Column-normal fracture features of the basaltic joints in Santa Maria Regla, Hidalgo State, Mexico

摘要

Columnar fractures are imposing geological structure observed at several locations around the world. They usually have a polygonal cross-section that vary in size from a few centimeters to meters. The columnar basalt flow at Santa Maria Regla in central Mexico shows a typical two-layer structure with an upper entablature and a lower colonnade. For the colonnade, the horizontal parting surface of basaltic columnar joints shows a polygonal pattern, which is most commonly five sided. In the upper part of the colonnade, the hexagonality (m) index changes from 0.94 to 1.42 with column height, indicating a progressive maturation of columnar joints with time. However, in the lowermost colonnade, the m value does not show such correlation. Three types of column-normal fractures are observed: plane, convex-upward and concave-upward, which can be explained by different stress states of contraction in individual columns. Radial markings and axisymmetric plumes can be observed on some columnar-normal fractures. The flaw points of the former are usually encountered in the polygon center, whereas those of the latter are located nearly at the edge of the polygon. These two types of column-normal fracture markings are formed after the columnar joints.