VOLGEO-04579; No of Pages 17
Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Emplacement dynamics of phonolite magma into maar-diatreme
structures — Correlation of field, thermal modeling and AMS analogue modeling data
Prokop Závada a,⁎, Petr Dědeček a, Karel Mach b, Ondrej Lexa c, Marcel Potužák d
a
Geophysical Institute, Academy of Sciences of the Czech Republic, Boční II/1401, 141 31 Prague 4, Czech Republic
Severočeské doly a.s., Doly Bílina, ul. 5. května 213, 418 29, Bílina, Czech Republic
Institute of Petrology and Structural Geology, Charles University,Prague, Czech Republic
d
CORNING Inc., SP-FR-04 R4S12F, Corning, NY 148 31, USA
b
c
a r t i c l e
i n f o
Article history:
Received 28 January 2010
Accepted 18 July 2010
Available online xxxx
Keywords:
phonolite
maar-diatreme
magma emplacement dynamics
thermal modeling
AMS analogue modeling
cavitation
a b s t r a c t
Emplacement mode and original shape and dimensions of a well exposed phonolite body in the České
středohoří Mountains (Czech Republic) were reconstructed using combined techniques of structural analysis
of magmatic fabrics and columnar jointing together with analogue and thermal mathematical modeling of
cooling for different shapes of experimental bodies. Phreatomagmatic rocks in the vicinity of some phonolite
stocks in the area of interest suggest that the phonolite bodies were likely emplaced into maar-diatremes.
Our modeling revealed that intrusion of magma into phreatomagmatic maar-diatreme craters can result in
cryptodomes, extrusive domes, lava lakes or branched intrusions. The fabric and columnar jointing pattern of
the selected phonolite body reveals best fit with an asymmetric extrusive dome emplaced into the maar
crater. The scaling analysis and thermal modeling also suggest that the phonolite extrusion could have
formed within 6–66 days and cooled to the background temperature after 10,000 years. Combined analogue
and thermal modeling also revealed that the phonolite extrusions into maar-diatreme craters are marked by
upper tier (collonade) of vertical columns and lower tier of curved and outward flaring columns. Both tiers in
the phonolite extrusions are divided by a subhorizontal suture.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Phonolite and trachyte bodies form expressive landmarks in
several Tertiary intra-continental volcanic provinces in Europe.
These provinces are associated with the rift systems of Rhein graben
(Germany), Eger rift in Bohemian Massif (Czech Republic) and
Limagne graben (France) that formed due to complex tectonic
processes in the foreland of the Alpine orogeny (Fig. 1). Phonolite
and trachyte magmas were emplaced during the last evolution stages
of these rift systems (Camus, 1975; Kopecký, 1978).
The emplacement mechanism and original shapes of phonolite and
trachyte bodies are often poorly constrained, because of unknown
paleosurface level and poor exposure of the host rocks in their
surroundings, although they likely represent eroded remnants of
cryptodomes, lava domes, laccoliths, lava flows or exposed volcanic
conduits (Varet, 1971; Camus, 1975; Kopecký, 1978; Ulrych et al.,
2000; Lorenz and Haneke, 2004; Závada et al., 2009b). A puzzling
evidence in the phonolite bearing volcanic provinces is that both lowaspect ratio (coulées or thin laccoliths) and high-aspect ratio bodies
(e.g. cryptodomes) can be found, which reflects either different local
⁎ Corresponding author. Tel.: +420 267 103313; Fax: +420 272 761549.
E-mail address: [email protected] (P. Závada).
environment of the magma/lava emplacement (e.g. flat surface versus
volcanic crater) or less likely the large variation in rheological
properties of magma controlled by crystal content and melt chemistry
(Dingwell et al., 1996; Saar et al., 2001; Giordano et al., 2004). Another
unresolved feature of phonolite magma emplacement is that some
individual phonolite bodies merge in plan-view with neighboring
bodies, while others form solitary monuments. Earlier detailed studies
of phonolites and trachytes attempted to define their emplacement
mode using flow kinematics deduced from internal fabrics and
fractures and considered the phonolite bodies as extrusion domes
or “squeeze-up mounds” of highly viscous magma on the paleosurface
(Cloos and Cloos, 1927; Varet, 1971; Jančušková et al., 1992; Arbaret
et al., 1993; Závada et al., 2009b). In some cases, slices of basement
rocks were reported to be uplifted by viscous drag of the magma along
the steep walls of the intrusions (Varet, 1971; Kopecký, 1978).
