GeolCarp_Vol62_No6_535

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, DECEMBER 2011, 62, 6, 535—546 doi: 10.2478/v10096-011-0038-3

Introduction

With  respect  to  magma  fragmentation  two  “end-members”
can be recognized. In a “dry” magmatic eruption the expan-
sion of magmatic volatiles in an overpressured environment
is  responsible  for  fragmenting  the  magma  explosively.  In
small-volume  mafic  magmatic  systems  scoria  cones  and
spatter cones are the typical volcanoes produced by the mag-
matic fragmentation style (Vespermann & Schmincke 2000).
On  the  other  end  of  the  magma  fragmentation  spectrum  is
the “wet” explosive interaction of magma with water result-
ing in phreatomagmatic eruptions that form maars, tuff rings
and  tuff  cones  (Lorenz  1973,  1986;  Wohletz  &  Sheridan
1983; Lorenz & Kurszlaukis 2007). Maar and tuff ring vol-
canoes  form  when  the  rising  magma  interacts  either  with
ground or surface water respectively (Lorenz 1986; Németh
et al. 2010; White & Ross 2011). The evolution of a phreato-
magmatic  volcano  is  commonly  related  to  1)  syn-eruptive
valley  systems  where  water  is  readily  available  below  the
surface along hydrologically active faults and fractures such
as  in  the  West  Eifel  Volcanic  Field  in  western  Germany
(Lorenz 1984; Lorenz & Zimanowski 2000), or 2) in low ly-
ing, well-drained siliciclastic sedimentary basins such as the
Pannonian  Basin  in  Central  Europe  in  which  the  Bakony—

Shallow-seated controls on the evolution of the Upper

Pliocene Kopasz-hegy nested monogenetic volcanic chain in

the Western Pannonian Basin (Hungary)

GÁBOR KERESZTURI

1,2,3

and KÁROLY NÉMETH

Volcanic Risk Solutions, CS-INR, Massey University, PO Box 11 222, Palmerston North, New Zealand; kereszturi_g@yahoo.com

Geological Institute of Hungary, Stefánia út 14, H-1143, Budapest, Hungary

Department of Geology and Mineral Deposits, University of Miskolc, Hungary

(Manuscript received November 19, 2010; accepted in revised form June 9, 2011)

Abstract: Monogenetic, nested volcanic complexes (e.g. Tihany) are common landforms in the Bakony—Balaton High-
land Volcanic Field (BBHVF, Hungary), which was active during the Late Miocene up to the Early Pleistocene. These
types of monogenetic volcanoes are usually evolved in a slightly different way than their “simple” counterparts. The
Kopasz-hegy Volcanic Complex (KVC) is inferred to be a vent complex, which evolved in a relatively complex way as
compared to a classical “sensu stricto” monogenetic volcano. The KVC is located in the central part of the BBHVF and
is one of the youngest (2.8—2.5 Ma) volcanic erosion remnants of the field. In this study, we carried out volcanic facies
analysis of the eruptive products of the KVC in order to determine the possible role of changing magma fragmentation
styles and/or vent migration responsible for the formation of this volcano. The evolution of the KVC started with
interaction of water-saturated Late Miocene (Pannonian) mud, sand, sandstone with rising basaltic magma triggering
phreatomagmatic explosive maar-diatreme forming eruptions. These explosive eruptions in the northern part of the
volcanic complex took place in a N—S aligned paleovalley. As groundwater supply was depleted during volcanic activity
the eruption style became dominated by more magmatic explosive-fragmentation leading to the formation of a mostly
spatter-dominated scoria cone that is capping the basal maar-diatreme deposits. Subsequent vent migration along a few
hundred meters long fissure still within the paleovalley caused the opening of the younger phreatomagmatic southern
vent adjacent to the already established northern maar. This paper describes how change in eruption styles together with
lateral migration of the volcanism forms an amalgamated vent complex.

Key words: volcanic hazard, phreatomagmatic, scoria cone, maar, vent migration, magma fragmentation, pyroclastic
density current.

Balaton Highland and Little Hungarian Plain Volcanic
Fields (BBHVF and LHPVF respectively) are located (Mar-
tin & Németh 2004).

A “sensu stricto” monogenetic volcano is defined as: “a

volcano that was active for a relatively short period of time,
days to years, and that erupted in many small-volume erup-
tions” (Lorenz 2007; Németh et al. 2010). However, a large
number  of  volcanoes  traditionally  viewed  as  monogenetic
seem to be actually complex volcanic edifices and their erup-
tion  histories  are  defined  by  multiple  eruptive  phases  or  is
even  polycyclic  and/or  polymagmatic  in  nature  (Brenna  et
al. 2010; Kereszturi et al. 2010; Needham et al. 2011). Vol-
canoes of this subtype can form well-distinguished volcanic
units generated by distinct eruptive phases or even by sever-
al  eruptive  episodes  (Németh  et  al.  2010).  Fragmentation
styles (e.g. phreatomagmatic vs. magmatic) can change dra-
matically  during  complex  monogenetic  volcano-forming
eruptions  and  are  commonly  associated  with  lateral  and/or
vertical  vent  migrations  (Auer  et  al.  2007;  Ort  &  Carrasco-
Nú

ez 2009). In addition eye-witness accounts and geologi-

cal records of recent maar-forming eruptions support such
vent migrations (Németh & Cronin 2011). The variability of
these processes can significantly control the architecture and
the shape of the resulting volcanic edifice.

536

KERESZTURI and NÉMETH

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GEOLOGICA CARPATHICA, 2011, 62, 6, 535—546

The primary goal of the present research is to reveal the

fundamental  role  of  eruption-related  processes,  such  as
changing fragmentation styles and vertical/horizontal move-
ments of the explosion locus in the course of the evolution of
a monogenetic volcano referred to here as the Kopasz-hegy
Volcanic  Complex  (KVC;  Fig. 1)  in  the  central  part  of  the
BBHVF.  The  main  characteristic  of  the  K

VC is the signifi-

cantly different shape that makes it unique within the BBHVF.

