Sequences climatiques de sols derives de roches volcaniques aux iles canaries

Abstract

Soil sequences derived from Tertiary and Quaternary volcanic rocks in the Canary Islands have been investigated according to their exposure being to the north or to the south and to altitude. Some of the soil characteristics are described, emphasising the evolution in the sequences and the conditions of genesis. The northern sequence is divided into two parallel catenas, the differences in which are due to the age of parent materials. From top to bottom, the old sequence includes ferrallitic soils, fersiallitic soils and finally vertisols. The Recent sequence, includes andosols and brown soils. Thus, the distribution is related with climatic altitudinal zonality: perhumid at the top, then contrasted (with alternating humid and dry seasons) and finally subarid near the coast; this zonality is also reflected by the plant geography. The relationships between soils and present-day climatic zones are evident for Recent formations: andosols and perhumid climate, brown soils and contrasted climate. These relationships seem equally plausible for the more ancient formations. The authors suggest a direct lineage between unsaturated andosols and ferrallitic soils on the one hand, and between brown soils and fersiallitic soils on the other. The southern sequence comprises, successively from the top downwards: andic brown soils, typic brown soils, fersiallitic soils, vertisols, cinnamonic soils and sodic soils. Because of a much lesser deposition of Recent volcanic material, only one sequence can be noted. The soil distribution is also related to a climatic and vegetational altitudinal zonality. Moreover, the sequence has a greater extension towards the arid pole, because the south slope is less rainy than the north. The relationships between soil genesis and existing climatic conditions still seem very plausible for the upper part of the sequence, especially the andic brown, the typic brown and the fersiallitic soils. In the lower part of the sequence, however, the authors suggest that the evolution is related to the fluctuations of Quaternary climate. At the time of the last fluctuation, after a more humid climate favoring clay formation, there followed an increasingly arid climate producing calcium carbonate accumulation, gypsum and finally sodification of soils as well as genesis of palygorskite.