For a group of phonolite stocks close to Most town in the České
středohoří Mountains in Bohemia (Fig. 1), Kopecký (1985) suggested
that these represent teardrop shaped intrusions into maar-diatremes,
while Fediuk (1985) proposed that these are only remnants of a
differentially eroded coulée of phonolite lava. The first explanation is
supported by outcrops of maar deposits in the vicinity of the phonolite
stocks (Kopecký, 2000). Intrusion of magma into the maar-diatremes
(phreatomagmatic volcanoes) is typical for the final stage of the
phreatomagmatic volcano evolution, when the influx of water
0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2010.07.012
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
2
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
Fig. 1. Geological map and a vertical profile of the studied area (simplified and modified after Kopecký (1991)). Position of the Eger rift in the structure of the European Variscides and
extent of the České středohoří volcanic centre in the Eger graben (simplified after Cajz et al., 1999) is also depicted (RG — Rhein graben, RM — Rhenish Massif, BM — Bohemian Massif,
AM — Armorican Massif, MS — Massif Central). Numbers designate specific phonolite bodies described in the text: 1 — Bořeň body, 2 — Želenický Hill, 3 — Zlatník, 4 — Červený Hill,
5 — Hněvín, 6 — Široký Hill, 7 — Ryzelský Hill, 8 — basanitic maar 1 km S of village Liběšice, 9 — Kaňkov Hill, 10 — A large diatreme crater with a phonolite body revealed by drilling
works during coal mining exploration, 11 — Špičák. Inset of the vertical profile depicts a hypothetical geological setting at the time of the Bořeň phonolite body emplacement.
Volcanic products of the 3rd volcanic phase (Cajz et al., 1999) are not present in the map.
triggering the phreatomagmatic explosions in the root zone of the
volcano is terminated or insufficient (Konečný and Lexa, 2003;
Lorenz, 2003; Martin and Németh, 2004, 2005; Auer et al., 2007).
The mechanical interaction between intruding magma and host maar
sediments or the tuff ring surrounding the phreatomagmatic
volcanoes was documented for basalts by Konečný and Lexa (2003)
and Martin and Németh (2005), respectively. However, possible
intrusion/extrusion shapes and emplacement dynamics of magma
into the weakly consolidated explosive deposits of the maar-diatreme
systems was not yet systematically evaluated, e.g. by numerical or
analogue modeling.
In this contribution, we constrain the shapes and emplacement
dynamics of phonolite bodies emplaced into phreatomagmatic craters
using a combined approach of field analysis of magmatic fabrics and
cooling fractures together with analogue modeling and thermal
modeling of their cooling. At first, we measured the spatial
distribution of fabrics and columnar jointing on exceptionally well
exposed Bořeň u Bíliny phonolite body, which was probably emplaced
into a phreatomagmatic crater. Several analogue models of magma
emplacement into phreatomagmatic craters were then created to test
the similarity of their relative dimensions and internal fabrics with
those on the Bořeň phonolite body. Finally, thermal models based on
the geometry of selected analogue models and equipped with
thermophysical parameters characterizing the rocks in the field
were constructed for comparison of the thermal models and the
thermal structure of the Bořeň phonolite body. The shapes of analogue
models ranged between teardrop shaped cryptodomes to lava domes
with outward flaring thick stems or lava lakes with flat bottom surface
depending mainly on the degree of initial overpressure of the
analogue material and also on the “thickness” of the intruding
material controlled by the mixing ratio of the analogue melt (water)
and analogue crystals (plaster of Paris). Our modeling implies that the
most probable geometry for the Bořeň body is an asymmetrical
extrusion, where the model magma intruded the maar-diatreme
structure through the center of the crater, partially pushed aside the
filling of the maar and extruded and extended laterally by inflation
and diffluence on the crater floor. In some cases, intrusion of the
analogue material branched within the model diatreme and resulted
in two subsequently emplaced extrusive bodies on the crater
periphery. Our results also suggest a new explanation for the
geological origin of both Devils Tower and Missouri Buttes phonolite
bodies (WY, USA), which will be further elaborated in a special paper.
