Page 330 - Caldera Volcanism Analysis, Modelling and Response

Structural Development of Calderas 305

Kumano (Japan; Miura, 2005). These collapses show a differential subsidence,
accommodated by ring faults (deeper part) and flexures (shallower part).
Therefore, the experimental structures are commonly observed in nature.
Moreover, their progression is also commonly found at several well-studied calderas
(as for example, Sabaloka, Ishizuchi, Grizzly Peak, Miyakejima, Bolsena, Guayabo).
At many calderas, these structural features may easily overlap. This may occur with
advanced subsidence, when older structures remain preserved, or with asymmetric
collapse, developing structures only apparently corresponding to different collapse
stages along the caldera.
This overview highlights the importance of defining the structure of a caldera
beyond its overall morphological expression, especially at young calderas. In fact,
only the precise assessment of certain structural features (ring faults, downsags)
should be employed as a diagnostic feature of collapse types.

6. Towards a New Caldera Evolution Scheme

The experiments and their matching to nature suggest an original revision of
the structure, mechanism of development and relationships of the established
caldera types described in the literature: downsag, piston-type, funnel, trapdoor and
piecemeal. In fact, the consistency among experiments and nature suggests that the
architecture and development of the caldera end-members described in the
literature may be contextualised within a subsidence continuum (Figure 12;
Acocella, 2006a, 2007). Along such a continuum, the exact geometry and style of
the established caldera types may be related to contributory factors, as roof aspect
ratio, symmetries, reactivation of pre-existing faults.
Downsag calderas, corresponding to experimental Stage 1 form under limited
subsidence and are the starting architecture to develop any other collapse. Most
collapses will pass through the downsag phase, explaining the moderate number of
downsag calderas in nature. Likely candidates are Buckhorn and Bracciano (Henry
and Price, 1984; Di Filippo, 1993).
The increase in subsidence results in the partial or full development of
a high angle reverse ring fault (Stage 2), whose surface expression in nature
is a rim scarp. In these calderas, the rapid decay of the overhanging reverse fault
scarp develops subvertical to inward dipping caldera rims, towards the outer
border of the ring fault. Therefore, the structural boundary of Stage 2 calderas
lies in an inner position with regard to the morphological rim. The fault may
bound piston or funnel collapses, with roof aspect ratios Ao1or AW1,
respectively (Roche et al., 2000; Figure 12). Likely candidates of Stage 2
calderas are Erta Ale, Kilauea, Krafla and Sierra Negra (Newhall and Dzurisin,
1988; Munro and Rowland, 1996; Acocella, 2006b; Gudmundsson and Nilsen,
2006).
Increasing the subsidence further forms an external downsag bordering the
reverse fault (Stage 3). This downsag is distinguished from that of Stage 1 by the
presence of the inner ring fault, formed at Stage 2. Likely candidates are Rotorua
and Bolsena (Barberi et al., 1994; Milner et al., 2002).