The KVC edifice closely resembles a volcanic chain or a
large phreatomagmatic volcano formed through a complex
eruption series from several eruptive vents.

Inspite of the young age of the KVC (around 2.8—2.5 Ma;

Balogh K. pers. comm.), its inner volcanic architecture and di-
verse pyroclastic successions are relatively well-exposed

along erosional escarpments in a circular array that provides
an excellent insight into the volcanic evolution of a complex,
“chain-like” eruption center.

Geological background

The KVC is located at the western boundary of the Kál

Basin (Fig. 1). The basement of this part of the Bakony Mts
and Balaton Highland mostly consists of Devonian schist
(e.g. Lovasi Schist Formation), Permian red sandstone (e.g.
Balaton Highland Sandstone Formation) and Triassic marl,
dolomite (Budai & Csillag 1998; Budai et al. 1999; Csillag
1999, 2003; Fodor et al. 2002). The Kál Basin is character-

Fig. 1. The location of the Kopasz-hegy Volcanic Complex in the BBHVF and its simplified geological map. Note: the stars are the
measured sections.

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ized  by  highly  degraded  syncline  and  anticline  structures
comprised  of  the  aforementioned  rocks  (Csillag  2003).  Ac-
cording to Csillag (2003), an anticline structure that formed
during  early  Alpine  structural  movements  from  the  Creta-
ceous to Middle Eocene is located directly below the volca-
nic  remnant  of  the  KVC.  Overlying  these  old  structures
unconsolidated  and  porous  Upper  Miocene  (Pannonian
stage) siliciclastic sediments from Lake Pannon can be found
(Budai et al. 1999; Csillag 2003). Their sedimentation ended
around  8 Ma  ago  in  the  study  area  (Magyar  et  al.  1999).
Southward  progradation  of  rivers  gradually  filled  the  shal-
low basin of the late stage Lake Pannon and transformed the
area into a broad coastal plain with rolling hills and streams
contained in longitudinal valleys. This environment provid-
ed  abundant  ground  and  surface  water  along  N-S-trending
stream valleys for the small-volume intracontinental basaltic
volcanism (Martin & Németh 2004).

According to K-Ar and Ar-Ar radiometric dating (Balogh et

al. 1986; Balogh & Pécskay 2001; Balogh & Németh 2005;
Pécskay et al. 2006; Wijbrans et al. 2007; Balogh et al. 2010),
the alkali basaltic volcanism was active over a period of nearly
6 Myr (Szabó et al. 2004). The onset of volcanism was about
~ 7.94 Ma at the Tihany Peninsula (Balogh & Németh 2005;
Wijbrans et al. 2007) while its last activity occurred about
~ 2.29 Ma at Bondoró (Balogh & Pécskay 2001; Kereszturi et
al. 2010). Volcanism in the BBHVF is characterized by a low
magma output rate, around 0.5 km

/Myr, combined with tec-

tonically controlled behaviour (Kereszturi et al. 2011).

Recent studies revealed that the volcanism of the BBHVF

was typical of monogenetic volcanic fields that erupted in en-
vironments  with  high  external  water  availability  and  which
were  producing  various  types  of  phreatomagmatic  volcanic
landforms such as maars and tuff rings that are commonly as-
sociated  with  scoria  cone  forming  events  in  their  late  stage
evolution (Németh et al. 2001; Martin & Németh 2004; Auer
et al. 2007; Csillag et al. 2008).

According to new K-Ar age determinations by Kadosa

Balogh (pers. comm.), the KVC was emplaced during the late
phase of volcanic activity of the BBHVF between 2.82 ± 0.36
and 2.59 ± 0.12 Ma ago. The KVC is composed of the erosion-
al  remnants  of  two  N-S  aligned,  oval-shaped  (in  map  view)
and  closely  spaced  eruption  centers  (northern  and  southern).
The  alignment  of  the  volcanic  vents  correlates  well  with  the
surface and sub-surface extent of the pre-volcanic siliciclastic
sediments (e.g. unconsolidated sand and silica cemented sand-
stone lenses) of the Kálla Gravel Formation that formed with-
in  a  N-S  elongated  paleovalley  system  during  the  Miocene
and/or Pliocene (Bence et al. 1999). Furthermore, the nearby
Kopácsi-hegy (about 1 km toward the NE; Fig. 1) with its bas-
al  maar  with  intra-crater  scoria  cone  and  valley  filling  pyro-
clastics  flow  deposits  inferred  to  be  initiated  from  this
phreatomagmatic  volcano  also  show  a  similar,  N—S-trending
elongation  (Németh  &  Martin  1999).  Two  other,  previously
identified  nearby  eruptive  centers  (Kopasz-hegy  north  and
south;  Fig. 1  and  Fig. 10)  and  a  third  vent  called  Harasztos
just south of the KVC may also be part of the aligned vents of
the KVC. On the basis of scattered surface deposits of unsort-
ed accidental lithic fragment and chilled volcanic juvenile par-
ticle dominated lapilli tuffs of Harasztos it is inferred to be a

deeply eroded phreatomagmatic volcano, where diatreme fa-
cies deposits are exposed and cross-cut by N-S trending basalt
dykes (Bence et al. 1987; Németh et al. 2003). In this study,
we focus only on the two northern, amalgamated maar struc-
tures forming the KVC and do not elaborate on the volcanic
relationship between the KVC and the southern vents.

