etna edifice on its clay-rich substrate, aided by magmatic pressure from feeder dykes intruded into individual faults, and by downward pressure from surface loading of erupted lava. This study clarifies the way in which the mechanism of gravitational spreading is maintained throughout the lifetime of this and other spreading volcanoes and rifts on Earth and Mars.

Highlights

Etna is the type specimen of a gravitationally spreading volcano, in which extensional structures at the summit are a consequence of outward gravitational spreading of the flanks (Borgia et al, 1992; Borgia and Murray, 2010; Merle and Borgia, 1996)
In a perfect elastic medium, dyke injection causes the ground to be raised both sides of the dyke, and slightly lowered over the dyke itself (Dieterich and Decker, 1975, Pollard et al, 1983, Okada and Yamamoto, 1991)
The north flank deformation during dyke injection has resulted in generally broader areas of fracturing and subsidence typically 2 km wide for the 7 dyke injection events described above (Fig. 3 left)

Summary

Introduction

Etna is the type specimen of a gravitationally spreading volcano, in which extensional structures at the summit are a consequence of outward gravitational spreading of the flanks (Borgia et al, 1992; Borgia and Murray, 2010; Merle and Borgia, 1996). Examination of the entire deformation record August 1975 – September 2018 reveals the existence of a graben with a floor that is sinking continuously, and bounding faults that are extending at variable rates The existence of this graben appears to have escaped notice partly because the fractures associated with its formation have been rapidly covered by up to 120 m of ash and lava from this most active volcano (Murray, 1980; Behncke et al, 2006), and perhaps because during the four decades for which data is available, it has subsided piecemeal rather than as a single unit, though the edges are well-defined on the map of cumulative movement. It has been undetectable by radar interferometry, because Etna's summit area lacks permanent scatterers (Bonforte et al, 2011)

Methods

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Discussion

Conclusion