Interplay between magmatic activity and sector collapse illustrated by Planchon volcano (Andean southern volcanic zone) and Stromboli volcano (Aeolian Arc)

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Tormey, Daniel۱; Tibaldi, Alessandro۲; Pasquare, Federico۳
Holding Date 03 September 2008

The interplay between magmatic activity and sector collapse depends on a series of feedback mechanisms controlled by magma physical chemistry, depth of magma storage, and structural evolution of the shallow crust. The linkages are best understood by integrated studies of field relations, geochemistry, petrology, structural geology, geophysical monitoring, and numerical and analogue modeling. The processes are well illustrated by studies at Planchon Volcano (Andean SVZ) and Stromboli Volcano (Aeolian Arc).
At Planchon Volcano, located in remote, steeply glaciated terrain on the crest of the Andes at 35° 15 ’S, there has been interplay between magmatic activity and sector collapse, where shallow-level magmatic activity may have facilitated sector collapse. Sector collapse then caused southward migration of volcanic activity, and the shallow-level magmatic activity temporarily ceased. Post-sector collapse magmatic activity created the Planchon II edifice, which was rapidly dissected; it had a larger proportion of near-vent pyroclastic deposits than Planchon; this structural weakness may have contributed to the rapid self-destruction of the cone by landslides. Detailed geochemical study of Planchon documents that most basalt and basaltic andesite lavas erupted from Planchon evolved at mid-crustal depths of approximately 12 to 18 km depth. Prior to the debris avalanche, however, a series of flows indicate shallow-level fractional crystallization and assimilation of shallow crustal wall rocks. The depth appears to be within the upper 3 km of the crust. Although the temporary development of a shallow magma body may have ultimately facilitated the debris avalanche, either through increased magma pressure and summit inflation, enhanced hydrothermal activity, or melting of the summit ice cap, the data do not allow a definitive conclusion.
Recent numerical and analogue modeling of the interplay between magmatic activity and sector collapse conducted at Stromboli Volcano in the Aeolian Arc allows the above conclusions to be extended with greater certainty. Stromboli has a comparable history of sector collapse and migration of magma centers. Numerical modeling evaluating stability under static conditions, seismic loading, magmastatic conditions, and magma overpressure conditions, indicate that a magma overpressure at shallow levels is required to initiate sector collapse. Analogue modeling arrives at the same conclusion. Post-collapse migration of eruptive location towards the sector collapse is also observed at Stromboli Volcano and may represent migration towards a zone of lower lithostatic pressure. Analogue modeling shows that stress reorientation following collapse tends to focus dyking towards the collapse zone. Numerical modeling further supports that debuttressing from the collapse depression affects the stress distribution , and thus magma paths, on the order of a few hundred meters from the active center; as observed at Planchon.