Page 344 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 319

When referred to as ‘paterae’ rather than calderas, as is common for Io, about 42%
of the calderas have straight or irregular margins (Radebaugh et al., 2001).
The mean diameter of calderas on Mars is somewhat smaller than on Venus, or
48 km compared with 68 km on Venus, but larger than the mean diameter (41 km)
of calderas on Io (Radebaugh et al., 2000, 2001). Perhaps the best known caldera
on Mars is the multiple and nested caldera on Olympus Mons (Mouginis-Mark and
Rowland, 2001), but many other calderas have been studied in detail (Crumpler
et al., 1996; Scott and Wilson, 2000). Most of the calderas on Mars appear to have
inward-dipping fault scarps (Scott and Wilson, 2000).
Calderas on Earth are much smaller than those on Venus, Mars and Io. Earth
calderas vary in maximum diameter from about 1.6 km to about 80 km, the largest
one being multiple or geometrically complex (Lipman, 2000) like the largest
calderas on other planetary bodies. Calderas on Earth can be classified in various
ways according to the types of volcanoes to which they belong (Francis, 1993;
Lipman, 2000; Radebaugh et al., 2001; Krassilnikov and Head, 2004; Cole et al.,
2005). Generally, however, calderas associated with basaltic shield volcanoes
(Hawaii, Galapagos) tend to be smaller than those associated with composite
volcanoes (including ‘ash flow’ calderas). Thus, the mean maximum diameter of
active calderas associated with shield volcanoes is 6–7 km, whereas that of calderas
associated with composite volcanoes is 18–19 km (Radebaugh et al., 2001).
Similarly, maximum diameters of Quaternary calderas range from about 1.6 to
50 km, with about 94% of them having maximum diameters of less than 20 km
(Lipman, 1997; Krassilnikov and Head, 2004).
Many calderas on Earth have been filled with lava flows to their rims. For
example, the Mokouaweoweo caldera at the Mauna Loa volcano, Hawaii, shows
clear evidence of lava flows having flowed out of the present caldera rims,
suggesting that when lava flowed the caldera was full. Similar results have been
obtained from the Kilauea caldera, suggesting that both calderas have been filled to
overflow during the past 1,000 years (Mouginis-Mark and Rowland, 2001). Also,
the Galapagos calderas are all of similar diameters (average 6 km) but the depths vary
from about 200 m (Sierra Negra) to about 920 m (Fernandina). These calderas
show clear evidence of having varied in depth over time, presumably partly as a
function of the degree of lava filling. Furthermore, there is evidence that some
calderas on Mars and Venus have become partly or completely filled with lava flows
(Mouginis-Mark and Rowland, 2001).
Partial and complete filling of calderas has implications for their mechanical
strength and stability and, therefore, for the dips of the ring faults. There is no
doubt that most ring faults are subvertical dip-slip faults (Figures 1, 2 and 4–9).
While it is likely that the dips of ring-fault segments vary depending on the local
stresses in individual layers, particularly in composite volcanoes (Figure 4 and 9), the
average dip is an important factor in any model of ring-fault formation
(Gudmundsson and Nilsen, 2006). Some studies indicate that the general dip of
the ring fault is outward from the centre of the caldera (Williams et al., 1970;
Branney, 1995; Cole et al., 2005); other studies indicate that the ring fault is generally
steeply inward-dipping (Macdonald, 1972; Filson et al., 1973; Aramaki, 1984;
Lipman, 1984, 1997, 2000; Newhall and Dzurisin, 1988; Gudmundsson, 1998a;