Page 381 - Caldera Volcanism Analysis, Modelling and Response

356 Thomas R. Walter

Various model geometries were considered, and tests were conducted for reservoir
radii of 5 and 10 km, yielding very similar results. For spherical chambers, a radius
of 5 km was used, while for elliptical chambers, 10 5 km was chosen. These
diameters agree with most known calderas (Newhall and Dzurisin, 1988). The
depth chosen for the chambers was 10 km to the reservoir center, thus between
5 and 7.5 km to the reservoir roof. Although realistic loading conditions and
geometries were applied for all models, the results are used mainly in a qualitative
way. The main goal of these model simulations is to emphasize the geometric
complexities of ring-dikes at various caldera systems.

3. Results

Three main sets of results are distinguished. First, deformation around a
depressurized magma chamber is described together with how local structures can
affect deformation. Second, the amount a ring-fault surrounding the depressurized
magma chamber opens is described in order to examine the potential locations of
dike intrusions. Third, models are designed to exemplify how processes external to
the caldera, such as an earthquake or intrusion, can affect the location of a ring-dike
intrusion. The results are shown in map view, cross-section, and side views as
displacement vectors and contour plots. A coordinate system is indicated in each of
the figures for orientation; x–y is used for the horizontal plane and z denotes the
vertical direction, so a view in the x–z plane is a side view.

3.1. Deformation around a depressurized magma chamber
3.1.1. Deflating spherical magma chamber
First, a simple scenario with a deflating magma chamber embedded in a uniform
elastic material is considered. The magma chamber is spherical, with a radius
r ¼ 5 km, located at a depth d ¼ 10 km below the surface (i.e. the depth of the roof
of the chamber is 5 km). The magma chamber is subjected to a pressure drop of
10 MPa; this is the only type of loading in these models. The model setup is shown
in Figure 2A, and results are shown in map view (Figure 2B) and along a cross-
section west to east (x–xu, Figure 2C, D). The horizontal displacement field, shown
by displacement vectors, indicates movement of the material towards the deflating
source. Contours in Figure 2B indicate the amount of vertical displacement (U z ),
which is negative and thus defines subsidence. The cross-sections indicate the
horizontal displacement field (U x , Figure 2C), showing that the material on the
west side of the chamber is displaced to the east, while the material on the east of
the chamber is displaced to the west (Figure 2D). At the surface, the maximum
horizontal displacement occurs at a slightly eccentric location above the edge of the
chamber. The vertical displacement field (U z ) shows that the material above the
chamber sinks downward (Figure 2D); maximum subsidence occurs in a bell-
shaped area just above the magma chamber. Below the chamber, a slight upward