Page 349 - Caldera Volcanism Analysis, Modelling and Response

324 Agust Gudmundsson

eruptions onto the caldera floor are supplied with magma through dykes (Figure 8),
the compressive stresses generated by the dykes would encourage reverse slip on an
otherwise normal ring fault and, thus, tend to lock the fault. Because of friction
along the fault plane (Figures 7 and 9), ring-fault slip is much more easily stopped
on an inward-dipping fault, after a certain displacement, than on an outward-
dipping fault (Figure 6).

3. Geometry of the Magma Chamber

Most ring faults are generated by the local stresses around crustal magma
chambers (Figures 1, 2 and 4–8). Field studies of plutons in deeply eroded
palaeovolcanic zones suggest that many crustal chambers, at least during the end
stages of their evolution, have shapes not far from ideal ellipsoids (Gudmundsson,
2006; Gudmundsson and Nilsen, 2006).
In Iceland, for example, there are many well-exposed crustal magma chambers
(plutons) at 1.5–2 km depth of erosion in extinct composite volcanoes (central
volcanoes, stratovolcanoes). Most of these plutons, representing the uppermost
parts of extinct shallow crustal magma chambers, are of gabbro (Gudmundsson,
2000a, 2006). But there are also many felsic plutons in deeply eroded roots of
extinct composite volcanoes in Iceland. Similarly, extinct crustal magma chambers
of various sizes and depths of exposure occur in other deeply eroded volcanic
regions on Earth, such as in Scotland (Upton, 2004).
Commonly, there are ring faults and ring dykes associated with the extinct
chambers. The caldera fault in the Tertiary Hafnarfjall Volcano in West Iceland
(Figure 9), for example, can be traced to a shallow gabbro pluton, the uppermost
part of an extinct magma chamber (Gautneb et al., 1989). Extinct, well-exposed
crustal magma chambers of this type, as well as geophysical studies of active magma
chambers, indicate that chamber geometries are commonly approximately similar
to ideal ellipsoidal bodies (Figure 10; Gudmundsson, 1998b, 2002, 2006). Shallow
chambers are normally located in crustal segments, which, during most unrest

Figure 10 Ideal magma chambers are ellipsoidal.Three main ellipsoidal geometries of magma
chambers are (A) a sphere, (B) an oblate ellipsoid, that is, a sill-like chamber, only half of which
is shown here and (C) a prolate ellipsoid (modi¢ed from Gudmundsson and Nilsen, 2006).