Page 347 - Caldera Volcanism Analysis, Modelling and Response

322 Agust Gudmundsson

as deep as 900 m. If it became filled with lava flows, as has occurred on many of
the Galapagos calderas, the extra load on the caldera floor would be 20–25 MPa
(Figure 6). That the caldera is able to sustain this load without slip has implications
for the likely dip of the ring fault.

Figure 7 Vertical loading of a caldera £oor by eruptive material such as lava £ows need not
cause slip on an inward-dipping ring fault, anymore than on graben faults. Because of the
inward dip, the vertical load generates compressive stresses across the ring fault, resulting in
resistance to vertical slip. Calderas associated with inward-dipping ring faults can thus be ¢lled
with eruptive materials without slipping.

Figure 8 Many caldera-£oor eruptions are fed by dykes, the overpressure of which generates
additional compressive stress on parts of the ring fault. For an inward-dipping fault, the compressive
stress would encourage reverse movements on an otherwise normal fault and thus tend to lock the
fault and make it non-slipping. By contrast, overpressured dykes would tend to encourage slip on
an outward-dipping ring fault (Figure 5).