**1. Introduction**

The tragic events on November 2019 at Whakaari (White Island; New Zealand), which killed over 20 people, highlight the need for a better understanding of the controls on phreatic eruptions. In particular, the behaviour of breccia-filled conduits remains a potentially controlling factor on the pressurization timescales and total pressure build-up. Many large volcanic eruptions begin with a

phreatic/hydrothermal eruption that is responsible for clearing a pathway for magma to reach the surface [1]. Similarly, shallow intrusions and economically significant mineral resources are frequently associated with hydrothermal breccia sheets and pipes [2]. The physical properties of these breccia filled conduits is thus critical to volcano monitoring and economic mineral exploration.

The physical properties of the altered materials that fill hydrothermal conduits directly control (1) magma outgassing e fficiency, (2) the build-up of pressure that can lead to explosive eruptions [3], and (3) subsequent fluid flow, hydrothermal alteration, and mineralization. Past studies of magmatic conduits and breccias reveal insights into magma flow, outgassing, and fragmentation processes [4–7]. Detailed studies of hydrothermal breccia filled conduits, however, are dispersed between the maar diatreme, hydrothermal mineralization, and petroleum basin literature [2,8–11] and are rarely applied to eruption models.

Until now, studies have described the physical properties of rocks collected from the crater floor of Whakaari to understand their mechanical behaviour and permeability [12,13] and to interpret fragmentation [14], however, these rocks may not be representative of the material filling the vent. Instead, the physical properties of volcanic ballistic samples can be used to provide insight into conduit processes [15]. Therefore, the ballistic field of the 2016 eruption [16] provides a unique opportunity to directly sample the pre-eruption conduit and the magmatic hydrothermal interface beneath the crater at Whakaari. Here, we combine the exceptional altered ballistic samples with established rock mechanical methods to make implications that can be directly applied to an active hazardous volcano.

Whakaari is an andesite stratocone, whose peak emerges 300 m from the Pacific Ocean 48 km o ff the east coast of New Zealand. The magmatism is the product of oblique western subduction of the Pacific plate under the Australian plate. Whakaari has been New Zealand's most active volcano in recent history. The most recent magmatic and hydrothermal eruption period has lasted from 1976 until present exhibiting a range of magmatic, phreatomagmatic, and hydrothermal eruptions [16,17]. Whakaari has undergone a complex sequence of unrest and minor eruptions since 2011. Geophysical [18–20], geochemical [21], and observational studies [22] reveal a complex interplay between a shallow magmatic system and surface hydrology with volcanic activity modulated by a variably permeable hydrothermal system. Magmatic gas is transferred [23] towards the surface via branching pathways to a series of vents and fumaroles [21]. This model is consistent with studies of hydrothermal vents elsewhere in New Zealand [24]. Quantifying the material properties and the gas transfer mechanisms is critical to developing realistic geophysical and fluid chemistry models and to understanding the hydrothermal magmatic interface with particular relevance to andesite volcanoes with well-developed hydrothermal systems.

Investigating the nature of the rocks that surround magma bodies allows for the interpretation of geophysical signals. For example, volcanic eruptions have been successfully forecast using volcanic earthquakes [25–29]. Earthquake types are inherently tied to the physical properties of the material they travel through [23,30] and the fluid within any cracks [31]. Similarly, models of deformation [20,32] and fluid flow [33] are reliant on the physical properties of conduits and volcanic edifice. During the current period of unrest at Whakaari, geophysical, geochemical, and geological phenomena seen at the surface are interpreted as a direct result of fluids travelling through variably altered volcanic rock.

