Field observation and laboratory studies reveal that extrusive lava and intrusive rocks emplaced in the near-surface setting have natural joint networks typically characterized by pentagonal-hexagonal columnar structures, whilst fracture networks in deeper emplaced granitic complexes exhibit a parallelepiped joint structure. Thermal cracking experiments, using pseudo-magmatic material (i.e. paraffin) have clarified the relationship between the anisotropy of (natural) crack networks and external pressure. The observed difference in joint network patterns and structure in shallow intrusive/extrusive complexes, compared to deep plutonic rocks, is most strongly influenced by (inferred) depth of formation, and effect of confining pressure. A numerical thermal conduction model was used to evaluate the anisotropy of joint formation in granite. Joints parallel to the surface of granite rock body are explained by 'Eggshell' and 'Uplift' models, corresponding to cooling effects along the brittle-ductile transition boundary and/or the removal of superincumbent load, during uplift.
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