Peer review of: Unexpected large eruptions from buoyant magma bodies within viscoelastic crust (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust. 

By Sigmundsson et al., 

- A review 

This paper is well written, topical, and of broad significance to the body of knowledge in the fields 
of volcanotectonics and volcano seismology. The manuscript presents a new model for how 
fractures in tension might interact with deep magma chambers to reveal how large eruptions may 
occur with lower-than-expected levels of warning. I have only some very minor comments and 
suggestions (below) after which the manuscript is suitable for publication in Nature 
Communications. 

Minor comments: 

1) Line 30: suggest “…magma body may decrease well…” 

2) Line 70: I’d suggest “neglected” rather than “ignored”. 

3) Line 165: Again, I’d suggest use “neglected” rather than ignored. 

A couple of more general points, for consideration: 

- Might the author write a line or two explaining how the model (whether spherical or elliptical) is 
reasonable given the likely complexities in the magma chamber? I fully realize that the proposed 
model dramatically simplifies the calculations, and in fact I agree that it is fine, but with the 
community now starting to see the inherent limits of the so-called “Mogi model” I wonder if the 
authors might comment on some of the limitations? 

- Figure 3 & 4, and also lines 151-154 & 160-165: might authors again comment (perhaps in the 
discussion) as to what the likely “stable geometry” of the “sustained channel” might be? This is 
especially important if one considers the local stresses, which might evolve from one set of 
conditions below the brittle-ductile level (and the deep chamber) to another set of conditions 
supporting the lateral dyke. 


Reviewer #2 (Remarks to the Author): 

Dear Authors, 
I have been reading with interest your contribution. The simple analysis you use is interesting and 
it is in my opinion contributing to stress that eruptions triggered by buoyancy could be preceded 
by little warning, which is very important for volcanic hazard assessment. 
I find the manuscript interesting, even if portions of it can be improved for clarity. I provided some 
suggestions in the annotated version, but I am not a native English speaker and I leave it to 
authors with better English skills than mine to clean up the text. 

I have to say that Figure 3 and 4 are not of the quality required for publication in Nature 
Communications. Figure 3 should not be schematic but rather show the influence of the different 
parameters of Equation 4 on the evolution of pressure during eruption. Figure 4 can be a summary 
figure but should be significantly improved. I know it is not an art context, but still... 

I have one major comment: What about volatiles? 

The Bardarbunga-Holuhraun magmas contained quite a lot of excess fluids. What si the impact of 
excess volatiles on the evolution of overpressure in the magma reservoir and on magma ascent 
within the dyke? 

1. The presence of excess volatiles in the reservoir that fed the eruption will increase magma 
compressibility, decreasing even further the overpressure generated by magma injection (see 
Kilbride et al., 2016, Nat.Comm). 
2. Excess fluids at depth will decrease magma density and thus increase buoyancy 
3. Can the progressive expansion of excess fluids help maintaining the dyke open? 

I think these aspects should be discussed quite in depth in the main text as they could reinforce 
your conclusions and otherwise bring quite some criticisms from readers.