Paka

Paka volcano lies within the rift, rises 600-700 m above the rift floor and covers 280 km2. General descriptions will be found in Hackman (1988) and Dunkley et al. (1993), the latter including a coloured geological map on a scale of 1:50,000. Contemporaneous faulting led to the formation of a north-trending rift zone extending down the northern flanks which controlled the dispersion of lavas and pyroclastic flows during the final stages of eruption and caldera collapse (Dunkley et al., 1993). Paka is surrounded by several smaller volcanic centres that are linked to the volcano by lines of basalt and trachyte cones and eruptive fissures. The general stratigraphy, from oldest to youngest, is Lower Trachytes, Lower Basalts, Upper Trachytes, Upper Basalts, pyroclastic deposits and, after caldera formation, intra-caldera trachyte and mugearite lavas. The Lower Trachytes form lobate or button-shaped flows and according to Hackman (1988), there is a large mugearite flow within the sequence. The Upper Trachytes, which form thick flows, are the most voluminous rocks on Paka and were erupted from the summit area and numerous satellite centres on the eastern and southern flanks. They contain up to 30% feldspar phenocrysts, and rare ones of alkali pyroxene set in a matrix of alkali feldspar laths, zoned aegirine-augite and aenigmatite; arfvedsonite and nepheline are rare but sodalite crystals are common (Key, 1987b). Hackman (1988) refers to the lowermost trachytes as being characteristically analcime-bearing. The basaltic units include picritic varieties as well as hawaiites. The summit and large parts of the northern and western flanks are mantled by trachytic pyroclastic deposits. In the summit area these consist of pumice lapilli tuffs, agglomerates and breccias with strongly welded glassy zones in the tuffs and breccias. On the flanks there are poorly consolidated pumice tuffs with thin zones of welding while tuffs and breccias of pyroclastic flow and mass flow origin are concentrated in stream sections. Eruption of these pyroclastic rocks seems to have triggered formation of the 1.5 km diameter caldera. Activity ceased after eruption of trachyte and mugearite flows within the caldera. The general chemical classification of the rocks is indicated on a TAS diagram by Dunkley et al. (1993, Fig. 13.1) and Sceal and Weaver (1971) present major and trace element data from which they discuss the origin of the salic rocks. Geothermal activity is covered in detail by Dunkley et al. (1993)