2. Geological setting
The Tertiary volcanic activity in the region of České středohoří
Mountains is associated with the SW–NE trending continental Eger
Rift corresponding to a reactivated first order Variscan tectonic
boundary (Plomerová et al., 2003; Hrubcová et al., 2005. The rifting in
this area was accompanied with mantle upwelling and several
tectonic phases of reactivation of deep seated regional fabrics and
faults (Špičák and Horálek, 2001; Uličný, 2001).
The volcanic activity in the region can be divided into three main
periods (Cajz et al., 1999): (1) low-differentiated, weakly crustalcontaminated upper mantle magmas — basanitic lavas (carrying
lherzolite xenoliths) and volcanoclastics, 36–26 Ma in age, (2) trachybasaltic magmas and volcanoclastics including trachytes and phonolites of bimodal tephrite/basanite–phonolite suite, marked by crustal
differentiation and assimilation of the primary mantle magma and by
the lack of lherzolite xenoliths, 31–25 Ma in age, and (3) flows of
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
basanites, geochemically similar to the first group, dated at 24 Ma. The
first group of volcanic products forms subvertical dykes or lava flows
along the major faults bounding the rift. The phonolites and trachytes
of the second phase intrude the Cretaceous sediments and the
volcano–sedimentary sequences of the first phase along minor faults
within the complex graben structure and are emplaced as shallow
intrusive laccoliths, extrusive domes or intrusions into maardiatremes (Kopecký, 1978; Ulrych et al., 2000). These bodies have
circular or elliptical form in the map and are aligned along the SW–NE
trending faults of the Eger rift system. The volcanic products of the
third phase are not present in the studied area and consist of basanite
lava flows close to Děčín town.
The best exposed phonolite body in the České středohoří
Mountains is the Bořeň u Bíliny forming a prominent landmark in
the vicinity of Bílina town (Fig. 1, body no. 1). The Bořeň phonolite is
classified as a nephelinic phonolite, because it contains abundant
nepheline (up to 5 vol.%) or simply as phonolite according to the USGS
chemical classification (Le Bas et al., 1986; Hrouda et al., 2005). Other
phonolite bodies, Želenický Hill and Zlatník, of similar composition
and texture are aligned behind the Bořeň Hill at a distance of 2 and
3 km, respectively, in the WSW direction. Other three large phonolite
bodies occur further west on the periphery of the Most town. Two of
them, Široký Hill and Hněvín, merge in the map on their circumference. Ryzelský Hill is the largest in areal extent and represents a
solidified coulée of phonolite lava (Kavka, 1981). Other three
subcircular bodies are present east and northeast from Most town;
Špičák (no. 11), Červený Hill (no. 4) and a phonolite plug revealed by
coal exploration drilling works under the Quaternary deposits 1 km
east from Červený Hill (Fig. 1, body no. 10). Other irregular patches of
3
phonolite bodies in the map probably represent remnants of small
intrusions (sills or laccoliths) or lava flows.
On the eastern foothill of Hněvín (Kopecký, 2000) and at the
northern foothills of Zlatník and Červený Hill, phreatomagmatic
deposits are found (Kopecký, 2000). These indicate that the adjacent
phonolite plugs were emplaced into phreatomagmatic volcanoes.
Extensive mantle of phreatomagmatic deposits was also revealed by
drill holes around the plug no. 10 (Fig. 1). Sample of phreatomagmatic
lapilli tuff at Zlatník Hill consists of angular fragments of phonolite,
basement orthogneiss and angular sideromelane shards with vesicles.
The sideromelane fragments indicate that the phreatomagmatic
volcanism in the area was active since the first volcanic phase
(Fig. 2A, C). Phreatomagmatic agglomerate at Hněvín reveals rounded
phonolite clasts with dark chilled margins encased in quartz-rich
tuffaceous matrix (Fig. 2B). Some of the phonolite blocks are up to
50 cm in diameter and some are cross-cut by fractures filled with fine
grained tuff (Fig. 2B). The only lithic fragments in the phreatomagmatic breccia samples are orthogneisses from the basement. Some
parts of the Hněvín outcrop reveal fine-grained phonolitic tuff layers
showing abundant fossiliferous relicts of plant roots that represent
the surficial facies of the maar-diatreme system (Kopecký, 2000;
Lorenz and Kurszlaukis, 2007). Phreatomagmatic tephra at Červený
Hill is formed by unconsolidated white sand to gravel sized material
with local rounded clasts of orthogneisses.
