THE ROLE OF POROSITY ON DOME EXTRUSION PROCESSES: EXPERIMENTAL INSIGHTS

Decompression of rising magmas causes gas exsolution and a concomitant increase in magma porosity. Slow ascent commonly results in cooling and crystallization of the residual melt on the same time scale. Thus, slow ascending magmas commonly produce domes that are typically at or below their Tg, or have undergone degassing-induced crystallization. These magmas can be either highly porous or nonporous. For example, high residual porosity indicates that the system had low permeability and was quenched before the porosity could be removed. This quenching of primary porosity could be induced by cooling of the melt to below Tg, or by crystallization of the melt to produce a solid framework.

Here, we demonstrate experimentally the effects of porosity on the strength and failure behaviour of dacite dome rocks. Our triaxial rock deformation experiments were run at confining pressures (Pc) of 0, 25, 50, and 75MPa, at room temperature and strain rates of ~1 x 10^-4 s^-1. Our starting material has both low (6-8%) and high (17-24%) porosities, a uniform bulk composition (65 wt% SiO2) and is either highly crystalline or has a glassy matrix. The low porosity dacite experiments show a progressive increase in peak strength (100-700 MPa) with increasing Pc and all cores show brittle behavior, characterized by a rapid stress drop. Run products contain macroscopic fractures with deformation extremely localized around the shear fractures. Experimentally deformed dacites show extreme grain size reduction and the production of gouge. In contrast, the high porosity dacites are 3-4 times weaker than low porosity dacite. The mechanism of deformation is dominated by distributed cataclastic flow rather than localized faulting. There is no stress drop, no discrete slip surface and no gouge production.

Our experiments suggest that domes with low residual porosity will extrude via brittle fault zones accompanied by microseismicity (e.g., Mt. St. Helens), and feature carapaces of cataclastic gouge (e.g., ‘whalebacks’) such as observed at Unzen, Montserrat (Watts et al. 2002) and Mount St. Helen’s (Cashman et al. 2009). Conversely domes extruded at or below Tg and having high porosity will lack microseismicity, deform by distributed cataclastic flow rather than localize faulting, and may produce more stable structures.