The nonwelded ash-flow tuffs of the 1.4-m.y.-old lower (Otowi) member of the Bandelier Tuff on the flanks of the Jemez Mountains erupted from as many as seven ring-fracture vents bounding the large Toledo Caldera at the center of the volcanic complex.

Field evidence for these multiple vents includes (1) broad radial distribution of large volumes of ash-flow tuff; (2) areal variation in the compositions of lithic inclusions, with basalt-andesite clasts more abundant in the western than in the eastern sectors; (3) areal contrasts in welding and zonation, with welding best developed in the western sections; (4) paleotopography; and (5) flow directions determined from field measurements of elongated pumice fragments and charred trees.

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