The presence of phreatomagmatic breccias in mantles of several
phonolite bodies and also a basanitic maar that was mapped 2 km
SSW from the Bořeň phonolite body (Fig. 1; Kopecký, 1991) suggest
that the area of interest was subjected to vigorous phreatomagmatic
activity during the first two volcanic phases in the Tertiary (Cajz et al.,
Fig. 2. Samples of phreatomagmatic lapilli tuff (A) found on N side of Zlatník Hill and phreatomagmatic agglomerate (B) found on E foothill of Hněvín. Note the chilled black margins
of rounded, fractured and altered phonolite fragments in (B). Modal analysis of traced clasts in a thin-section of the Zlatník phreatomagmatic lapilli tuff reveals predominance of the
volcanic material (C). Smallest fragment traced in the fine-grained matrix has a diameter of 0.2 cm. Scale bar in (A) is 10 cm.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
4
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
1999). Abundance of orthogneiss basement fragments encased in the
breccias also reflects that the phreatomagmatic activity was established in hard-rock environment (Lorenz, 2003). Occurrence of
phreatomagmatic tephra in the vicinity of Zlatník Hill, conspicuous
alignment trend of Zlatník, Želenický Hill and Bořeň phonolites
following the major fault system active in the area during Tertiary
(Kopecký, 1978, 1991) and circular plan-form of these bodies in planview suggest that all these bodies were emplaced into phreatomagmatic volcanoes that formed on a zone of structural weakness with a
joint aquifer (Auer et al., 2007; Lorenz, 2003). This is also supported
by their high-aspect ratio together with exposure of orthogneiss
basement rocks on the western foothill of Bořeň. The mantles of
phreatomagmatic tephra around these bodies can be buried under the
talus of Holocene phonolite block accumulations.
3. Structural and fabric data of the Bořeň phonolite body
The Bořeň phonolite is a circular body in the map with an average
diameter of ca. 500 m. On western side of the body, it is in contact
with basement orthogneisses. Eastern side of the body is surrounded
by thick Quaternary accumulations of blocky phonolite talus and
outcrops of Cretaceous sediments covered by basanite lavas of the first
volcanic phase. Correlation of the exposure levels of Cretaceous
sediments and basanite lavas in the surroundings of Bořeň suggests
that each of these units covering the basement were approximately
100 m thick at the time of Bořeň phonolite body emplacement (see
inset of profile in Fig. 1). In the central-SE part of Bořeň, steep walls
confined by columnar joints encircle an elevated plateau (Fig. 3A),
which forms a natural fortress. NW side of the body is shielded by a
steep wall rising from the lowest exposure level of the body up to the
toe of the fortress, where it forms a small plateau.
The magmatic fabric in the massive and homogeneous green to
grey (altered) phonolite is discernible only locally from the alignment
of tiny (0.5–1 cm) sanidine crystals. No macroscopic fold or shear
structures can be observed on the outcrops. Orthogneiss xenoliths up
to 8 cm in diameter are sparsely distributed in the phonolite. The body
is affected by two jointing systems. Orientation of the first set of joints
is subparallel to the magmatic fabric and defines an asymmetric onion
like internal structure of the cupola (Fig. 3C) well known from other
phonolitic or trachytic bodies (Cloos and Cloos, 1927; Varet, 1971;
Závada et al., 2009b). This system is analogous to flat lying joints (Fjoints) known from high structural levels of plutons (Cloos, 1922).
Second system is represented by columnar joints that define steep
vertical columns up to 3 m wide in the central-SE part of the body
(Fig. 3B, D). Vertical columns are about 30–50 m high and their dip
directions diverge as their dip angle decreases on the periphery of the
body, where the columns are only 0.5–1 m wide (Fig. 3B, D).
Individual joint growth increments typical with transverse bands on
the columns well known from solidified basalt lava flows (DeGraff and
Aydin, 1987) were detectable only in the NW margin of the body. In
this location, joints confining horizontal, 1 m wide and few meters
long columns terminate in the vertical wall of massive phonolite.
Looking on the Bořeň body from the SE on bright day one can see that
the wide and vertical columns encircling the elevated part of the body
are underlain by a set of narrower columns about 20 m below the top
of the vertical walls and 50 m below the summit of Bořeň (Fig. 4). This
discontinuity is interpreted as a suture between the “upper” and
“lower colonnades” (Spry, 1962) that formed during cooling from the
upper and lower boundaries of the original phonolite body. Detailed
relief model of the Bořeň body was created using photogrammetric
technique from aerial photographs and used for plotting and
interpretation of the structural data.
