Master fracture surfaces show a thin band with an evolved composition at the fracture surface mineral textures in this band also show evidence of quenching of this material. The entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling. Cube-jointing is a more densely fractured version of entablature, which likely forms when more coolant enters the hot lava.
The interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature. These are: striae-bearing, column-bounding fractures and pseudopillow fracture systems that themselves consist of two different frac-ture types-master fractures with dimpled surface textures and subsidiary fractures with curved striae. A number of different fracture types are described in entablature outcrops from the Búrfell lava and older lava flows in Þjórsárdalur, southwest Iceland. It is thought to form when water from rivers dammed by the lava inundates the lava flow surface, and during lava-meltwater interaction in subglacial settings. Monitoring acoustic emission microseismic activity in seismically active zones could potentially be used as a means of detecting stress corrosion in rocks and the nucleation of earthquakes.Entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows. The stress corrosion index for Westerley granite was between 35 and 39 for tests in air and liquid water. The stress corrosion index for Black gabbro was between 32 and 36 for tests in air or in liquid water at high stress intensity factors, but fell to 29 for tests in liquid water at low stress intensity factors. The critical stress intensity factors for Black gabbro and Westerley granite were respectively 2.88 MN.m −3/2 and 1.74 MN.m −3/2.
The acoustic response, especially the amplitude distribution of events, is a sensitive indicator of the micro-mechanism of crack extension, which in turn depends upon the stress intensity factor, crack velocity and degree of “humidity“ at the crack tip. The rate of emission can be used as an indirect measure of crack velocity. Both these materials give significant acoustic emission, probably due to extension of the macrocrack, at even the slowest crack velocity (10 −9m.s −1) studied. The acoustic response (event rates, ring-down count rates and amplitude distributions) were continuously monitored during the experiments. Stress corrosion of Black gabbro and Westerley granite in air and liquid water at 20☌ has been studied using single cracks in double torsion specimens.
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