VERTICAL DENSITY VARIATIONS IN CLIMACTIC TUFFS OF THE YELLOWSTONE PLATEAU VOLCANIC FIELD, SOUTHEAST IDAHO

The climactic eruptions of the Yellowstone volcanic plateau consist of the voluminous (2450 km³) Huckleberry Ridge Tuff at 2.1 Ma, which formed the Yellowstone I caldera; the smaller (280 km³) Mesa Falls Tuff at 1.3 Ma, which produced the Yellowstone II caldera; and the Lava Creek Tuff (1000 km³) at 0.64 Ma, which generated the Yellowstone III caldera. We measured vertical density variations in portions of these tuffs to investigate their eruption histories; specifically, the numbers of individual ash sheets that built each tuff, the relative temperatures of emplacement, and the durations between eruptions.

One member of the Huckleberry Ridge Tuff formed from at least five individual ash sheets—the second sheet was particularly cool and the last three sheets were relatively hot. Short periods (hours to weeks) separated these eruptions. Several thick, hot deposits of this member welded so intensely that the depositional record was erased. The Mesa Falls Tuff formed as a single cooling unit from at least four ash sheets. We infer that sheet 1 was emplaced hot; after months to years, sheet 2 was emplaced at about the same temperature; after at least a few days, sheet 3 was emplaced at significantly cooler temperature; and, after days to months, sheet 4 was emplaced at intermediate temperature. Member B of the Lava Creek Tuff formed from at least five ash sheets. The first three sheets were relatively hot and were followed by two cooler sheets. Short periods (hours to weeks) separated these eruptions.