Compositional modes in the ash fraction of some modern pyroclastic deposits: their determination and significance |
| |
Authors: | R De Rosa |
| |
Institution: | (1) Dipartimento di Scienze della Terra, Università della Calabria, I-87036 Arcavacata di Rende (Cosenza), Italy e-mail: derosa@unical.it, IT |
| |
Abstract: | This paper illustrates some problems involved in the quantitative compositional study of pyroclastic deposits and proposes
criteria for selecting the main petrographic and textural classes for modal analysis. The relative proportions of the different
classes are obtained using a point-counting procedure applied to medium-coarse ash samples that reduces the dependence of
the modal composition on grain size and avoids tedious counting of different grain-size fractions. The major purposes of a
quantified measure of component distributions are to: (a) document the nature of the fragmenting magma; (b) define the eruptive
dynamics of the eruptions on a detailed scale; and (c) ensure accuracy in classifying pyroclastic deposits. Compositional
modes of the ash fraction of pyroclastic deposits vary systematically, and their graphical representation defines the compositional
and textural characteristics of pyroclastic fragments associated with different eruptive styles. Textural features of the
glass component can be very helpful for inferring aspects of eruptive dynamics. Four major parameters can be used to represent
the component composition of pyroclastic ash deposits: (a) juvenile index (JI); (b) crystallinity index (CrI); (c) juvenile
vesicularity index (JVI); and (d) free crystal index (FCrI). The FCrI is defined as the ratio between single and total crystal
fragments in the juvenile component (single crystals+crystals in juvenile glass). This parameter may provide an effective
estimate of the mechanical energy of eruptions. Variations in FCrI vs JVI discriminate among pyroclastic deposits of different
origin and define compositional fields that represent ash derived from different fragmentation styles.
Received: 15 January 1998 / Accepted: 8 February 1999 |
| |
Keywords: | Pyroclast types Methodological approach Modal analysis Eruption style Compositional indices |
本文献已被 SpringerLink 等数据库收录! |