School of Earth Sciences

Black and white: Characterising a silicic invasion

of a basaltic magma chamber

Supervisors: Dr J. Gottsmann and Professor J Blundy
Project partners: Dr R Sulpizio and Professor Piero Dellino, University of Bari, Italy

Models of explosive volcanic eruptions of intermediate and silicic magmas invoke the intrusion of fresh mafic magma in a magma chamber with resident evolved magma as a key component for eruption triggering [1,2].

There are, however, cases where the chemical stratification of pyroclastic deposits is at odds with such a model, indicating the opposite: silicic magma invading resident mafic magma prior to eruption.

The Pollara eruptive sequence on Salina island in Italy is one such example [3]. The eruption was initially fuelled by mafic magma, which transgressed into a blend of mafic and silicic magmas with the most evolved rhyolitic magma erupted last in the sequence of the Lower Pollara pyroclastics (LPP). The current model of eruption triggering of the LPP involves the arrival of rhyolitic magma in a compositionally zone mafic magma chamber, inducing a mechanical instability which eventually led to magma roof failure and eruption initiation. A hiatus in activity was followed by the eruption of mixed andesite and rhyolite magmas [4,5] forming the Upper Pollara pyroclastics (UPP). In contrast to the LPP, which is interpreted to have been dominantly produced by tephra fall, the UPP is interpreted to result from complex deposition of an unstable eruptive column dominated by pyroclastic density currents [5].

Both eruptive successions are rich in lithics derived from different subvolcanic lithologies including basement rock and cognate cumulates, providing vital insights on the structure of the sub-volcanic plumbing system, magma chamber conditions and their role on eruption dynamics.

Using the Pollara eruptive sequence as a case study, this project aims at

1. reconstructing the subvolcanic architecture prior to the two eruptive episodes by combining field-based investigations with detailed petrographic and petrological characterisation of juvenile as well as lithic components;
2. assessing and comparing fragmentation processes in both eruptive sequences in view of establishing criteria that led to the development of a stable eruption column to form the LPP series as opposed to an unstable column for the UPP series and
3. investigating the role and mechanics of silicic rejuvenation of a resident mafic magma chamber as a trigger of explosive eruption and related eruption dynamics

The project is suitable for a candidate with strong interests in physical volcanology and magmatic petrology. Prior experiences with field work in volcanic areas, the investigation of pyroclastic successions and numerical skills are of advantage.

The student will receive training in field volcanology, experimental petrology, analytical techniques such as microprobe and scanning electron microscopy, and theoretical and experimental investigations of the dynamics of explosive eruptions.

Field work will be conducted in Salina. Part of the research will be conducted with the project partners at the University of Bari.

Further reading:

[1] Murphy, M.D., Sparks, R.S.J., Barclay, J., Carroll, M.R., and Brewer, T.S., 2000, Remobilization of andesitic magma by intrusion of mafic magma at the Soufriere Hills Volcano, Montserrat, West Indies: Journal of Petrology, v. 41, p. 21 – 42.

[2] Bachmann, O., Dungan, M.A., and Lipman, P.W., 2002, The Fish Canyon magma body, San Juan volcanic field, Colorado: Rejuvenation and eruption of an upper crustal batholith: Journal of Petrology, v. 43, p. 1469 – 1503.

[3] Calanchi, N., Rosa, R., Mazzuoli, R., Rossi, P., Santacroce, R., and Ventura, G., 1993, Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing: an example from Salina (Aeolian islands, Southern Tyrrhenian Sea): Bulletin of Volcanology, v. 55, p. 504 - 522.

[4] Donato, P., H. Behrens, et al., 2006, Crystallization conditions in the Upper Pollara magma chamber, Salina Island, Southern Tyrrhenian Sea: Mineralogy and Petrology 86(1): 89 - 108.

[5] Perugini, D., Ventura, G., Petrelli, M., and Poli, G., 2004, Kinematic significance of morphological structures generated by mixing of magmas: a case study from Salina Island (southern Italy): Earth and Planetary Science Letters, v. 222, p. 1051 - 1066.

[6] Sulpizio, R., De Rosa, R., and Donato, P., 2008, The influence of variable topography on the depositional behaviour of pyroclastic density currents: The examples of the Upper Pollara eruption (Salina Island, southern Italy): Journal of Volcanology and Geothermal Research, v. 175, p. 367 - 385.