Emuruangogolak (Emurua Kirig West, Emuruagiring)

Emuruangogolak is a Quaternary basalt-trachyte volcano located within the rift (Suguta Trough) in the central part of the Maralal area (No. 085-00-011). The main volcanic edifice rises 700 m above the rift floor and covers 33x20 km but extends to the north for 9 km in the form of a chain of fissure-controlled scoria cones, tuff rings and maars (Dunkley et al., 1993). After construction of an early trachytic shield two periods of caldera collapse occurred, each preceded by explosive pyroclastic activity followed by alternating eruptions of basalt and trachyte (Weaver, 1977). However, the presence of an earlier caldera (Weaver, 1977; Williams et al., 1984) is disputed by Dunkley et al. (1993). The early shield essentially comprises lavas, but there are minor thin trachytic ignimbrites, which were followed by pumice tuffs that were ejected from pyroclastic cones on the upper flanks of the volcano. According to Weaver (1977) a caldera of 9x7.5 km then developed near which are sited small cones from which long flows of hawaiite and olivine basalt emanated. The caldera was filled with trachytic pumice tuffs that in places contain blocks up to 0.5 m in diameter of trachyte and syenite. A second smaller caldera (3.5x4.5 km) then formed within the first with later eruption of basalt lavas and large volumes of trachyte from four lava cones. Copious basalts then emanated from fissures over much of the volcano and extend southwards across the rift floor to the Silali volcano (No. 085-00-026) with some extrusion of trachyte lavas within the basalt series. The youngest flow is a trachyte blocky lava. The basalts are commonly olivine- (Fo87-80) and plagioclase- (An82-62) phyric with the plagioclase in some aphyric varieties andesine indicating hawaiite (Weaver, 1977), which is confirmed by the rock chemistry. Opaque phases are abundant in some types and reflected in relatively high TiO2 values while picritic varieties are rich in cumulate olivine and clinopyroxene phenocrysts. The trachyte lavas include 5-30% alkali feldspar phenocrysts (Or10-38) and phenocrysts of clinopyroxene zoned to aegirine and fayalite (Fa92-96) in a groundmass of alkali feldspar, poikilitic ferrorichterite zoned to arfvedsonite, sodic pyroxene and aenigmatite with up to 5% quartz. Obsidians comprise glass with phenocrysts of alkali feldspar, green clinopyroxene and fayalite. Whole rock analyses, including some trace element data, are given by Weaver (1977) who discusses at length the bimodality of the volcanic suite i.e. no rocks with SiO2 values between 49 and 59% were found amongst 80 analysed. Major, trace element and U-Th-Ra isotope analyses for numerous basalts and comenditic and pantelleritic trachytes are given by Black et al. (1998). Sr, Nd and Pb isotopic data for a basalt and several trachytes are in Norry et al. (1980). A detailed account of the geothermal activity at Emuruangogolak is given by Dunkley et al. (1993) with an accompanying 1:50,000 coloured geological map; their general account includes a number of coloured photographs of the complex.