The 151 ka Sutri Formation, erupted from the Vico Volcano, Central Italy, represents a caldera forming eruptive sequence consisting of associated pyroclastic fall and ignimbrite deposits and voluminous coarse, proximal lithic, spatter and pumice-rich breccia facies. After an initial limited Plinian fallout phase (Sutri A) and a partial eruption column collapse event that generated a small volume pyroclastic flow (Sutri B), the later stages of the Sutri eruption produced a southerly-dispersed lithic clast-rich breccia (Sutri C) that grades vertically into a fines poor ignimbrite (Sutri D). Sutri C is interpreted to be a co-ignimbrite lag breccia involving an element of lateral flow produced during vent widening. Vent widening in the south presumably occurred in response to vent wall rock instabilities and increased magma discharge rate during the early Plinian phase that destroyed a large portion of the southern side of the pre-existing Vico stratovolcano/shield edifice. The vertical gradation into an overlying fines poor ignimbrite (Sutri D) reflects changing source conditions at the vent, a progressive increase in magma discharge rate and cessation of vent-widening activity. A succeeding, more extensive and complex association of contemporaneous proximal lithic-rich and spatter-rich breccias that are distributed radially around the present day caldera (Sutri E4) are also considered. No stratigraphic relationship exists between the Sutri E4 breccia facies and the lower breccia facies (Sutri C) due to limited outcrop. Three subdivisions of Sutri E4 are recognised based on field analysis, grainsize studies and petrographic characteristics. All occur at the same stratigraphic level radially around the caldera and are distinguished as separate facies from Sutri C and D based on componentry: Sutri E4 (sbx) — spatter-rich breccia (north–east); Sutri E4 (lsbx) — boulder size lithic clast-rich breccia with associated spatter component (southeast–south) and Sutri E4 (flsbx) — spatter-rich breccia with associated lithic clast component (southwest–west). Mixtures of dense, boulder size lithic clasts, spatter and pumice clasts were most likely ejected up along a widening conduit in the north as well as through newly created, radially distributed fissures formed by steadily increasing magma discharge rate in response to progressive collapse of the magma chamber roof into the chamber. Eruption column collapse occurred and lithic, spatter and pumice rich co-ignimbrite breccias were emplaced radially around the vent (Sutri E4 (sbx, lsbx) signalling the onset of caldera collapse, shortly followed after by emplacement of the Tufo rosso a scorie nere (Sutri E4 (flsbx) and E5) and final caldera collapse. This study is concerned with the mechanism responsible for mixing of large variably vesiculated spatter, highly vesiculated pumice and abundant lithic clasts preserved in both these proximal breccias (i.e. in the conduit and/or eruption column or during transport and emplacement) and considers the implications for the eruption styles produced. The juvenile clast types are mixed in each breccia facies together with voluminous quantities of conduit wall rock lithic clasts. We concluded that they were generated contemporaneously within a single volcanic conduit that likely widened over time into concentrically distributed fissures. The degree of vesiculation and resulting morphology of the juvenile clasts likely reflect variations in magma rise rate whereby fast magma rise rate or faster than bubble nucleation and coalescence produced spatter and slower magma rise rates promoted bubble growth and coalescence producing pumice.

BEAR A., N., CAS R. A., F., Giordano, G. (2009). The implications of spatter, pumice and lithic clast rich proximal co-ignimbrite lag breccias on the dynamics of caldera forming eruptions: the 151 ka Sutri eruption, Vico Volcano, Central Italy. JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 181(1-2), 225-255.

The implications of spatter, pumice and lithic clast rich proximal co-ignimbrite lag breccias on the dynamics of caldera forming eruptions: the 151 ka Sutri eruption, Vico Volcano, Central Italy

GIORDANO, Guido
2009-01-01

Abstract

The 151 ka Sutri Formation, erupted from the Vico Volcano, Central Italy, represents a caldera forming eruptive sequence consisting of associated pyroclastic fall and ignimbrite deposits and voluminous coarse, proximal lithic, spatter and pumice-rich breccia facies. After an initial limited Plinian fallout phase (Sutri A) and a partial eruption column collapse event that generated a small volume pyroclastic flow (Sutri B), the later stages of the Sutri eruption produced a southerly-dispersed lithic clast-rich breccia (Sutri C) that grades vertically into a fines poor ignimbrite (Sutri D). Sutri C is interpreted to be a co-ignimbrite lag breccia involving an element of lateral flow produced during vent widening. Vent widening in the south presumably occurred in response to vent wall rock instabilities and increased magma discharge rate during the early Plinian phase that destroyed a large portion of the southern side of the pre-existing Vico stratovolcano/shield edifice. The vertical gradation into an overlying fines poor ignimbrite (Sutri D) reflects changing source conditions at the vent, a progressive increase in magma discharge rate and cessation of vent-widening activity. A succeeding, more extensive and complex association of contemporaneous proximal lithic-rich and spatter-rich breccias that are distributed radially around the present day caldera (Sutri E4) are also considered. No stratigraphic relationship exists between the Sutri E4 breccia facies and the lower breccia facies (Sutri C) due to limited outcrop. Three subdivisions of Sutri E4 are recognised based on field analysis, grainsize studies and petrographic characteristics. All occur at the same stratigraphic level radially around the caldera and are distinguished as separate facies from Sutri C and D based on componentry: Sutri E4 (sbx) — spatter-rich breccia (north–east); Sutri E4 (lsbx) — boulder size lithic clast-rich breccia with associated spatter component (southeast–south) and Sutri E4 (flsbx) — spatter-rich breccia with associated lithic clast component (southwest–west). Mixtures of dense, boulder size lithic clasts, spatter and pumice clasts were most likely ejected up along a widening conduit in the north as well as through newly created, radially distributed fissures formed by steadily increasing magma discharge rate in response to progressive collapse of the magma chamber roof into the chamber. Eruption column collapse occurred and lithic, spatter and pumice rich co-ignimbrite breccias were emplaced radially around the vent (Sutri E4 (sbx, lsbx) signalling the onset of caldera collapse, shortly followed after by emplacement of the Tufo rosso a scorie nere (Sutri E4 (flsbx) and E5) and final caldera collapse. This study is concerned with the mechanism responsible for mixing of large variably vesiculated spatter, highly vesiculated pumice and abundant lithic clasts preserved in both these proximal breccias (i.e. in the conduit and/or eruption column or during transport and emplacement) and considers the implications for the eruption styles produced. The juvenile clast types are mixed in each breccia facies together with voluminous quantities of conduit wall rock lithic clasts. We concluded that they were generated contemporaneously within a single volcanic conduit that likely widened over time into concentrically distributed fissures. The degree of vesiculation and resulting morphology of the juvenile clasts likely reflect variations in magma rise rate whereby fast magma rise rate or faster than bubble nucleation and coalescence produced spatter and slower magma rise rates promoted bubble growth and coalescence producing pumice.
2009
BEAR A., N., CAS R. A., F., Giordano, G. (2009). The implications of spatter, pumice and lithic clast rich proximal co-ignimbrite lag breccias on the dynamics of caldera forming eruptions: the 151 ka Sutri eruption, Vico Volcano, Central Italy. JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 181(1-2), 225-255.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11590/114346
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