the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2025
Assessment of possible volcanic hazards in Germany with regard to repository site selection
Abstract. In the context of selecting a site for the long-term disposal of radioactive, heat-generating waste in deep geological formations, the potential for future volcanic activity within the next 1 million years must be systematically evaluated. This assessment draws upon an integrated analysis of geological, geochemical, and geophysical datasets, as well as isotopic measurements of crustal and mantle-derived gases. Relevant data sources include teleseismic imaging, long-term seismic and microseismic monitoring—particularly deep earthquake patterns—and geodetic observations of vertical crustal movements. Additional insights are provided by geological and mineralogical studies that inform the spatial distribution and petrogenesis of volcanic rocks. When combined with geophysically derived mantle anomalies and radioisotopic age data for volcanic centers, these datasets enable the delineation of areas with an elevated probability of future volcanism. Special focus is given to the Quaternary volcanic provinces of the Eifel and Vogtland, which are identified as regions with a significantly increased likelihood of renewed activity. The outermost volcanic centers in these regions are used to define preliminary hazard perimeters. A conservative safety buffer of 25 km beyond these limits is adopted to define the exclusion zone boundary for deep geological repositories. In the Vogtland region, known for its characteristic earthquake swarms, seismic epicenters are equated with volcanic centers to delineate zones of potential recurrence. The extent of this area is adjusted accordingly based on seismic swarm distribution and geophysical data. A major secondary hazard associated with volcanism in the Eifel is the potential damming of the Rhine River within its narrow Middle Rhine Valley by lava flows or tephra deposits. Prolonged blockage of the river would result in extensive upstream flooding, affecting the Upper Rhine Graben and its tributary valleys. Two regions in Germany—north of the Westerwald and east of the Black Forest—are classified as having a low probability of future volcanism within the next 1 million years. In the Tertiary volcanic fields, no volcanic activity is expected within the next 1 million years due to their advanced age and normal mantle and gas compositions.
Competing interests: I declare that neither I nor my co-authors have any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 07 Oct 2025)
- RC1: 'Comment on sand-2025-2', Andreas Klügel, 27 Aug 2025 reply
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CC1: 'Comment on sand-2025-2', Axel Schmitt, 03 Sep 2025
reply
Community comment of Schreiber and Jentzsch (submitted)
The manuscript by Schreiber and Jentzsch (Sch&J, submitted) addresses Germany’s volcanic hazard potential in the context of long-term nuclear waste storage. By its own description, it represents an integrated analysis of existing research. This includes the distribution, timing, and eruptive styles of late Quaternary volcanic fields, with particular emphasis on the Eifel. Within this scope, it is essential that sources are cited comprehensively and accurately, and that data and conclusions from prior work are represented fairly and correctly.
For time reasons, I primarily concentrate on sections 4.3–4.5, where I am most familiar with the current state of research. In these chapters, I find that Sch&J (submitted) frequently omit essential references to original work, misrepresent published findings, and rely on untested interpretations. In many places, nearly every other sentence requires a citation. A few of the most critical issues are highlighted below:
Line 253: No source cited for “Remnants of Eocene volcanic activity.”
Line 257: Tables S3 and S4 lack proper attribution in the text. Tables and figures are frequently reproduced at low-quality from published work, with Table S1 not even being translated into English. Captions were in part verbatim adopted form the originals without proper attribution. Overall, efforts in synthesising published data are minimal.
Lines 257/258: Numbers given for “256 and 100 volcanic centers for East and West Eifel” lack a source.
Line 268: The term “current geochronological dataset” is unclear. Mertz et al. (2015), the only cited study, covers only a limited number of sites.
Line 270: The phrase “substantially predate these ages” requires quantification. As existing geochronological methods are incapable of resolving annual, decadal, and even centennial age differences within the relevant timescale, this statement is nonsense.
Line 271: “One notable outlier—a lava flow near Gerolstein—has been dated at 32 ± 13 ka”: attribution missing.
Line 272: “Modern techniques” are invoked without specification. As Mertz et al. (2015) is the sole reference provided, then only ~3% of West Eifel volcanoes would be reliably dated, not 15%.
Line 278: The Late Pleistocene is incorrectly defined as “<100 ka.”
Line 282: No source for the “100–80 ka eruption age for Hohe Buche” is provided. It is concerning when unsubstantiated statements like this are propagated here and in popular media. Such unfounded claims diminish serious research efforts in geochronology.
Line 288: The section on supercritical fluids is speculative and disconnected from the chapter title. It fails to explain why maar craters are not universally produced. Again, substantive recent studies on this topic are ignored (e.g., Rausch et al., 2015; Schmincke et al., 2025).
Line 290: The claim regarding lava flows in “half of these cases” lacks attribution.
