This large, 15 km diameter volcano was built by early explosive activity which produced a central cone of nephelinitic and ijolitic pyroclastic rocks, which were followed by extrusion of lavas that occur mainly on the western side of the volcano. Sodic pyroxene, nepheline, magnetite and carbonate are the commonest minerals in the pyroclastics and blocks include nephelinite, phonolite and rarer ijolite, melteigite and nepheline syenite. The later lavas include nephelinite, melanephelinite, phonolitic trachyte, analcitite and augitite, one variety of which contains large olivine phenocrysts. The last two rock types grade into each other and contain some biotite and nepheline and prominent accessories include perovskite, apatite and calcite. Dykes of similar composition to the lavas cut the pyroclastic rocks. Three rock analyses, including trace element and Sr, Nd and Pb isotope data, are in Paslick et al. (1995).