*3.1. Lithologies*

We focus our analysis on relatively unaltered lava from the surface, ballistics (unaltered lava, altered lava, altered tu ff, and hydrothermal breccias), and sulfur cemented tu ff from surface fumaroles [13]. The relatively unaltered lava blocks had some combination of very thin alteration rinds (Figures 2a and 3a) and central portions that were unaltered allowing unaltered cores to be subsampled (Figure 2b). Textural and compositional analyses reveal low porosity with unaltered phenocrysts of pyroxene, plagioclase, and iron oxides, and microlites of plagioclase and pyroxene, typical of Whakaari lava [56]. Porosity is observed to be mainly created by open macro- and microcracks (Figure 3a).

**Figure 3.** Scanning Electron Microscopy EDS (Energy Dispersive X-ray Spectroscopy) element maps of (**a**) relatively unaltered lava; (**b**) altered lava; (**c**) altered lava; (**d**) altered lava. Note that the scale of each image is the same as in (**c**).

Altered lava cores were drilled from yellow or white ballistics, from partial or complete alteration rinds thicker than 2 cm (Figure 2b). Alteration of the albite microlite rich lava groundmass has resulted

in a crystallised microporous groundmass of silica polymorphs. Macrofractures and macropores are enlarged by dissolution and completely replaced, forming anhydrite and alunite veins, pockets (Figure 3b,d), or more diffuse areas partially replaced with alunite (Figure 3c). Larger pores contain coarser recrystallized grains of anhydrite (Figure 3b).

Ballistics of pyroclastic rocks were also found: altered ash tuff, lapilli tuff and tuff breccia (Figure 2c,d). These ballistics were completely altered to a yellow or white colour rather than just on their surfaces. Such pervasive alteration makes clasts and matrix hard to distinguish in hand specimen, due to their similar texture and mineralogy. The matrix/cement consists of <1 mm recrystallized silica polymorphs (Figure 4a), large areas of alunite (Figure 4a), and local areas of anhydrite (Figure 4b) and some clay minerals identified in XRD. Clasts are tuff, or occasional recrystallized coarser grained lava with albitic patches (Figure 4b). Clasts and matrix are relatively porous with pores existing between recrystallized grains (Figure 4c,d).

**Figure 4.** Scanning Electron Microscopy EDS (Energy Dispersive X-ray Spectroscopy) element maps and backscatter images, respectively, of (**a**) and (**c**) ash tuff matrix of a hydrothermal breccia from block 2-3; (**b**) and (**d**) ash tuff block 3-3.

Many ballistics consist of large clasts with only patches of matrix and were significantly mineralised and as such are described here as hydrothermal breccias. A large proportion of these samples are recrystallized and are frequently made up of more than 50% hydrothermal minerals such as alunite or anhydrite (Figure 2d,e and Figure 5b,c). Elemental sulfur is rare but does occur in small patches (Figure 5b). Texturally characteristic cristobalite with fishscale style fractures also occurs, while porous cristobalite is also seen in some vein margins (Figure 5c,d). Multiple generations of alunite and anhydrite veins are seen producing in-situ brecciation of clasts. Fractures are also observed within veins, or parallel to pre-existing fractures, and at the boundary between veins and stronger silicic clasts (Figure 5f). It is also worth noting the contrasting porosity in the vuggy anhydrite, the microporous lava, and the characteristic fishscale cracked cristobalite.

**Figure 5.** Scanning Electron Microscopy EDS (Energy Dispersive X-ray Spectroscopy) element maps of (**a**) EDS stacked image of sulfur cemented ash tuff; (**b**); EDS stacked image of alunite cemented portion of hydrothermal breccia (**c**) EDS element map of hydrothermal breccia; (**d**) same sample as (**c**) in backscatter to highlight porosity distribution (**e**) EDS stacked image of hydrothermal breccia (**f**) EDS stacked image of open vein in hydrothermal breccia.

These samples are additionally compared against data and textural observations of sulfur encrusted and cemented ash tuff from the surface. These yellow rocks are dominantly matrix supported and cemented with sulfur and contain angular ash and lapilli sized clasts of silica polymorphs and albite and irregular rounded pores resembling vesicles (Figure 5a).