Line 295: A relevant publication on magma ascent in the Eifel based on hydrogen diffusion (Denis et al., 2013) is omitted.
Line 307: This section neglects scoria cones and maars, especially those surrounding Laacher See. It only briefly mentions important evolved complexes (Rieden, Wehr). Wehr is important as it shares compositional similarities with Laacher See, and its eruption ages approach the inferred onset of Laacher See mafic magma accumulation (Sturm et al., 2024; Bourdon et al., 1994). Instead, this section recycles an unpublished opinion regarding the Laacher See eruption volume, while published data on the timing of magma accumulation at Laacher See are overlooked or misrepresented (Bourdon et al., 1994; Schmitt F. et al., 2023).
Line 311: The interpretation of structures as collapse calderas lacks reference.
Line 314: Recent references for the precise age of the Laacher See eruption are absent (Reinig et al., 2021; Warken et al., 2025).
Line 321: More recent work on zircon crystallisation and magma residence conditions (Rout & Wörner, 2018; Rout & Wörner, 2020; Schmitt F. et al., 2023) is not cited.
Line 332: Mismatched reference: Demoulin et al. (2009) is cited in the text, while Demoulin & Hallot (2009) appears in the reference list. Demoulin & Hallot (2009) cite “estimated ~0.05–0.1 mm/yr displacement rates during the upper Pleistocene and the Holocene” for boundary faults of the Rhenish Massif. In addition to the unclear attribution of the stated 0.06–1.7 mm/yr displacement rates in Sch&J (submitted), it is difficult to see how regional faulting far removed from Laacher See can be related to viscoelastic magma emplacement, where long-lasting magmatism has modified the thermal and rheological structure of the crust for over 200,000 years.
Line 341: “Collapse volume at Laacher See is only ~0.5 km³.” A citation is given (Viereck & van den Bogaard, 1986), but the content is misrepresented. Viereck & van den Bogaard (1986, p. 70, translated from German) state:
“…the volume deficit of ca. 6 km³ was only to a small degree compensated by the Laacher See caldera (based on the dimensions of the collapse structure to a maximum of 10–20%), which is another indication for possible replenishment of the partially erupted magma chamber by ‘fresh’ basanite magma.”
Thus, a collapse volume of 0.5 km³ appears incorrectly attributed. Moreover, the cited work already presents an explanation for the difference between eruption and collapse volumes. Sch&J (submitted) either overlook or ignore this, as they do with existing research that demonstrated late basanite magma recharge into the Laacher See system (Wörner & Wright, 1984; Sundermeyer et al., 2020).
Line 443: “Topographic analysis suggests that a lava-induced barrier could reach over 200 m above sea level.” The reader is left uninformed about what this topographic analysis entails. Proposing such a scenario (which also features prominently in the abstract) without explanation and attribution to existing research on the damming of the Rhine River after the Laacher See eruption is unacceptable (Park & Schmincke, 2020a; 2020b).
In summary, the parts in Sch&J (submitted) reviewed here fall short of scholarly standards in both form and content. The pervasive omission of references and misrepresentation of prior research, while introducing oversimplified, untested, and in some cases already refuted hypotheses, undermines its credibility as a scholarly work.
Lastly, although not the focus of this community comment, I am surprised how state-of-the-art research on quantitative hazard assessments for distributed volcanism could be ignored in such a study (e.g., Bertin et al., 2019; Ang et al., 2020; Alohali, 2022, and references therein). I perceive this as a lack of methodological rigor. The conclusions by Sch&J (submitted) are thus at best qualitative guesswork and should be disregarded as basis for policy decisions for nuclear waste disposal.
Axel Schmitt
Curtin University
Additional references
Alohali, A., Bertin, D., de Silva, S., Cronin, S., Duncan, R., Qaysi, S., & Moufti, M. R. (2022). Spatio-temporal forecasting of future volcanism at Harrat Khaybar, Saudi Arabia. Journal of Applied Volcanology, 11(1), 12.
Ang, P. S., Bebbington, M. S., Lindsay, J. M., & Jenkins, S. F. (2020). From eruption scenarios to probabilistic volcanic hazard analysis: An example of the Auckland Volcanic Field, New Zealand. Journal of Volcanology and Geothermal Research, 397, 106871.
Bertin, D., Lindsay, J. M., Becerril, L., Cronin, S. J., & Bertin, L. J. (2019). MatHaz: a Matlab code to assist with probabilistic spatio-temporal volcanic hazard assessment in distributed volcanic fields. Journal of Applied Volcanology, 8(1), 4.
Bourdon, B., Zindler, A., Wörner, G., & Widom, E. (1994). U–Th systematics of a young basanitic volcano from the Eifel, Germany: Implications for melting and melt transport. Earth and Planetary Science Letters, 128(1–2), 1–17.
