Cooling Fractures in Lavas: Mechanisms and Environments of Formation

Forbes, Anne (2013). Cooling Fractures in Lavas: Mechanisms and Environments of Formation. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f113

Abstract

The focus of this study is cooling fractures in lavas that are associated with environments containing ice, snow or liquid water. Two main fracture types feature in this study: columnar jointing and pseudopillow fractures. This thesis addresses how and why these fractures form in particular environments.

Columnar jointing in rhyolite lava is particularly common in subglacial environments and rare in subaerial or purely subaqueous environments. Five subglacial rhyolites are studied with the presentation of the first set of measurements from subglaical rhyolite columns, showing considerably smaller column sizes and striae widths than in basalts. Some simple modelling was undertaken in order to explain these differences.

Pseudopillow fractures consist of a large, metre-scale, master fracture with many smaller, centimetre-scale, subsidiary fractures perpendicular to the master fracture. They are found in lava compositions from basalt to rhyolite. All documented occurrences are in lavas that have been inferred to have interacted with liquid water, ice or snow. The term pseudopillow fracture system is proposed to describe the consistent package of two different fracture types occurring together. Pseudopillow fracture systems were studied in two different trachyandesite lava flows. Three different master fracture types were identified on the basis of fracture surface textures displaying either chisel marks, cavitation dimples, or river lines and rough/smooth textures. Two types of subsidiary fractures were identified on the basis of their morphology: polygonal and planar subparallel.

The fractures in entablature, a formation common in basalt lava flows that have been inundated with water, were studied. Entablature was found to contain pseudopillow fracture systems and columnar jointing, which interact to form chevron fracture patterns. Master fractures form by ductile fracture of evolved residual melt, and show evidence of rapid cooling, related to coolant ingress. Two end members were identified: cube-jointing and column-bearing entablature, resulting from faster cooling in cube-jointed entablature.