Alteration a ffects the physical properties of volcanic rocks [34–37]. Alteration may drive the loss of porosity and permeability via mineral precipitation in pores and cracks [3,37,38] and/or cementation [39]. Reductions in porosity due to alteration can promote brittle behaviour [39]. Conversely, alteration may drive dissolution and pore creation with measurable porosity and permeability increase [40,41] and promote a ductile failure mode [38,42,43]. Additionally, microcracking is a critical textural consequence of alteration related processes. Fractures in volcanic environments typically form in response to tectonic, intrusive, hydration, or thermal forces [44–46], however, they can also form in response to specific thermally-driven mineral reactions and drive porosity/permeability changes [47,48]. Cracks are an important textural consideration in volcanic conduits, and their influence on, for example, the physical

properties of rocks may be highly dependent on lithology and confining pressure, particularly in altered volcanic rocks [37,46,49–52].

Here, we present textural and petrological characteristics of volcanic ballistic samples from the 27 April 2016 hydrothermal eruption of Whakaari. We then present strength, porosity, and permeability variations as a function of confining pressure (i.e., depth). These data are compiled into a conceptual model in which transient cracks control fluid flow in an altered breccia-filled conduit.

#### **2. Materials and Methods**

#### *2.1. Sampling and Sample Preparation*

Ballistics were collected from various sites 15–200 metres from the rim of Whakaari's crater lake (Figure 1a) during several brief visits following the 27 April 2016 eruption by Kilgour, Farquhar, and Christenson on 2 May, 2 June, and 9 June, 2016 [53]. Three short sampling missions from three di fferent scientists minimised individual exposure following protocol outlined in [54]. A quantitative analysis of a statistically relevant sample set of ballistic lithologies in the field via photo survey was not possible due to the partially buried and discoloured nature of the ballistics and the limited time for systematic sampling within the Health and Safety Risk considerations of GNS Science and University of Canterbury (Figures 1c and 2) [16]. Further, our sampling was biased towards ballistics that were suitable for cutting and drilling (>10 cm max. diameter) (Figure 2). We also include some additional (nonballistic) measurements made on samples of lava, ash tu ff, and sulfur cemented tu ff collected from surfaces exposed in 2015 (location details provided in [12,13].

**Figure 1.** (**a**) Whakaari's crater lake with ballistic collection area highlighted; (**b**) location of Whakaari north-east of the central North Island of New Zealand; (**c**) typical ballistic crater showing discoloration and partial burial of the ballistic.

**Figure 2.** Photographs showing examples of collected ballistic samples from the 2016 eruption at Whakaari cut in two to reveal interior, (**a**) Unaltered lava, (**b**) altered lava, (**c**) Ash Tuff, (**d**) Hydrothermal breccia, (**e**) hydrothermal breccia of altered and unaltered lava.

The collected blocks were subsampled by drilling 20 mm diameter cylinders nominally 20–40 mm long. After porosity and permeability were measured on these intact samples, electrical tape was wrapped around the cores and they were loaded diametrically in compression until the appearance of a through-going tensile macrofracture (i.e., a "Brazilian" indirect tensile test). The tape ensured that the two sample halves remained in contact upon failure, so that the permeability of the fractured samples could be measured in the laboratory (following the method described in [49]).

An ideal methodology would have allowed all samples to have been analysed using all methods. However, this was not possible, due to (1) the timing of availability of samples and experimental facilities across four institutions, three countries, and three student projects, (2) the destructive nature of some of the methodologies (e.g., XRD and accidental sample destruction during drilling), (3) available time for analysis. Our focus was on characterising the permeability and porosity of the rocks that comprised the conduit and encompassing the major breccia lithologies. We made 60 permeability measurements at 3 MPa confining pressure following the standard procedure at University of Canterbury, and a further 25 measurements were done at 1 MPa (following the standard procedure at the University of Strasbourg); all of these had a corresponding connected porosity measurement (Table S1 supplemental data). Nine samples were selected for the two series of time-consuming variable confining pressure experiments: one series without fractures and then the same samples used in the second series with tensile fractures (Table S2 Supplemental data). A subset of 17 of these were chosen for uniaxial compressive strength (UCS) tests (Table S3 Supplemental data). Individual sample numbers and lithological groupings are reported in the supplemental data.

