Pressure Controlled Permeability in a Conduit Filled with Fractured Hydrothermal Breccia Reconstructed from Ballistics from Whakaari (White Island), New Zealand

Round 1

Reviewer 1 Report

See uploaded report.

Comments for author File: Comments.pdf

Author Response

Response to reviewer 1

The fatal December 2019 hydrothermal eruption of Whakaari will generate additional interest in this timely paper that explores vent dynamics revealed by the 2016 eruption. Overall the analytical approaches are robust, manuscript well written and Figures 7 and 8 are key to supporting the model produced which helps to explain spatial and temporal variations in behaviour that occur in this hydrothermal magmatic system.

We thanks the reviewer for the constructive comments and we have made the adjustments as recommended, which has resulted in an improved manuscript.

Major comments:

1. This contribution appears to be one of a variety of reasonably closely aligned papers out of the research group and greater emphasis on the unique contribution of this manuscript would be useful.

Ok, our third paragraph in the intro was supposed to be highlighting the novel aspect of the samples and volcano, however we take the reviewers point that it is the combination of samples volcanic scenario with the technique that is novel here as a result we added “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.”

2. There is little introduction to Whakaari and its style of volcanism, magma composition etc. The paper is currently very Whakaari centric and some increased introductory material about how this systems sits in the international volcanic context and discussion about the international context of findings and transferability of the model would increase its international relevance.

 

To address this we have added the following sections

Whakaari is an andesite stratocone, whose peak emerges 300 m from the Pacific Ocean 48 km off 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 (Houghton and Nairn, 1991; Kilgour et al., 2019) with particular relevance to andesite volcanoes with well-developed hydrothermal systems.

3. In Section 2.1 and the subsequent sections the analytical sampling was not well explained. I got the impression not every method was performed for every sample. Better explanation about why (and what) subsets were used for some techniques would be useful and perhaps a table could be included within Section 2, or at very least the table in the supplementary material expanded to track the range of analyses that were performed for every sample presented. If was not clear how the samples shown in Figure 2-5 relate to the physical measurements. Some of the information from the opening sentences of 3.1 would have been appropriate in 2.1. Table 1 could be integrated within a more comprehensive table.

To address this, we have now added this explanation at the start of the methods and we have additionally cleaned up the supplementary data table to allow easier navigation.

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 measurement were done at 1 Mpa (following the standard procedure at Strasbourg) all these had a corresponding connected porosity measurement (Table 1 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 2. supplementary data). A subset of 17 of these were chosen for UCS strength tests (Table 3 supplemental data). Individual sample numbers and lithological groupings are reported in the supplemental data.

15 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.

 

There are 8 intact lava measurements shown in Figure 6b at 3 MPa which are relatively tightly clustered in and only 7 in Figure 6a for 1 MPa which are far more scattered. Are these the same samples (perhaps not given the vertical line on 6a) and if so what is reducing the scatter under increased confining pressure even for the intact lava?

The lava measurements in 6b are all from one relatively homogeneous lava block representative of the volcanic edifice, whereas the data presented in 6a are from various unaltered lava portions of ballistic. figure caption to clarify the 1Mpa and 3Mpa data.

Figure 6. Permeability as a function of porosity at (a) confining pressure 3 MPa (this study) and 1 MPa (Heap et al. 2017) 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)."We also adjusted the paragraph before Figure 6 also needs to be updated in some way. Here is one suggestion:

"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^-12 m² at 1 and 3 MPa; Figure 6a, b)."

 

4. In section 2.2 and 2.3 a lot of detail was presented but very few citations made. This is not my area of expertise but I wondered if all of this was necessary. Particularly if these are standard techniques more could be cited away in a short summary in the main body of the text with the detail moved to the supplementary material. If the analysis is novel/heavily customised more justification as to why this approach was taken would be useful.

After discussion with the rock mechanic focussed coauthors, we are of the opinion this level of methodological description is appropriate, the methods here are relatively new and despite international standards existing for some of this, some methods do vary slightly between laboratory and it is therefore important to report.

5. That permeability increases with increasing porosity (lines 349-351) is not a surprise and is a relationship that should be recognised goes beyond volcanic rocks. Similarly for permeability vs confining pressure and a broader range of literature can be used to contextualise these findings.

To address this we have added the following text and references

"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. Farquharson et al., 2015; Wadsworth et al., 2016; Kushnir et al., 2016) and porous sedimentary rocks (e.g. Bourbie and Zinszner, 1985)."

"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. Vinciguerra et al., 2005; Nara et al., 2011) and microcracked granite (e.g. Darot et al., 1992) as a function of increasing confining pressure."

Bourbie, T., & Zinszner, B. (1985). Hydraulic and acoustic properties as a function of porosity in Fontainebleau sandstone. Journal of Geophysical Research: Solid Earth, 90(B13), 11524-11532.

Darot, M., Guéguen, Y., & Baratin, M. L. (1992). Permeability of thermally cracked granite. Geophysical Research Letters, 19(9), 869-872.

6. The discussion of overpressure in the paragraph starting line 411 confused me about whether it was overpressures(between hydrostatic and lithostatic) promoting brittle failure preferentially in the altered conduit zone or whether we are talking about the exceedance of confining pressure that essentially causes ‘fracking’.

We removed the term overpressure as this is often used differently by the geothermal and volcanological communities (Peacock et al., 2017). Here we imply pore pressure > confining pressure, but this only causes fracking if it > tensile strength of the rock.  Here we discuss the possibility of fluid flow when pore pressure > confining pressure but not necessitating it to also be > confining pressure + tensile strength of rock because the rock is already broken  We additionally added the following sentence to help clarify.

