Ultra-distal Tephra Deposits from Supereruptions: Examples from Toba and New Zealand

Volcanic ash ejected during two of the largest eruptions in the Quaternary, the Younger Toba Tuff and the Whakamaru eruption, was dispersed over wide areas and would have affected large parts of the globe. The ~ 74 ka Youngest Toba Tuff (YTT) eruption, Toba, Sumatra, deposited ash over the Bay of Bengal and the Indian subcontinent to the west of the Toba caldera. The ~ 340 ka Whakamaru eruption from the Taupo Volcanic Zone (TVZ), New Zealand, deposited the widespread Rangitawa Tephra, dominantly to the southeast (in addition to occurrences northwest of vent), extending across the landmass of New Zealand, and the South Pacific Ocean and Tasman Sea. These supereruptions involved ~ 2500 km³ and ~ 1500 km³ of magma (dense-rock equivalent; DRE), respectively. Ultra-distal terrestrial exposures of YTT were investigated at two localities in India, Middle Son Valley, Madhya Pradesh, and Jurreru River Valley, Andhra Pradesh, at distances of > 2000 km from the source caldera. An ~ 4 cm thick basal 'primary' ashfall unit is observed at the sites, although deposits containing reworked ash are up to ~ 3 m. Exposures of Rangitawa Tephra on the Chatham Islands, > 900 km from the source caldera, are ~ 15 - 30 cm thick. At more proximal localities (~ 200 km from source) the Rangitawa Tephra is ~ 55 - 70 cm thick and characterized by a crystal-rich basal layer and normal grading. These ultra-distal tephra deposits are typically crystal-poor and characterized by fine and very-fine ash (with high PM₁₀ fractions). Glass chemistry, stratigraphy and grain-size data for these distal tephra deposits are presented with comparisons of their correlation, dispersal and preservation. Using field observations, ash transport and deposition were modeled for both eruptions using a semi-analytical model (HAZMAP), with assumptions concerning average wind direction and strength during eruption, column shape and vent size. Model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout. Modeling based on observed YTT distal tephra thicknesses indicate a relatively low (< 40 km high), very turbulent eruption column, consistent with deposition from a co-ignimbrite cloud extending over a broad region and depositing ~ 1500 km³ DRE. Similarly, the Whakamaru eruption was modeled as producing a predominantly Plinian column (~ 45 km high, ~ 420 km³), with dispersal to the southeast by strong prevailing winds. More proximal tephra deposits (at ~ 200 km from source) can be replicated in the modeling by co-ignimbrite deposition. Significant ashfall out of the main dispersal direction, to the northwest of source, cannot be replicated in our modeling, and is most likely due to a significant change in wind direction during the eruption. The widespread dispersal of large volumes of fine ash from both eruptions may have had global environmental consequences, and would have acutely affected areas up to thousands of kilometers from vent.