Columnar joints in intermediate and felsic volcanic rocks

Abstract

Columnar joints are among the most spectacular structures in various types of igneous rocks. In contrast to basaltic rocks, very few studies systematically examine this structure in other types of igneous rocks. Here, I focus on columnar joints in intermediate and felsic volcanic rocks (IFVRs) which outcrop in eastern and southern China through detailed field observations and measurements. The interpretations and analyzed results show that: (1) Columnar joint is not rare in IFVRs which have similar internal structures to columnar jointed basaltic lavas, but the high viscosity usually inhibits their formation potential of columnar joints in intermediate and felsic lavas (ratio of formation probabilities between IFVRs and basalts is ~ 1/29 in eastern and southern China). (2) Columnar joints in IFVRs usually display curved structure that are likely formed by the slightly downslope of solidified top part and the local rotation of the isotherms, which can be classified as entablature. (3) 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 most possibly are controlled by their emplaced styles, surrounding environments and rock textures. (4) High effusion rates of lavas and/or pyroclastic flows emplaced in flat areas (i.e., Basin floor, canyon, lava lake, caldera interior) through stagnant cooling processes (~ 1.4–35°C/h) would be the key mechanisms to form columnar joints in IFVRs.