Chapter 7 Structural Development of Calderas: A Synthesis from Analogue Experiments

Abstract

A number of analogue models studying caldera architecture and development have been recently performed under different conditions (apparatus, materials, scaling parameters, stress conditions). An overview of the experiments reveals a consistent scenario for caldera structure and development, regardless of imposed boundary condition. In fact, a complete collapse can be summarised through four main stages, proportional to the amount of subsidence, progressively characterised by a: (1) downsag; (2) reverse ring fault; (3) peripheral downsag and (4) peripheral normal ring fault. A brief comparison to natural cases shows that all these experimental structures, as well as their development, are commonly observed, even at various scales. Such a consistency between models and nature suggests a general applicability of experimental results. The four evolutionary stages adequately explain the architecture and development of the established caldera end-members (downsag, piston, funnel, piecemeal, trapdoor) along a continuum, where one or more end-members may correspond to a specific stage. While such a continuum is controlled by progressive subsidence, specific collapse geometries result from secondary contributory factors (roof aspect ratio, collapse symmetry, pre-existing faults). The proposed evolutionary scheme incorporates not only the geometric features of calderas, but more importantly, also their genetic features.