Fifteen thin sections were selected for SEM analysis, and 18 samples were chosen for XRD analysis. These samples were selected to represent the range of lithologies and styles of alteration and allow some overlap of techniques on the same sample. We focus our presentation here on representative samples and textures.

#### *2.2. SEM and XRD Preparation and Methods*

Thin sections were prepared to dimensions of 25 × 47 mm, ground to 30 μm thick, and mounted without cover slips. Most samples contained variability in alteration degree, and whenever possible, thin sections were prepared so as to include both sides of these mineralogical boundaries. Thin sections of eight samples were analyzed using a JEOL-IT300 Scanning Electron Microscope (SEM). This was to determine the mineral composition of the rock and the infill in open pore spaces. Backscatter Electron Detector Contrast (BED-C) images and Energy Dispersive X-ray Spectroscopy (EDS) analyses were obtained with images collected at various scales ranging from 60× to 600× magnification, although our results here display all 100× images. The weight-percent ratios of elements within points of each thin section were measured using the EDS detector in order to calculate mineralogy. Compositional element maps (EDS) and lithological structures (BED-C) were produced using the computer program AZtec.

The mineralogy of rock powders and also clay separates were analysed using a Philips PANalytical X'Pert Pro X-ray di ffraction (XRD) machine, using X-rays at 2θ angles of 7–70◦. Dislodged electrons were received through a 1⁄4◦ slit. Data were received into the PANalytical computer software Data Collector and then processed to determine mineralogy in the PANalytical computer software High Score.

Two factors were combined in order to determine the most likely mineralogy of each sample. (1) The d-spacing peaks of the minerals within the sample were cross-referenced within the mineral database in High Score to generate a list of possible minerals. (2) Their likelihood of presence within the volcanic-hydrothermal system in combination with existing SEM data. The most abundant peaks for each sample (>50%) were considered a dominant mineral.

In order to determine which clays are present in the ballistic samples, the clays were physically separated from powders onto glass slides. Ten grams of each sample were measured on a precision scale and mixed with 200 mL of distilled water. This was mixed in a Waring blender for two minutes at high power. After mixing, the liquid and fine fraction of the mixture was rapidly decanted into a beaker. This sample was then placed into a 50 mL centrifuge tube and centrifuged at 750 rpm for two minutes. The centrifuged sample was quickly decanted into a second 50 mL tube, which was also centrifuged. This liquid was decanted, and the clay fraction at the bottom of the tube was suctioned out by pipette and placed onto a glass slide. This was placed in the oven at 90 ◦C for at least one hour, until the sample was dry and ready for XRD analysis. When running clay separation slides, 2θ angles of 5–65◦ and a 1⁄8◦ slit were used.

#### *2.3. Physical Laboratory Measurements*

Skeletal volume measurements were undertaken using an AccuPyc II 1340 pycnometer. We used helium and ultra-high purity nitrogen for the measurements performed at the Institut de Physique du Globe at the University of Strasbourg (IPGS; France) and the University of Canterbury (New Zealand), respectively. These values were used to calculate connected porosity using the bulk sample volume.

Samples (20 × 20 mm) of unaltered intact lava (WI20 series) and fractured lava (WI20 series) and high-permeability ballistic samples (20 mm in diameter and nominally 20–40 mm in length) were measured at the IPGS. Permeability was measured using a benchtop gas (nitrogen) permeameter (see Farquharson et al., 2016; Heap and Kennedy, 2016). The oven-dry samples were first allowed to equilibrate for one hour at the target confining pressure of 1 MPa (the confining fluid used was also nitrogen gas). We used the pulse-decay technique to measure the intact lava samples and the steady-state method to measure the fractured lava and porous ballistic samples. For the pulse-decay measurements, a pore pressure di fferential of 0.2 MPa (measured using a Keller pressure transducer) was first imposed on the sample for a duration of one hour. The valve to the gas bottle was then closed and the decay of the upstream pore pressure (within a known volume) across the sample was recorded as a function of time. These data were then used to assess permeability using Darcy's law and to check for any ancillary corrections, such as the Forchheimer and Klinkenberg corrections, which were applied on a case-by-case basis [55]). These pulse-decay measurements required the Klinkenberg correction. For the steady-state measurements, steady-state volumetric flow rates were measured using a Bronkhorst gas flowmeter for six di fferent pressure di fferentials. Steady-state was ensured by waiting for the volumetric flow rate and di fferential pressure to stabilize before measurements were recorded. These data were then used to assess permeability using Darcy's law and to check for any ancillary corrections, such as the Forchheimer and Klinkenberg corrections, which were applied on a case-by-case basis. These steady-state measurements required the Forchheimer correction.

