mineralogical and fabric characterization and classification of ...

- 11 -In general, soil zones of the weathering profile are comparatively shallow in volcanicrocks, usually less than 15 m thick, in contrast to the deep profiles that occur over thegranitoid rocks. However, thick soil profiles of up to 35 m have also developed, particularlyin the coarser grained volcanic rocks (Figure 3). Deeper soil profiles have been recorded, butthese are considered to be confined to faults and closely fractured zones. Corestonedevelopment, which is characteristic of the medium- and coarse-grained granites, is notcommon in weathering profiles developed over volcanic rocks, except in coarse ash tuffs andlapilli tuffs (Plates 1 and 2).Cataclasis and shearing of the rocks by tectonic processes and granite intrusions and theassociated hydrothermal alteration are a major influence in profile development and soilformation at many locations in Hong Kong. Hydrothermal alteration generally results inweakening of rocks and deeper penetration of weathering (Irfan, 1996a; Irfan & Woods,1997). Some volcanic rocks are more resistant to weathering (e.g. lavas) than others, resultingin well-defined escarpments where resistant rocks cap and protect the softer more easilyweathered tuffs. For example, in the western New Territories, the slightly metamorphosedtuffs form thin weathering profiles with corestones, but strongly metamorphosed tuffsgenerally form resistant ridges (Langford et al, 1990).The general weathering profile in lithologically uniform volcanic rocks is similar to thatof granitoid rocks, and consists of four major layers grading from residual soil at the top tofresh rock at depth. In many localities in Hong Kong, the transitional rock and soil zone isusually relatively thin, except in coarse-grained volcanic rocks which have a similar profiledevelopment to medium to coarse-grained granites (Plate 1). In bedded sequences of varyinglithology, the rock mass may consist of relatively unweathered bands or beds alternating withdecomposed bands or beds. In some cases, the soil layer may be underlain by the fresh rock,with intermediate layers not present (Plate 3).The residual soil layer over the volcanic rocks is not very well developed, as is the casewith granites also. It is generally underlain by the transitional soil layer containing relict rockfabric but no structures. This zone is up to 3 m thick at many locations even on gently tomoderately sloping ground.3.2 Weathering Grade Classification3.2.1 Classification of Weathered Rock MassAs the weathering profiles developed over the volcanic rocks are similar to those of thegranitic rocks, the same weathering grade classification scheme is generally applicable to bothrock types. The six-fold mass grading scheme recommended by BS 5930 (BSI, 1981) hasalso found favour for classifying various components of the weathering profile developed overvolcanic rocks (Powell & Irfan, 1987; GCO, 1990; Irfan, 1994) (see Table 1). The zonalscheme proposed by GCO (1988) is particularly suitable for corestone-forming, generallycoarse-grained volcanic rocks (e.g. coarse ash tuffs). However, it is less well suited forfine-grained rocks which show gradual loss of strength with weathering without forming anysignificant soil material.The schemes proposed by GCO (1988) and BSI (1981) and the most recent classificationschemes proposed by the Geological Society of London (1995) fail to cater adequately in