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Isopach maps produced by applying a cubic-B spline model to thickness data from: Cerro Negro 1995 (Hill et al. 1998); El Chichon 1982 Layer B (Carey and Sigurdsson 1986); Mount St. Helens May 18th 1980 (Sarna-Wojcicki et al. 1981); Askja 1875 Layer D (Sparks et al. 1981); and Taupo Plinian Pumice (Walker 1981). Red markers denote reported locations where no ash was observed
