Thick silicic volcanic ash layers commonly observed hundreds of kilometres from potential source areas have resulted from large-magnitude explosive eruptions that have no historical equivalents. We have developed a model that predicts the duration of these eruptions from the vertical size grading of feldspar phenocrysts near the base of deep-sea tephra layers. The size grading is a function of the release time of the particles, their settling velocity, the water depth at the site of the ash layer, and the duration of the eruption. The model has been tested on two layers of the Worzel D ash in the eastern equatorial Pacific. This ash layer is the distal counterpart of the rhyolitic Los Chocoyos ash-flow tuff and H-tephra layer associated with the formation of the Lake Atitlán caldera in Guatemala. The estimated duration of the eruption is 20 to 27 d. Calculations using published estimates of the volume of erupted material yield an average magma-discharge rate of about 240,000 m3/s. This rate is approximately equivalent to that recorded in the 1956 eruption of Bezymianny and the 1912 eruption of Katmai.