Characterization of columnar- jointed intermediate to felsic volcanic rocks in eastern and southern China

Columnar joints are widespread in basaltic lavas, where they have been extensively studied. Although rarer, they can also be found in intermediate and felsic volcanic rocks (IFVRs). In eastern and southern China, columnar joints are about 30 times more likely to be found in mafic lavas than in IFVRs, raising the question of their formation mechanisms. Detailed field observations and measurements are presented here for 11 Chinese intermediate to felsic volcanic fields. The comparatively high viscosities (i.e. relatively lower emplacement temperatures) usually reduce the formation probabilities of columnar joints in IFVRs. The formation of columnar joints is influenced by the cooling rate (or strain rate) which primarily controlled whether melts cross the glass transition temperature. The largely variable size (side length ∼ 10–161 cm) and ordering (Hexagonality Index ∼0.68–1.5) of columns in IFVRs are not directly associated with their major element compositions, but possibly are controlled by their textures, emplacement styles, and surrounding environments. High effusion rates of lavas and/or pyroclastic flows emplaced in flat areas (i.e., basin floor, caldera interior) through stagnant cooling processes (i.e. welding and/or devitrification) would be more likely to form columnar joints in IFVRs.