Caldera structure, amount of collapse, and erupted volumes: The case of Bolsena caldera, Italy

Abstract

Calderas are common on volcanoes, but their structure is seldom visible. The 19-kmwide Bolsena caldera, Italy, formed between 0.6 and 0.2 Ma. The largely preserved structural rim and subsurface data make Bolsena ideal to investigate caldera structure in relation to the subsidence and erupted volume. In this paper, we use remote sensing, fi eld analysis, and available subsurface data. At the surface, the caldera passes from a down-sag (south rim) to a narrow and densely faulted area (north rim), with outer normal and inner reverse faults. The caldera structure on the widely faulted east rim appears to be scale dependent, with a staircase-like fault zone (larger scale), horst-and-graben-like structures (intermediate scale), and domino-like structures (smaller scale). Subsurface data indicate asymmetric collapse, with a northward increase in subsidence, ranging from diffuse (to the south) to focused (to the north) deformation at the surface. The collapse rate, constant between ca. 490 and 175 ka, was more than magma output between ca. 330 and 130 ka, highlighting signifi cant (~200 m) and prolonged (~200 ka) posteruptive subsidence. As the nearby Latera caldera (west rim of Bolsena) was mostly active between ca. 265 and 160 ka, much of the subsidence at Bolsena may be related to this activity, suggesting a common magmatic reservoir. The subsidence-related structural variations along the caldera rim and the signifi cant posteruptive subsidence found at Bolsena have not been found in other calderas. © 2012 Geological Society of America

Calderas and magma reservoirs

“…Giordano and Dobran, 1994;Watkins et al, 2002). As is typical for other stratovolcano eruptions, none of these caldera complexes has undergone post-collapse resurgence; instead, post-collapse subsidence, intracaldera volcanism and sedimentation are common (Giordano et al, 2006;Acocella et al, 2012).…”

“…The QRMP comprises four major caldera complexes that have produced recurrent eruptions of tephritic to tephri-phonolitic ignimbrites with DRE (dense rock equivalent) volumes of 1-50 km 3 ( Fig. 5; Giordano et al, 2006;Boari et al, 2009;Masotta et al, 2010;Freda et al, 2011;Vinkler et al, 2012;Acocella et al, 2012). The recurrence times for caldera-forming eruptions at each caldera complex are 40 to 50 ka, and caldera areas range from 30 to 300 km 2 .…”

Structural analysis and thermal remote sensing of the Los Humeros Volcanic Complex: Implications for volcano structure and geothermal exploration

“…Caldera collapse starts and mainly occurs on steeply outward-dipping reverse ring faults (Yoshida, 1984;Komuro, 1987;Martí et al, 1994;John, 1995;Geshi et al, 2002;Hardy, 2008;Geyer and Martí, 2014). Following the gradual increase of subsidence, outer inward-dipping normal ring faults may form during the collapse, enlarging the caldera perimeter (Geshi et al, 2002;Cailleau et al, 2003;Lavallèe et al, 2004;Holohan et al, 2005;Michon et al, 2009;Acocella et al, 2012). With an increase of the magmatic overpressure, doming and resurgence may occur before and after the caldera collapse, generating concentric and radial inward-dipping faults and fractures (e.g.…”

On the role of hydrological processes on the water balance of Lake Bolsena, Italy

“…The history of this volcanic district is both complex and uncertain, with its formation attributed to a long series of explosive phases and collapses of volcanic structure that gave rise to the entire Bolsena Lake basin (Acocella et al . ; Mazza et al . ).…”