Page 331 - Caldera Volcanism Analysis, Modelling and Response

306 Valerio Acocella

Figure 12 Evolution of natural calderas geometries, summarized in four stages, which derives
from the merging of the experimental and natural data.To the right, the classic caldera end-
members (Lipman,1997; Cole et al., 2005) and related conditions to form.

When the normal ring fault, peripheral and concentric to the reverse fault,
reaches surface, it forms two nested collapses (Stage 4). Such caldera pairs, common
in nature, may relate to the same collapse episode, not necessarily to two distinct
collapse events (Marti et al., 1994; Acocella et al., 2001). Likely candidates are Campi
Flegrei, Latera, Long Valley, Valles, Miyakejima, La Garita, Fernandina and Rabaul
(Self et al., 1986; Carle, 1988; Barberi et al., 1994; Munro and Rowland, 1996; Orsi
et al., 1996; Lipman, 1997; Geshi et al., 2002; Bai and Greenlangh, 2005).
In this evolutionary frame, additional remarks concern the trapdoor and
piecemeal end-members. The experiments suggests that trapdoors may develop as a
result of asymmetries in the chamber analogue (Acocella et al., 2001), in the roof
load (Kennedy et al., 2004) or of small other heterogeneities (Roche et al., 2000).
In general, they may be associated with very small roof aspect ratios (thin crust
compared to chamber width), where their formation may be enhanced by an
asymmetric load distribution above the magma chamber. Trapdoor calderas show,
in addition to a portion of ring fault, a downsagged side. Trapdoors may therefore
represent specific asymmetries during Stages 2 to 4. With lower subsidence, a
downsag structure forms on one side and a (reverse) fault on the opposite (Stage 2
caldera). Higher subsidence may induce (reverse) faulting on the flexured side and
form an additional (normal) fault at the periphery of the reverse fault (Stage 4
caldera). However, fully developed Stage 4 calderas should not be trapdoor, as the
downsag should be completely replaced by the outer ring fault.