Abstract
The strength of rocks in the subsurface is critically important across the geosciences, with implications for fluid flow, mineralisation, seismicity, and the deep biosphere. Most studies of porous rock strength consider the scalar quantity of porosity, in which strength shows a broadly inverse relationship with total porosity, but pore shape is not explicitly defined. Here we use a combination of uniaxial compressive strength measurements of isotropic and anisotropic porous lava samples, and numerical modelling to consider the influence of pore shape on rock strength. Micro computed tomography (CT) shows that pores range from sub-spherical to elongate and flat ellipsoids. Samples that contain flat pores are weaker if compression is applied parallel to the short axis (i.e. across the minimum curvature), compared to compression applied parallel to the long axis (i.e. across the maximum curvature). Numerical models for elliptical pores show that compression applied across the minimum curvature results in relatively broad amplification of stress, compared to compression applied across the maximum curvature. Certain pore shapes may be relatively stable and remain open in the upper crust under a given remote stress field, while others are inherently weak. Quantifying the shape, orientations, and statistical distributions of pores is therefore a critical step in strength testing of rocks.
Highlights
Background and Methods2.1 Kilauea Pahoehoe Lava Small volume tholeiitic pahoehoe lavas are emplaced as non-channelized, inflated sheets on the subhorizontal (1-2°) south flank of Kilauea Volcano
We show that pore geometry – not just the scalar quantity of porosity – 53 provides a fundamental control on rock strength
Large oblate pores in the basal zone are generally well-aligned sub159 horizontally (Figure 3E); the contribution of smaller pores with aspect ratios >0.40 has the 160 effect of increasing the mean aspect ratio for basal zone samples
Summary
Background and Methods2.1 Kilauea Pahoehoe Lava Small volume tholeiitic pahoehoe lavas are emplaced as non-channelized, inflated sheets on the subhorizontal (1-2°) south flank of Kilauea Volcano. Careful characterisation of several lavas exposed in the fault footwall reveals a distinctive 3-zone physical stratigraphy based on the total volume and geometry of vesicles and the scale of joint patterns: (1) a top of 18-31% porosity, with sub-spherical vesicles up to 4 mm in diameter; (2) a core of 12-13% porosity, with sub-spherical vesicles up to 1.5 mm in diameter; and (3) a base, of 15-19% porosity, with oblate or amalgamated vesicles up to 15 mm in diameter The thickness of these three zones scale proportionally with the thickness of a lava, and representative samples were targeted for each zone. Olivine phenocrysts are typically euhedral up to 1.00-1.25 mm in size
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