#### *3.2. Porosity and Permeability*

Porosity for the entire sample set varies from a couple of percent up to ~60%, and permeability varies from ~4 × 10−<sup>19</sup> to ~4 × 10−<sup>15</sup> m<sup>2</sup> (Figure 6). Although there is a general trend of increasing permeability with increasing porosity, as observed in previous studies on the permeability of volcanic rocks [57,58], there is also substantial scatter within and between lithologies (Figure 6). For example, the permeability of samples with a porosity of ~40% can vary from ~2 × 10−<sup>16</sup> to ~4 × 10−<sup>15</sup> m<sup>2</sup> (Figure 6a). The relatively unaltered lava ballistics generally have lower porosity and permeability than the altered lava, altered ash tuff, and sulfur flow (Figure 6a). Compared to rocks collected from the surface (data from [13,59]; shown in grey on Figure 6a), the ballistic samples (yellow symbols on Figure 6a) generally have lower porosity and a narrower range of permeability. The samples with experimentally created tensile fractures are 4–5 orders of magnitude more permeable than the

unfractured rocks at both confining pressures of 1 and 3 MPa (Figure 6a,b). Irrespective of the initial permeability, the permeabilities of the fractured samples are very similar at low confining pressures (~10−<sup>12</sup> m<sup>2</sup> at 1 and 3 MPa; Figure 6a,b).

**Figure 6.** Permeability as a function of porosity at (**a**) confining pressure 3 MPa (this study) and 1 MPa, and (**b**) confining pressure 1 MPa showing the wide variability of matrix permeability (solid black points are unaltered intact lava from the edifice; yellow are altered intact ballistic samples collected at 3 MPa confining pressure) and low variability of fracture permeability (open points).

As confining pressure increases, the matrix permeability of the rocks decreases (Figure 7). A decrease in permeability as a function of increasing confining pressure has been previously reported in laboratory studies on volcanic rocks (e.g., [60,61]). The intact (i.e., without tensile fracture) altered samples have a higher permeability than the unaltered lava (Figure 7). An increase in confining pressure from 1 to 30 MPa leads to a decrease in permeability of ~1 order of magnitude in the unaltered lava and fractured unaltered lava, a decrease of 1-2 orders of magnitude in the unfractured altered lava, and a decrease in permeability of 2–4 orders of magnitude in the fractured altered lava (Figure 7). Importantly, our data show that the permeability reduction as confining pressure is greater in the fractured altered samples than in the fractured unaltered samples (Figure 7).

**Figure 7.** Permeability as a function of confining pressure showing the different impacts of confining pressure on altered and unaltered, fractured and unfractured samples.

Generally, our data show that uniaxial compressive strength (UCS) varies greatly over the range of porosity we tested. When combined with the other data from Whakaari [12] a clearer trend becomes evident in which UCS 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]). The observed decrease in UCS as a

function of porosity is not linear (Figure 8). The compressive strength data show a consistently low strength for samples containing >10% porosity (Figure 8). Low porosity unaltered lava is the strongest sublithology, while altered rocks are both more porous and weaker, and the sulfur cemented tuff is significantly weaker than the lava samples of similar porosity.

**Figure 8.** Uniaxial compressive strength (UCS) as a function of porosity for samples from this study and from [13,59].

## *3.3. Results Summary*

The ballistics erupted from Whakaari are andesitic lavas, breccias, and tuffs. These rocks are all variably altered, containing silica polymorphs, alunite, and anhydrite. Many clasts show concentric alteration rinds. Anhydrite and alunite are common in pockets and veins, and they contain intercrystalline porosity, evidence for multiple cracking, veining, brecciation events, and cristobalite precipitation. In the tuff ballistics, it is difficult to distinguish matrix and clasts as both are recrystallized with silica polymorphs and alunite producing an overprinted microporosity. XRD results (Table 1) support the SEM interpretations, and additional analysis from the surge deposit shows dominantly cristobalite and plagioclase with little evidence of anhydrite and alunite. Consistent with previous data from surface rocks, tuff and altered lava ballistics are more porous and permeable than the less altered lavas. The permeability of the altered outer rind is generally higher when compared to the unaltered core of the same sample. When tensile fractures were created in cores, permeability increased by 4-6 orders of magnitude. However, the permeability of the fractured altered rocks decreases more than that of the fractured unaltered lava as confining pressure increases. Our results hint at the critical role fractures may play in the permeability of the altered rock that likely dominate the conduit at Whakaari.