Denis, C. M., Demouchy, S., & Shaw, C. S. (2013). Evidence of dehydration in peridotites from Eifel Volcanic Field and estimates of the rate of magma ascent. Journal of Volcanology and Geothermal Research, 258, 85–99.
Park, C., & Schmincke, H. U. (2020). Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming II. Journal of Volcanology and Geothermal Research, 397, 106791.
Park, C., & Schmincke, H. U. (2020). Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I. Journal of Volcanology and Geothermal Research, 389, 106688.
Rausch, J., Grobéty, B., & Vonlanthen, P. (2015). Eifel maars: Quantitative shape characterization of juvenile ash particles (Eifel Volcanic Field, Germany). Journal of Volcanology and Geothermal Research, 291, 86–100.
Reinig, F., Wacker, L., Jöris, O., Oppenheimer, C., Guidobaldi, G., Nievergelt, D., ... & Büntgen, U. (2021). Precise date for the Laacher See eruption synchronizes the Younger Dryas. Nature, 595(7865), 66–69.
Rout, S. S., & Wörner, G. (2018). Zoning and exsolution in alkali feldspars from Laacher See volcano (Western Germany): constraints on temperature history prior to eruption. Contributions to Mineralogy and Petrology, 173(11), 95.
Rout, S. S., & Wörner, G. (2020). Constraints on the pre-eruptive magmatic history of the Quaternary Laacher See volcano (Germany). Contributions to Mineralogy and Petrology, 175(8), 73.
Schmincke, H. U., Sumita, M., Chakraborty, S., & Hansteen, T. H. (2025). Origin of maar clusters at the type locality Eifel (Germany): H2O or CO2? Bulletin of Volcanology, 87(3), 1–8.
Schmitt, F. H., Schmitt, A. K., Gerdes, A., & Harvey, J. C. (2023). Magma accumulation underneath Laacher See volcano from detrital zircon in modern streams. Journal of the Geological Society, 180(1), jgs2022-064.
Sturm, A., Schmitt, A. K., & Danišík, M. (2024). Updating the Eifel record: Zircon double-dating of tephras from Wehr volcano (East Eifel, Germany) as marker horizons for the European Pleistocene loess stratigraphy. Quaternary Science Reviews, 338, 108810.
Sundermeyer, C., Gätjen, J., Weimann, L., & Wörner, G. (2020). Timescales from magma mixing to eruption in alkaline volcanism in the Eifel volcanic fields, western Germany. Contributions to Mineralogy and Petrology, 175(8), 77.
Warken, S. F., Schmitt, A. K., Scholz, D., Hertwig, A., Weber, M., Mertz-Kraus, R., ... & Sigl, M. (2025). Discovery of Laacher See eruption in speleothem record synchronizes Greenland and central European Late Glacial climate change. Science Advances, 11(3), eadt4057.
Wörner, G., & Wright, T. L. (1984). Evidence for magma mixing within the Laacher See magma chamber (East Eifel, Germany). Journal of Volcanology and Geothermal Research, 22(3-4), 301-327.
Citation: https://doi.org/10.5194/sand-2025-2-CC1 -
CC2: 'Reply on CC1', Ulrich Schreiber, 05 Sep 2025
reply
Comments on A. Schmitt's Statement
Basis of the Work and Number of Citations
The basis of the discussed paper is a 150-page report on the assessment of potential volcanic activities in Germany over the next 1 million years. For publication, a summary of the key arguments within a maximum of 30 pages, including references, was required. This led to the avoidance of in-depth discussions on detailed questions that would not have altered the outcome. As a result, several pages of citations had to be omitted.
On Certain Details:
The remnants of the lava flow of the Hohe Buche still lie on gravel from the younger middle terrace (Meyer, 1988; Mangartz, 1998). Therefore, it cannot have the K/Ar age of 366,000 ± 40,000 years determined by Schmincke/Mertes (1979), but is more likely around 100,000 years. Age determinations are only approximations and must be considered as probabilities due to the limited sample and analysis numbers. Different methods always yield varying results. For high probabilities, a large number of measurements would need to be conducted, which is understandably rarely feasible.
Supercritical Fluids and Eruption Mechanisms:
The section on supercritical fluids is based on physicochemical data. From the deep drilling at Windisch-Eschenbach, we know that significant amounts of water with high salt concentrations can also be found at depths of 6,000 meters. Strike-slip faults provide fast ascent paths for magma, and water can infiltrate from above. The necessary temperature of 374.12 °C for the supercritical state is immediately reached in contact with magma at depth. The required pressure of at least 221 bar may suffice to about 1,000 meters without ascent. Maintaining critical pressure beyond that, at the tip of the rising magma, is a logical consequence of the tectonic stress conditions. Only the depth of the initial eruption cannot be specified, as the local tectonic conditions must be known for this.
