title: Near-vent processes of the 273 ka Poris eruption (Tenerife)
Creators: name: Smith, Natasha
Publisher: name: University of Liverpool Repository
Year: 2012

description: Near-vent processes during caldera-forming explosive eruptions are poorly known, as are relations between proximal and counterpart distal ignimbrites. Consequently, understanding of the full course of these eruptions and how to analyse their deposits for purposes of hazard assessment has been limited. This study aims to address the limitations by using a lithofacies architecture approach to investigate a proximal succession that has a closely studied distal counterpart. The Poris Formation (273 ka) of Tenerife is well exposed in the Diego Hernandez wall of Las Cañadas and detailed interpretations of its distal-flank ignimbrites have been published. In this study, Poris proximal lithofacies architecture is described, geochemical and stratigraphic analyses are used to correlate the proximal and distal successions, and a coherent model for the Poris eruption is presented. The proximal Poris succession records an opening Plinian phase, followed by the generation of a pyroclastic density current (PDC) that was partly blocked by near-vent topography. The upper part of the current surmounted the barrier and deposited an ignimbrite veneer. During a widespread hiatus in flow, ash and various ash aggregates were deposited from a hybrid cloud that included parts of both the Plinian and co-ignimbrite plumes. Phreatomagmatic explosivity then generated an erosive PDC rich in fine lithics; initial waxing was followed by pulsatory current waning, recorded in repeated sets of inversely graded tuff. Hybrid deposition ensued, as coarse pumice fallout and ballistic material entered PDCs to become deposited as a stratified pumice-block tuff. A main phase of PDC activity then occurred, recorded by 9 m of massive lapilli tuff with evidence of widespread scour and bypass, and including lithic-rich layers that record incremental edifice disruption. Laterally variable stratification and load structures show that the PDC was highly non-uniform and unsteady at this time. Climactic caldera collapse is recorded proximally as a lithic-block layer in erosive contact with the underlying ignimbrite, which marks widespread bypass to distal slopes. A pumice-block breccia 10 m thick records the eruption waning. Published accounts of the distal Poris stratigraphy are described and correlation with the proximal record is made based on physical characteristics and geochemical trends. The occurrence of distinctive grey accretionary pellets in an ash layer above the proximal phreatomagmatic unit and in the distal Magua Member (of Brown and Branney, 2004) is regarded as an ‘entrachron’, marking temporal correlatives. The influx of lithics that accompanied climactic collapse is recorded at the main lithic-block layer proximally and in distal lithic-breccia deposits; these constitute another entrachron. Geochemical analysis finds that two petrogenetically distinct magmas were continuously available throughout the Poris eruption: a high-Zr series and a low-Zr series. Both the proximal hybrid unit and a distal Plinian unit (the Caballos Member of Brown and Branney, 2004) have a relatively low Zr signature, linking the two and providing evidence of increased mingling between the magma series at this stage in the eruption. The low-Zr magma was tapped extensively following climactic collapse; the predominance of black and banded pumice blocks in the resultant deposits is a further correlative marker linking the proximal and distal records. The Poris eruption involved 8 distinct phases: Plinian deposition, first PDC generation, phreatomagmatism, hybrid fall and flow, main stage PDC activity and ignimbrite deposition with scour and bypass, caldera collapse, waning PDC, and waning fallout. In contrast to the model founded on the distal succession, which invokes four PDCs, it is found that only two were generated during the Poris eruption: the first, relatively small, and the second, large and sustained. Distal flow hiatuses occurred during waning following the phreatomagmatic and hybrid phases, but flow continued in the proximal zone at these times. This work is relevant to studies of ash aggregates, as it highlights differences between proximal and distal deposits and those at different levels in the stratigraphy, and it is relevant to studies of Plinian deposits. The evidence of widespread hybrid deposition emphasises that the use of isopach maps can only ever provide minimum estimates of Plinian deposition, with the possibility of misleading dispersal information, because the amount of Plinian fallout material deposited along with ignimbrite is impossible to quantify. The new information presented here concerning the frequency and behaviour of PDCs during the Poris eruption may have implications for the planning of risk mitigation during future explosive eruptions on Tenerife. The effect of topographic interaction is a particularly important consideration, as similar effects due to the Cañadas landscape may be anticipated during future eruptions.