Page 370 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 345

magma can be driven out of the chamber when its excess pressure is supposed to
be zero or even negative (Equations (3) and (4)). Theoretical and experimental
results indicate that it is unlikely that the excess pressure in the chamber can
become negative while magma continues to be driven out of the chamber.
3. Several new conceptual models as to ring-fault initiation and development are
presented in the paper. These indicate that most ring faults are likely to be of
somewhat irregular dips (Figure 4), but that the general fault plane is mostly
vertical or steeply inward dipping (Figures 1, 2 and 7–9). While outward-
dipping ring faults may exist, calderas associated with such faults are likely to be
mechanically very unstable and unlikely to become filled with lava flows without
slip, as is common in many calderas on Earth and other planets (Figure 5).
Formation of outward-dipping ring faults would normally result in emptying of
the associated magma chamber and, therefore, in very large eruptions. Many,
perhaps most, caldera slips and associated eruptions, however, are small.
4. The main results of numerical models of ring-fault formation (Figures 14–19)
are as follows: (a) Excess pressure and underpressure in a shallow chamber
normally favour dyke injection rather than ring-fault formation. (b) For doming
or tension, a spherical magma chamber favours dyke injection except when the
layer hosting the chamber is soft (10 GPa) or one with recent dyke injections, in
which case the surface stress field favours ring-fault formation. (c) For an oblate
chamber in a 20 km wide crustal segment, a ring fault can be generated either by
tension or tension and doming; for a 40 km wide segment, doming alone is
sufficient to generate a ring fault. (d) The individual layers in a volcano may
develop different local stresses; it follows that stress-field homogenisation
through all the layers between the chamber and the surface is a necessary
condition for ring-fault formation. (d) Because the mechanical properties of the
layers that constitute basaltic edifices are more uniform than those that constitute
composite volcanoes, stress-field homogenisation and, thus, ring-fault formation
or slip is more commonly reached in basaltic edifices than in composite
volcanoes. (e) The stress fields most likely to initiate ring faults in all volcano
types are those generated around oblate ellipsoidal chambers subject to tension,
doming or both.

ACKNOWLEDGEMENTS

I thank Isabel Bivour, Steffi Burchardt, Gabriele Ertl, Nadine Friese, Kristine Nilsen and Sonja Geilert
for help with figures and for running some of the numerical models. I also thank A. Folch and an
anonymous reviewer for very helpful comments. Part of the work reported here was supported by a
grant from the European Commission through the project ‘Prepared’ (EVG1-CT-2002-00073).

REFERENCES

Acocella, V., Cifelli, F., Funiciello, R., 2000. Analogue models of collapse calderas and resurgent
domes. J. Volcanol. Geotherm. Res., 104, 81–96.