What is interesting about this process is the change in the solubility of water for polar substances when it becomes supercritical. The solubility for salts is lost, and they essentially precipitate out, becoming incorporated into the magma (NACHRICHTEN - Forschungszentrum Karlsruhe Year 33 1/2001 p. 59-70). If there is insufficient water at depth, no scH2O-triggered initial eruption will occur. This relationship has not yet been considered and is an important factor to take into account due to its own dynamics.
Why not every eruption point in the Eifel was addressed has been explained above.
The Laacher See Eruption:
Statements regarding the eruption volume of Laacher See have already been presented and extensively discussed at conferences (Görlitz, EGU). The entire discussion regarding the caldera, magma chamber volume, and fictitious supporting basanite intrusion is based on data from a doctoral thesis, which incorporated data from diploma theses. From this, an eruption volume was derived, which has been cited in all subsequent publications with small deviations (> 6 km³, 18 km³ due to pumice formation). According to the author, this result only represents an initial approximation. However, it has never been critically questioned, despite numerous inconsistencies. For example, there is no geophysical evidence (using various methods) of a relictic magma chamber of the size that would be required for the erupted volume. Models of a magma chamber have been proposed without a tectonic basis.
Request for Explanation:
How do you interpret the opening of the magma chamber of at least 9 km³ (including remnants) in the brittle zone of the crust within the few tens of thousands of years you have determined? The tectonic movement rates are not nearly sufficient for this.
Attempts at Explanation:
Instead of addressing the root cause of the problems, namely the original collection of tephra, explanations were offered that are not substantiated (e.g., influx of 6 km³ of magma during the eruption phase of a few weeks; as evidence, basanitic streaks in phonolitic tephra, which are very common and do not provide any indication of the volume of the intrusion). Lava output volumes, for example, at Etna range between 10 and 20 m³ per second. This amounts to 1,728,000 m³ per day with a 20 m³ discharge rate. To fill a 6 km³ hole, this would take slightly over 9.5 years.
The reflection seismic data from the Laacher See sediments, by the way, show undisturbed stratification (Bahrig's thesis), which does not suggest turbulent developments of magma intrusions in the underground.
How Valid is the Data Foundation?
Two examples illustrate why the studies of Laacher See volcanism are based on a false foundation: A large part of the magma volume was calculated based on distal fans. One of these fans extends far to the south, beyond the Alps. Only one point, located near Turin, was cited. The citation given was Schneider 1978. However, a search for evidence of Laacher See tephra in this work is unsuccessful. On the contrary, it is stated on page 65, third paragraph:
"To establish the chronological transition II/III in the northern Italian region, we are dependent on radiocarbon dating, as to date no Alleröd-period volcanic ash from the Laacher See area (Germany) has been found in this region that could be used as a welcome chronological marker north of the Alps." (Original: „Um den Übergang II/III im norditalienischen Raum zeitlich festlegen zu können, sind wir auf 14C angewiesen, da bis heute in diesem Gebiet keine allerödzeitliche Vulkanasche aus dem Laachersee-Gebiet (Deutschland) gefunden werden konnte, die nördlich der Alpen als willkommene Zeitmarke genutzt werden kann.“)
Further points concern unverified occurrences that were adopted from the literature. For example, there is an isopach for the total thickness of proximal Laacher See tephra that extends southeast beyond the Mosel. Only one site is cited, from Stöhr 1963, west of Waldesch. Stöhr's sketch clearly shows that these tephra must be older than those from Laacher See, as ice-wedge pseudomorphs are documented. The remnants of Laacher See tephra are found in a debris fan at the start of the adjacent steep valley (own investigations), where layers of only a few centimeters have been washed together.
Furthermore, it was not considered that the Laacher See tephra, due to the lack of vegetation, remained exposed for many years, and any significant storm could have swept away material. The resulting deposits can hardly be distinguished from the primary ones. There is no consideration of error analysis. The isopach maps used for calculations are based on free interpretation by the author and are substantiated by few or no data over most of their extent. They do not meet the required scientific standards.
For the assessment of future volcanic development in Germany, a reliable data foundation is a prerequisite. Since this is not provided for the Laacher See volcano, against which all other magma volumes (East Eifel) are compared and assessed, works based on this foundation have been noted but not always given full consideration.