3.1. Microstructural data acquisition
3.1.1. Magmatic fabric and orientation of cavities
The phonolite trachytic texture is defined by parallel alignment of
groundmass fine sanidine laths (20–100 μm) and nepheline columnar
crystals (20–100 μm) enclosing phenocrysts of sanidine, nepheline,
clinopyroxene (aegirine–augite) and sodalite that range in size from
Fig. 3. Contours of elevation in meters above sea level (A), contour diagram of column width (B), contour diagrams with structural marks of dip angle and dip direction of F-joints
(C) and plunge angle and plunge direction of the columns (D) for the Bořeň cupola.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
5
Fig. 4. View on the Bořeň body from the SE revealing the suture between the thick columns of the upper colonnade and thinner and curved columns of the lower colonnade.
0.5 to 1 cm (Fig. 5). The phonolite groundmass is disrupted by
microscopic cavities that are about 0.5–1 cm long and have lenticular
to sigmoidal shapes with smooth or irregular ragged boundaries
(Fig. 5B). Cavities are aligned in a specific direction and are typically
developed in the vicinity of phenocrysts or sometimes even crosscut
the phenocrysts (Fig. 5B). The cavities are similar to those described
by Smith et al. (2001) in dacite lava and are typically filled with
secondary minerals such as zeolite and natrolite. Phonolite from the
Želenický Hill revealed euhedral crystals of aegirine augite growing
into the cavities (Fig. 5C).
For the purpose of microstructural analysis of magmatic fabric, we
at first attempted to employ the AMS (anisotropy of magnetic
susceptibility). Unfortunately, this technique had to be discarded,
because in the phonolite rocks of interest, the AMS has no relationship
with the flow-induced magmatic fabric or this relationship is difficult
to interpret. Microscopic and microprobe analysis revealed that the
main carriers of the magnetic susceptibility are titanomagnetite
grains (10–50 μm) that originated probably from decomposition
reaction of residual amphibole crystals and relatively large inclusions
of magnetite (250–400 μm) in large sodalite crystals. Population of
small titanomagnetite grains in the groundmass is absent in the
studied phonolites, in contrast to other trachytes and phonolites in
the České středohoří Mountains (Ulrych et al., 2000; Závada et al.,
2009b). This can be explained by gravity driven separation of the
magnetite crystal fraction in the magmatic reservoirs before ascent
and groundmass crystallization of the phonolite magma, so that only
magnetite crystals enclosed within light sodalite grains or amphibole
pseudomorphoses were carried up.
The crystallographic preferred orientation (CPO) of groundmass
sanidine and nepheline crystals was measured from oriented thin
sections on a scanning electron microscope CamScan S4 on Faculty of
Sciences (Charles University, Prague). Acquired EBSD patterns were
recorded and indexed using the Channel5 software of HKL Technology
(Schmidt and Olensen, 1989). Crystallographic models of Scambos et
al. (1987) and Simmons and Peacor (1972) were used for sanidine
and nepheline, respectively. Each thin section was calibrated using a
silicone monocrystal and only measurements with angular deviation
below 1 were accepted. CPO was measured at least from 120 crystals
throughout the thin-section in 12 samples located on E–W and N–S
profiles across the cupola. All data were rotated to geographic
coordinates and are presented on lower hemisphere stereographic
projections. Rotations, plotting and calculation of the eigenvalues and
eigenvectors of the CPO data was done using the software developed
by D. Mainprice (ftp://www.gm.univ-montp2.fr/mainprice//CareWare_Unicef_Programs/).
Fabric of the phonolite reveals numerous lenses to sigmoidal
shaped cavities (Fig. 5) that are similar to those described by Smith
(2000) and Smith et al. (2001) in trachyte and dacite. Cavities were
traced and digitized from micrographs in ArcView GIS environment
using extension Poly and statistically evaluated using the PolyLX
Matlab toolbox (Lexa et al., 2005). Orientation of cavities was
reconstructed from their alignment in three mutually perpendicular
thin sections.
