Numerical Experiments on the Welding Processes in the Pyroclastic Flow Deposits

Abstract

Prosesses of welding within pyroclastic flow deposits are simulated numerically. Basic equations to simulate the welding are the equation of heat conduction, the relationship between porosity and viscosity, and the relationship between viscosity and temperature. As the relationship between porosity and viscosity, a theoretical equation derived by Murray and other investigators is employed : ln φ=ln φ_o-(3/4)・(P/η)・t. These equations are solved iteratively by a high speed electronic computer. Standard model of the pyroclastic flow deposits is chosen as follows ; initial temperature is 650℃, thickness of deposits is 75 m, viscosity at 650℃ is 10¹⁵ poise, and physical properties of the deposits are nearly the same as those of pumices. Calculations are performed for the standard and various modified models. Results indicate that : (1) degree of welding is mainly controlled by viscosity, intial temperature, and thickness of the pyroclastic flow deposits ; (2) welding is ceased within the first several years after deposition ; (3) on the other hand, duration of cooling of the deposits is continued to tens to hundreds years, nearly ten times greater than that of welding ; (4) position of the most densely welded part is situated at about two third depht of the deposits regardless wide variation of the parameters of the models ; (5) if the value of the viscosity becomes less than that of the experimental data by the effect of high vapour pressure, welding is possible even if the initial temperature is lower than 600℃ ; (6) within the range of the parameters employed in this study, welding both in the upper 10 m of the deposits and in the layer thinner than 10 m seems improbable.