The initiation and development of a caldera-forming Plinian eruption (172 ka Lower Pumice 2 eruption, Santorini, Greece)

摘要

The rhyodacitic 172 ka Lower Pumice 2 (LP2) eruption terminated the first magmatic cycle at Santorini (Greece), producing a proximal <50 m thick succession of pyroclastic fall deposits, diffusely -stratified to massive ignimbrites and multiple lithic breccias. The eruption commenced with the development of a short-lived precursory eruption column, depositing a <15 cm blanket of 1-2 cm sized pumice fragments at near vent localities (LP2A1). The precursor deposits are conformably overlain by a <30 m thick sequence of reversely-graded/ungraded pumice fall deposits that reflect opening and widening of a point -source vent, increasing mass discharge rates up to 10(8) kg s(-1), and the development of a 36 km high Plinian eruption column (LP2-A2, A3). The progressive increase in maximum vesicle number density (N-vF) in rhyodacitic pumice, from 3.2 x 10(9) cm(-3) in the basal fall unit of LP2-A2-1 to 9.2 x 10(9) cm(-3) in LP2-A3, translates to an increase in magma decompression rate from 18 to 29 MPa s(-1) over the course of the initial Plinian phase. This is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface. Such interpretations are supported by the increase in lithic clast abundance vertically through LP2-A, and the occurrence of basement -derived (deep) lithic components in LP2-A3. The increasing lithic clast content and the inability to effectively entrain air into the eruption column, due to vent widening, resulted in column collapse and the development of pyroclastic density currents (PDCs; LP2-B). A major vent excavation event or the opening of new vents, possibly associated with incipient caldera collapse, facilitated the ingress of water into the magmatic system, the development of widespread PDCs and the deposition of a <20m thick massive phreatomagmatic tuff (LP2-C). The eruption cumulated in catastrophic caldera collapse, the enlargement of a pre-existing flooded caldera and the discharge of lithic-rich PDCs, depositing proximal <9 m thick lithic lag breccias (LP2-D). (C) 2017 Elsevier B.V. AM rights reserved.