Eruption Style, Emplacement Dynamics and Geometry of Peralkaline Ignimbrites: Insights From the Lajes-Angra Ignimbrite Formation, Terceira Island, Azores

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.

The recent volcanism of Flores Island (Azores): Stratigraphy and eruptive history of Funda Volcanic System

“…However, it is well known that estimating tephra volumes on islands is challenging as the deposits are often incompletely preserved due to the reduced subaerial area of deposition. It is therefore reasonable to consider that a significant, but unknown, portion of pyroclastic material is deposited offshore, as commonly reported for other explosive eruptions on small islands (e.g., Andronico and Pistolesi, 2010;Pimentel et al, 2015Pimentel et al, , 2021Kueppers et al, 2019). In the case of Flores, and as it happens in tropical and subtropical climates with strong winds and/or intense rainfall regimes, this is aggravated by the high syn-and postdepositional reworking of subaerial exposures (Bolós et al, 2021), and thus the estimates presented here must be considered as minimum erupted volumes.…”

Peralkaline silicic extrusive rocks: magma genesis, evolution, plumbing systems and eruption

“…There have been ten young (<5 ka) sub-Plinian eruptions of comenditic trachyte [Jeffery et al, 2016], the latest occurring in 1761 ce [Pimentel et al, 2016]. Pimentel et al [2021] describe the potentially devastating effects on the 55,000 inhabitants of a future ignimbrite-forming eruption of Pico Alto volcano, Terceira.…”

Eruption Sequences and Characteristics of Weizhou Island Volcano, Guangxi Province, South of China

“…Understanding the history, sequences, and characteristics of eruptions that occurred in Weizhou island is necessary for volcanic hazard assessment. Small oceanic islands are particularly vulnerable to explosive eruptions and the risk is potentially aggravated by their geographical isolation and the weak socioeconomic conditions of local communities (Pimentel et al, 2021). Therefore, detailed studies are needed to widen our understanding of Weizhou island eruptions and their potential impacts.…”