Time-dependent compaction creep in tuffs from Krafla volcano (Iceland)

Volcanic rock can be subjected to high and fluctuating pressures and stresses associated with volcanic activity and geothermal production. When subject to a high constant stress in the brittle regime (shallow depths), strain and porosity increase as a function of time, eventually leading to macroscopic failure—a process called brittle creep. In the ductile regime (deep depths), rare experiments have shown that strain accumulates and porosity decreases at a constant stress. During this process—called compaction creep—the rates of strain accumulation and porosity reduction decrease as a function of time. Here, we performed triaxial constant strain rate experiments and triaxial compaction creep experiments on three samples of tuff sampled from boreholes drilled into Krafla volcano (Iceland). The tuffs differ in terms of their source depth (~395, ~505, and ~690 m), macroscopic texture (grain size and distribution), and mineral content (different quantities of clay minerals/chlorite). The connected porosities of the tuffs, however, are very similar (0.29–0.35). We first performed X-ray computed tomography on each tuff in order to provide a quantitative description of their microstructure (grain size and distribution, and pore size, distribution, shape, and orientation). Triaxial constant strain rate experiments were then performed at different effective pressures to map out the yield cap for each tuff. Finally, triaxial compaction creep experiments were performed at effective pressures corresponding to the same position on the yield cap for each tuff. The constant differential stress used in these experiments was selected as the same proportion between the onset of inelastic compaction and the inflection point in the stress-strain curve from the constant strain rate experiment performed at the same effective pressure. All three tuffs accumulated strain and lost porosity as a function of time under a constant stress, although the rates of strain accumulation and porosity reduction, and therefore the maximum strain and porosity loss achieved at the end of the experiment, were different. For example, the porosity loss at the end of the experiments (after 100 hours) for the three tuffs was 0.014, 0.015, and 0.023. Because the connected porosity of the three tuffs is the same, differences in their compaction creep behaviour can be explained by differences in their microstructure and mineral content. The time-dependent compaction of porous volcanic rocks, demonstrated here for tuffs, has implications for volcano stability and geothermal production.