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. Heap et al., 2015) and if combined with a decompression event can drive fragmentation and eruption (Mayer et al., 2016).

Additional minor comments:

Line 18 ‘and’ transmitting…  fixed

Line 24 If just reading the abstract in isolation the comparison of ‘cracked’ altered samples compared to intact unaltered samples seems a bit strange and more explanation about why this is an important comparison to make is needed. We agree with the reviewer however we don’t think this is realistic within the 200 word limit

Line 42 ‘frequently associated’ – better supported with multiple, including newer, references  OK added two more recent references

Line 59 A few descriptive words about the nature of the 2016 eruption would be relevant.  Added the word hydrothermal

Line 60 vs 70-71 magmatic hydrothermal vs hydrothermal magmatic  ok fixed as magmatic-hydrothermal

Line 72 ‘improves’ instead of ‘allows’  ok

Line 75 materials ‘rupturing’? opted to stay with original wording

Line 88 ‘Additionally, microcracking is a critical textural consequence of alteration related processes’ - citation needed.

Line 106 ‘minimised individual exposure’ to?  Ok added to volcanic hazards

Line 111. A few words about how ballistics from this eruption were identified, tied to Figure 1c, would be appropriate. Ok added “Figure 1c shows an impact crater indicating a ballistic origin containing a ballistic sampled.”

Figure 1b – more could be done in this figure to explain the setting of Whakaari Figure 2a – white box of annotation cropping bottom of text  We think the additional text explaining the tectonic situation of New Zealand from comment 2 above is sufficient without the need to add to this

Figure 2e – typo of ‘viened’ in title. Ok fixed

Figure 3 0 colour legends are very small but appear to be uniform – I suggest replacing with a single legend. Ok increased size of colour legends this was easier to do than replace all with one single legend

Figure 3c has two scale bars, 3d one and 3a & b none. Standardise scale and present common scale bar. Extra scale bars removed

Table 1 – clearer row separation and moving the footnote to be part of the caption would make this clearer. We are attempting to follow format of journal happy to change during proof stage with journal typesetter

Figure 4 – misnumbered as Figure 1 in caption. Not sure about this

Figure 5d is at a different scale than the others. Particular as this is the same sample as 5c I think it should be standardised and a common legend and scale for the group. White boxes around annotations without clearance of text in multiple cases e.g. left hand side of 5d “Fish”  ok fixed a bunch of minor things on this fig. but didn’t change the scale of 5d as this was intentionally at a higher magnification

Line 301. At this point I noted the question ‘but did the samples all fracture under similar conditions or was there a strength variation’. Relook at the order the analysis were presented to see if this could work with presenting the UCS i.e. creating the fractures before you present the effect of having the fractures on permeability. The tensile fractures we created are not the same as the shear fractures reported later in the UCS as is described in the methodology . To clarify we added The tensile fractures we created are not appropriate to report the tensile strength of the samples as the dimensions required for the core permeability are non-standard for tensile strength tests (e.g. Heap and Kennedy, 2016).

Figure 6a – Caption says it is a confining pressure of 1 or 3 MPa whereas the figure annotation says 3 MPa. We fixed this above

Line 309 Both Figure 6 and 7 support this statement. Fixed

Line 321 replace ‘porosity’ with ‘samples’. The relationship to porosity is established in the following sentence.  ok

Line 333. This is the first time you have stated Whakaari is andesitic! Fixed this with point 2 above

Line 358 – missing citations after the e.g. fixed

Line 360 – missing citation after the e.g. fixed

Line 364-5 – I noted a question here about the fines you acknowledged in sampling were missing. You address this in the following paragraph but wonder if the information in 361-374 should be merged into a single paragraph of the closing sentence in 364-5 delayed until after the fines discussion has been made. Ok paragraph merged

Figure 9 – do the yellow colours have meanings that need to be defined in the caption and can the representation of cracks/veins between the conduit and surrounding carry more meaning with a legend or enhanced caption. Ok added

Line 403 diffusely  ok

Line 413. Exceed hydrostatic pressure? I think this got addressed in response to point 6 above

Reviewer 2 Report

Congratulations, this is a very interesting and complete manuscript. Although long, it is easy to follow, leading to the conclusions.

I have only minor comments on it:

 

• Figure 1: the quality of the figure could be improved.
• Line 228: then deformed
• Line 235: you do mean unaltered lava ballistics, right?
• Line 357: “as concluded by previous studies.” Please cite these studies.

It is not clear to me why last section is called “Implications for fluid flow monitoring and eruption”. What are the exact implications for monitoring?

Author Response

Reviewer 2

 

We thanks the reviewer for the positive review and minor additions that will improve the manuscript.

 

Figure 1: the quality of the figure could be improved. Ok we have improved the quality and labeling of this figure

 

Line 228:”then deformed”- not sure on what the reviewer means here, this may have been related to some extra spacebars? If so that is now fixed

Line 235: you do mean unaltered lava ballistics, right? We clarified lava versus ballistics in this paragraph

Line 357: “as concluded by previous studies.” Please cite these studies. Ok we have added these.

It is not clear to me why last section is called “Implications for fluid flow monitoring and eruption”. What are the exact implications for monitoring?

Ok fair point the implications are for interpretation of volcano monitoring, e.g. the interpretation of VLPs or rapidly changing degassing signals, so we adjusted the title to indicate this

Implications for interpreting fluid flow and volcano monitoring