The causes and impacts of highly productive silicic caldera systems: A case study of Witori Volcano, Papua New Guinea

Witori volcano, on the island of New Britain in the south-west Pacific, is a highly active silicic caldera system. Throughout the Holocene it has produced at least five Plinian eruptions (VEI ≥ 6), with eight smaller VEI 3–5 eruptions in the past ~ 1000 years. Tephra from the larger Plinian eruptions has recently been identified on the New Guinea mainland, up to 700 km downwind of Witori, necessitating re-evaluation of eruption magnitudes and output rate of this volcano. Since 6.4 ka > 38.8 km 3 DRE of magma has been erupted at Witori. Uncertainty in the size of numerous eruptions means this volume may be > 60 km 3 DRE, more than twice that of similar systems like Taupō. Comparison of deposit characteristics with Santorini and Tambora suggests the largest eruptions from Witori (W-K1–6.4 ka; W-K2–3.4 ka) potentially reached magnitudes of VEI 7. These eruptions involved crystal-poor, high-silica (~73–76 wt% SiO 2 ) rhyolites. The SiO 2 content of Witori magmas decrease with declining eruption magnitude and increased eruption frequency, indicating a direct relationship between eruption size and timescales of fractionation. Witori magmas are low-K tholeiites, produced by high-degrees of partial melting within the mantle. The high eruption rate observed at Witori results from rapid transit of magma through the crust, facilitated by widespread crustal fracturing/faulting in central New Britain associated with the rotation of the South Bismarck microplate. Archaeological records demonstrate individual eruptions caused significant impacts with abandonment periods of multiple generations; however, humans repeatedly returned following each eruption.