Zircons are useful because of what they are: crystals smaller than the width of a human hair, grown in magma before it erupts, carrying uranium that decays to lead isotopes at a known rate, which is what makes precise dating possible [6]. The age is what this work leans on. Ages from the two English boreholes, drilled 65 kilometres apart and cored 80 years ago, match zircons from the Kinnekulle tephra, the thick ash layer that covers Scandinavia and the Baltic region [5][7]. Contemporaneity, a buried caldera system, and a plausible flight path across the Tornquist Sea of the Ordovician together point to a source for the ash [8]. Volume is the quantity that remains unresolved.
That gap is where the disagreement sits. The analysis is presented as showing for the first time how far the ash travelled and therefore how big the eruption really was [9]. Michael Poland, of the Cascades Volcano Observatory and the Yellowstone Volcano Observatory, and not involved in the work, told Live Science that it obviously must have been a very large eruption but that the volume would be very difficult to calculate given the poor preservation of the deposits [3]. Worth noticing that the term volcanologists offer instead carries the same arithmetic burden: Peter Rowley of the University of Bristol would call this an ultraplinian eruption [12], and ultraplinian is defined by ejecting more than 1,000 cubic kilometres of material [13]. Rowley also says the geological record is far from complete and that eruption volumes are very hard to pin down for very old volcanoes [18]. So the preference tracks the vagueness of the class, with no firmer number underneath it. Stephen Self of the University of California, Berkeley, puts the constraint most plainly: eruptions get recognised through their deposits, while the volcano itself leaves no such record [16].
The preservation problem scales with age, and the age here is the point. Rowley's nearest analogue in size is the Oranui event at Taupo in New Zealand, more than 26,000 years ago [11]. The Ordovician ran from 488.3 to 443.7 million years ago, a window of 44.6 million years [8][21]; take its youngest edge and the English deposits are still at least roughly 17,000 times older than that analogue [23]. Very little of the ejected material survives, most of it eroded or buried [17]. Poland notes the Kinnekulle bed has itself degraded over time [19]. The boreholes sit in some of England's flattest landscapes, with no trace of the eruptions at the surface [20]. The second, earlier eruption the team dates to 453.7 million years ago falls 10 million years before the close of the Ordovician [20][22].
What would move the magnitude claim from inference to measurement is thickness mapping of surviving tephra, and that is the same material Poland describes as degraded [19]. The correlation looks like the durable part of this: three decades of work to reconstruct a structure that leaves nothing to see [1] has produced an age link across a vanished sea. The size, for now, is a floor argued from where the ash landed, and "huge" is the description the evidence supports, while a cubic-kilometre figure would overreach it [2].
