Late Cenozoic columnar-jointed basaltic lavas in eastern and southeastern China: morphologies, structures, and formation mechanisms

Abstract

Late Cenozoic basaltic lavas in eastern and southeastern China commonly display spectacular columnar joints. In four volcanic fields in this region, we have observed and classified five types of colonnade (vertical, inclined, horizontal, fanning upwards, and fanning downwards) that provide information on the cooling regime and emplacement mechanism of the lava flows. In total, we analyzed the geometry and morphology of joints across ten different lava flow areas in the Changle volcanic field (CVF), Luhe volcanic field (LVF), eastern Zhejiang volcanic field (EZVF), and Niutoushan-Linjin Island-Nanding Island volcanic field (NLNVF). Field observations show that columnar-jointed alkaline basalts commonly exhibit narrow and irregular shapes indicating fast cooling. Tholeiites with columnar joints, in contrast, show wide and regular polygons, indicating slower cooling. Vertical columnar joints and other types (horizontal and fanning shaped) have different mature patterns, with the implication that lavas flowed on relatively flat land and in paleo-valleys, respectively. This study suggests a simple qualitative description of different types of colonnade can be made based on their alignment patterns, which represents a direct and easy way to derive solidification processes after lava eruption from colonnaded lavas in outcrop.