Timescales of Human Interest in the Geological Records of Rhyolite Eruptions

Abstract:

Prehistoric eruptions are generally treated as discrete events, separated from preceding and following events by periods defined by ¹⁴C or radiometric dating as centuries or longer. Modern events such as Monserrat (1995-?) or Mount St. Helens (1980-86; 2004-2008) blur this distinction with ‘eruption episodes’ versus ‘separate eruptions’ becoming a semantic issue. For rhyolite eruptions, where historic events are scarce, discerning episodicity within, or close spacing between, eruptions becomes important. Time gaps of a few decades or less are challenging to recognise and quantify in eruption products, but important in terms of modelling future events and considering the consequent hazards, economic impacts and associated societal responses. We here consider the geological record for the timing of prehistoric rhyolite eruptions over a range of sizes. In some, such as the Bishop Tuff, there is no evidence in the eruptive stratigraphy to suggest any significant hiatus in activity, and an overall timing of days for the main activity has been inferred. In contrast, evidence from other deposits indicates a prolongation of geologically distinct eruptions, such as the Oruanui (over several months) and Huckleberry Ridge Tuff (over likely over a few decades). Such evidence is in the form of syn-eruptive erosion and reworking, or breaks long enough for successive units to have contrasting welding histories but for the earlier units to still be hot and fumarolically active when the later ones were emplaced. Some eruptions in the young record from Taupo volcano were sourced from widely separated vents, yet have only subtle evidence for time breaks between them, suggestive of years to 1-2 decades. The youngest event at Taupo consisted of the catastrophic 232±5 CE explosive activity (eruption Y), which was followed by a break of 10±5 years before effusive activity (eruption Z) occurred under the re-formed Lake Taupo. The lengths of time breaks within and between rhyolite eruptions are overlapping, and present major challenges for linking field and petrological evidence and considering whether or not an eruption has ended.