Sigmoidal particle density distribution in a subplinian scoria fall deposit

Abstract

A general expression to describe particle density distribution in tephra fall deposits is essential to improve fallout tephra mass determination and numerical modelling of tephra dispersion. To obtain particle density distributions in tephra fall deposits, we performed high-resolution componentry and particle density analyses on samples from the 2006 subplinian eruption of Tungurahua volcano in Ecuador. Six componentry classes, including pumice and scoria, have been identified in our sample collection. We determined the class of 300 clasts in each 0.5ϕ fractions from −4.5ϕ to 3.5ϕ and carried out water pycnometry density measurements on selected size fractions. Results indicate that the mean particle density increases with ϕ up to a plateau of ~2.6 g/cm³ for clasts finer than 1.5ϕ. The density of scoria and pumice increases between −3 and 1ϕ, while dense particle density is sub-constant with grainsize. We show that the mean particle density μ of the vesicular fractions is a function of grainsize i (ϕ scale) given by a sigmoidal law: $\mu (i)={{{K+\beta }} \left/ {{\left( {1+\alpha {e^{-ri }}} \right)}} \right.}$, where K, β, α and r are constants. These sigmoidal distributions can be used to determine accurately the load of each componentry class and should be applicable to many tephra deposits and for modelling purposes.