Ulrich Schreiber
Citation: https://doi.org/10.5194/sand-2025-2-CC2 -
CC3: 'Reply on CC2', Axel Schmitt, 09 Sep 2025
reply
The scientific method relies on data and hypotheses being critically evaluated through peer review. When peer-reviewed data are incompletely or in a biased manner attributed, as in Sch&J (submitted), this must be criticized. The timely reply is appreciated, but it does not invalidate this criticism for the following reasons:
Basis of the Work and Number of Citations
Sch&J (submitted), even if derived from a longer report, does not absolve the authors of the duty to properly attribute prior research. Scholarly diligence also extends to relevant references that appeared between completion of the report and submission of the manuscript. Space would not be an issue had Sch&J (submitted) concentrated on established data. Furthermore, a report available online (Schreiber and Jentzsch, 2021, 132 p.) shows the same disregard for existing research as Sch&J (submitted). For example, none of the additional references listed in CC1 were included in that report.
Age of Hohe Buche Scoria Cone and Lava
The stated age of 100,000 years is speculative. Re-dating of Hohe Buche is underway as part of ongoing research (DFG Project number 450662246; Sturm et al., in prep.).
Supercritical Fluids and Eruption Mechanisms
The model advocated is neither quantitative, nor substantiated by observations relevant to the Eifel, nor validated through peer review.
Volume of the Laacher See Eruption
Sch&J (submitted, and reply to CC1) call for downsizing the Laacher See eruption volume, arguing that 1) published data foundations are faulty, 2) basanite recharge is “fictitious,” and 3) accumulation of the reported magma volume is physically impossible within allowable timescales.
Although some of these allegations are more than a decade old (e.g., Schreiber & Berberich, 2013, EGU2013-5908), none have been published following peer review. Introducing them into a report on volcanic hazards in relation to nuclear waste storage circumvents scholarly diligence.
Moreover, the arguments by Sch&J (submitted, and reply to CC1) are easily refuted:
- The presence of Laacher See tephra in northern Italy has been confirmed (Finsinger et al., 2008). Even if this were not the case, any resulting volume change would be negligible, owing to the logarithmic scaling of tephra thickness with distance and the dominance of the NW fan and proximal deposits. Alleged discrepancies with published data for one site out of 643 in the Laacher See tephra distribution database (https://www.tephrabase.org/structure.html), even if valid, are inconsequential.
- Petrologic evidence for basanite recharge is unequivocal (e.g., Sundermeyer et al., 2000). The mass ratios of basanite parent to differentiated phonolite magmas are quantitatively constrained in previous peer-reviewed studies (Wörner et al., 1983; Wörner & Schmincke, 1984a, b). Similar parent-to-evolved magma mass relations have since been documented in numerous other studies. Inferring basanite recharge durations from comparison with Mt. Etna lacks justification. Nonetheless, documented output rates there reach 100 m³/s (Harris et al., 2011), sufficient to fill a Laacher See-sized system in less than two years. As an aside, a 6.3 km³ phonolite eruption volume for Laacher See is consistent with published scaling relationships between caldera diameter and eruptive volume in other volcanoes (Geshi et al., 2014). Likewise, crater volumes smaller than erupted volumes are common (e.g., the 2 km³ lake filling the crater from the 30 km³ DRE Millennium eruption of Changbaishan; Wei et al., 2013).
- The exclusive focus on tectonic opening in a putative pull-apart structure ignores other mechanisms such as viscoelastic deformation, uplift, and stoping, which permit magma accommodation in the upper crust. Integrated filling rates of 0.0006 km³/year (combining the basanite volume in Wörner and Schmincke, 1984a, with the onset of basanite-phonolite differentiation at Laacher See from Bourdon et al., 1994) are well within the range of eruption rates reported for intraplate mafic volcanic fields worldwide (van den Hove et al., 2017).
This response maintains that Sch&J (submitted), intended as a volcanic hazard assessment for nuclear waste disposal sites, disregards substantial literature and misses its target by repeatedly advancing idiosyncratic opinions untested by peer review while ignoring essential research by others. Sch&J (submitted) is therefore unsuited for publication in its present form.
Additional references
Finsinger, W., Belis, C., Blockley, S. P., Eicher, U., Leuenberger, M., Lotter, A. F., & Ammann, B. (2008). Temporal patterns in lacustrine stable isotopes as evidence for climate change during the late glacial in the Southern European Alps. Journal of Paleolimnology, 40(3), 885-895.
Geshi, N., Ruch, J., & Acocella, V. (2014). Evaluating volumes for magma chambers and magma withdrawn for caldera collapse. Earth and Planetary Science Letters, 396, 107-115.
Harris, A., Steffke, A., Calvari, S., & Spampinato, L. (2011). Thirty years of satellite‐derived lava discharge rates at Etna: Implications for steady volumetric output. Journal of Geophysical Research: Solid Earth, 116(B8).
van den Hove, J. C., Van Otterloo, J., Betts, P. G., Ailleres, L., & Cas, R. A. (2017). Controls on volcanism at intraplate basaltic volcanic fields. Earth and Planetary Science Letters, 459, 36-47.
