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of these volatiles, respectively to intensive fluidization proces-
ses (Clement 1982, Clement and Reid 1989, Kilham et al. 1998,
Kirkley et al. 1998, Field and Scott Smith 1999, Scott Smith
1999). The individual magmatic models vary quite remarkably.
Nevertheless, it has to be realized that, regardless of the model
on the formation of these maar-diatreme volcanoes, any model
has to comply with the laws of physics and should be supported
by respective experiments. With respect to the phreatomagmatic
model, experiments show that kimberlite or carbonatite melts
can interact explosively with water (Kurszlaukis et al. 1998,
Lorenz et al. 1999). Experiments supporting the magmatic
model for the formation of maars and diatremes have not been
performed yet.
Hard-rock and soft-rock environments of maar-dia-
treme volcanoes
Two principally different environments exist with respect to
the source of groundwater interacting with the dyke magma:
groundwater in hard-rock environments and groundwater in
soft-rock environments. Several combinations of these envi-
ronments are possible as will be discussed below.
Hard-rock environment
Joint aquifers. Hard rocks as such are more or less imperme-
able with respect to the groundwater flow required for start-
ing and supporting the phreatomagmatic explosive activity
of maar-diatreme volcanoes at the level of the top end of the
respective feeder dyke. Hard rocks, however, are cut by joints
and faults, many of which are hydraulically active. Thus hard
rocks represent the so-called joint aquifers. Hydraulic activity
of these zones of structural weakness varies in respect of their
orientation relative to the respective stress field (Lorenz 1973,
Lorenz and Büchel 1980, Büchel 1984, 1993). In regions where
hard rocks are uplifted, the zones of structural weakness are
more hydraulically active and more readily eroded into val-
leys (Lorenz 1973, 1982b, Lorenz and Büchel 1980, Büchel
1984). This is not to suggest that joints and faults outside the
valley floors are not hydraulically active, but they are less so
than those underlying valley floors. The particular zones of
structural weakness that result in valley formation are used
preferentially by water from many ordinary springs and from
thermal, mineral, and CO
2
springs on their way to the surface.
Hydrogeology, balneology and economic use of CO
2
make use
of this relationship. In karstic areas like the Swabian Alb in
southern Germany, most sinkholes/dolines are located in valley
floors also pointing to preferential hydraulic activity beneath
the floors of former water courses (Lorenz 1982a, Lutz et al.
2000).
The classic maar region of the world, the West Eifel, is
underlain by Devonian shales, slates, sandstones, greywackes,
limestones, dolomites, and Lower Triassic sandstones, thus
hard rocks of various kind. Because of the Tertiary and es-
pecially Quaternary uplift, the Eifel is highly dissected by
more or less deep valleys (Illies et al. 1979, Fuchs et al. 1983).
Especially in areas underlain by Lower Devonian rocks there
is not enough groundwater supply available for the local com-
munities in the area. Nevertheless, out of the c. 270 (except for
3 phonolites) ultrabasic alkalibasaltic to foiditic monogenetic
volcanoes of the Quaternary West Eifel Volcanic Field with
the exception of 3 maars all other 67 maars are cut into val-
ley floors, many even at the head of a valley, no matter if the
eruptive fissure at depth followed the respective valley trend
or cut across the valley. The available exposures in most of
these maars show that phreatomagmatic eruptions were active
in these maar-diatreme volcanoes from the beginning to the end
of their activity; at least this is what the tephra rings preserved
from erosion indicate. It is very rare that magmatic eruptions
were active between the phreatomagmatic eruptions (Lorenz
and Zimanowski 2000). This relationship in the groundwater-
poor West Eifel also points to the fact that groundwater was
available underneath the valley floors but it also demonstrates
that the structural zones of weakness lying underneath the re-
spective valleys were hydraulically active enough to support
the phreatomagmatic activity of the maar-diatreme volcanoes
from the very beginning to the very end of the local eruptive
activity.
Other areas where hard rocks form the country rocks and
where maar-diatreme volcanoes are frequently localized in val-
leys are, e.g., volcanic fields in the Massif Central in France
(Camus 1975), the Swabian Alb in southern Germany (Fig. 3;
Lorenz 1982a, Keller et al. 1990, Lutz et al. 2000), and the kim-
berlites in Lesotho, southern Africa (Nixon 1973, 1995). Despite
the fact that the valleys in their present depth and shape are
younger than the diatremes, the present valleys are successor
valleys of earlier valleys cut down on hydraulically active zones
of structural weakness due to regional uplift. Thus if the present
valleys are the result of erosion on hydraulically active zones of
structural weakness then the maar-diatremes at their time of for-
mation in all probability formed on the same zones of structural
weakness.
The Ukinrek East Maar erupted in 1977 on the saddle be-
tween two shallow little valleys following the trend of a fault
zone exposed in the crater walls of the maar (Büchel and
Lorenz 1993). And the Nilahue resp. Carran Maar in southern
Chile erupted in 1955 in a valley and even collected intermit-
tently surface water from a small stream in its crater (Müller
and Weyl 1956, Illies 1959). It has to be stated, however, that
there do not have to be valleys in order for maar-diatreme
volcanoes to form, it is the availability of groundwater in
hydraulically active zones of structural weakness which is re-
quired. Nevertheless, it is in uplifted areas that most of these
hydraulically active zones of structural weakness get shaped
into valleys by erosion.
In many of the Tertiary volcanic fields of Central Europe
which formed in diverse hard-rock environments (granitoids,
gneisses, schists, basalts, sandstones, limestones, etc.) there exist-
ed a great many basic to ultrabasic maars and also initial maars,
the latter with either scoria cones or lava lakes (Bussmann and
Lorenz 1983, Lorenz 1985, 1998, Keller et al. 1990, Suhr and
Goth 1996, 1999, Suhr 2000, Cajz et al. 2000). In the Quaternary
volcanic field of the Chaîne des Puys in the Auvergne, maars
and initial maars are associated with the basic magmas (Camus