INVESTIGATION INTO THE FORMATION OF LARGE PUMICE BLOCKS FROM THE POST-232 AD TAUPŌ ERUPTION AND THEIR IMPLICATION OF A TRANSITIONING MAGMA SYSTEM

Lake Taupō is an active caldera located in New Zealand’s Taupō Volcanic Zone (TVZ): an area characterized by large, rhyolitic eruptions. The caldera was formed during the 232 AD eruption (Eruption Y), which evacuated over 35 km³ of material, making it one of the largest eruptions on record in the last 5,000 years. On the eastern shore of Lake Taupō, there are blocks of massive pumice ranging from 1 m to 17 m in diameter. These large blocks are hypothesized to have erupted from a subaqueous rhyolitic dome (Eruption Z) at the Horomatangi reefs, which formed 20-30 years after Eruption Y. We compare the Eruption Y ignimbrite to the Eruption Z blocks and observe a difference in microlite abundance and whole rock composition, indicating a rearrangement of the magma system in the short period between these eruptions.

Using x-ray fluorescence spectrometry, we analyzed the whole-rock compositions of the Eruption Z pumice blocks and the Eruption Y ignimbrite. We found that Eruption Z has an average SiO₂ of 73.3%-- lower than that of Eruption Y with a 74.1% average SiO₂. Additionally, we found a higher average amount of MgO and Al₂O₃ in Eruption Z (0.38% and 13.7%, respectively) than Eruption Y (0.33% and 13.5%, respectively). We hypothesize that this shift towards a more mafic composition indicates that the massive, caldera-forming Eruption Y triggered a rearrangement of Lake Taupō’s magma system. To compare the mechanisms of these two magmas as they moved up the conduit, we analyzed the frequency and size of microlites and micro-phenocrysts, as larger and more abundant microlites are characteristic of a slower cooling magma. Using electron backscatter diffraction, we found more microlites in the blocks than the ignimbrite. Additionally, textural analysis of a single block revealed a higher amount of microlites in the core, indicating an extended period of cooling relative to the quenched rims. These shifts in the composition, texture, and conduit behavior of Lake Taupō provide insight into the period after caldera-forming eruptions during which transitions in magmatic and eruptive behavior take place.