Volcanic architecture

The KVC is an elongated volcanic chain composed of two

erosional volcanic edifices each about 800—1000 m in diam-
eter (Fig. 1). The highest point of the edifice is 300 m a.s.l.
at the northern part of KVC with its elevation systematically
decreasing  towards  the  south.  Along  the  present  erosional
remnant of the KVC, we examined seven outcrops in detail
(Fig. 1). The eruption sequences of KVC are represented by
14 sedimentary facies that are separated on the basis of bed-
ding characteristics, structures, grain-size, and composition.
The  terminology  of  pyroclastic  deposits  such  as  ash,  lapilli
and  blocks  is  based  on  Fisher  &  Schmincke  (1984)  and
White & Houghton (2006). According to Ingram (1954) the
bed  thickness  is  defined:  thinly  laminated  < 0.3 cm;  thickly
laminated 0.3—1 cm; very thinly bedded 1—3 cm; thinly bed-
ded  3—10 cm;  medium  bedded  10—30 cm;  thickly  bedded
30—100 cm and very thickly bedded  > 100 cm.

Northern edifice

Medium bedded lapilli tuff (LT1)

Description: This facies LT1 is widespread in the outcrops

of  the  KVC,  mostly  in  proximal  position  just  about  300 m
from the eruptive center. However, this facies type can be oc-
casionally identified in more distal position up to 800 m away
from  the  eruptive  center  (Fig. 1).  These  layers  are  a  few  dm
thick  and  mostly  composed  of  grey  to  brownish  lapilli  tuff
(Figs. 2  and  3).  The  massive  lapilli  tuff  shows  a  high  abun-
dance of angular to sub-angular, accidental lithic clasts (cm to
dm  sized)  sourced  from  the  underlying  country  rocks,  espe-
cially of sandstone fragments from the Pannonian sediments.
Some  of  them  have  a  mm-wide  chilled  margin.  The  upper
parts of the outcrops comprise more Permian sandstone frag-
ments  than  the  lower  parts.  Below  the  larger  lithic  blocks,
there  are  no  impact  sags  in  these  beds.  Individual  layers  are
parallel bedded and normal graded. Cauliflower bombs occur,
but they are rare.

Interpretation: The location at the base of the succession,

bedding structures and the abundance of accidental lithics in-
dicate that these beds were formed during a vent clearing
event that were triggered by phreatomagmatic eruptions
(Lorenz 1986).

The contact zone between the pre-eruptive sediment and

the pyroclastic rocks of the KVC has not been exposed, but
Pannonian  sand  and  sandstone  outcrops  can  be  found  near
the  base  of  the  KVC  about  200—210 m  a.s.l.  (Budai  et  al.
1999). This evidence implies that the bottom of the outcrop
( ~ 220—230 m  a.s.l.)  is  close  to  the  very  basal  part  of  pyro-
clastic  succession.  The  abundance  of  Pannonian  sandstone

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lithic  fragments  suggest  that  these  explosions  occurred  in
this rock unit along a hydraulically active fault system. The
increasing  amount  of  fragments  from  deep  seated  rocks  in
higher  sections  in  the  pyroclastic  sequence  indicates  the
downward  migration  of  the  explosion  chambers  in  time
(Lorenz 1986; Lorenz & Kurszlaukis 2007). Lack of impact
sags beneath larger clasts and the bedding characteristics of
the  pyroclastic  rock  units  suggest  that  larger  blocks  were
transported by relatively dry, pyroclastic density current in a
lateral direction (Sohn & Chough 1989).

Thinly laminated tuff (T1) and

lapilli stone (LS1)

Description: This facies (T1) is

exposed  in  the  eastern  slope  of  the
KVC (mostly in the Section 2 in the
Fig. 1). It consists of white to grey,
thinly  laminated  coarse-  and  fine-
ash  beds  (Figs. 2,  3  and  4).  It  is
commonly  parallel  bedded  and
sometimes  cross-laminated  without
any  larger  blocks,  impact  sags  or
any  eye-visible  accidental  lithic
clasts.  However,  the  matrix  com-
prises  abundant  particles  derived
from siliciclastic sediments. The max-
imum  thickness  is  about  20—30 cm.
This facies is mostly intercalated by
medium  and  very  thinly  bedded
(LT1 and LT2) and undulating bed-
ded  lapilli  tuffs  (LT3).  Upward  in
the  section  (Fig. 2),  the  facies  T1
gradually  becomes  a  more  lapilli-
dominated,  but  still  relatively  well-
sorted bed (facies LS1).

Interpretation: The size of the

pyroclasts  (mostly  ash  and  lapilli-
dominated LS1 facies) and its cross-
laminated  nature  suggest  that  this
facies  may  have  been  produced  by

Fig. 2. Log of Section 2 (GPS coordinates: N 46°52

’25” and E 17°32’51”) and Section 4

(GPS coordinates: N 46°52

’21” and E 17°32’52”) exposed on the eastern slope of the KVC.

base surge rather than fallout (Sohn & Chough 1989). The fine
grain size (i.e. lack of visible lithics) may represent highly ef-
ficient  explosive,  phreatomagmatic  fragmentation  of  the  as-
cending  magma  under  relatively  ‘optimal’  magma  to  water
ratio to gain maximum explosive energy release (Wohletz &
Sheridan 1983). The source of external water is inferred to be
the slightly water-saturated Pannonian sediments close to the
surface as the deposit contains a large number of fine-grained
particles from the pre-volcanic succession (Martin & Németh
2004).  This  could  also  be  a  result  of  the  relatively  shallow
level  of  magma  fragmentation  where  magma  interacted  with
“soft substrate” hosted aquifer. Both, the lack of impact sags
and any deformation of the beds as well as the sharp bedding
contact suggest that the depositional system was relatively dry
(i.e. no excess water/moist in the system to cause plastic bed
deformations). The lack of impact sags demonstrates that the
country  rocks  were  easy  to  fragment,  friable  and/or  poorly
consolidated.

Very thinly bedded tuff (T2) and lapilli tuff (LT2)

Description: These two facies occur in the most distal posi-

tion only on the northern slope of the KVC (Section 1 in the
Fig. 1). The very thinly bedded lapilli tuff (LT2) displays al-
ternating layers of coarse ash and lapilli tuff with mostly nor-
mal grading (Figs. 2 and 4). The thickness of individual layers
is about several cm to few dm. The very thinly bedded tuff
(T2) comprises more ash than the LT2 (Fig. 4). Both facies
LT2 and T2 are grain-supported.

