Page 345 - Caldera Volcanism Analysis, Modelling and Response

320 Agust Gudmundsson

Figure 4 When the mechanical properties (particularlyYoung’s modulus) of the layers that
constitute a composite volcano change abruptly from one layer to another, the local stresses in
the layers may be very di¡erent. Consequently, the local stresses that favour outward-dipping
ring-fault segments in some layers may favour vertical or inward-dipping segments in other
layers (Gudmundsson, 2007).

Geshi et al., 2002). It should be noted that Glencoe in Scotland is often taken as an
example of an outward-dipping ring fault but is actually a tilted ring fault — part of
the fault dips outward and part inward — that was originally with a dip close to
vertical (Anderson, 1936).
The conceptual model of a steeply inward-dipping ring fault is supported by the
well-documented collapse of the Fernandina caldera in the Galapagos Islands in
1968; the collapse occurred on a ring-fault dipping inward at about 801 (Simkin
and Howard, 1970). More recently, the collapse of the Miyakejima caldera in Japan
in 2000 was primarily on an inward-dipping ring fault (Geshi et al., 2002). Also,
many ring faults are occupied by dykes, most of which are vertical or steeply inward
dipping (Oftedahl, 1953; Almond, 1977).
Lava flows that erupt on the floor of a caldera indicate, first, that there is an
active (that is, at least partly fluid) magma chamber beneath the caldera (Figure 5).
Clearly, if there was no magma chamber beneath the caldera there would be no
source for the lava flows to be erupted inside the caldera. In fact, eruptions inside
calderas are very common, both on Earth (Newhall and Dzurisin, 1988) and