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1975). In addition, Kilian Crater is a trachytic maar (with a small
spine of trachyte in its centre), and a number of the trachytic
domes of the Chaîne des Puys became emplaced, in a second,
magmatic phase, inside an initial trachytic maar (Camus 1975).
Karstic limestones. Special remark has to be made on
karstic limestone environments. The Miocene olivine melilitite
Swabian Alb Volcanic Field in southern Germany formed on
karstic Upper Jurassic limestones (Fig. 3) which were karstic
already in Miocene time (Lorenz 1979, 1982a). Out of about
350 monogenetic volcanoes there formed almost only maar-
diatreme volcanoes. The Eisenrüttel in all probability was
a lava lake occupying an initial maar (Keller et al. 1990). And
the Grabenstetten dyke could have supplied scoria cones and
some lava flows (Lorenz 1982a). All other localities represent
maar-diatreme volcanoes (Lorenz 1979, 1982a), thus there was
so much karst groundwater that the ultrabasic magma had hardly
a chance to reach the surface without interacting explosively
with groundwater. Other karstic limestone areas which support
maar-diatremes are found in the northern Hegau Volcanic Field
(Keller et al. 1990), in the Causses, Massif Central, France, and
in several kimberlite volcanic provinces in China (Zhang et al.
1989). The diamondiferous Mbuji Mayi kimberlite maar-dia-
treme volcano in Kasai, Congo, also formed within a sedimentary
series containing karstic limestones (Demaiffe et al. 1991).
Late intrusives and extrusives. Magnetic traverses across
most maars in the West Eifel (Büchel 1984, 1987, 1988, 1993),
however, demonstrate that about 40 % of the maars are associ-
ated with local magnetic anomalies of higher intensity than the
rest of the near-surface diatreme fill and crater fill. These highs
point to local magmatic deposits underneath the present crater
floors indicating that, at the end of the phreatomagmatic maar-
diatreme activity, magma must have intruded these diatremes
to near-surface levels. In many instances magma probably also
erupted magmatically onto the former crater floor, the scoria or
lava lake being now covered by post-eruptive deposits (Büchel
1984). The gravimetric and magnetic investigation of the
Pulvermaar by Diele (2000a, b) also points to a late magmatic
activity in that particular crater.
Maar lakes. After the eruptions ended, the maar craters
filled with groundwater and surface water because the craters
undercut the previous level of the valley floors (Fig. 1). Only
the youngest maars still contain a lake, especially when they
are large enough and cut off from a potential influx of fluvial
sediments. The older and the smaller younger maar-crater lakes
are filled by post-eruptive sediments of various kinds and the
maar craters ultimately change into “dry” maar craters.
Scoria cones or lava lakes with an initial maar. Zones
of structural weakness also exist underneath the valley slopes
and on Tertiary and Pleistocene plateau areas between the val-
leys, as, e.g., in the Eifel (Lorenz and Büchel 1980, Büchel
1984). Judging from the volcanoes formed on these zones
they were less hydraulically active than those underneath the
valleys. When in the West Eifel ultrabasic magma rose along
or cut across hydraulically active zones of structural weakness
outside the valleys, this also resulted frequently in phreatomag-
matic activity and formation of maar-diatreme volcanoes. At
almost all of these localities (with the exception of the three
above mentioned maars), an initial maar-forming phreatomag-
matic phase was followed by a magmatic phase which pro-
duced a scoria cone in the initially formed maar (Fig. 4). This
volcano type was called scoria cone with an initial maar by
Lorenz and Büchel (1980). Some initial maars are relatively
Fig. 3. A diagram showing the relationship between centres
of diatremes and valley pattern (perennial streams or
dry valleys) in the Swabian Alb. The Swabian Alb is
underlain by karstic limestones of Upper Jurassic age.
The centres of diatremes are taken from Maeussnest
1974.
Fig. 4. Tephra-ring deposits from the Hasenberg volcano in
the Quaternary West Eifel Volcanic Field (Lorenz
and Büchel 1980). Typical maar tephra rich in Lower
Devonian rock clasts (sandstones, slates) formed in
an initial phreatomagmatic phase and is overlain by
scoria formed by the following magmatic phase. Thus
a scoria cone formed within an initial maar. Hammer
for scale.