Fig. 3. Lithofacies T1 intercalation between LT1 in the Section 2 in
the northern part of KVC. The arrows indicate small displacement
along a possibly syn-eruptive fault.

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Fig. 4. Field photo and simplified sedimentary log of distal phreatomagmatic units of the KVC
in the Section 1 (GPS coordinates: N 46°52

’34” and E 17°32’30”). Legend is in the Fig. 2.

Fig. 5. Closer view of an increased scoriaceous-rich lapilli bed (A)
and the overview photo of the transitional layers exposed in the
Section 4 (B).

Interpretation: Facies LT2 is interpreted as the result of a

low concentration, dry base surge based on the grading and
the segregation of lapilli and coarse ash (Sohn & Chough
1989). Facies T2 may represent the subsequent fall-out from
an elutriation ash-cloud related to this turbulent and low con-
centration base surge (Sohn & Chough 1989).

Undulating bedded lapilli tuff (LT3)

Description: Undulating bedded lapilli tuffs (LT3), in

general, occur in distal positions in the northern slopes of the
KVC  (Fig. 3).  Facies  LT3  is  situated  on  the  top  of  the
phreatomagmatic  rock  units  exposed  at  the  base  of  the  out-
crop.  These  layers  are  characterized  by  slightly  undulating
and  normal  graded  bedding  structures  and  rare  block-sized
fragments.  LT3  is  comprised  of  homogeneous,  grey  to
brownish  tuffs  and  lapilli  tuffs.  Individual  layers  are  a  few
cm to dm thick.

Interpretation: This facies may be interpreted as a lateral

equivalent of the LT1, and inferred to be generated by rela-
tively ‘dry’ base surges on the basis of its sharp, stratified and
normal  graded  style  of  bedding.  In  addition,  the  undulating
appearance  of  facies  LT3  is  also  in  agreement  with  a  pyro-
clastic  density  current  depositional  origin  (Crowe  &  Fisher
1973). This facies is inferred to have been deposited within
the N—S paleovalley but at a significantly different distance
from  the  source  vent  and  in  a  different  position  within  the
paleovalley than facies LT1. The undulated bedding charac-
teristics indicate that the pyroclastic density current was low
in particle concentration (Vazquez & Ort 2006). Such condi-
tions  are  expected  along  the  shoulder  of  a  valley  thus  LT3

most likely represents the pyroclastic
deposits  accumulated  in  an  ‘over-
bank’ location in medial or distal po-
sition from the vent, similarly to the
phreatomagmatic  pyroclastic  flow
deposits  of  Kopácsi-hegy  nearby
(Németh & Martin 1999).

Medium bedded, scoria-rich lapilli

tuff (LT4)

Description: This facies mostly

crops  out  in  the  upper  part  of  the
northern  volcanic  remnant.  Facies
LT4  is  richer  in  juvenile  fragments
than  LT1.  In  general,  these  juvenile
fragments  vary  in  vesicularity  (from
poor  to  moderate),  and  they  are
mostly grey to black or reddish in co-
lour (Figs. 2 and 5). Individual layers
of facies LT4 are a few dm in thick-
ness and show normal grading.

Interpretation: The stratigraphic

position indicates that this facies was
deposited during the late stage of the
eruptive sequence and is the result of
explosive eruptions with transition
from phreatomagmatic to magmatic

fragmentation of the rising magma. Such fragmentation style
changes are inferred to be controlled by near surface geolog-
ical  conditions  such  as  the  water  supply  fluctuations  or  a
variable  magma  supply  rate  into  the  root  zone  (Lorenz  &
Kurszlaukis  2007).  The  magma  fragmentation  responsible
for the formation of LT4 is interpreted as the result of dryer
but still phreatomagmatic eruptions. This facies is a precur-
sory facies followed by subsequent deposition of more mag-
matically fragmented pyroclastic deposits higher in the rock
sequence (Fig. 2).

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Thickly bedded, scoriaceous pyroclastic breccias (PB1)

Description: Scoriaceous pyroclastic breccias are wide-

spread in the upper section of the edifice (Figs. 1, 2 and 6).
Layers  of  these  facies  are  poorly  defined.  It  is  generally  a
succession of a light grey to black, highly to moderately ve-
sicular basaltic breccias and rarely lapilli which are interca-
lated  with  thin  lava  layers  (Facies  LR1;  Fig. 6).  These
deposits are highly to moderate welded and agglutinated and
form  four  mound-shaped  hills  on  the  top  of  the  northern
eruption  center  that  breached  toward  the  south  (above
~285 m a.s.l.; Fig. 1).

Interpretation: On the basis of textural (e.g. high vesicu-

larity)  and  bedding  characteristics,  we  interpret  these  pyro-
clastic rocks to be eruptive products from weakly Hawaiian to
Strombolian-style  eruptions  (Vespermann  &  Schmincke
2000; Agustín-Flores et al. 2011) in the final stage of the ac-
tivity of the KVC. The distribution of facies PB1 reveals that
these eruptions produced a scoria cone over the basal shallow
maar crater that was surrounded by a low tephra ring. In spite
of its young age, the scoria cone itself is characterized by poor
preservation  (i.e.  almost  invisible  morphology  in  the  field,
breaching  of  the  crater),  which  can  indicate  some  degree  of
truncation during syn- or post-eruptive time.

Lava rocks (LR1 and LR2)

Description: Two types of lava rocks have been identified

at the KVC (LR1 and LR2). Facies LR1 comprises a coherent
texture but shows still recognizable few cm- to dm-sized clast
outlines  suggesting  that  the  rock  was  originally  fragmented.
Most  of  these  facies  dip  toward  the  center  of  the  preserved
volcanic edifice (Fig. 1). Vesicles are rare. This facies occurs
only in the Section 5 in the SE part of the KVC. Facies LR2 is
characterized by a grey to black colour, and a moderately ve-
sicular  coherent  texture  with  rare  bomb  and  block-rich  hori-
zons (Fig. 7). Generally these rocks form an outward dipping
(about 5—10 degree) layered rock unit. A small quarry exposes
LR2 in the eastern edge of KVC (Fig. 1).

