**4. Discussion**

Our new laboratory data show that the permeability of our ballistic samples increases as a function of increasing porosity (Figure 6a), as observed in previous studies on the permeability of volcanic rocks (e.g., [57,58,65]) and porous sedimentary rocks (e.g., [66]). We also observe that a single tensile fracture in a laboratory sample can increase permeability by many orders of magnitude (Figure 6b). Similar to previous experiments [49] showed similar increases to permeability following the formation of a macroscopic fracture. Experiments performed at elevated pressures show that permeability decreases as a function of increasing confining pressure (Figure 7). We interpret this here as a result of the closure of pre-existing microcracks and high-aspect ratio pores within the samples, as concluded by previous studies that measured the permeability of microcracked volcanic rocks (e.g., [60,61]) and microcracked granite (e.g., [67]) as a function of increasing confining pressure. Finally, our uniaxial data show that the uniaxial compressive of our ballistic samples decreases as a function of increasing porosity (Figure 8), in agreemen<sup>t</sup> with previous studies on the strength of volcanic rocks (e.g., [62–64]).

Our new data allow us to assess the structure and composition of the breccia-filled conduit at Whakaari. The ballistic samples illustrate four distinct lithologies: (1) generally relatively unaltered lava blocks with thin alteration rinds, (2) completely or dominantly altered lava blocks, (3) completely altered tuffs, and (4) hydrothermal breccias. These lithologies represent the polylithic and variable breccia that comprise the conduit (Figure 9a).

**Figure 9.** Proposed model of Whakaari conduit (**a**) near the surface and (**b**) eruption.

During an eruption, porous, weaker rocks are fragmented into smaller grain sizes [68] and therefore may be underrepresented in the block-sized ballistic samples. Soon after the 2016 eruption, the ash-sized component of the surge deposit had a generally yellow colour; however, our XRD analysis of the surge deposits indicates a dominantly altered cristobalite-rich composition. This indicates that the most fragmented component of the surge had a dominantly cristobalite mineralogy, and this likely comprised the finer grained dominantly tuff matrix of conduit-filling breccia which fragmented to form the surge and ballistics. Cristobalite was also observed in a dacitic bomb erupted at Whakaari in 1999 and is an inevitable product of shallow vapour phase alteration [69].

Several ballistic samples show evidence of brecciation and hydrothermal veining, with many samples also displaying anhydrite and alunite-bearing altered rims (Figures 3 and 5). The ballistic sample lithologies parallel those discussed in previous studies of the edifice of Whakaari: (1) altered tuffs composed of dominantly silica polymorphs and alunite and (2) relatively unaltered coherent lavas and breccias dominated by plagioclase and pyroxene (e.g., [12,14]. However, our data adds a third and fourth additional lithology, less common at the surface but dominant in the ballistics: (3) altered lavas dominated by anhydrite, and (4) hydrothermal breccias. We only found trace amounts of sulfur precipitation in the ballistics, indicating that sulfur-precipitated tuff may not be common in the subsurface.

We thus propose a model of a breccia-filled conduit containing clasts of dominantly altered lava and tuff in a matrix of relict ash-sized pyroclasts now replaced by silica polymorphs, alunite and anhydrite (Figure 9a). Similar to [13], we envisage that the conduit is surrounded by less altered coherent beds of lava, lava breccia, and tuff. Textural evidence in the ballistics and observations at the surface sugges<sup>t</sup> that macrofractures are ubiquitous within the conduit and throughout the edifice, although these may be locally filled with alteration minerals such as anhydrite and alunite to form veins (Figure 9b). We acknowledge that some of the fractures present in the ballistics may be created during eruption, however, the overprinting of multiple fracture events recording in veining and mineralisation (Figure 5) show that fractures also develop pre-eruption in the conduit. Therefore, conduit itself may locally resemble a mineralised cockade breccia [35] but also contain facies similar to the heterolithic unbedded country rock breccia of maar diatremes [9] and the conduit beneath Unzen volcano [70]. Several vents were identified in the 2016 eruption [16] which support a model of a breccia filled vent system that diverges at shallow levels similar to that envisaged by [24].

#### **5. Implications for Fluid Flow Monitoring and Eruption**

At Whakaari, degassing occurs at distinct fumaroles, beneath the lake [21,22] and di ffuse across the crater floor [71]. Macro- and microcracks are common in rocks collected at the surface [13]. Based on our data and previous studies [49], we estimate a fractured rock mass permeability of ~10−<sup>13</sup> to ~10<sup>12</sup> m<sup>2</sup> at the surface, away from open fumaroles. Our data also show that, for fractured samples, permeability in altered material is highly confining pressure-dependent, whereas the permeability in unaltered material is much less confining pressure-dependent. This relationship implies that permeability in the (highly fractured, altered) conduit is more confining pressure-dependent than the surrounding (less fractured, unaltered) edifice.

The e ffective pressure on any given part of the subsurface varies and is an interplay between depth (confining pressure) and pore pressure. Volcanoes in a state of unrest have variable pore pressures that frequently exceed confining pressures driving fluid flow, if pore pressure is greater than the tensile strength of the rock this can cause tensile failure (e.g., [12]) and if combined with a decompression event can drive fragmentation and eruption [14]. This is seen dramatically at Whakaari, which has been in an extended period of unrest and minor eruption since 2011. Outgassing and lake properties (temperature, chemistry, and level) vary significantly [21] and frequently shift on timeframes of hours to days, implying highly variable pore pressure, including periods where pore pressure exceeds confining pressure.

