Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 2. Application to the Pinatubo 1991 eruption

The granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic Plinia n phase of the 1991 eruption at Pinatubo. The tephra fall deposits are comp osed of two units: layer C-1, which corresponds to the first half of the cl imactic phase, and layer C-2 the second half. The granulometric estimates o f the expansion rate of the umbrella cloud for layers C-1 and C-2 are 7 x 1 0(10) and 3 x 10(10) m(3)/s, respectively, which agree with the observation s of satellite images. These estimates indicate that the magma discharge ra te decreased from 9 x 10(8) to 3 x 10(8) kg/s during the eruption. The grai n-size distribution at the top of the eruption column is characterized by d epletion of coarse clasts, suggesting a distinct deceleration in the gas th rust region down to several tens of meters per second, particularly during the second half of the climactic phase. The total amount of layers C-1 and C-2 is estimated to be 3 x 10(12) kg, and the very fine particles, which di d not deposit in the accessible on-land area, may occupy up to 60% of the t otal ejecta. The effective duration of the eruption estimated by the presen t methods (a few hours or less) is substantially shorter than a previous es timate from the real-time observation based on infrasonic data (similar to 10 hours). This discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and/or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous Plinian activity and generation of pyroc lastic flows. It is also suggested that the intensity of eruption was fluct uating with time so that the instantaneous observations such as cloud heigh t in satellite images do not always show a quantitative agreement with the time-averaged features predicted by the present methods.