3.1.2. Results of microstructural analysis
Preferred orientations of poles to (010) and (001) planes of
sanidine crystals in rocks with trachytic texture reliably indicate pole
to flow planes (magmatic foliation) and flow direction (magmatic
lineation), respectively, of the lava (Závada et al., 2009b). Sanidine
groundmass fabric is typical with well clustered poles to (010) and
girdle-like distribution of poles to (001) planes (Fig. 6). Nepheline
crystals have hexagonal columnar shape and preferred orientation of
their poles to (0001) planes well corresponds with clusters of poles to
(001) planes of sanidines for all but two samples (Fig. 6), therefore
this crystallographic direction of nepheline is also considered as a
good indicator of magmatic lineation.
Diagrams of the measured CPO fabric that were rotated into
geographic coordinates (Figs. 6 and 7) reveal that the magmatic fabric
is subhorizontal in the elevated, southern and eastern part of the
cupola (samples B82, B92, and B94) and at the eastern toe of this
elevated plateau (sample B83). Samples B41 and B43 collected in the
central part of the cupola below the elevated plateau (at the toe of
vertical columns), reveal folded fabric marked by several maxima of
(010) directions (Závada et al., 2009b). The trachyte fabric defined by
alignment of (010) sanidine faces dips vertically on western and
northern part of the body and strikes parallel with the margin
(samples B37 and B97). On northern and southern slope of Bořeň
(samples B65 and B93), the fabric dips at moderate angles to the north
and south, respectively. Eastern slope of Bořeň is marked by
alternating steep (sample B24) and flat fabrics (sample B96).
Magmatic lineations indicated by V1 eigenvectors of poles to (001)
planes of sanidines and poles to (0001) of nephelines trend
approximately in N–S direction in samples collected on the N–S
profile through the cupola (samples B65, B41, B43, B92 (only
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
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P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
shallowly to SW, cavities within the shear zone are steep. Similarly, on
the eastern margin (sample B122), 0.1 cm wide subvertical shear
zones with N–S strike and west side-up sense of shear divide domains
with subhorizontal fabric and cavities along large faces of phenocrysts
dipping shallowly to the east. On one locality on the northern slope of
the Bořeň body (close to sample locality B65, Fig. 7) the trachyte fabric
is disaggregated into separate domains up to several centimeters in
diameter with discordant internal fabrics.
4. Analogue modeling of magma emplacement into
maar-diatreme structures
Fig. 5. Micrographs of the Bořeň phonolite thin-sections from samples collected in the
central (A, sample B43) and western peripheral part (B, sample B96) of the cupola.
Sample from the northern side of the quarry on phonolite body Želenický Hill
(C) reveals cavities with inward grown euhedral aegirine–augites (indicated by
arrows). Note close spatial relationship between the cavities (white) and the
phenocrysts (grey), cavities crosscutting phenocrysts in the lower right corner of
image (B) and sigmoidal cavities in sample B43 (A).
nepheline), B94) and also in its eastern flat part (sample B82). On
northern and eastern margins (samples B37 and B97), lineations are
subhorizontal and parallel to the strike of the vertical fabrics.
Cavities in the phonolite lava typically dip at higher angles than
the sanidine fabric (or in opposite direction for steep fabric in sample
B97) and follow a similar strike direction (Fig. 7). Cavities dip at
moderate to steep angles on the margins of the exposed body and
strike subparallel with the nearest margin. On the periphery of the flat
and elevated part of the cupola (samples B24, B43, B92, and B83),
cavities are sigmoidal, with their central and tail parts oriented at high
angle and subparallel, respectively, to the trachytic fabric. In central
part of the elevated plateau (sample B46), vertical cavities cut through
horizontal fabric and strike E–W.
In southern flat part of the cupola (sample B94), the horizontal
fabric is disrupted by microscopic shear zones dipping shallowly to
the north. While the cavities outside the 0.5 cm wide shear zones dip
Detailed analysis of fabrics and fracture systems throughout the
Bořeň phonolite body motivated the second part of this complex
study, which combines the analogue modeling and finite element
numerical modeling of cooling for selected geometries of magma
intrusions into model maar-diatreme volcanoes. Our earlier studies
revealed that the understanding of internal structure and fabric
pattern of eroded volcanic bodies can be conveniently resolved using
analogue modeling (Závada et al., 2009a,b). Flow induced fabrics can
be analyzed from disrupted color layering of the analogue material
corresponding to flow planes, and AMS fabric, which records the local
deformation/flow shapes, flow planes and flow directions from the
particles of admixed magnetite dust (Závada et al., 2009a).