During recent unrest there were a range of surface phenomena, with several episodes of lake draining, mud fountaining and star bursting, ash venting [22] (Edwards et al., 2018), directed ballistic bursts and surges [16,18] (Chardot et al., 2014; Kilgour et al., 2019), and the appearance of a lava dome. Geophysics and satellite imagery show minor localised deformation and slope instability [32], significant tremor, and very long period (VLP) earthquakes [18,19]. Taken as a whole, these data reveal a model of shallow intrusions contributing time-variable heat and mass to the surface, along fumaroles and through eruptive vents. This model is consistent with the interpretation of gas advection as a mechanism to explain measured VLP activity [23]. Our interpretation of the eruptive vents as hydrothermally-cemented altered breccias with confining pressure-dependent permeability provides a mechanism to facilitate some of this time-dependent cycling behaviour.

Our observations indicate that cracks and veins are common in the ballistic rocks—something that we would expect from a conduit environment subjected to changing compressive and tensile stresses associated with intrusion and fluid movements. Our data sugges<sup>t</sup> that pre-existing cracks in weaker hydrothermally mineralised zones easily close at confining pressures relevant for conduit processes. In unaltered rocks fractures may not fully close as asperities on crack surfaces may prop them open, maintaining permeability even at higher confining pressure. On long timescales of weeks to months these fractures allow eventual alteration and mineral replacement along these fluid flow pathways, and minerals may eventually crystallise within them to form veins. In contrast in the altered breccia-filled conduit, the weaker strength of hydrothermally mineralised fracture margins allow the asperities to be crushed, thus allowing closure under increased confining pressures, even on short timescales. This allows cracks to open and close as pressurized pore fluids open cracks and travel towards the surface (fluid advection; [23]). Once a fluid pulse has passed, pore pressures are reduced and the confining pressure recloses the fractures. In this manner, fractures can close instantaneously and reopen on timescales associated with gas advection. Hence, the opening and closing of the cracks is controlled by the accumulation of gas, su fficient to overcome confining pressure and generate a pore pressure. This allows time-variable advection of fluids, explaining the rapidly changing surface phenomena outlined in [22] and also the time variable tremor and very long period earthquakes described in ([18,23].

E ffective pressure-controlled crack closure and permeability reduction are unlikely to build su fficient pore pressure to drive Whakaari's explosive events. Our data provide ample additional evidence for repeated fracture creation and mineral precipitation. Hydrothermally altered material in the conduit is generally weaker than the edifice forming rocks (Figure 8) and with tensile strengths of 3–5 MPa [59]. The generally weak and porous nature of the altered tuff and hydrothermally altered breccia implies it has a low fragmentation threshold [72] and would be susceptible to fragmentation due to phase-change related pressure fluctuations [14]. The dominance of the cristobalite in the surge matrix implies that it is composed of altered ash recycled from fine fragmentation of a porous tuff matrix [14]. There have been several different locations of vents on the lake floor [16] and the conduits are likely branching close to the surface with variable depths and types of mineralisation and resultant fragmentation histories similar to those envisaged at Okaro [24]. The frequency of explosive processes is on the order of weeks to years [18], which is sufficiently long to allow minerals to precipitate in veins and between breccia clasts in the conduits, resealing them. The exact conditions for this process are explained in [59]. Similar timescales for pressurisation as a result of alteration-induced reductions to permeability were proposed by [3]. Similar processes have been envisaged at other volcanoes [73]. However, here we offer an explanation for geophysical, geochemical, and visible changes that can occur in seconds to hours.

In conclusion, we provide evidence for a breccia-filled conduit of altered lava and tuff clasts dominantly cemented by alunite and anhydrite. The permeability of this altered material is susceptible to rapid variation in effective pressure allowing highly time-variable fluid advection, outgassing, and geophysical changes at the surface.

**Supplementary Materials:** An excel spreadsheet with all our data used for Figures 6–8 are available online at http://www.mdpi.com/2076-3263/10/4/138/s1.

**Author Contributions:** Conceptualization, B.M.K., M.C.V. and M.J.H.; methodology, B.M.K., M.C.V. and M.J.H.; formal analysis, A.F., R.H., S.M., M.C.V., T.R. and M.J.H.; investigation and experimentation, A.F., R.H., S.M., M.J.H., and T.R.; writing—original draft preparation, B.M.K., A.F., R.H., M.C.V., and M.J.H.; writing—review and editing, B.M.K., A.F., M.C.V., M.J.H., G.K., A.J., and B.C.; supervision, B.M.K., M.C.V. and M.J.H.; project administration, B.M.K.; funding acquisition, Sampling on the island was carried out by A.F., G.C., B.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by New Zealand Ministry of Business, Innovation and Employment (MBIE) Strategic Science Investment Fund. M.J.H acknowledges an Erskine Teaching Fellowship awarded by the University of Canterbury.

**Acknowledgments:** We would like to acknowledge and thank White Island Tours, Frontiers Abroad, and the Green Room in aiding with the good times of data collection. Stephanie Gates and Ame McSporran also contributed useful discussion and data not directly used in the paper. Geoff Kilgour Bruce Christenson and Art Jolly were supported by the New Zealand Ministry of Business, Innovation and Employment (MBIE) Strategic Science Investment Fund. Shaun Mucalo assisted with SEM analysis.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
