Cyclic fracture mechanisms in cooling basalt

Striae on the fracture surface of columnar joints in basalt may be rationalized in the light of experimental fracture mechanics. Morphologic similarity between experimentally produced and natural fracture surfaces suggests that the striae in basalt were formed during incremental crack advances accompanying cyclic stress buildup and release in the cooling body. Fracture surfaces in a Hawaiian flow (Boiling Pots) are striated by alternating smooth (∼2.5 cm wide) and rough (∼ 1.0 cm wide) zones normal to the column axis, which suggest elastic and nonelastic failure, respectively. By analogy to laboratory studies of cyclically fatigued samples, the rough band is interpreted as representing successive stopping positions of the crack tip during fracture. Seismic studies of thermal cracking in Kilauea Iki lava lake (Hawaii) indicate that thermal fracture in cooling basalt is a discrete event, characterized by a sudden period of crack advance. Comparison between cooling units shows that thin bodies have narrow striae and column faces, while thick bodies have wide striae and column faces. This suggests the importance of relative temperature gradients (that is, cooling rates) in driving the crack advance. Observations of the sense of shear on column faces [as revealed by orientations of small platelets (fracture lances) within striae] permit a kinematic interpretation of polygon development and suggest that purely tensile opening frequently is coupled with antiplane shear during crack propagation. Striations are thus a record of the crack advance in cooling basalt; not only should they provide a new tool for the field geologist, but their interpretation should lay the basis for models that examine the coupling of the thermal and mechanical behavior in subsolidus cooling basalt.