Wei, H., Liu, G., & Gill, J. (2013). Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions. Bulletin of volcanology, 75(4), 706.
Citation: https://doi.org/10.5194/sand-2025-2-CC3 -
CC4: 'Reply on CC3', Ulrich Schreiber, 11 Sep 2025
reply
Hohe Buche
The lava of the Hohe Buche is exposed along the Rhine, overlying gravels of the Lower Middle Terrace. The outcrop is easily accessible and clearly identifiable. In this respect, I agree with you that the age determination involves a speculative component, since the given value only represents a maximum age with reference to the outcrop. Due to Rhine erosion and Roman quarrying, it remains unknown how far the lava flow extended into the Rhine. This implies that the lava may in fact be significantly younger. It is therefore most welcome that a dating effort is planned—please share the results once available.
Initial Eruption of the Maar Volcano
Which contradictory values regarding supercritical water as a trigger of the initial eruption are available to you that lead you to question the model?
Laacher See
To clarify the situation once again: for the Laacher See tephra we are discussing a volume of 18 km³. This corresponds to a column with a base area of 2.5 km × 2.5 km (approximating the caldera diameter) and a height of 3000 meters. Such a value does not align with the relatively low tephra thicknesses in the immediate vicinity of the Laacher See, where the maximum of proximal deposits should be expected.
For the refilling of the Laacher See system (caldera with a volume of 0.5 km³ or magma chamber with 6 km³?), you suggest a timespan of two years (assuming high, but realistic, flux rates). However, the eruption cycle of the Laacher See volcano has been estimated by Schmincke and others to last only a few days to weeks. What, then, occurred in the interim with the >6 km³ void in the crust?
At this point it is unclear whether you indeed defend the scenario of refilling the >6 km³ magma chamber as an argument supporting subsidence. If so, the implications would need to be explained. According to your own investigations, the initial filling required several tens of thousands of years, during which the magma evolved towards a phonolitic composition. If, however, a replenishment occurred 13,000 years ago, then continued magma evolution should still be expected today, given that no subsequent eruption has taken place. Pure cooling without further magmatic differentiation would not yet be completed. Does this imply that there is currently an active magma chamber beneath the Laacher See, the activity of which remains undetected? However, no geophysical measurements provide evidence for this, nor do other indicators exist.
A simple estimate of caldera depth, derived from the erupted volume, would result in depths exceeding 1000 meters for the given diameter. This is clearly unrealistic, since at the same time the caldera area would have significantly expanded laterally due to normal faulting. Neither of these processes is observed.
The determined depth of magma generation clearly lies within the brittle failure domain of the upper crust, which is not a matter of speculation. The lateral extent of the postulated magma chamber is limited for thermal reasons. A viscoelastic deformation model to explain the space problem is excluded at this depth.
Uplift? In which area, by what magnitude, how documented?
Erosion? Of what, in which area, how documented?
Involving structural geologists in clarifying these questions would be highly beneficial.
Data Density
643 sites over an area exceeding 500,000 km² are of primarily statistical significance. They suffice for a qualitative statement. It is unrealistic to assume that a value at a single locality is uniformly valid across the entire area up to the next point with equal thickness. These may instead represent very locally constrained wind plumes transporting material. Turbulent wind transport is far too dynamic, particularly since it cannot be established whether these are late storm deposits occurring long after the eruption ended. The involvement of meteorologists in such discussions would therefore be essential.
At that time, based in Bonn and in cooperation with the Geological Survey of Rhineland-Palatinate (Mainz), we mapped the Westerwald sheets of Westerburg, Selters, and parts of Montabaur and Meudt. With considerable personnel effort, several hundred 1-meter soil profiles were recorded and the Laacher See pumice layers identified. The results are documented in diploma theses. It quickly became apparent that distribution was highly variable, with morphology-dependent differences: some areas contained no or only very minor proportions. With few exceptions, the profiles fitted within the 1-meter fan of van den Bogaard, as can be seen on the map below. No occurrence exceeded 0.5 m thickness, except for one site representing an alluvial fan. In the southeast, the map shows the only point that led to extending the 1 m contour significantly further SE—an erroneous interpretation as explained above.
This reveals a fundamental methodological problem: the size of the study area requires a large number of investigators in order to obtain reliable conclusions regarding tephra distribution. Inferring the size of the Laacher See magma chamber from the much-cited isopach map is therefore not tenable.