The Bořeň body represents a remnant of an eroded phonolite body
that was emplaced close to the paleosurface and probably intruded a
maar-diatreme structure. To evaluate the similarity of the analogue
models with the original (Bořeň body), at first, the shape of Bořeň and
its relative dimensions in the probable maar-diatreme crater together
with its internal fabric pattern on NW–SE and SW–NE profiles were
projected over the vertical sections of the sliced analogue models.
Although an unknown volume of the phonolite was already eroded,
the original vertical dimension of Bořeň is indirectly indicated by the
level of the suture between the two collonades of columns that formed
during cooling of the body. This suture should correspond to the halfheight of the original body, if we assume that the rate of cooling from
its top and bottom boundary was similar. In the second step, for
models that fulfill the dimensional preconditions and reveal similar
fabric pattern as the Bořeň body, thermal modeling of cooling was
employed to further test the similarity between the models and the
original. The columnar jointing pattern developed on Bořeň was
compared with the cooling patterns given by the thermal models. A
rough preliminary outline of the original shape for the Bořeň phonolite
can be already estimated, considering the fact that the joints confining
the columns grow perpendicular to the margins of original volcanic
bodies during conductive cooling (Jaeger, 1961; DeGraff and Aydin,
1987). The “inverted fan” like columnar jointing pattern should form
during cooling of a body with flat roof and a base that dips moderately
to the vertical axis of the body. In other words, it should resemble a
body with outward flaring funnel shaped stem and a horizontal roof.
4.1. Experimental apparatus and analogue materials
To model emplacement of magma into maar-diatreme structures,
we employed a modified hydraulic squeezer apparatus used earlier
for modeling of lava domes (Závada et al., 2009a) and plaster of Paris
as an analogue of magma. Plaster suspensions are good analogues for
magmas that consist of low viscosity melt and large volume
proportion of crystals. Suspensions of plaster powder and water are
characterized by similar viscosities when their yield strength is
exceeded regardless of their mixing ratio (Závada et al., 2009a).
However, increasing mixing ratio of the plaster powder and water
(the thickness) is marked by increasing yield strength of the
suspension. Physical characteristics and preparation of the analogue
material with admixed magnetite dust for tracing of AMS fabric and
coloring are described in our earlier experimental work (Závada et al.,
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
7
Fig. 6. CPO (crystal preferred orientation) data for sanidine and nepheline measured from the thin-sections of samples collected throughout the Bořeň phonolite cupola. Contours are
spaced at 0.5 multiples of uniform distribution for (001) direction of sanidine and (0001) direction of nepheline and multiples of uniform distribution for (010) direction of sanidine.
Number of CPO measurements per sample (N) is also indicated. Contours in the map correspond to elevation in meters above sea level. Positions of NW–SE and SW–NE profiles are
shown for later reference.
2009a). During the experiment, the plaster of Paris suspension is
squeezed out from a container at the bottom of the apparatus by a
hydraulic squeezer force transmitted over a steel frame and a loading
board with a central hole (Fig. 8). The plaster then intrudes into a steel
cone that is mounted on a vertical feeding conduit (20 cm long and
4 cm wide) and filled with a mixture of sand and solid angular plaster
chunks (b1 cm in diameter). This loose material represents the
phreatomagmatic lapilli tuff or agglomerate, which is poorly consolidated and agitated several times by subsequent phreatomagmatic
explosions shooting debris jets from the root zone towards the surface
(Lorenz, 1986; Zimanowski, 1998; Ross et al., 2008). The tapering
angle of the cone of 70° was chosen to mimic the diatreme slope in
hard rock environment (Lorenz, 2003; Schulz et al., 2005). Above the
30 cm long cone, a broader crater was modeled from kaoline clay to
reflect relatively smaller dip angle of phreatomagmatic crater in softrock environment (Lorenz, 2003; Auer et al., 2007) like the Cretaceous
marlstones in the surroundings of the Bořeň phonolite body. For five
of the experiments, a “lid” of clay and sand layers was created at the
top of the sand filled steel cone and in the lower part of the broader
crater to study the mechanical effect of stratified maar sediments on
emplacement mode and shapes of the intrusions/extrusions into the
maar-diatremes (Table 1). Experimental runs were controlled
manually keeping the extrusion rate as slow as possible except for
first four models. The last 15 runs were captured as movies on a digital
camera. The measured parameters are the initial load of the squeezer
preceding each experiment, extrusion duration and maximum load of
the squeezer at the end of each experiment. The three last
experiments (D-31, D-32, and D-33) were motivated by the result
of one earlier experiment (D-18), where the intrusion of plaster
branched in the diatreme (Table 1, Fig. 9). In these experiments, the
source container was filled with three different plaster portions of
different mixing ratio. A cylindrical cake of the thick plaster (mixing
ratio of plaster powder:water = 2.5) placed in the middle of the
source container was surrounded by an annulus of more diluted
plaster portion (2.3) and the thinnest (2.2) filling the corners of the
rectangular container outside of the annulus. This setup was designed
to mimic extrusion of magma from a magmatic chamber that is
stratified in terms of increasing crystal content in its shallower levels.
