Page 395 - Caldera Volcanism Analysis, Modelling and Response

370 Thomas R. Walter

deformation only on one side of the fault. Thus, dike intrusion occurs only on one
part of the circle: the numerical models support the evolutionary hypothesis
and structural development proposed for the Erongo complex (compare Figures 3
and 4).
Many caldera systems and ring-dikes form in areas that are subject to tectonic
deformation. The Askja volcanic complex is located at the divergent plate boundary
in northern Iceland and hosts the 8 km wide Askja caldera (Sturkell and
Sigmundsson, 2000). In 1875, dike intrusion occurred in the south-eastern margin
of the Askja caldera, associated with a Plinian eruption and a parasitic 4.5 km wide
¨
caldera that is now filled by Lake Oskjuvatn. The dike was part of a larger rifting
event, and reactivated the south-eastern flank of the Askja caldera ring-fault. The
models presented in this paper show that these parts of the ring-fault are subject to
opening if a remote extensional deformation source is applied, for example, a
tectonic earthquake or rifting. Dike opening due to tectonic extension is also
proposed for large-scale eruptions in Mexico (Aguirre Diaz and Labarthe
Hernandez, 2003). The so-called fissure ignimbrites are probably not directly
related to caldera collapse, but may be related to tectonic unclamping of a main dike
(Figure 10g).
A well-exposed ring-dike is known from Loch Ba at the igneous center of Mull,
in northwestern Scotland (Figure 10f ). Ring-dike emplacement occurred
during the last major intrusive event of the volcanic center, intruded directly into
a ring-fault. The dike is rhyolitic and contains up to 20% mafic inclusions, implying
significant magma mixing (Walker and Skelhorn, 1966; Sparks, 1988). The ring-
dike is thought to have intruded the ring-fault during a rapid caldera collapse
associated with the violent eruption of a welded tuff from a strongly zoned magma
chamber (Sparks, 1988). The dike is mostly vertical and dips 70–801 outward in the
north-western segment (Bailey et al., 1924). However, the ring-dike does not form
a complete ring; instead, it intrudes with a variable thickness from 0 to 400 m along
a ring 8 km in diameter. In areas where the dike did not intrude, a ring-fault is
exposed. This suggests that the ring-dike intruded an existing ring-fault where
variable opening of the ring-fault may be related to small variations in its
orientation (strike and/or dip) or magma chamber geometry.
Volcanic activity outside of a caldera system may affect the location and timing
of ring-dike intrusions. The Galapagos volcanoes are case examples for ring-dike
intrusions that are surrounded by radial dikes in the periphery (Chadwick and
Howard, 1991). Due to alternating radial and circumferential dikes, the Galapagos
volcanoes developed a typical morphology, reminiscent of an ‘‘inverted soup bowl’’
(Chadwick and Dieterich, 1995). In 1995, a radial dike intrusion occurred on the
south-west flank of Fernandina volcano (Figure 10h, just outside of the 5 km-wide
caldera (Jonsson et al., 1999). The volcano did not erupt until an eruptive dike
intruded the ring-fault in the south-western part of the caldera in 2005. The 2005
dike is a ring-dike, and its location matches the location of maximum opening as
suggested by the numerical models in this study (compare Figure 9).
The locations of ring-dike intrusions are probably consistent with areas of
hydrothermal activity, alteration, and ore deposition (Stix et al., 2003). For the
caldera of Campi Flegrei, uplift was suggested to be largely influenced by