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CC3: 'Reply on CC2', Axel Schmitt, 09 Sep 2025
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CC2: 'Reply on CC1', Ulrich Schreiber, 05 Sep 2025
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CC5: 'Comment on sand-2025-2', Lothar Viereck, 17 Sep 2025
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I read the paper and found many points to be commented, e.g. references cited in the text are missing in the reference list, e.g. Kreemer et al 2020, but the major point is, that the authors pretend that the old volcanic fields in Germany are underlain by "normal" mantle. This is not the case! All volcanic fields up to an age of 70 ma in question are underlain by "anomalous" mantle (Legendre et a 2012 and Meier et al. 2016). This indicates that the probability of renewed volcanic activity is given for all Cenozoic volcanic fields. Or would anyone have 1 Ma ago predicted that volcanism would start again in the old fields of the Eifel or Vogtland? No! The most likely fields would have been Lower Hessia or the Hegau.
The other point is, that not only in the volcanic field of the East Eifel the Rodderberg, Bonn, is located about 30km away from the Laaher See. The Dike at Sandebeck near Horn-Bad Meinberg, most probably part of a volcanic system, occurs 30 km north of the northernmost group of volcanic edifizes in the volcanic field of Lower Hessia near Warburg. Thus, a probability radius of 10 km is too short ! This short distance may be correct in direction of the strongest extension but not in the direction of elongation of the field, NW-SE (variscan A-C cleavage) in the case of the Eifel and the mesozoic "eggische" NNW-SSE direction in the case of the NW-part of the Lower Hessian volcanic field.
ps: if the authors are interested in the text with all comments, please let me know and I send a pdf
Citation: https://doi.org/10.5194/sand-2025-2-CC5
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General comments
This manuscript is a very nice summary of the extent of Quaternary volcanism in Germany, along with related features such as CO2 degassing, underlying mantle structures, and tectonic framework. These aspects are discussed to clarify possible causes for volcanic activity, and are projected to the next 1 million years to assess potential volcanic hazards relevant to repository site selection. The authors show that volcanic hazards exist not only for the active volcanic fields in the Eifel and Vogtland, but also along the upper Rhine River due to potential damming, and to a minor degree for the area between Stuttgart and Lake Constance.
Overall this manuscript is very well written, well organized, and I enjoyed reading it. It is accompanied by a number of figures and tables as electronic supplement, all of which were taken directly from the literature. What I am missing, though, is a more detailed map of the West and East Eifel volcanic fields in the main manuscript, as this region makes up much of the text. I am also missing an appropriate consideration of the review of Eifel volcanism by Schmincke (2007), because a considerable part of the present manuscript is dealing with aspects discussed in detail by him (see below). Apart from this, I have only a few specific and technical comments, and suggest the manuscript to be published after a minor revision.
Specific comments
Lines 192-194: I think the conclusions that 1) the formation of a new volcanic field within the next million years is unlikely, and 2) there will be only minor lateral shifts in volcanic activity within the established Quaternary fields, are valid and important. In this context it could be noted that the spatial orientation and rough age progression of the west and east Eifel volcanic fields (younger volcanism to the southeast) do not agree with the direction of plate motion. I would also add the important observation that Cenozoic volcanic fields in Central Europe in general do not show any age progression related to plate motion, as was shown in the review and synthesis by Schmincke (2007). This review touches much of the current contribution, and therefore is an indispensable source that should really be considered and cited in this work:
Schmincke H.U. (2007): The Quaternary Volcanic Fields of the East and West Eifel (Germany). In: Ritter, J.R.R., Christensen, U.R. (Eds.) Mantle Plumes. - Springer, 241-322.
Lines 269-270: this is a bit confusing, as the early (explosive) and the final stage of a volcanic eruption are typically no more than weeks to months apart. If a phreatomagmatic maar formation substantially predates the age of a spatially related lava flow then we essentially speak of two distinct eruptions. But do you know of any example in the Eifel where earlier maar deposits substantially predate an overlying scoria cone? Usually, as judged from the deposits, both reflect one single eruption without a prolonged break.
Chapter 4.4, Volcanoes of the West Eifel: actually, more than half of this short chapter deals with general processes related to magma ascent, but unspecific to the West Eifel. On the other hand, much of the preceding chapter describes the age distribution of West Eifel volcanoes. This could be re-organized: I suggest to move the West Eifel stuff of chapter 4.3 into 4.4 and move the more general part of 4.4 (which applies to young Eifel volcanism in general) to 4.3.
I also suggest to add recent findings on the existence of fluid-filled lenses beneath the West Eifel on the base of the old DEKORP seismic reflection data, as this is highly relevant for this contribution: Eickhoff D., Ritter J.R.R., Hlouaek F., Buske S. (2024) Seismic Reflection Imaging of Fluid-Filled Sills in the West Eifel Volcanic Field, Germany. Geophysical Research Letters 51, e2024GL111425. doi:10.1029/2024GL111425.
