Page 348 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 323

Figure 9 Ring faults tend to be subvertical, but are often with somewhat variable dip. Here
is a part of the ring fault (indicated) of theTertiary Hafnarfjall central volcano inWest Iceland.
The vertical displacement across the fault is estimated at several hundred metres. A person
(indicated by a white arrow) provides a scale.

There have been many discussions about the overall dip of collapse calderas
(Figures 4–9); the main data are summarised by Gudmundsson and Nilsen (2006).
Clearly, an outward-dipping fault that extends into a shallow magma chamber, as it
must do when the chamber is the source for new lava flows erupted at the bottom
of the caldera, would tend to slip as soon as the lava pile inside the caldera reached a
moderate thickness. This follows from the load generated by the lava pile and the
extension (and thus lack of friction) between the subsiding caldera block and the
ring-fault wall resulting from the load-generated displacement (Figure 6).
For an inward-dipping ring fault, by contrast, the load-generated vertical
displacement does not result in any extension across the ring fault (Figure 7). But
there is, of course, extension across the fault due to the tensile stress concentration
(because of doming, horizontal tension or both) as discussed below. Also, when