Relationship between caldera collapse and magma chamber withdrawal: An experimental approach A. Geyer ⁎ , A. Folch, J. Martí Institute of Earth Sciences Jaume Almera (IJA-CSIC), Lluís Solé Sabarís s/n, 08028 Barcelona, Spain Received 25 October 2005; received in revised form 10 April 2006; accepted 1 May 2006 Available online 3 July 2006 Abstract Collapse calderas have received considerable attention due to their link to Earth's ore deposits and geothermal energy resources, but also because of their tremendous destructive potential. Although calderas have been investigated through fieldwork, numerical models and experimental studies, some important aspects on their formation still remain poorly understood. One key issue concerns the volume of magmas involved in caldera-forming eruptions. We perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction. The experimental device consists of a transparent box (60 × 60 × 40 cm) filled with dry quartz sand and a water-filled latex balloon as a magma chamber analogue. Evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure. The experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults. We study the appearance and development of specific brittle structures, such as surface fractures or internal reverse faults, and correlate each different structure with the corresponding removed magma chamber volume fraction. We also determine the critical conditions for caldera onset. Experimental results show that, at any stage of caldera developments, the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve. It implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios (shallow and wide) than for chambers with high aspect ratios (deep and small). These results are in agreement with natural examples and previous theoretical studies. © 2006 Elsevier B.V. All rights reserved. Keywords: volcanology; caldera collapse; analogue models; ring faults 1. Introduction It is generally accepted that collapse calderas form by subsidence of the roof of a magma chamber, commonly during the course of an eruptive event (e.g. Williams, 1941; Druitt and Sparks, 1984; Lipman, 1997; Martí et al., 2000; Roche and Druitt, 2001). Although collapse calderas have been the subject of multiple studies due to its huge destructive potential and to their link to many of the Earth's ore deposits and geothermal energy resources (e.g. Lipman, 1992; Guillou-Frottier et al., 2000), some important aspects on caldera dynamics and structure still remain uncertain and controversial. Experimental studies have improved notably the knowledge of the different collapse mechanisms and have explained the origin of different caldera morphol- ogies observed in nature (e.g. Komuro et al., 1984; Journal of Volcanology and Geothermal Research 157 (2006) 375 – 386 www.elsevier.com/locate/jvolgeores ⁎ Corresponding author. Fax: +34 934011112. E-mail address: ageyer@ija.csic.es (A. Geyer). 0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.05.001