Line 323: there are more recent works that shed light into the temporal evolution of the Laacher See magma chamber. These suggest periodic recharge events throughout the magma reservoir’s entire lifespan of ~24 ky. As these works are relevant to the main topic of this contribution, their main findings should be included here:
- Sundermeyer C., Gätjen J., Weimann L., Wörner G. (2020) Timescales from magma mixing to eruption in alkaline volcanism in the Eifel volcanic fields, western Germany. Contributions to Mineralogy and Petrology 175, 77. doi:10.1007/s00410-020-01715-y.
- Rout S. S., Wörner G. (2018). Zoning and exsolution in alkali feldspars from Laacher See volcano (Western Germany): constraints on temperature history prior to eruption. Contrib. Mineral. Petrol. 173, 95. doi: 10.1007/s00410-018-1522-x
- Rout S. S., Wörner G. (2020). Constraints on the pre-eruptive magmatic history of the Quaternary Laacher See volcano (Germany). Contrib. Mineral. Petrol. 175, 73. doi: 10.1007/s00410-020-01710-3
Lines 335-336: very interesting discussion on the volcanological assessment of the Laacher See system, yet I think it needs to add two important points. (1) The growth model and extreme crustal deformation invoked here are based on gradual accumulation of magma at crustal levels. However, the works by Sundermeyer, Rout and co-workers show that magma can accumulate at much deeper levels and gradually fill a shallower reservoir during rare and discrete recharge events. Hence, large present-day deformation would not necessarily be observed. (2) The discussion seems to imply that the Laacher See eruption will be followed by a similar scenario, preceded by re-filling of the reservoir. This may happen at some time, but not necessarily now. The preceding volcanic phases (Rieden and Wehr) have shown that large eruptions were commonly followed by volcanic activity producing scoria cones in the wider vicinity. Again, in that case, large present-day deformation would not necessarily be observed.
l.538-539: It is not clear why a constant helium concentration observed in mantle-derived gases over a few years is sufficient to infer a decoupling between gas transport and active magmatism. What precisely do we know about this active magmatism, how frequent are potential recharge events by primitive melts from depth? If these occur at timescales of thousands to ten thousands of years, then there is little change in active magmatism and degassing in the last years/decades, hence no decoupling is evident. This casts doubt on the conclusion that the mantle source may be either waning or no longer actively replenished in this area. I suggest to remove this conclusion or provide a more conclusive reasoning.
Technical corrections and comments
Generally: a number of local town names appear in the text, but not all are shown or referred to in a map. This is particularly evident for the Eifel volcanic fields, of which a detailed map is lacking. Maybe the existing maps can be modified accordingly?
l.63: explosive, effusive, or a combination of both
l.124-125: for better clarity you could change "helium isotope composition ..." to "3He/4He ratio (R) ..."
l.129-134: I assume that most R/Ra data listed here were measured as part of CO2-dominated gases; maybe this could be explicitely stated. As for the highest value at Glees, it would be useful to state the kind of gas emission: was it diffuse CO2 degassing from the ground?
l.180, "magmatic conditions in the underlying mantle": I am not sure what is meant by this expression: the proportion of partial melt in the mantle? Or the melting conditions?
l.184, "...uplifted to a depth of approximately 50 km": please provide reference.
l.185: suggest to change to "the plume top is encased by lithospheric mantle", as it is not the entire conduit that is meant here.
l.189: it is confusing to write "forming a separate intrusion" here, as this may be understood by less specialized readers as a magmatic intrusion. Some readers may even think that a plume consists of melt rather than ductile crystalline material... I suggest to remove this subclause.
l.256, l.280, and others: I suggest to replace all references to the two popular textbooks by Schmincke ("Volcanism" and "Vulkane der Eifel") by his 2007 review, which is a much more appropriate source.
Line 349: why not providing the source for the 300 ka dating of Rodderberg: Paulick H, Ewen C, Blanchard H, Zöller L (2009) The Middle-Pleistocene (~300 ka) Rodderberg maar-scoria cone volcanic complex (Bonn, Germany): eruptive history, geochemistry, and thermoluminescence dating. Int J Earth Sci 98:1879–1899.
l.510: sentence is incomplete: "is located"?
l.515-516: I don't think that a popular textbook (Schmincke 2009) is a citeable source for the initial consideration of a mantle plume. I suggest to remove this subclause.
l.553. repetition of l. 547.
l.557: for clarity change to "elevated 3He/4He ratios of mantle-derived helium."
l.562: do you mean the volcanic / volcanological evolution?
l.594-607: this is a duplicate of lines 580-593; delete.
l.559, geophysical modelling: this is quite broad; it would be helpful to the reader to specify this briefly in the sentence: modelling of gravity data, deformation data...?
Figure 2, caption: for the reader not familiar with the Walker et al. (2005) paper it would be helpful to explain the abbreviations of this figure (e.g. APM, Pn) and also the grey shaded areas in the caption.