TITLE:
Ash Features from Ordinary Activity at Stromboli Volcano
AUTHORS:
Chiara Benedetta Cannata, Rosanna De Rosa, Paola Donato, Jacopo Taddeucci
KEYWORDS:
Ash, Basaltic, Strombolian, Tachylite, Sideromelane, Ordinary Persistent Activity
JOURNAL NAME:
International Journal of Geosciences,
Vol.5 No.11,
October
27,
2014
ABSTRACT: In this work we focus our attention on micro-scale textural observations
and chemical analysis of the surface of ash particles erupted from ordinary
activity at Stromboli volcano (Aeolian Islands, Italy). Ashes are related to
three vent systems: NE1, NE2 and SW showing different
eruptive styles. Samples were collected during three campaigns: 1-5 September
2008; 20, 22 September 2008 and 18 June 2009, with two different methods:
collection on a clean surface downwind and use of remotely controlled, electrically
propelled aeromodels, which penetrate directly in the eruptive plume and
collect ash samples that are less affected by transport-related sorting
processes. A micro-textural approach has been conducted working at step: from
preliminary observations under binocular microscope on a medium number of 700
particles, to thin sections and sub-micron investigations, using high
resolution instruments. Micro-texture and morphology of ash particles were
observed under FE-SEM, at high magnification (30,000/50,000×)
measuring the main shape parameters (area, perimeter, max length, compactness,
Feret’s diameter, Heywood’s diameter and elongation). Ashes are made up by a
wide spectrum of fragments whose end-members are represented by two main
textural types with different colour, shape, internal textures and vesicularity
named “Type a” and “Type b”. “Type a” shows great similarities with fragments
called “tachilite” while “Type b” fragments are very similar to the named “sideromelane”
fragments. Fragments with intermediate textural features are
present and named as “Type a1” and “Type b1”,
respectively. The relative abundances of each type of fragments greatly vary in
samples collected from different groups of vents. Chemical analysis of the
external surfaces indicates that all the different grain types underwent
alteration phenomena and precipitation of neo-formation minerals on their
surfaces. The alteration is dependent from the atmospheric conditions around
the vent and not from the type of fragments. During the periods of sampling,
ash erupted from individual explosions of different vents, does not show big
differences in morphology and chemistry of surface features, but mainly in
terms of relative proportion of fragments. “Type b” fragments predominate in
the ash erupted from typical strombolian activity whereas “Type a” fragments
are ubiquitous.