Bilibino

The extinct Bilibino volcano reaches a height of 390 m above the adjacent river valley. It is of central type, forms a truncated cone which is elliptical in section and has a basal diameter of over 1 km. The structure of the upper part of the cone is clearly exposed and formed by alternating layers of dense, dark grey and reddish brown vesicular basalts and poorly cemented explosive products. The lavas are porphyritic with phenocrysts of olivine in a matrix of labradorite (An60-65), clinopyroxene, Ti-Fe oxide minerals and glass. The chemistry indicates them to be nepheline normative (about 5-6% ne) mildly alkaline olivine basalts, but the occurrence has not been thoroughly studied and more alkaline types may occur. For a detailed mineralogical account see Dovgal and Chasovitin (1965).