Forum Reply Time-evolving surface and subsurface signatures of Quaternary volcanism in the Cascades Arc

We Dave Sherrod his constructive Comment (Sherrod, 2021). The Cascades arc Quaternary extrusion volume we incorrectly attributed to Sherrod and Smith (1990) was derived from preliminary calculations using the post–Crater Lake rate of 1.3 km 3 /km/m.y. presented in Sherrod and Smith (1988) for central Oregon. We converted extrusion rate to volume by multiplying this rate by a segment length of 260 km and 2 m.y., giving 676 km 3 in central Oregon and a total Cascades extruded volume of 2520 km 3 (mistyped as 2570 km 3 in our paper; O’Hara et al., 2020). The 5100 km 3 arc total volume suggested by Sherrod falls between the 4540–6100 km 3 that arise from converting 3–6 km 3 /km/m.y. extrusion rate (Sherrod and Smith, 1990) to volume, including off-axis volcanism. Being the most recent comprehensive compilation of ongoing Cascades volcanic fieldwork, we used the ~6400 km 3 estimated by Hildreth (2007) for Cascades Quaternary extrusion in our paper. As such, the erroneous representation of Sherrod and Smith (1990) does not impact our quantita-tive

We thank Dave Sherrod for his constructive Comment (Sherrod, 2021). The Cascades arc Quaternary extrusion volume we incorrectly attributed to Sherrod and Smith (1990) was derived from preliminary calculations using the post-Crater Lake rate of 1.3 km 3 /km/m.y. presented in Sherrod and Smith (1988) for central Oregon. We converted extrusion rate to volume by multiplying this rate by a segment length of 260 km and 2 m.y., giving 676 km 3 in central Oregon and a total Cascades extruded volume of 2520 km 3 (mistyped as 2570 km 3 in our paper; O'Hara et al., 2020). The 5100 km 3 arc total volume suggested by Sherrod falls between the 4540-6100 km 3 that arise from converting 3-6 km 3 /km/m.y. extrusion rate (Sherrod and Smith, 1990) to volume, including off-axis volcanism.
Being the most recent comprehensive compilation of ongoing Cascades volcanic fieldwork, we used the ~6400 km 3 estimated by Hildreth (2007) for Cascades Quaternary extrusion in our paper. As such, the erroneous representation of Sherrod and Smith (1990) does not impact our quantitative results, but does show how dispersed deposits can alter volume estimations. Differences between published edifice volume estimates (e.g., O'Hara et al., 2020, our table S1; Sherrod and Smith, 1990, their table 1) that arise from how erosion is accounted for and how the edifice basal contour is defined highlight continued challenges in quantifying long-term eruptive output, particularly for methods that leverage digital elevation models. This is an opportunity for future work.
Our estimation that edifice volumes represent ~50% of total extruded volumes is an average over the entire arc. We agree with Sherrod that partitioning between edifices and distributed deposits likely varies alongside the physical controls on volcanism, depending both on the scale of measurement and environment. For example, Grosse et al. (2020), comparing mafic monogenetic cone volumes to associated lava flows in the Andes, found a 1:1 relationship between volumes. However, seismic imaging of submarine volcanoes in the South China Sea implies that edifices are 3-50% of total erupted products (Sun et al., 2019). In the central Oregon Cascades, a 50% value for edifice volumes is likely an overestimate. Considering a 3-6 km 3 /km/m.y. extrusion rate for central Oregon (2010-3570 km 3 , including Newberry), comparison with volcano volumes in the same area (985 km 3 ) suggests that edifices represent at most 27-49% of total extruded volume for this region.
Finally, from the standpoint of reconciling upper crustal geophysical data with geologic observations, it is interesting to compare the metrics G and  in our paper, measuring subsurface volcanic influence and volumeweighted edifice vent distribution, respectively, to along-arc Quaternary extrusion rates. Taking across-arc summations of these quantities, we see that high  co-locates with high extrusion rates from Sherrod and Smith (1990) (Fig. 1). The summation of G, however, highlights a dichotomy between the northern and southern sections of the arc. South of 46°N, G is large, suggesting extensive subsurface magmatic modulation, and probably Basin and Range tectonic influence. North of 46°N, G is low, except near major stratovolcanoes, suggesting localized crustal magma transport.
Highest G values occur in central Oregon, corresponding to the peak in extruded volume noted by Sherrod and Smith (1990). This provides a framework for future interrogation of the physical processes responsible for spatially variable surface/subsurface magmatic connections at an arc scale.
In conclusion, we again thank Sherrod for the opportunity to correct our misstatement and to re-articulate exciting scientific opportunities that remain in Cascades volcanology.