Page 346 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 321

Figure 5 Many calderas become ¢lled with eruptive materials, mainly lava £ows, without
slipping. Active volcanism inside a caldera indicates that there must be a £uid magma chamber
beneath the caldera. If the caldera fault were outward dipping, one would expect slip to occur
due to the extra loading from the lava £ows inside the caldera (Figure 6). Dyke-fed eruptions
inside a reverse-fault caldera would encourage slip since the overpressure associated with a dyke
generates horizontal compressive stress on part of the ring fault, and this stress favours reverse
movement along the fault (cf. Figure 8).

Figure 6 Eruptive material that accumulates on the £oor of an existing caldera generates load
(Figure 5). For an outward-dipping ring fault, the vertical load has the e¡ect of opening up the
fault, resulting in very little friction along the fault walls, particularly if there forms or exists
a non-solidi¢ed ring dyke along the fault. It is thus not clear, for an outward-dipping fault, how
new lava £ows can pile up in the caldera without ring-fault slip.
elsewhere in the solar system (Scott and Wilson, 2000; Mouginis-Mark and
Rowland, 2001). Second, when lava flows fill the caldera and overflows its rims the
indication is that the caldera is able to sustain the extra load without a
corresponding slip on the existing ring fault. The present Fernandina caldera is