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small in size and others are comparatively larger which implies
that at the various sites there was relatively shorter or longer
availability of groundwater and thus shorter- or longer-lived
explosive phreatomagmatic activity prior to a change over to
magmatic activity which occurred when magma still kept ris-
ing while little or no groundwater remained available. In the
West Eifel, outcrop-dependent, out of about 200 scoria cones,
approximately two thirds show an initial maar phase. The large
number of maars and initial maars (and phreatomagmatic phas-
es within scoria cones) in the West Eifel – and also in the East
Eifel Volcanic Field with a smaller number of maars but a large
number of initial maars (Schmincke 1977) – points out that the
shear availability of groundwater in hydraulically active zones
of structural weakness is very conducive to phreatomagmatic
explosive activity when magma rises along or intersects such
zones of structural weakness.
Root zones in hard-rock aquifers. The root-zone explo-
sion chambers discussed above are formed by shock waves
generated by the thermohydraulic explosions. Thus the energy
imparted to the hard rocks surrounding the site of explosion
produces intensive fragmentation of the surrounding country
rocks. Radially outward, the newly generated fractures will
decrease in number per unit rock volume but the existing joints
and faults may become slightly widened because the shock
waves seem to lead to a slight decrease in rock density in the
immediate surroundings of diatremes (Diele 2000a, b) imply-
ing a slight reorientation of blocks with respect to each other.
Opening of other hydraulically active zones of structural weak-
ness in the near vicinity of the main zone of structural activity
will give access to new sources of groundwater even when the
amount of water is not large. Thus the thermohydraulic explo-
sions create new access of groundwater to the explosion sites
(Lorenz 2000a, c, Lorenz and Zimanowski 2000, Lorenz and
Kurszlaukis in press). At the maars on the valley floors, the
hydraulically active zone of structural weakness underneath
the valley floor in all probability will still represent the main
conduit for groundwater to the explosions, as at these maars the
thermohydraulic explosions are active from the beginning to
the end of the eruptive activity. Away from the valleys, howev-
er, there is almost always a change over from phreatomagmatic
maar eruptions to magmatic scoria production.
Soft-rock environment
A soft-rock environment implies unconsolidated sediments
which are water-saturated up to, or close to, the surface. The
lack of cement makes all coarser sediments (sands, pebble beds,
unconsolidated breccias) a pore aquifer with a high permeabil-
ity. Syn-sedimentary or post-sedimentary but pre-diagenetic
volcanic activity in areas underlain by water-saturated uncon-
solidated sediments occurs in continental rift zones, molasse
basins, late orogenic basin-and-range provinces or coastal sedi-
mentary deposits.
Well-known late orogenic basin-and-range-type grabens
exist in southeastern, central, western and southwestern Europe
in the late Variscan basin-and-range province (Lorenz and
Nicholls 1976, 1984). These grabens contained more or less
thick accumulations of water-saturated unconsolidated sedi-
ments and most grabens also saw syn-sedimentary volcanism
Fig. 5. Typical bedded upper-level pyroclastic deposits from
the Rödern diatreme in the Carboniferous-Permian
Saar-Nahe Basin, SW Germany (Lorenz 1971, 1972).
Juvenile clasts are well visible. The matrix between
the juvenile clasts contains many individual mine-
ral grains derived from the Carboniferous-Permian
country rocks which at the time of volcanism were
water-saturated and unconsolidated. Thus the Carboni-
ferous-Permian sediments at the time of synsedimen-
tary volcanism in the Saar-Nahe Basin represented
a typical soft sediment environment. The left tephra
became oxidized and the right tephra became reduced
during the diagenesis. In the left tephra, imbrication
of juvenile clasts indicates deposition by base surges.
Fig. 6. A block of feldspar-bearing sandstone which synerup-
tively subsided in the Rödern diatreme in the Carbo-
niferous-Permian Saar-Nahe Basin, SW Germany (Lo-
renz 1971). The inside of the block contains dykelets
and stringers of tephra containing juvenile ash grains
and lapilli (bright in colour) showing that the present-
ly indurated sandstone block was not indurated yet
at the time the diatreme formed in the Carboniferous-
Permian times. Thus, at the time of diatreme forma-
tion, the block subsided in the diatreme as a block of
unconsolidated sand.