GeoLines
15 (2003)
72
GeoLines
15 (2003)
73
Introduction
Occurrence of maar-diatreme volcanoes. After the intermedi-
ate, basic and ultrabasic scoria cones, monogenetic maar-dia-
treme volcanoes are the second most common volcano type on
continents and islands (Wohletz and Heiken 1992). The majority
of the maar-diatreme volcanoes represent the phreatomagmatic
equivalent of the magmatic scoria cones and their associated
lava flows (Lorenz 1985, 1986, 1998). Most maar-diatreme vol-
canoes occur in volcanic fields of basic to ultrabasic composition
which comprise several tens to several hundreds of individual
monogenetic volcanoes – scoria cones, their lava flows, and
maar-diatremes. In addition, maar-diatremes are associated with
large polygenetic volcanoes and occur in the foot plains and cal-
deras of shield volcanoes, stratovolcanoes and caldera volcanoes.
Finally but more rarely, they also form as a phreatomagmatic
equivalent of acidic to intermediate domes.
Scoria cones. Basic to ultrabasic magma in the upper crust
mostly flows through dykes of less than, to in excess of, one
metre in thickness. Flow direction is either vertical or more or
less lateral, as increasingly demonstrated by AMS studies (e.g.,
Ernst and Baragar 1992). Very close to the Earth’s surface, the
vertical flow component becomes dominant and has a velocity
of several m/s to in excess of 10 m/s. Scoria cones form when
magma close to the Earth’s surface rises through a dyke and,
during its final approach to the surface accelerates because
of the formation and growth of vesicles and fragments due to
several processes such as hydrodynamic fragmentation in free
air, vesiculation in the still fluid state and brittle fragmentation
caused by magma flow exceeding the critical shear strength
(Zimanowski 1998, Morissey et al. 2000). Associated lava flows
either form because of a high production rate resulting in less ef-
fective cooling and coalescence on the ground of still fluid clasts
or because of quiet effusion. Scoria cones are known to erupt
for weeks, months or even years. The scoria cone of Paricutin
in Mexico, finally 424 m high, and its associated lava flow field,
finally 24,8 km
2
in size, erupted for about 9 years (Feb. 20, 1943
to March 4, 1952; Luhr and Simkin 1993). Also Jorullo in its
neighbourhood, which reached a final height of about 350 m
and formed a lava field about 1.25 km
2
in size, erupted for about
15 years (1759–1775; Luhr and Simkin 1993). According to
Luhr and Simkin (1993) most of the eruptions giving rise to
scoria cones are over within one year or less.
Size of maars. The maar-diatreme volcano in principal con-
sists at the surface of the maar crater, which is cut into the pre-
eruptive land surface, the tephra ring surrounding the crater, the
more or less cone-shaped diatreme, which underlies the maar
crater, the irregular-shaped root zone surrounding and underly-
ing the lower end of the diatreme, and finally the narrow feeder
dyke at depth (Fig. 1). The maar crater is less than 100 m to over
2 km in diameter (measured from the crest of the tephra ring)
and several tens of metres to 300 m deep (measured also from
the crest of the tephra ring). The tephra ring is several metres to
perhaps over 100 m high. Its inner slope dips towards the inte-
rior of the crater at about 33°
(natural angle of rest)
and the outer
slope dips outward at a much shallower angle (e.g., 5–10°), both
angles depending on the total volume ejected, the pre-eruptive
topography, and, during and immediately after the eruptions of
the volcano, also on the state of moisture content of the tephra
and erosion processes (e.g., slumping). The tephra ring is built
up by possibly a few tens to over of 1000 tephra beds, the ma-
jority being only a few mm, cm to 1–2 dm thick. These thinly
Maar-Diatreme Volcanoes, their Formation, and their Setting
in Hard-rock or Soft-rock Environments
Volker LORENZ
Institut für Geologie, Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany
ABSTRACT. Maar-diatreme volcanoes mostly form when rising magma in basic to ultrabasic volcanic fields interacts explosively
with groundwater. Less commonly, there also exist maars associated with intermediate to acid magmas. The formation and growth
of the maar-diatreme volcano type, the second most common volcano type on continents and islands, is reviewed applying the phre-
atomagmatic model of its formation. The site of explosions is the root zone which penetrates downward on its own feeder dyke.
Because of repeatedly developing mass deficiencies in the root zone, the overlying cone-shaped diatreme and the maar crater are
the consequent collapse/subsidence features. Prolonged downward penetration of the root zone leads to repeated collapse phases
of both the diatreme and maar, thus both grow in size the longer the maar-diatreme volcano is active.
Two contrasting environments exist with respect to groundwater availability for the phreatomagmatic explosions: the hard-rock
environment which is a joint aquifer and the soft-rock environment which is a pore aquifer. In the hard-rock environment, the tephra
of maar-diatreme volcanoes contains large volumes of rock clasts originally derived from the hard country rocks formerly occupy-
ing the root zone resp. the diatreme and maar crater. In the soft-rock environment, the tephra contains large amounts of the indi-
vidual minerals and pebbles from the sediments but hardly any rock clast consisting of indurated sediments. The two environments
are frequently combined in areas where unconsolidated, water-saturated sediments overlie diagenetically indurated sediments
and/or crystalline basement rocks or in areas where unconsolidated sediments contain interbedded solidified and jointed sills and
lava flows.
Maar-diatreme volcanoes have typically formed in areas characterized by rather normal groundwater conditions. In contrast, are-
as characterized by highly permeable water-saturated rocks or pebble beds just below the Earth’s surface give rise to tuff-rings or
tuff-cones. Tuff-cones also form in the shallow sea or in lakes.
KEY WORDS: maar, diatreme, phreatomagmatism, hydrogeology, hard rocks, soft rocks.