Page 343 - Caldera Volcanism Analysis, Modelling and Response

318 Agust Gudmundsson

Figure 3 Calderas are not con¢ned to the Earth but occur on several of the solid planets and
satellites.The calderas on Mars,Venus and Io (a satellite of Jupiter) tend to be considerably
larger than most calderas on Earth. Many, perhaps most, extraterrestrial calderas are not related
to explosive eruptions but rather to a mechanism similar to that which generates shield-volcano
calderas such as on the islands of Hawaii and Galapagos (modi¢ed from Radebaugh et al.,
2000).

290 km (Leutwyler, 2003). Since many of the large calderas are multiple, and some
large calderas, particularly on Venus, contain extensive sets of concentric fracture
systems, the maximum diameter of the largest calderas is partly a matter of
definition. There is no doubt, however, that many caldera diameters on Io and
Venus exceed 200 km and the largest ones approach 300 km (Figure 3; Radebaugh
et al., 2000; Leone and Wilson, 2000; Wilson, personal communication, 2006).
In a study of 97 calderas on Venus, Krassilnikov and Head (2004) found the
average diameter to be about 68 km. Other studies indicate that the calderas are
commonly 40–80 km in diameter (Head et al., 1992; Crumpler and Aubele, 2000;
Pace and Krassilnikov, 2003). Most of the calderas are: (1) simple depressions, rather
than located at the top of major volcanic edifices; (2) comparatively shallow, with
depths of a few hundred metres to a maximum of 1.5 km; (3) unrelated to major rift
zones and (4) if volcanically active then the activity followed (rather than being
prior to) the collapse (Pace and Krassilnikov, 2003; Krassilnikov and Head, 2004).
Analysis of 292 calderas on Io gave a range in diameter from 2.5 to 203 km
(Radebaugh et al., 2000). Although the mean caldera diameter is about 40 km, the
mode (most common) diameter is close to 6 km. As indicated above, at the multiple
caldera Tvashtar Catena is presumably the largest on Io and has a maximum
diameter of at least 290 km (Leutwyler, 2003; Wilson, personal communication,
2006). Many calderas on Io have one or more comparatively straight sides and are
thus clearly controlled by tectonic structures; some calderas on Io in fact appear to
be related to regions of pull-apart (McEwen et al., 2000; Radebaugh et al., 2000).