Weathering mechanisms and indices of the igneous rocks of Hong Kong

Abstract

Weathering mechanisms of volcanic and granitic rocks in Hong Kong have been studied by chemical analysis, optical microscopy on thin sections and magnetic susceptibility measurements. The mobility of elements of calcium, sodium, iron, potassium, magnesium, silicon, aluminium and constitutional water is investigated in detail using the Mobility Index. Chemical weathering processes in Hong Kong are dominated by decomposition of feldspars and biotite, leaching of alkali and alkaline earth metals, and enrichment of ferric oxide under prolonged subtropical climate conditions. Data for some granitic residual soils from five neighbouring regions in Southern China are also gathered and examined. It is found that the residual soils in Hong Kong are abnormal and immature with respect to its latitude. By examining various quantitative chemical indices and comparing them with the current six-grade material classification scheme, WPI , LOI and I mob are found to be good indicators of the degree of weathering for both volcanic and granitic rocks in Hong Kong. Regarding the optical microscopy, the micropetrographic index ( I p ) is only suitable for granitic rock but not for volcanic rock due to the presence of fine-grained minerals and small voids. For the magnetic susceptibility measurements, consistent results with weathering grades for volcanic rocks are obtained by applying a moving average technique. However, due to the low content of Al 2 O 3 and Fe 2 O 3 in granitic rock, SOC and the magnetic susceptibility index, which are appropriate for quantifying the degree of weathering of volcanic rock, are not quite suitable for the granitic rock.