Interpretation: Facies LR1 and LR2 are inferred to have

resulted from short-lived Hawaiian-type lava fountain erup-
tions (Head & Wilson 1987). According to the stratigraphi-
cal position, facies LR1 accumulated between the foot of the
scoria cone and the inner crater wall of the tephra ring sur-
rounding  the  shallow  maar  crater.  The  ballistically  ejected
spatter  was  large  and  the  magma  output  rate  was  high
enough to retain the heat of lava clots upon landing and for
quick  accumulation  to  form  thin  rootless  lava  flow(s).  In
contrast, the gently outward dipping of the facies LR2 indi-
cates that this facies was accumulated on the outer flank of
the scoria cone as a small-scale spatter-fed lava flow.

Southern edifice

Disorganized lapilli tuff (LT5) and tuff breccias (TB1)

Description: This facies (LT5) is widespread around the

southern vent (Section 6 in the Fig. 1). This facies is mostly
massive, but sometimes weakly bedded, poorly-sorted, nor-

mal graded and matrix-supported. The rock contains grey to
brownish and yellowish lapilli with common block of basaltic
cognate xenoliths (LT5; Fig. 8). The accidental lithic clasts
are heterogeneous in composition and in size (up to a few
dm in diameter) and include minor portions of Permian red

Fig. 6.  Capping  scoria  cone  units  with  dominant  highly  vesicular,
black  tuff  breccias  (PB1)  intercalated  with  small  lava  horizons
(LR2)  in  Section 3  (GPS  coordinates:  N 46°52

’18” and

E 17°32

’48”).

Fig. 7. Field photo of a small-scale rootless lava flow (in the Sec-
tion 5; GPS coordinates: N 46°52

’10” and E 17°32’48”) generated

by an intermittent Hawaiian-stage in the eruption history of the KVC.

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sandstone,  Pannonian  sandstone,  sub-rounded  pebbles  of
quartz  and  angular  basaltic  clasts.  This  outcrop  contains  a
large  proportion  of  basaltic  and  Devonian  schist  fragments
(Fig. 8).  On  occasions  these  accidental  lithics  are  enclosed
by  a  thin,  chilled  basaltic  rim.  The  largest  fragments  of  the
lava  clots  reach  a  diameter  of  up  to  60—80 cm.  In  addition,
few  mantle-derived  xenoliths  (lherzolit)  and  amphibole  xe-
nocrysts a few mm in size have been recognized.

The TB1 is often normally graded and contains fine, ma-

trix-poor  tuff  breccias  (Fig. 8).  The  main  characteristics  of
LT5 and TB1 are the presence of lapilli and block-sized co-
herent basalt fragments up to  ~ 60—80 cm in diameter. These
blocks are mostly black, angular, many of them with signifi-
cant surface cracks and rarely welded and vesiculated in tex-
tures. The geographical distribution of these basalt lithics is
limited to the rim of the southern eruption center. Neverthe-
less, in a more distal position (800—900 m from the inferred
source vent) only a few cm-sized particles can be seen in the
pyroclastic  units.  The  facies  LT5  occasionally  contains
rounded  boulders  of  Permian  red  sandstone.  A  few  meters
below  the  Section 6  a  small  dyke  with  a  thickness  of  about
30—50 cm  is  exposed  (Fig. 8).  This  dyke  intruded  into  the
previously  described  pyroclastic  rock  unit  and  has  a  sharp
margin with the fragmented host rocks. The dyke dips steep-
ly toward the interior of the southern eruption center.

Interpretation: Like the northern eruption center, the

southern  vent  was  also  formed  by  phreatomagmatic  erup-
tions,  which  produced  the  LT5  and  TB1.  Evidence  for  this
type  of  eruptions  includes  the  fine,  matrix-supported  do-
mains and the weakly bedded appearance of the pyroclastic

successions rich in glassy juvenile pyroclasts
and abundant accidental lithic fragments. Due
to differences in the facies architecture of the
pre-volcanic  country  rock  assemblages  the
volcanic processes were inferred to be slight-
ly different from the northern vent as it is in-
dicated  by  the  great  variety  of  accidental
clasts  including  the  dominant  basaltic  and
schist fragments. This abundance of acciden-
tal  lithic  clasts  from  various  country  rock
sources supports the model of subsurface ex-
plosive  eruptions  that  excavated  and  mixed
these lithologies. These lithics such as Permian
red  sandstone  are  sometimes  rounded  to  sub-
rounded,  and  are  inferred  to  have  originated
from  a  N-S  aligned  active  stream  valley  in
which  both  the  northern  and  southern  vents
erupted.  In contrast, large basaltic blocks were
formed by magmatic fragmentation without in-
teraction with external water.

The coarse fragmentation of magma, the

matrix-poor  texture  of  the  preserved  pyro-
clastic rocks and the high abundance of mag-
matic clots exposed around the southern vent
suggest that the available water supply in the
capping  Pannonian  sediments  was  limited
(i.e. it had a variably low groundwater reflux
into  the  root  zone).  In  addition,  a  significant
proportion of those basaltic fragments show a

Fig. 8. Section 6 (GPS coordinates: N 46°52

’00” and E 17°32’18”) exposed near

the “overlapping” part of the KVC and expose weakly to massively bedded LT5 and
TB1 with high proportions of Devonian schist from the underlaying strata and basal-
tic fragments from the simultaneously active scoria cone forming eruptions of the
northern eruption vent, respectively. Legend is in the Fig. 2.

well-developed  crack  system  on  the  surface  suggesting  that
the  fragments  suffered  rapid  cooling  during  the  eruption.
The  origin  of  large  basalt  fragments  could  originate  from  a
simultaneously active nearby northern vent.

