Andosolization of Soils on a Strombolian Cone at Mount Bambouto, Cameroon

Morphology, mineral and geochemical investigations were carried out on two selected soil pedons (ZA and TO) developed on a late Quaternary-aged strombolian cone to better understand andosolization processes in Mount Bambouto, Cameroon. Both pedons have A-BC-C horizonation. They have thick surface (A) horizon with bulk density -3, Alo + 0.5Feo about 2% and P retention >90%. Mineral association consists predominantly of kaolinite, gibbsite, goe- thite, organometal complexes and trace amount of ferrihydrite and allophane. The Ki values between 1.1 and 1.6, the low TRB (45 to 67 mg kg-1), the important IMob (20% to 24%), the CIA between 60% and 70% and the EFs above 1.16 for Al and Fe, and below 0.6 for Si indicate sparingly hydrolysis process in subsoil during which released Al, Fe and Si form allophanic or ferrihydrite minerals undergo crystallization into kaolinite, gibbsite and goethite, respectively. At topsoil, part of released Al (and Fe) is organically bounded with organic acids to form organometal complexes.

Ash Features from Ordinary Activity at Stromboli Volcano

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.