Discussion

Abstract

Dr K azuaki N akamura said that the description of several thin pyroclastic flows (author’s base surge) being intercalated near the source within air-fall deposits of Plinian type was very interesting. He said he had the impression that an apparent correlation existed between the non-wind circumstances and the occurrence of the pyroclastic flows in the air-fall deposits. Was this a correct impression? If such a correlation were established, it offered a very interesting view regarding both the mechanism of formation of pyroclastic flows, and the possible control of volcanic eruption by external factors. The expected mechanism under such circumstances would be that the simple overloading of ejecta in the eruptive column could form the pyroclastic flow. This mechanism further suggested that the author’s base surge might be somewhat different from the original base surge as defined by J. Moore (1967). The A uthors replied that they did not have enough data to know if the suggested correlation was true or not, but they agreed that it was a very interesting possibility that must be borne in mind as further work is done. They agreed that the simple overloading of ejecta in the eruptive column was the most likely mechanism for production of the pyroclastic flows, and not the base surge as defined by J Moore Dr R. A. O ld asked Dr W alker how individual ash bands had been identified in the field as they showed considerable variation in composition and thickness. Dr W alker replied that provided that sampling stations were closely spaced (it