Abstract
Thin sections of about 200 horizons or layers, representing 60 soil profiles in the coastal area and on the windward slopes of Isla Santa Cruz (Galápagos Islands) were analysed. Based on the fabric and the composition of the groundmass and the presence of pedofeatures in individual layers or horizons, clusters of similar material are made and 7 units and 3 subunits distinguished. Plotting these units on a survey map gives a good insight in the spatial distribution of soil materials, expressing different combinations of parent material and precipitation. In the coastal area and the lowest slopes, with a summer dry climate, respectively reddish and greyish and brown materials with a porphyric c/f related distribution pattern and striated b-fabrics, and often with fragmented illuvial clay coatings are observed. The coarse material contains mainly holocrystalline basalt fragments, unweathered in the coastal area, or basalt derived individual minerals. The micromass has a halloysitic-smectitic composition. On the higher slopes, with a permanent moist climate, materials have also a porphyric or fine monic c/f related distribution pattern, but the b-fabric is undifferentiated and no illuvial features are present, gibbsitic features sometimes occur and the micromass consists mainly of halloysite and gibbsite. Subunits are distinguished here according to the microstructure, and the quantity and type of coarse material. Mesocrystalline basalt, often vesicular, dominates over holocrystalline, pointing to the influence of scoria. Soils on the higher slopes are more strongly weathered than those on the drier lower slopes and the coastal area. In depressions in the higher areas, materials with a yellowish or brownish grey micromass with a striated b-fabric, and well developed limpid illuvial clay coatings and impregnative iron oxide nodules occur. The micromass has a halloysite-smectite composition. Contrary to existing hypotheses the reddish soils in the coastal area are not considered as palaeosoils (roots of tropical soils), but as modern soils developed in colluvium on the lower slopes, which was deposited on totally eroded surfaces (marine terraces?). On the slopes the distribution of units is not only determined by hypsometric zones, as suggested in literature, but is rather clustered according to types of parent material. The soils on Santa Cruz are supposed to be formed after the last interglacial period, different from the red soils of San Cristóbal which are older.
Highlights
The study of the fauna, flora and geology of Galápagos Islands is rather popular amongst scientists, this is not the case for soils (Sabau 2008)
A preliminary micromorphological study of some profiles was presented during the Third International Working Meeting on Soil Micromorphology in Wroclaw (Stoops 1972), and some additional observations were published by co-workers (Eswaran et al 1973; Morrás 1974, 1976)
Plotting Bc and Bm on a schematic map (Figure 2) shows that holocrystalline basalt is the main type of rock fragments found in the coastal area and the lower slopes, whereas Bm occurs on the middle and upper slopes, often mixed with Bc
Summary
The study of the fauna, flora and geology of Galápagos Islands is rather popular amongst scientists, this is not the case for soils (Sabau 2008). The aim of this paper is to present a global interpretation of the micromorphology of all samples collected 50 years ago, most of them not yet studied, according to modern concepts. This will allow to get a better insight in the formation and evolution of these soils from a micromorphological point of view, and to select profiles for further detailed chemical and mineralogical studies
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