Massive bedded (LT6) and slightly undulating lapilli tuff

(LT7)

Description: Both of these facies are exposed only in the

southern  edge  of  the  KVC  and  comprise  massive,  poorly
bedded (LT6) and undulating, thinly stratified (LT7), normal
graded, matrix-supported, grey to yellow lapilli tuff with rare
large blocks (Fig. 9). This part of the pyroclastic succession
of  the  KVC  contains  accidental  lithic  clasts  (i.e.  basalt  and
Triassic marl fragments), significantly smaller in size (only a
few  cm)  and  better  sorted  compared  to  the  LT5.  Some  ash
laminas show “mud cracks”.

Interpretation: LT6 and LT7 are inferred to be the same

layers  of  pyroclastic  units  that  were  observed  in  the  Sec-
tion 6, but in probably more medial to distal positions from
the vent. Their matrix-supported appearance and their slightly
undulate  and  sometimes  thinly  stratified  bedding  suggest
deposition from pyroclastic density currents (Vazquez & Ort
2006). But this pyroclastic rock unit is located in the south-
ern extremity of the KVC, and has no cross- or dune-bedded
layers, so therefore its depositional environment is probably
situated closer to the rim of the maar crater of the southern
vent system. The presence of mud cracks in the fine-grained
tuff  beds  suggests  short  (minutes  to  hours  or  even  days)
pauses in the volcanic activity.

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3D distribution of facies associations

The KVC consists of two, relatively small (600—800 m in

diameter) nearly circular shaped (in map view) volcanic edi-
fices, the northern and southern eruption centers (Fig. 1). The
fundamental  problem  with  respect  to  the  evolution  of  the
KVC  is  the  proximity  of  these  two  eruption  centers  to  each
other  both  from  a  temporal  (shown  by  K-Ar  ages  range  in  a
narrow  time-frame  from  2.82 ± 0.36 Ma  and  2.59 ± 0.12 Ma)
and a spatial point of view (the two edifices are coalescing and
form a narrow volcanic chain from N to S). This chain-like ar-
rangement is not as common in the case of maar diatreme vol-
canoes, as they are among monogenetic scoria cones that form
narrow volcanic chains commonly along fissures with several,
relatively small volcanic edifices, for example the Mt Amaril-
la  chain  in  Tenerife,  Canary  Islands  (Clarke  et  al.  2009).
Chain-like  phreatomagmatic  volcanoes,  however,  are  known
from  regions  where  structurally  strongly  controlled  and  frac-
tured deep rock units are covered by unconsolidated water-sat-
urated  porous  media  (sand,  silt,  tillite)  such  as  is  the  case  in
the Pali Aike Volcanic Field in Argentina (Ross et al. 2011) or
the  Hverfjall  eruptive  fissure  in  Iceland  (Mattsson  &
Höskuldsson 2011).

On the basis of our study, we present the distribution of

the major types of deposits including basal phreatomagmat-
ic, transitional as well as capping Hawaiian/Strombolian-

type pyroclastic rocks in the KVC. Both the northern and the
southern  eruption  centers  are  characterized  by  basal  pyro-
clastic rocks representing pyroclastic deposits originally de-
rived  from  pyroclastic  density  currents  and  subsequent
ash-falls  that  are  preserved  mostly  in  proximal  and  rarely
medial/distal  positions.  The  proportion  of  scoriaceous  frag-
ments  is  increasing  upward.  The  basal  pyroclastic  rocks  on
the southern side of the edifice are mostly built up by unbed-
ded or weakly bedded pyroclastic units with a great variety
of  accidental  lithics  including  Devonian  schist,  Permian
sandstone, Pannonian sand/sandstone and Triassic marl. The
most significant feature of the southern eruption center’s py-
roclastic  successions  (mostly  in  proximal  positions  to  the
vent) is the abundance of basaltic fragments from a few cm
up to 1 m in diameter (Fig. 8). The origin of these fragments
cannot  be  explained  by  applying  a  purely  transitional  erup-
tion (as the case of the northern edifice), because in the case
of the southern eruptive center, we have not found any sign
of  gradual  changing  of  textural  characteristics  (increasing
number  of  vesicle-rich  lapilli)  similar  to  those  in  structures
such as are preserved in the northern eruption center. In the
lithofacies LT3 and LT4 the increased proportions of scoria-
ceous  lapilli  and  blocks/bombs  can  be  found  as  a  sign  of  a
change  in  the  style  of  eruptions.  The  northern  side  of  the
KVC hosts a deeply truncated and degraded scoria cone rem-
nant  which  is  only  preserved  in  the  form  of  four  small
mounds (Fig. 1). The inferred location of the crater is proba-
bly  in  the  center  of  the  northern  edifice  surrounded  by  the
abovementioned small mounds (Fig. 1). The crater is signifi-
cantly breached towards the south.

Briefly, this proximal location of the eruption centers may

have caused several anomalies in the architecture of both
phreatomagmatic volcanoes and in the architecture of the
northern capping scoria cone.

Discussion

Shallow-seated geological structure beneath the Kopasz-
hegy Volcanic Complex

The pyroclastic successions exposed in the examined out-

crops contain a high abundance of accidental lithic fragments
that were derived directly from the underlying pre-eruptive
formations. A similar diversity of country rock xenoliths was
documented by Auer et al. (2007) from the adjacent volcanic
complex of Fekete-hegy, but the Fekete-hegy covers a signifi-
cantly larger area and is located on a large fault system.

In the case of the KVC, these accidental lithic fragments are

comprised of Devonian schist, Permian red sandstone, Trias-
sic marl and Pannonian sandstone. They vary gradually from
the northern eruption center to the southern one (i.e. lack of
Triassic marl in the northern edifice). In addition, a recent de-
tailed volcano-sedimentary study of a further eruption center,
Harasztos (Fig. 10) also found a high proportion of Triassic
marl lithics and only a low abundance of Pannonian sandstone
(Németh et al. 2003).

