Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff

Abstract

Pyroclastic fl ows produced by large volcanic eruptions commonly densify after emplacement. Processes of gas escape, compaction, and welding in pyroclastic-fl ow deposits are controlled by the physical and thermal properties of constituent material. Through measurements of matrix porosity, permeability, and electrical conductivity, we provide a framework for understanding the evolution of pore structure during these processes. Using data from the Shevlin Park Tuff in central Oregon, United States, and from the literature, we fi nd that over a porosity range of 0%-70%, matrix permeability varies by almost 10 orders of magnitude (from 10 -20 to 10 -11 m 2 ), with over three orders of magnitude variation at any given porosity. Part of the variation at a given porosity is due to permeability anisotropy, where oriented core samples indicate higher permeabilities parallel to foliation (horizontally) than perpendicular to foliation (vertically). This suggests that pore space is fl attened during compaction, creating anisotropic crack-like networks, a geometry that is supported by electrical conductivity measurements. We fi nd that the power law equation: k 1 = 1.3 × 10 -21 × ϕ 5.2 provides the best approximation of dominant horizontal gas loss, where k 1 = permeability, and ϕ = porosity. Application of Kozeny-Carman fl uid-fl ow approximations suggests that permeability in the Shevlin Park Tuff is controlled by crack-or disklike pore apertures with minimum widths of 0.3 and 7.5 μm. We fi nd that matrix permeability limits compaction over short times, but deformation is then controlled by competition among cooling, compaction, water resorption, and permeable gas escape. These competing processes control the potential for development of overpressure (and secondary explosions) and the degree of welding in the deposit, processes that are applicable to viscous densifi cation of volcanic deposits in general. Further, the general relationships among porosity, permeability, and pore geometry are relevant for fl ow of any fl uid through an ignimbritic host.

Relating Topological and Electrical Properties of Fractured Porous Media: Insights into the Characterization of Rock Fracturing

“…Setting the Dirichlet conditions Φ * 0 and Φ * 1 to 1 V and 0 V, respectively, implies that the effective electrical conductivity in one direction can be defined as σ = J 1 with J 1 the electric flux leaving the right side of the domain. This definition is consistent with laboratory experiments that have been conducted on rock samples (e.g., [25,[33][34][35]), and the reader should refer to [50] for a numerical multi-directional evaluation of σ. Note that, in this work, our DDP approach is always used with 21 grid-cells in each direction and that the numerical convergence of these results has been verified through tests using a finer discretization.…”

“…In the latter case with low-porosity rocks, the relationship F = τ/φ, where τ is defined as the electrical tortuosity, is also used (e.g., [17,42,43]), and corresponds to a particular case of the extended Archie's law F = aφ −m (e.g., [44,45]) with geometric constant a = τ and m = 1. As the electrical parameters m and τ provide meaningful and complementary information on the considered systems, they are used to characterize the topological properties of the fractured rock samples, sometimes independently of the relationship between F and φ (e.g., [22,25,[33][34][35]46]). More precisely, m is related to the pore geometry of the system and characterizes how the electric current samples the interconnected porosity (e.g., [22,29]), whereas τ is related to the irregularity of the electrical flow pathway and characterizes the complexity of this path through the system (e.g., [22,42,47]).…”

“…This has been the objective of numerous laboratory experiments (e.g., [22][23][24][25]) and modeling approaches (e.g., [26][27][28][29][30]) conducted for a wide variety of porous and fractured rocks. Whereas these studies defined and characterized the corresponding petrophysical relationships for porous media (e.g., [16,31,32]), some experimental results obtained for fractured-rock samples remain unexplained because the impact of the fracture-network properties on the resistivity response is unclear (e.g., [25,[33][34][35]). From a numerical modeling standpoint, multiple-porosity model formulations show how the presence of vugs and fractures impacts the effective electrical properties of rocks (e.g., [14,26,36]).…”

Causes of fragmented crystals in ignimbrites: a case study of the Cardones ignimbrite, Northern Chile

“…The degree of welding can be quantified by measuring the density and porosity of the matrix, the aspect ratios of pumice clasts (fiamme), and the deformation of glass shards (e.g. Peterson 1979;Quane and Russell 2005;Wright and Cashman 2014). Despite the large literature on welding and compaction of ignimbrites, however, there are no studies of crystal deformation during compaction.…”

“…These processes are controlled by the residence time of the pyroclastic material above the glass transition temperature, the deposit thickness, the compactional load (Smith 1960;Riehle et al 1995;Quane and Russell 2005;Wright and Cashman 2014) and rates of volatile absorption by glass components (Sparks et al 1999). The degree of welding can be quantified by measuring the density and porosity of the matrix, the aspect ratios of pumice clasts (fiamme), and the deformation of glass shards (e.g.…”