18. March 2003
Dr. Henrik Svensen
Physics of Geological Processes (PGP)
PO Box 1048 Blindern
University of Oslo
0316 Oslo, Norway
Email: hsvensen@geologi.uio.no
Editorial office of Geology,
Ben A. van der Pluijm
Carol Traynor
Dear Editors,
Thank you for the constructive comments on our manuscript “Subsurface combustion in
Mali: Refutation of the active volcanism hypothesis in west Africa”, and that you will
consider it for publication in Geology.
We have taken the comments of both reviewers into consideration, and made
corrections and clarifications to the manuscript accordingly (see details below). In
addition we have rewritten the introduction to make clearer the significance and broad
implications of our work (as you emphasized in the decision letter). The details of our
changes are given below.
The comments by Reviewer #1:
Reviewer #1 considers the descriptions of the combustion process as the most important
aspect of our manuscript. However, he also says that the phenomenon is very unusual,
and should be regarded as a curiosity. In some ways this is true, but we want to make
some comments on this issue to emphasize that we are dealing with something more
than a curiosity. Subsurface fires as we describe them fall in between two other
phenomena: coalmine fires and ground fires. These phenomena are widespread, and
may sometimes be reported in local news. Coal mine fires in China made the headings
in BBC in February, following the AAAS meeting in Denver (see
http://news.bbc.co.uk/2/hi/in_depth/sci_tech/2003/denver_2003/2759983.stm). The
EARS in the Netherlands (Environmental Analysis & Remote Sensing) are involved in
a project to monitor the fires, and write that:
The People's Republic of China (PRC) is the largest coal producer and exporter in the world. The coal
basins are widely distributed across the country, but are mainly exploited in the northern part. Many coal
seams are burning as a result of spontaneous combustion. The estimated coal losses are 100-200 million
ton per year, which far exceeds the annual export. The CO2 emission of these coal fires is estimated at 23% of the world CO2 production from fossil fuels. (http://www.ears.nl/EARShome/projects/txtco.htm)
Similar coal fires are known from South Africa, India, Indonesia, USA, Svalbard, etc.
This is not only the case for coalmine fires, but also for ground peat fires (e.g., USA,
Botswana, South Africa). Thus these types of fires are of great significance for release
of CO2 to the atmosphere. However, studies of the initiation and evolution of these fires
is almost non-existing in the geological literature. Furthermore, studies of the products
of these types of fires in the geological record are lacking, which makes it almost
impossible to address the importance of these types of fires throughout the geological
history. We don’t solve all these important issues, but we claim that our data from Mali
is a good starting point for focusing on the role of subsurface fires as a geological
process.
Specific comments:
a) We have shortened the text several places, and moved the previous Figs. 2A, 2B,
and 4 to a data repository. The length is now within the limits of four pages. We
have also eliminated the gray shading in the figures. The figure fonts and other
figure details are modified according to the comments by Eberle.
b) The introduction has been rewritten, and is now more general and – we hope –
more interesting for the general reader.
c) All localities mentioned in the text are in the figures.
d) We agree that our presentation of the “igneous daounites” was unclear. This has
been corrected. We tried to find the dikes during fieldwork, but didn’t succeed.
Thus we do not present any new data on the dikes but argue that the so-called
daounites are produced by the subsurface fires. The arguments are, in short: i)
The presence of nepheline in the dikes (Monod and Palausi, 1961) has not been
confirmed by reanalysis of the original samples (El Abbass et al., 1993). ii) The
high temperature minerals (cristobalite and mullite) are found in both “lava” and
baked diatomite. iii) We have observed one of these high temperature minerals
(mullite) in our samples of diatomite baked by subsurface fires. iv) The
geometry of the “dike networks” as described (Monod and Palausi, 1961) is very
similar to fracture networks and holes we observed that were produced by
subsurface fires. v) The mixed contents of sand, “lava” and baked and unbaked
diatomite (Monod and Palausi, 1961; Sauvage and Sauvage, 1992) suggests that
the daounites are merely mixtures brought together in the fractures and holes by
wind and annual floods and that have consolidated with time.
e) Dashes versus hyphens have been corrected by Geology.
f) To our knowledge, no heat flow data are available from our study area or nearby
regions. We are not aware of any scientific boreholes in the area where these
types of measurements could have been made. Furthermore, we believe that our
conclusions are too strong to validate the initiation of a study aiming at
measuring heat flow in the Lac Faguibine area. The reviewer also mentioned
this.
g) We have clarified the relationship between red diatomite and subsurface fires.
The localities we have visited are all situated on old lake floor of the Lake
Faguibine. The old lake floor is mostly covered by diatomite ooze. Thus the fires
oxidize the cover diatomite ooze. Several samples collected from the trench we
dug at the Haribibi locality have been studied by Prof. B. Stabell at the
University of Oslo, which verified that the near-surface sediments are indeed
diatomite ooze. The main diatom species is Aulacoseira granulata, which is a
typical fresh water species.
Comments by reviewer #2:
Reviewer #2 has three main comments:
1) Reviewer #1 points out that The Smithsonian Institution catalogue of Active
Volcanism hasn’t listed the studied region as active. We know that, but we don’t
think that it really makes a big difference to our story. We document the
phenomena that have been associated with possible incipient volcanism, and
discuss all relevant literature arguing for incipient volcanism.
2) The Cameroon volcanic line is not mentioned in our manuscript. We are careful
to place Mali and our study area in west Africa, which normally doesn’t include
Cameroon. We don’t regard it as necessary to explicitly state that our study area
differs from the geological setting in Cameroon.
3) We have rewritten the part of the discussion that mentions the dynamics of fire
fronts, and included a reference to a study by reviewer #2 on the migration of
volcanic fumaroles.
We hope that our revised manuscript, including the data repository, will be found
satisfactorily and suitable for publication in Geology without further reviews.
A CD with all files (text, figures, and data repository), together with scannable copies of
the figures, has been sent to Boulder by courier (on the 20th March). The text is written
in Microsoft Word 2000, the figures are in CorelDraw 9.0 (and as EPS), and the data
repository is made using Corel Ventura 10.0.
Sincerely,
Henrik Svensen