However, the distribution and type of dominant lithic frag-

ments show a high variability within a small area (4 km

Fig. 9. Example of distal outcrops (Section 7; GPS coordinates:
N 46°51

’73” and E 17°32’30”) of highly concentrated pyroclastic

density current deposits.

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For  example,  the  pyroclastic  units  of  the  northern  edifice
mostly  contain  Permian  and  Pannonian  sandstone  lithics
while  the  southern  edifice  mostly  exposes  basalt  fragments
and older Devonian schist fragments (up to a few dm in di-
ameter). Based on the distribution, type, dominance and rela-
tionship  to  the  local  and  regional  country  rock  structural
architecture,  we  can  draw  a  model  shown  on  Figure 10  to
characterize  the  hosting  geological  structures  beneath  the
KVC. In general, the Kál Basin as well as the whole region
of the BBHVF is characterized by small-sized anticline and
syncline  structures  of  mostly  Devonian  schist,  Permian  red
sandstone and Mesozoic carbonates (Csillag et al. 1998; Bu-
dai  et  al.  1999;  Csillag  2003,  2004;  Németh  et  al.  2003).
Based on the distinct spatial distribution of accidental lithic
fragments,  we  suggest  a  small-sized  (1 1 km)  anticline
structure  beneath  the  KVC.  This  hypothesis  coincides  with
other  regional  geological  observations  (Csillag  et  al.  1998;
Budai et al. 1999; Csillag 2003, 2004).

Changing fragmentation styles

In the evolution of the KVC a gradual change can be seen in

the nature of volcaniclastic deposits, which suggests a transi-
tion from phreatomagmatic to subsequent magmatic fragmen-
tation styles. However, this change in fragmentation style can
only be seen in the pyroclastic successions of the northern edi-
fice. In this case, the deepening of the conduit system resulted

Fig. 10. General model for the formation of a chain-like, nested monogenetic volcano, the KVC in the BBHVF. Older basement rocks formed
small, anticline and syncline structures capped by thin siliciclastic sediments and rocks of Pannonian age (sand/silt/mud and their diagenized
varieties). The formation of the KVC was characterized by the initial downward movement of the explosion loci (black arrow) as well as the
subsequent lateral migration of the volcanism (dashed arrow) along the boundary of the water-rich, unconsolidated sediment and the water-
poor, older schist and sandstones rocks beneath the KVC and the along the N-S aligned fracture system. The thickness of underlying rocks is
not to scale. Source of geological data: Budai et al. (1999), Csillag (2003, 2004), Csillag, G. (pers. comm.).

in a slight inverse distribution of accidental lithic fragments in
the preserved phreatomagmatic rock units, which indicates a
classical movement of the explosion-locus to a deeper posi-
tion during the course of the volcanic activity (Lorenz 1986;
Lorenz & Kurszlaukis 2007).

In the basal parts of these phreatomagmatic units, fine tuff

and lapilli-dominated layers are common and reflect a higher
degree of fragmentation of the ascending magma. These fine
ash  and  lapilli  intercalations  are  deposited  at  variable  strati-
graphic levels, indicating a complex history of magma—water
interaction over time (Fig. 2). The presence of fine Pannonian
siliciclast  particle-rich  laminated  tuff  (T1)  suggests  that  the
possible  source  of  phreatomagmatic  explosions  was  well-lo-
calized (probably not thicker than 100 m), but water-saturated
during  the  eruptions.  However,  the  diversity  and  complexity
of the phreatomagmatic deposits suggest that the water supply
varied  over  time.  This  intermittent  and  limited  supply  of
groundwater is altogether responsible for the formation of the
capping  scoriaceous  breccias  (e.g.  PB1),  intercalated  with
spatter-dominated units (e.g. LR1 and 2). These conditions al-
lowed  the  construction  of  a  spatter-dominated  scoria  cone
within the previously built maar crater (Fig. 1). Such changes
in  the  fragmentation  and  eruptive  style,  with  no  significant
time break, can be explained due to the gradual drying of the
capping  water-saturated  Pannonian  deposits  and  the  underly-
ing aquitard Permian and Devonian basement. Such situation
is  common  in  phreatomagmatic  volcanic  eruptions  and  has

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even  been  documented  during  the  historic  maar  eruption  of
Nilahue maar, Chile (Müller & Veyl 1957). Alternatively, the
groundwater  ejected  during  a  phreatomagmatic  eruption  was
more than the recharge rate of the porous aquifers, especially
if the volcanic activity was short-lived only (possibly days to
weeks).  The  deep  geological  setting  may  locally  also  have
controlled  the  fragmentation  style,  because  the  karst  water-
rich carbonates were partly missing beneath the KVC that oth-
erwise could have been able to provide substantial and quickly
rechargeable  water  to  fuel  sustained  phreatomagmatism  dur-
ing  the  entire  time  span  of  the  eruption  (Csillag  et  al.  1994;
Budai  et  al.  1999).  Instead  of  carbonates  the  water-poor  and
aquitard Permian sandstone and Devonian schist can be found
beneath the KVC (Budai et al. 1999).

Shallow-seated geological control on vent migration of the
Kopasz-hegy Volcanic Complex

In the KVC, two types of eruption loci migration took

place on the basis of sedimentary evidence (Fig. 10). In the
northern  eruption  center,  the  most  general  eruption  loci
movement  can  be  interpreted  as  downward  migration  of
the root zone of the diatreme as a commonly referred mode
inferred  for  many  maar-diatreme  volcanoes  worldwide
(Lorenz  1986;  Németh  et  al.  2001;  Lorenz  &  Kurszlaukis
2007).  Conditions  that  favour  downward  migration  may
have existed until the explosion loci reached the (physically
as  well  as  hydrologically  different)  older  basement  rocks
(i.e. Permian red sandstone; Fig. 10).

