Open AccessReconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 2. Application to the Pinatubo 1991 eruption
Author(s) -
Koyaguchi Takehiro,
Ohno Marekazu
Publication year - 2001
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jb900427
Subject(s) - tephra , geology , ejecta , magma , volcano , explosive eruption , lapilli , dense rock equivalent , grain size , effusive eruption , vulcanian eruption , rock fragment , mineralogy , phreatic eruption , pyroclastic rock , petrology , seismology , geomorphology , quartz , paleontology , physics , quantum mechanics , supernova
The granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic Plinian phase of the 1991 eruption at Pinatubo. The tephra fall deposits are composed of two units: layer C 1 , which corresponds to the first half of the climactic phase, and layer C 2 the second half. The granulometric estimates of the expansion rate of the umbrella cloud for layers C 1 and C 2 are 7×10 10 and 3×10 10 m 3 /s, respectively, which agree with the observations of satellite images. These estimates indicate that the magma discharge rate decreased from 9×10 8 to 3×10 8 kg/s during the eruption. The grain‐size distribution at the top of the eruption column is characterized by depletion of coarse clasts, suggesting a distinct deceleration in the gas thrust 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 3×10 12 kg, and the very fine particles, which did not deposit in the accessible on‐land area, may occupy up to 60% of the total ejecta. The effective duration of the eruption estimated by the present methods (a few hours or less) is substantially shorter than a previous estimate from the real‐time observation based on infrasonic data (∼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 pyroclastic flows. It is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time‐averaged features predicted by the present methods.