Fjölrit RALA - 15.06.2004, Síða 23

Micromorphology of the volcanic ash soils of the COST-622 reference
profiles
G. Stoops1 and M. Gérard2.
1 Laboratorium voor Mineralogie, Petrologie en Micropedologie, Universiteit Gent, Belgium;
2 Institut de Recherches pour le Développement (IRD), Bondy, France
In the frame of the COST-622 action, undisturbed and oriented samples were taken from 17
reference profiles on volcanic ash. Thin sections were prepared in three different laboratories,
and systematic micromorphological descriptions made according to Stoops (2003).
The least developed profiles are found on Iceland and Santorini. More developed profiles
occur in the Massif Central (France), and southem and central Italy, whereas the most
developed ones were sampled on Tenerife and the Azores.
The Iceland profiles (N7-N9) are characterised by the presence of high amounts of weakly
decomposed organic material, mainly subhorizontally oriented organ residues. An altemation
of organic and inorganic layers is noticed. Within the inorganic layers a microstratification,
often showing different mineral parageneses, occurs. These features point to a sequential
deposition on the surface. The inorganic material is always unweathered, varying from
pumice, over different types of glass to pyroclasts with single grains of feldspar and pyroxene.
The presence of peat-Uke organic layers and diatom skeletons indicates moist conditions. A
higher micromass content in some layers points to a possible erosion product of more evolved
soils. Locally an isoband stmcture latter points to repeated freeze-thaw processes.
The Santorini profiles (N13-N15) (only Ah horizons sampled) are merely coarse volcanic
deposits of different composition, some pumice rich, other dominantly composed of
pyroclasts, with only a low degree of weathering. Feldspars and pyroxenes are general angular
and fresh. The small amount of micromass, occurring as coatings or aggregates, has an
undifferentiated b-fabric. The microstmcture varies from coarse monic to enaulic and
chitonic.
The Italian profiles (N1 and N2 Naples, N3 and N4 north of Rome) are all characterised by
a bimodal granular microstmcture in the topsoil, grading with depth to a blocky one with a
weakly separated granular intrapedal microstmcture. The coarse material of N1 and N2
contains variable amounts of pumice, pyroclasts and volcanic minerals, whereas in N3 and N4
pyroclasts dominate. Compared to the profiles of Iceland and Santorini the amount of
micromass has considerably increased, but the b-fabric remains undifferentiated. Rock
fragments, especially pumice fragments, show a micromass coating. Infillings and excrements
point to a high biological activity. In the lower part of some profiles tmncated, more evolved
palaeosols, with anisotropic clay coatings, appear, pointing to polygenetic soils.
The French profiles (N16-N17, Massif Central) (only Ah and Bw horizons sampled) have a
granular microstmcture, sometimes weakly separated forming angular blocky aggregates in
the Bw horizon. The coarse material is a mixture of mainly microlithic pyroclasts, pyroxenes
and feldspars, with a dominance of pumice in the Bw of N16. Rock fragments are surrounded
by a coating of micromass with undifferentiated b-fabric, sometimes with a lighter colour than
the rest of the micromass in the same horizon.
Two profiles were sampled on the Azores, one on Fayal (N5) and one on Pico (N6).
Angular blocky microstmctures prevail, except in the Ah were granular microstmcmres occur.
Microlithic pyroclasts and vesicular vitric lapilli dominate the coarse material, although fresh
pumice is found in the top of N5. Most of the vitric vesicular lapilli show a pellicular
weathering to an orange isotropic material composed of Si, A1 and some Fe. The often
dominant micromass has an undifferentiated b-fabric. Hypocoatings and nodules of iron
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