Sunday, May 17, 2009

Weathering of basalt and clay mineral formation in Leyte, Philippines

Weathering is the physical, chemical, and biological alteration of minerals in rocks, sediments, and soils at or near the Earth’s surface. It is an important link in the global rock cycle and is also an essential process for the formation of soils and landforms. Chemical weathering of silicate minerals which compose over 90% of the Earth’s crust, removes CO2 from the atmosphere so it helps regulate the Earth’s climate over long time scales. Basalts are among the more easily weathered crystalline rocks thus, weathering of these rocks acts as a major CO2 sink. Chemical weathering of rocks likewise releases nutrient elements for use by the biota in the ecosystem and also produces clay minerals which are the central components of soils.

We studied the weathering of basalt by evaluating the gain and loss of elements, stream water composition, weathering indices, and clay mineral formation in the soil derived from basalt under the humid tropical conditions (average annual rainfall of 2700 mm and average temperature of 28oC) in Leyte, Philippines. The study site is located in the rain forest on the lower western slope of Mt. Pangasugan having an elevation of 100 m asl. The weathering profile studied is about 4 meters deep, heavy clay, acidic and yellowish red soil classified as Alisol (or Ultisol).

Results revealed that much of the basic cations Ca, Mg, K, Na and part of Si have already been lost from the weathering product (saprolite and soil). This was however accompanied by the accumulation of Al, Fe, C, and H2O. The extent of weathering as indicated by the loss of elements based on the total elemental composition of fresh rock and of saprolite and soil was closely related to the cation composition of the stream water in the study site. Relative rates of loss of bases and silica revealed the sequence: Ca>K>Na>Mg>>Si for the soil, and Ca>Na ≥ Mg>K for the stream water. The ratio Na : (Na+Ca) of the stream water indicated that its major source of cations was rock weathering.

Results also showed that the intensive basalt weathering has resulted in the formation and abundance of kaolinite and halloysite clay minerals (see above TEM micrograph) as well as goethite in the highly weathered soil. The idea that weathering moves to a system composed of SiO2, Al2O3, Fe2O3, and H2O (residua hypothesis of Chesworth) appears to be supported by the results of this study.

Reference
Asio VB and R Jahn. 2007. Weathering of basalt and clay mineral formation in Leyte, Philippines. Philippine Agricultural Scientist 90 (3): 204-212.

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