Paleomagnetic secular variation provides the timescale for the eruptive sequence of the 45 ka Shikotsu caldera-forming eruption

Abstract Our new paleomagnetic data provide the timescale for the eruptive sequence of the 45 ka Shikotsu caldera-forming eruption of VEI 7. The duration of the entire sequence is estimated to be centuries, which is considerably longer than previously thought. The studied volcanic sequence, located at ~ 10 km from the caldera rim, includes five units of pyroclastic flow deposits (units B3, C1, C2, D, and E in ascending order) and an uppermost pyroclastic surge deposit (unit F2). The ash matrix samples of the pyroclastic deposits, predominantly composed of juvenile material, were collected into aluminum and plastic cubes, which were precisely oriented using an originally designed tool set. As a result, the obtained paleomagnetic directions have high precision parameters (k = 200–1400) and small 95% confidence intervals (α 95 = 2–4°). These paleomagnetic directions determined from the sequence of the six units demonstrate a curve of paleomagnetic secular variation, which has a total angular distance of 14.4 ± 4.1°. The observed distinguishable paleomagnetic directions indicate that the earlier four units (B3, C1, C2, and D) were formed by four distinct eruptions over a period of 240 ± 70 years with repose times of decades between the eruptions. The indistinguishable paleomagnetic directions indicate that the three late-stage units (D, E, and F) were erupted in a short period, decades or less. Our new paleomagnetic data, combined with the reported petrological change in pumice, suggest that the extractions of crystal-poor rhyolitic melt from the magma chamber occurred in multiple eruptions over a considerable period (more than a hundred years). Graphical Abstract