Abstract
Ignimbrites are relatively uncommon on ocean island volcanoes and yet they constitute a significant portion of the stratigraphy of Terceira Island (Azores). The Lajes-Angra Ignimbrite Formation (ca. 25 cal ka BP) contains the youngest ignimbrites on Terceira and records two ignimbrite-forming eruptions of Pico Alto volcano that occurred closely spaced in time. Here, we present the first detailed lithofacies analysis and architecture of the Angra and Lajes ignimbrites, complemented by petrographic, mineral chemical, whole rock and groundmass glass geochemical data. The two ignimbrites have the same comenditic trachyte composition, but show considerable variability in trace element and groundmass glass compositions, revealing complex petrogenetic processes in the Pico Alto magma reservoir prior to eruption. The Angra Ignimbrite has a high-aspect ratio and is massive throughout its thickness. It was formed by a small-volume but sustained pyroclastic density current (PDC) fed by a short-lived, low pyroclastic fountain. Overall, the PDC had high particle concentration, granular fluid-based flow conditions and was mostly channelled into a valley on the south part of Terceira. By contrast, the Lajes Ignimbrite has a low-aspect ratio and shows vertical and lateral lithofacies variations. It was formed by a sustained quasi-steady PDC generated from vigorous and prolonged pyroclastic fountaining. The ignimbrite architecture reveals that depositional conditions of the parent PDC evolved as the eruption waxed. The dilute front of the current rapidly changed to a high particle concentration, granular fluid-based PDC that extended to the north and south coasts, with limited capacity to surmount topographic highs. Contrary to what is commonly assumed, the low-aspect ratio of the Lajes Ignimbrite is interpreted to result from deposition of a relatively low velocity PDC over a generally flat topography. This work highlights that the geometry (aspect ratio) of ignimbrites does not necessarily reflect the kinetic energy of PDCs and thus should not be used as a proxy for PDC emplacement dynamics. Although the probability of an ignimbrite-forming eruption on Terceira is relatively low, such a scenario should not be underestimated, as a future event would have devastating consequences for the island’s 55,000 inhabitants.
Highlights
Ignimbrites are formed by sedimentation of hot mixtures of ash and vesiculated juvenile clasts from pyroclastic density currents (PDCs) generated during explosive volcanic eruptions (Sparks et al, 1973; Cas and Wright, 1987; Branney and Kokelaar, 2002)
Most of the older ignimbrites share physical and geochemical similarities with the Lajes-Angra Ignimbrite Formation (LAI), suggesting that the processes involved in ignimbrite-forming eruptions have been recurrent in the last ca. 100 kyr, with significant implications for volcanic hazard assessment
The Angra Ignimbrite was formed by a small-volume sustained PDC that was fed by a short-lived, low pyroclastic fountain, which generated a high particle concentration, granular fluid-based PDC
Summary
Ignimbrites are formed by sedimentation of hot mixtures of ash and vesiculated juvenile (pumiceous) clasts from pyroclastic density currents (PDCs) generated during explosive volcanic eruptions (Sparks et al, 1973; Cas and Wright, 1987; Branney and Kokelaar, 2002). Description: Massive lapilli-ash (mLA) is the most common lithofacies and typically constitutes the main part of the Angra and Lajes ignimbrites It is matrix-supported, ranging from well sorted to very poorly sorted (Angra 1.5 < σφ < 4.1 vs Lajes 2.7 < σφ < 5.0; Self, 1974), with variable proportions of juvenile and lithic clasts in a light grey to dark grey matrix of vitric ash and abundant feldspar crystals (up to 3 mm in length) (Figures 5A,B). Description: Diffuse-stratified lapilli-ash (dsLA) is found locally in the Angra and Lajes ignimbrites It consists of matrix-supported, poorly sorted deposits, with variable proportions of juvenile and lithic clasts in a light grey to dark grey ash matrix rich in feldspar crystals (up to 3 mm in length). Eu/Eu* values show a negative correlation with Zr concentrations (Figure 13H)
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