Conclusions

Due to the high temperature gradient and low lithostatic stress, thermal cracking may be an important process in controlling fracture in Icelandic crust. In the absence of confining stress, such cracking starts in the temperature range $300^\circ$ to $400^\circ$C in fresh basalt. Thermal cracking leads to increased fluid permeability above about $300^\circ$C, with the permeability increasing very non-linearly with temperature. Thermal cracking also appears to lead to significant decreases in mechanical strength and resistance to crack propagation of PHT specimens when tested after cooling at room temperature.

However, a somewhat different picture emerges when both PHT and NHT specimens are actually tested at elevated temperatures. There appears to be an increase in tensile strength with increasing temperature. We attribute this to enhanced microplasticity at highly stressed crack tips effectively blunting cracks.