Page 300 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 275

However, there are also some important limitations to the application of
experimental models. First, they do not provide a means to quantify important
variables such as stress or pressure. They also offer little flexibility in establishing
chamber geometries and in varying mechanical properties of the host rock
analogues. Finally, there is until now no link between solid and fluid mechanics
(the physical properties of magma). We are thus restricted in our knowledge on
magma chamber pressure and the amount of magma withdrawal before collapse.
We must be aware that experimental models offer a good approach to gain a basic
understanding of caldera-collapse processes, yet these models only provide answers
to parts of the entire spectra of dynamics.

5.2. On the use of theoretical models
In contrast to experimental models, theoretical models offer the possibility of
parameterising the variables in each model. They also permit the definition semi-
quantitatively of the general conditions for fracture and faults formation. Moreover,
they can provide a link to incorporate magma properties into simulations of collapse
and are sufficiently flexible so that a parametric study should be straightforward.
However, there are still some important limitations: insofar as there is lack of
knowledge about the system under consideration (rock and magma rheology,
geometry, boundary conditions), theoretical models may provide results, which
may lead to erroneous conclusions. Increasingly complex theoretical models require
operators to not only use the software but also users with the necessary background
knowledge to distinguish between realistic or unrealistic results. However,
theoretical models are still not able to simulate fracture and fault propagation,
which still represents a serious numerical challenge.
Despite their limitations, theoretical models offer the unique possibility to
understand when and why caldera collapse occurs. This complements the informa-
tion obtained from experimental models referring to how caldera subsidence takes
place. Theoretical models on collapse calderas provide the necessary information
to determine the stress conditions that favour collapse and also those in which
such catastrophic events will not occur. In agreement with experimental results,
numerical models also show that the occurrence of caldera collapse is strongly
dependent on the geometry of the subvolcanic system (shape and volume of the
magma chamber, chamber depth) and on the physical properties of the host rock
and magma. One of the main results from theoretical models is that only very
particular stress configuration around a magma chamber will favour collapse. We
must hence conclude that collapse will only rarely occur over the whole history of
a volcanic system. Theoretical models also indicate that ring faults controlling
caldera subsidence will form at the margins of the magma chamber. As a
consequence, the area covered by the resulting caldera depression will be of the
same order as the surface projection of the underlying magma reservoir. This
excludes, in agreement with experimental results, the formation of wide calderas
from relatively narrow magma chambers and vice versa.