Abstract
AbstractMost studies of thermally induced cracking in rocks have focused on the generation of cracks formed during heating and thermal expansion. Both the nature and the mechanism of crack formation during cooling are hypothesized to be different from those formed during heating. We present in situ acoustic emission data recorded as a proxy for crack damage evolution in a series of heating and cooling experiments on samples of basalt and dacite. Results show that both the rate and the energy of acoustic emission are consistently much higher during cooling than during heating. Seismic velocity comparisons and crack morphology analysis of our heated and cooled samples support the contemporaneous acoustic emission data and also indicate that thermal cracking is largely isotropic. These new data are important for assessing the contribution of cooling‐induced damage within volcanic structures and layers such as dikes, sills, and lava flows.
Highlights
We present a generic study on the fracture development during heating and cooling of volcanic rocks, which is important as most studies to date have focused only on the heating part of a heating and cooling cycle [e.g., Fredrich and Wong, 1986; Richter and Simmons, 1974; Simmons and Cooper, 1978; Meredith et al, 2001; Vinciguerra et al, 2005]
Seljadalur basalt (SB) produces a significantly higher rate of acoustic emissions (AEs) energy output at higher heating and cooling rates, this effect is less apparent for Nea Kameni Dacite (NKD)
The lowest AE rates occur consistently during the hold periods at maximum temperature, which suggests that thermal equilibration is reached and that any thermal stress is minimized
Summary
We present a generic study on the fracture development during heating and cooling of volcanic rocks, which is important as most studies to date have focused only on the heating part of a heating and cooling cycle [e.g., Fredrich and Wong, 1986; Richter and Simmons, 1974; Simmons and Cooper, 1978; Meredith et al, 2001; Vinciguerra et al, 2005]. While we do not attempt to model volcanic systems, the thermal stresses generated in such systems, like mechanical stresses, are likely to be generated cyclically [Heap et al, 2013b] through new, hot magma received from time to time by the chamber and repeated intrusion and extrusion of magma. Such cyclicity may gradually produce additive rock damage, pushing the magma chamber toward failure, that is, rupture [Browning et al, 2015]. In addition to these processes, it is likely that cooling-related fractures increase the permeability and rate of degassing of magma in shallow conduits [Tuffen and Dingwell, 2005; Tuffen et al, 2003] as well as at the surface in viscous domes and lava flows [Cabrera et al, 2011; Gaunt et al, 2016]
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