This project will develop and test a methodology for the identification and analysis of morphological features on crystalline and glassy juvenile tephra from two calssic pyroclastic deposits: 1) the 1980 products of Mount St. Helens, and 2) the A.D. 79 deposits of Vesuvius. A wide range of deposits from lahars to plinian falls were produced by these two events. The stratigraphy and eruptive phenomena of both deposits have been well studied and characterized. Collective features of primary grain-surface morphology will be interpreted in terms of the successive changes in the character of these two eruptions. The relative effects of mechanical and chemical processes on grain surfaces will be traced through the eruption, transportation, deposition and cooling of the tephra. Data sets of sufficient pyroclasts from every major stratigraphic level of these deposits will be examined to identify key textures useful for interpretation of environmental histories. In addition, experimental studies will be done to investigate the morphology of hot fragments quenched in water, and the textures produced by abrasion of primary clasts in a specially designed device. Development of a methodology for pyroclast morphologic investigation and verification of the results using computer analysis are integral parts of this study. Various techniques for analysis of tephra morphologies will be tested to determine which are best suited to determine the history of the deposits from the characteristics of the individual components.
