Contrasting eruptive dynamics and hazards of two Holocene andesitic sub-Plinian to Plinian eruptions, Lower and Upper Citlaltépetl, at Pico de Orizaba, Mexico
摘要
Pico de Orizaba (Citlaltépetl) stratovolcano, eastern Mexico, preserves a complex Holocene explosive record, yet the eruptive dynamics, source parameters, styles, and hazard implications of individual eruptive episodes remain poorly constrained. Here, we examine the 9600–9500 cal BP Lower Citlaltépetl and 9200 cal BP Upper Citlaltépetl explosive eruptions by integrating field stratigraphy from 85 outcrops, lithofacies analysis, granulometry, componentry, petrography, geochemistry, and eruption source-parameter estimates to reconstruct their eruptive sequences, magma processes, and hazards. The Lower Citlaltépetl records onset dome-collapse/vent-clearing pyroclastic density currents, evolving into a sustained sub-Plinian fallout phase, and ending with complete column collapse. Its main fallout layers were dispersed southeastward from a ~25-km-high column at average mass eruption rates of 1.1 × 108 kg s−1. The total fall and pyroclastic density current dense-rock-equivalent volume was 0.1 km3, corresponding to magnitude 4.4. Juvenile products include dominant banded scoria and subordinate pumice. Petrography and geochemistry indicate that magma mixing between a shallow evolved high-Si andesite melt and an ascending mafic magma (both spanning 56–62 wt.% SiO2) drove the climactic phase. The Upper Citlaltépetl comprises a larger, more complex eruption that progressed from vent-clearing activity into successive sub-Plinian and Plinian fallout phases, followed by partial column collapse, waning fallout, and final complete collapse. The onset explosions produced a ~21-km-high sub-Plinian column dispersed northeastward, whereas the climactic eruption generated a 29–30-km-high Plinian column dispersed eastward at mass eruption rates of 3.5 × 108 kg s−1. The total eruption reached 0.3 km3 dense-rock-equivalent and magnitude 5. Upper juvenile products are dominated by scoria and dense lava, with no clear juvenile pumice. Their narrower andesitic compositions (58–60 wt.% SiO2), clinopyroxene-rich assemblage, crystal aggregates, rare olivine, and strong scoria microtextural heterogeneity (from microvesicular to microcrystalline to sheared) suggest a dominantly mafic-andesitic system driven mainly by rheological contrasts within the magma, strong bubble-melt coupling, rapid magma decompression and ascent, and magma shearing, rather than magma mixing. Both eruptions define contrasting pathways toward sub-Plinian and Plinian activity at Pico de Orizaba and provide the first detailed pyroclastic density current dispersal maps for the volcano. Hazard implications are significant: fall deposits draped areas of 400–1500 km2, whereas pyroclastic currents ranged from dome-collapse-related block-and-ash-flows to broad and highly mobile dilute currents, and confined, concentrated column-collapse-related currents, with volumes of 0.006–0.05 km3. These results show that Pico de Orizaba hazards are more diverse than previously represented and require style-specific, multi-phase eruptive scenarios for realistic hazard assessment.