Saturday, 15 July 2006

Soils Developed on Volcanic Materials in a Mountainous Environment (Mt. Teide, Canary Islands, Spain).

Carmen D. Arbelo1, J. Asterio Guerra1, Antonio Rodríguez-Rodríguez2, Bayanor Santana1, Jesús S. Notario1, and Juan L. Mora2. (1) Univ of La Laguna, Avda. Astrofísico Fco. Sánchez, La Laguna (Canary Is.), Spain, (2) Univ of La Laguna, Avda. Astrofísico Fco. Sánchez, La Laguna (Canary Is.), Spain

The mount Teide is placed on the Las Cañadas National Park in Tenerife (Canary Islands), which lies over 190 km2 and rises between 2000 and 3718 m a.s.l. It is made up of a great volcanic caldera over which rises the Teide-Pico Viejo strato-volcano complex. The caldera started to form about 3.5 million years ago and kept on its activity until 180.000 years ago. Then the making of the Teide-Pico Viejo complex started, which is still active. The volcanic materials released have been rather diverse in type and in nature, comprising basaltic, trachybasaltic, trachytic and phonolythic lava flows and basaltic, trachytic and phonolythic pyroclasts. The temperatures in the zone are rather contrasted, with a daily range of 15ºC, minimum values of –15ºC in winter, and maxima of 30ºC in summer. Average rainfall is 500 mm.yr-1, 17% of which occur as snow. Twenty three soil profiles developed from different parent materials aged between 3.5 Myr and less that 20 kyr, and under a sparse mountain scrub vegetation have been studied in the area, so as to determine their properties and to establish soil forming processes under the Canarian mountain scrub. The following soil types have been identified:Over basaltic and trachybasaltic lava flows On slopes lower than 20%:Lithic Xeropsamments (<20 kyr);Andic Dystroxerepts (20-180 kyr).Elsewhere: Lithic Cryopsamments (<20 kyr);Lithic Xerorthents (180-3500 kyr) Over recent parent materials, soils are scarcely weathered (CEC = 1.4-5.0 cmolckg-1), clay-poor (10-40 gkg-1), organic matter-poor (0.4-11 gkg-1), and acidic (pHH2O 5.8-6.9) Entisols in which Ca+2 predominates in the exchange complex and that show certain andic character (Alo =0.1-1%; P retention=50%). On ancient materials and gentler slopes, a cambic horizon may develop with a CEC =28-32 cmolckg-1, a higher clay content (about 260 and also a higher amount of organic C in the surface (11-15 Soil pH is still acidic (5.5), but the andic character is more marked (Alo = 0.6-0.9 %, P-retention = 70-75%). On steeper slopes, the characteristics are similar, but water erosion avoids the formation of cambic horizons.Over salic (trachytic and phonolythic) lava flows: On slopes lower than 20%:Lithic Xerorthents (<20 kyr)(Trachytes);Litjhic Haploxerepts (180-3500 kyr) (Phonolytes). Elsewhere: Lithic Cryorthents (<20 kyr) (Trachytes). Weathering of salic parent materials is hindered in such bioclimatic conditions, and soils are less developed (Lithic Entisols over recent materials). Only over the ancient phonolites at Las Cañadas Wall (180kyr – 3.5 Myr) an early cambic horizon can be observed (CEC =13.2 cmolckg-1, clay = 200 gkg-1, organic carbon = 24 gkg-1). However, a slight andic character appears in all cases (Alo = 0.3-1.0%, P-retention = 26-54%, Bulk density = 0.6-1.0 MgM-3).Over basaltic and trachybasaltic pyroclasts:On slopes lower than 20%:Humic Vitrixerands (<20 kyr);Lithic Vitrixerands (20-180 kyr):Typic Vitrixerands (20-180 kyr).Elsewhere:Typic Vitrixerands (<20 kyr). Basic pyroclastics rocks are more weathereable, even in such extreme conditions, whereby they give rise to Vitrandic Andisols, regardless their age and topographic position. These soils show a variable clay content (30-274 gkg-1), which tends to increase with the age of the parent material, an available water retention capacity higher than 15%, and a bulk density ranging between 0.3-0.8 MgM-3. Soil pH is neutral (7.1-7.4) and the CEC also varies (7-46 cmolckg-1), also increasing with the age and the weathering degree of the parent material (Alo = 0.2-1.0% y P-retention = 39-68%).Over trachytic pyroclasts and pumiceous tuffs:mOn slopes lower than 20%:Lithic Vitrixerands (<20 kyr) (Pumice);Typic Vitrixerands (20-180 kyr) (Pumice. Elsewhere: Vitrandic Xerorthents (<20 kyr) (Trachyte);Lithic Vitrixerands (20-180 kyr) (Pumice). The features of the soils formed on these materials (Vitrandic Andisols) are similar to those described above. The degree of weathering is lesser than in basaltic pyroclasts, as shown by the lower clay content and a higher gravel proportion (pumice and trachyte grains), lower CEC values (3-25 cmolckg-1) and Alo percentages (0.1 to 0.3%). Only trachytic pyroclasts on steep slopes give rise to coarse textured and shallow Vitrandic Entisols, as a consequence of water erosion and the nature of the parent material, less weathereable.

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