Page 369 - Caldera Volcanism Analysis, Modelling and Response

344 Agust Gudmundsson

outward-dipping ring fault (Figures 5 and 6). Consequently, slip on outward-
dipping faults and the associated eruptions would be expected to be very large
because the rock block bounded by the ring fault (the piston) would tend to subside
to the bottom of the chamber and thus drive out much of, or all, its magma. But we
know that many terrestrial caldera slips and associated eruptions are small, and it is
thought that many, perhaps most, collapse calderas on the planets and satellites are
not associated with large eruptions.
These results are also in agreement with the numerical models (Figures 14–19).
In these models, slight doming of the crustal segment hosting the chamber,
horizontal tension of that crustal segment or both can initiate ring faults. By
contrast, underpressure does not really generate a stress field that is likely to initiate
typical ring faults (Figure 16).
Ring dykes are associated with many ring faults (Figures 5, 6, 8 and 12). This
relationship obviously needs further exploration, since the timing of the ring-dyke
emplacement may be crucial in reducing the friction along the fault plane and, thus,
to allow slip more freely. For most caldera collapses, the sequence of events is also
unclear. One possibility is that the ring dyke initiates at the surface of the magma
chamber and then meets with a downward-propagating ring fault. Another
possibility is that the ring fault propagates, either from the free surface or from some
layers in the roof of the chamber, down to the top part of the magma chamber and,
on meeting the chamber, is injected by magma to form a ring dyke.

8. Conclusions

1. Although the details of the mechanics of formation of ring faults are still not fully
understood, it is known that most ring faults of collapse calderas are primarily
shear fractures, the initiation and development of which depend on the state of
stress in the host rock. Also, it is known that the state of stress in a volcano is
primarily controlled by the mechanical properties of its rock units and structures
(such as existing contacts, faults and joints), as well as by the loading conditions,
in particular, the geometry and magma pressure of the associated chamber.
Field studies of active and extinct calderas show that ring faults are generally
dip-slip faults, although many are partly faults (shear fractures) and partly ring
dykes (extension fractures). While slip on existing ring faults is much more
common in basaltic edifices (shield volcanoes) than in true composite
volcanoes, in both types of volcanoes most caldera unrest periods do not result
in ring-fault slip.
2. The paper provides a review of some popular models of ring-fault formation, in
particular, the underpressure (lack of magmatic support) model. While this
model is very appealing in its simplicity (the erupted magma leaves an empty
cavity into which the roof subsides, forming a caldera), there are several
difficulties with this explanation (Gudmundsson and Nilsen, 2006). Apart from
lack of volume correspondence between the magma leaving the chamber and the
volume of the collapse, perhaps the main mechanical problem is to explain how