For the lateral vent migration, the Quaternary Tecuitlapa

maar complex (Trans-Mexican Volcanic Belt) is probably the
best recently recognized example (Ort & Carrasco-Nú

2009).  In  the  crater  wall  sequence  of  the  Tecuitlapa  maar
abundant  accidental  lithic  fragments  document  explosive
events  that  “sampled”  various  levels  of  the  substrata  (Ort  &
Carrasco-Nú

ez 2009). The lateral vent migration process at

the Tecuitlapa maar has been explained by the high physical
and hydrological contrast between the underlying unconsoli-
dated and fractured bedrocks providing irregularities of water-
saturation level as well as mechanical character changes of the
country rocks (Ort & Carrasco-Nú

ez 2009). This explanation

can be partly adopted to interpret the lateral migration of the
volcanism documented in the KVC due the very similar geo-
logical conditions (Fig. 10). In the KVC, the upper pre-volca-
nic  deposits  were  loose,  unconsolidated  Pannonian  sand  and
silt and coherent sandstone lenses (Csillag et al. 1998; Budai
et al. 1999; Csillag 2004). In contrast, the deeper seated hard-
rocks such as the Permian red sandstone and Devonian schist
have  different  hydrological  and  mechanical  properties
(Gondár & Gondárné Sőregi 1999; Németh et al. 2001) simi-
lar to the country rocks at the Tecuitlapa maar. Additionally,
the  N-S  aligned  fault-system  beneath  the  KVC  (Fig. 10)  has
also helped the propagation of magma towards the south pro-
ducing a complex, closely spaced phreatomagmatic chain.

Scoria cone breaching caused by vent migration

The shape of a typical scoria cone is commonly character-

ized by some breaching. For example, the shape of 27 % of

the  monogenetic  flank  cones  of  Mt  Etna  is  disturbed  by
breaching (Corazzato & Tibaldi 2006). However, breaching of
a scoria cone is frequently a consequence of the effusion activ-
ity and/or tectonic settings during and in the late stage of the
course of an eruption (Corazzato & Tibaldi 2006) causing raft-
ing  events  that  can  remove  large  sections  of  the  cone  flank
(Németh  et  al.  2011).  Breaching  observed  at  the  KVC  does
not  relate  to  any  lava  flows,  but  may  have  been  associated
with the closely spaced additional eruptions and lateral migra-
tion of the volcanism from N to S. This newly formed vent has
likely effected the growth of subsequent adjacent scoria cones
as  well  as  the  stability  of  existing  landforms  in  the  northern
side of the capping, intra-maar scoria cone.

Conclusion

(1) The eruption history of the Kopasz-hegy Volcanic Com-

plex is characterized by phreatomagmatic eruption periods,
which built up two intercalating maar structures and a capping
intra-maar scoria cone (Figs. 1 and 10). The eruption took
place between 2.82 and 2.59 Ma ago according to K-Ar radio-
metric datings by Kadosa Balogh.

(2) The evolution of the KVC was predominantly

phreatomagmatic in origin, but the late stage eruptions formed
a small scoria cone on the top of the northern part of the com-
plex (Fig. 10). This scoria cone was a result of the changing
fragmentation style from phreatomagmatic to more magmatic
in a relatively short time frame. The probable reason for the
formation of a scoria cone was most likely the local exhaus-
tion of water supply from the Pannonian siliciclastic deposits.

(3) The formation of the younger phreatomagmatic volcano

in  the  southern  edge  of  KVC  was  inferred  to  be  a  result  of
phreatomagmatic  eruption  triggered  by  newly  intruded  mag-
ma  (Fig. 10)  and  the  motion  of  the  explosive  loci  towards
south within the small paleo-valley cut into the thin Pannonian
sediments. The reason for the lateral migration of the volcan-
ism was probably (i) the various hydrological properties of the
underlying basement rocks of the BBHVF that were unable to
support  magma/water  interactions  due  to  their  limited  dis-
charge  rate  (e.g.  the  Pannonian  sediments)  and  aquitard  be-
haviour  (e.g.  Permian  red  sandstone)  and  (ii)  extended  N-S
aligned fault system beneath the complex.

(4) In the case of the KVC, the travel path of small-volume

pyroclastic flow and surges as well as the direction of the lat-
eral migration of the volcanism are significantly governed by
the alignment of a N-S trending paleo-valley, which hosted
and controlled the entire formation of the eruptive vents.

(5) The scoria cone breaching was closely related to erup-

tions of the southern edifice. The magmatic fragments in the
pyroclastics succession were probably derived from the coeval
erupting  scoria  cone  vent  of  the  northern  edge  of  the  KVC.
These  simultaneous  magmatic  explosions  of  the  northern
scoria cone may have partly fuelled with large lava clots and
basalt blocks the concentrated pyroclastic density current as-
sociated  with  the  phreatomagmatism  of  the  southern  vent.
Due  to  the  load  from  the  northern  vent,  these  density  cur-
rents  were  more  concentrated  in  particles  than  pyroclastic
surges in general.

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(6) Magma fragmentation style changes and lateral migra-

tion with phreatomagmatic phases have a significant volcanic
hazard aspects (Lorenz 2007). A change in fragmentation
style can lead to the formation of intra-crater scoria and spatter
cones that can be destabilized by a newly opened gradually or
abruptly shifted phreatomagmatic vent in its vicinity posing
extra and unexpected hazards during a volcanic eruption.

Acknowledgment:  The present research has been funded by
the Department of Geology and Mineral Deposits, University
of Miskolc, Hungary and GK would like to thank the Institute
of Natural Resources (INR), Massey University, New Zealand
for the PhD Research Fellowship they offered. Field work for
KN was supported by the Massey University Leave and Ancil-
lary Appointments’ granted travel fund (LAAC10/37) and an
ISAT/IMF  Hungary—New  Zealand  Science  and  Technology
Cooperation  Fund  (RM14757).  The  authors  would  like  to
thank K. Balogh for the additional K-Ar measurement on sam-
ples collected from the KVC and G. Csillag for the helpful ad-
vice and suggestions in the field. The excellent comments and
the careful review by the Journal Reviewers, V. Lorenz and S.
Kurszlaukis, are also acknowledged.

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