Samples (20 × 20 mm) of unaltered intact lava (WI20 series) and fractured lava (WI20 series) were also measured at IPGS up to a confining pressure of 30 MPa. These experiments used either the pulse-decay (for low-permeability measurements) or the steady-state method (for high-permeability measurements), using the techniques described above. For these measurements, water was used as the confining fluid and argon gas was used as the pore fluid. The confining pressure was only increased to the next increment after the permeability of the sample was measured to be the same on two consecutive days.

Low-permeability 20 mm diameter by nominally 20–40 mm length cores of lava, altered lava, altered ash tu ff, hydrothermal breccia, and fumarolic sulfur flow (Table 1) were measured in a Core Laboratories Pulse Decay Permeameter-200 and analyzed using the software PDP V2.75 at the University of Canterbury Rock Mechanics Laboratory. Each oven-dried core was slid into a permeameter sleeve, and a hand pump was used to move Penrite ISO46 hydraulic oil into a surrounding chamber which set the confining pressure to 3 MPa. This pressure corresponds to the confining pressure that the rock experienced within the shallow subsurface [13]. The system was then left to equilibrate for at least thirty minutes. The calculations of connected porosity, mass, dimensions of the core, and atmospheric conditions such as temperature and barometric pressure values acted as a necessary input into the PDP software before running each set of tests. After entering these data, the computer automatically pressurized the chamber from above with high purity nitrogen gas, creating a pressure di fferential of 10 PSI between the top of the sample and the bottom. The rate at which the sample equilibrated to matching pressures above and below the sample was used by the permeameter calculator to determine the permeability of the sample, as described by [40].


**Table 1.** X-ray di ffraction data for the samples studied.

\* Clay found using clay-separation method, not performed on all XRD samples.

The permeability of the fractured low porosity rock cores was measured in the steady-state gas permeameter at the University of Canterbury. The cores dwelled at 1 MPa confining pressure for 30 min in a Hoek cell and a compression frame as high purity nitrogen gas flowed through the sample at 4 bar to equilibrate the pressure. The volumetric gas flow rate was increased five times without exceeding 500 mL/min to reduce the risk of potential damage to the flow meter from broken fragments

of the core entering the downstream tube and to reduce the deformation of the fractures themselves. The Forchheimer correction was applied to correct for flow inertia (as described in [55]).

Nine samples were tested under varied confining pressure, which was increased to mimic the environment of increasing depth of the conduit of Whakaari. These variable pressures simulate di fferent hydrostatic pressures at Whakaari from about 300 m to 3 km depth and follow the same methodology as above and described in [40]. The Forchheimer correction was applied at each of these confining pressure steps.

#### *2.4. Mechanical Laboratory Measurements*

Select oven dried core samples from a wide range of lithologies and degrees of alteration (Table 1) were placed in a Tecnotest KE300/ECE compression loading frame to measure their uniaxial compressive strength (UCS), where σ1 > 0 and σ2 = σ3 = 0.1 MPa (atmospheric). To measure UCS, a 20 mm diameter x 40 mm long core sample was placed between the machine's two platens, which then deformed the sample at a constant displacement rate of 0.03 mm/s. Macroscope failure of the sample was signalled by the formation of a through-going fracture, corresponding to a rapid decrease in stress. The peak axial stress achieved during the experiment was taken as the UCS.
