Page 342 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 317

of subsidence and general caldera stability. For example, it is known that many
calderas have, subsequent to their formation, been filled with eruptive materials
without slipping. The conceptual models help explain the conditions under which
this is possible.
The principal aim of the numerical models is to clarify the stress conditions for
ring-fault initiation so as to help forecast the likelihood of caldera collapse during
future unrest periods. Various loading conditions and magma-chamber geometries
are considered. In particular, I consider circular and sill-like chambers in layered
crustal segments where the layers have widely different mechanical properties. The
loading conditions in the models include magma-chamber underpressure and
horizontal tension and slight doming of the segment hosting the chamber.

2. Collapse Caldera Structures

A collapse caldera is characterised by a ring fault (Figures 1 and 2). The ring
fault is concentric and normally steeply inward dipping (Figure 2). Most ring faults
are not perfect circles in plan view but rather elliptical, that is, radius a in Figure 2 is
larger than radius b. For example, in Iceland the average a/b radius of collapse
calderas in the active volcanic zones is about 1.4 (Gudmundsson and Nilsen, 2006).
Acocella et al. (2003) discuss elliptical calderas in the Ethiopian Rift in Africa, and
Holohan et al. (2005) provide a general review of elliptical calderas in various
tectonic settings. Some calderas have rectangular margins, many of which are
described by Spinks et al. (2005), for calderas on Earth, and by McEwen et al.
(2000), Radebaugh et al. (2000, 2001) and Leutwyler (2003) for calderas on Io.
In the following paragraphs, I give a brief description of the caldera structures
on Io, Mars and Venus, and compare them with calderas on Earth. A general
outline of volcanic activity and landforms on the solid planetary bodies, however, is
beyond the scope of the present paper. Fortunately, there are many recent reviews
on extraterrestrial volcanism, including the relevant chapters in Sigurdsson (2000)
and Lopes and Gregg (2004). Much general information is also provided by Frankel
(2005). Physical data on the planetary bodies discussed here, as well as maps
and photographs of many volcanoes and calderas, can be obtained from Lodders and
Fegley (1998), Greeley and Batson (2001), Leutwyler (2003) and Miller and
Hartmann (2005), as well as from the books and papers cited above and below.
There is a significant difference in caldera sizes on Earth and other planetary
bodies (Figure 3). Since many calderas are multiple or form clusters, deciding the
(maximum) diameter of a caldera may not be as straightforward as it might seem.
This is, no doubt, the main reason for the different maximum sizes for calderas
quoted by various authors. Nevertheless, Radebaugh et al. (2000, 2001) have
analysed the sizes of calderas on Venus, Earth, Mars and Io. The size (diameter)
distributions are negative exponential laws rather than normal (Gaussian) laws. The
authors found that the mean diameter of calderas on Venus is about 68 km, on Mars
48 km and that on Io 41 km. Calderas on Venus reach up to 225 km in diameter
(Frankel, 2005), and those on Io (the multiple caldera Tvashtar Catena) up to