After each experimental run, solidified model bodies were excavated,
sliced vertically and photographed. Outlined geometries of the models
and traces of their flow planes from disrupted colored plaster is
presented in Fig. 9.
4.2. Experimental results
Experiments with relatively thick plaster of mixing ratio 2.5
(weight of plaster powder/water) that were squeezed up rapidly
resulted in emplacement of asymmetric extrusions with a thick stem
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
8
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
Fig. 7. Schematic map displaying the eigenvectors indicating the magmatic lineation from the CPO of groundmass sanidine and nepheline and magmatic foliation from alignment of
(010) planes of sanidine crystals together with preferred orientation of cavities reconstructed from three mutually perpendicular thin-sections. Note that for samples B24, B43, B83
and B92, orientation of both centers and tails of sigmoidal cavities are indicated. For sample B94, CPO data for both the magmatic fabric (subhorizontal) and a discrete shear zone
cutting this fabric are shown.
that have partially pushed aside the clastic filling of the model
diatreme (models D-2 and D-4, Fig. 9, Table 1). Medium initial
overpressure (model D-15) and slow extrusion of plaster resulted in
thick stem in the upper part of the crater and emplacement of two
large extrusive lobes. Experiments without the initial overpressure
and slow pressure build up either failed to extrude or produced single
extrusive dome emplaced close to the periphery of the crater (models
D-7 and D-8, Fig. 9). In one of the slow pressure build-up experiments,
the plaster was apparently stuck in the upper part of the conduit and
suddenly released to inflate above the crater to final diameter of
Fig. 8. Experimental apparatus used for analogue modeling of intrusions into maardiatreme structures. Width and depth of the model diatreme are 22 and 30 cm,
respectively. Liquid and layered plaster is injected into the model diatreme (cone filled
with clastic material) by the force of the hydraulic squeezer through a narrow
cylindrical conduit during the experiment. For some experimental runs, several up to
1 cm thick alternating clay and sand layers were superposed on the diatreme clastic
filling to mimic the layered maar sediments.
40 cm in 1–2 s after the squeezer attained a load of 120 bar (model D11). This body pierced a thin cover of clay and sand layers imitating
the maar sediments. In two experiments, dried clay layers above the
model diatreme worked as a competent “lid” that was only slightly
uplifted by a plaster plug that rose along one wall of the steel cone.
The plaster then extruded laterally from below the rotated lid (models
D-9 and D-12, Fig. 9). In other two experiments, where initial loading
started at 20 bar, a large teardrop shaped cryptodome formed
(models D-6 and D-14, Figs. 9 and 10), which pushed upwards and
sideways the entire filling of the crater and reached an aspect ratio
(height to radius of the dome above the diatreme crater) of 1.3 (model
D-6). Both cryptodomes have similar shapes, although one (D-14) had
to pierce a thin and incompetent lid of model maar sediments. These
experiments, together with experiments D-9 and D-12 lasted for 3–
5 min and were terminated at maximum load of 125 bar.
Extrusions of plaster slurries prepared with mixing ratios ranging
between 2.15 and 2.4 were squeezed slowly, without the initial
overpressure and lasted from 12 to 240 s. The maximum load of the
squeezer at the end of each experiment did not exceed 75 bar.
Experiments with diluted plaster of mixing ratio 2.15 and 2.2 (models
D-16 and D-17) formed a shallow lava lake fed by a narrow conduit
4 cm wide and an extrusion with flat-roofed top and a stem 7 cm
wide, respectively. In another experiment (model D-18, Fig. 9),
growth of one extrusive dome on crater periphery with diameter of
10 cm stopped although the load was continuously increasing and
another dome formed on the opposite side of the crater after some
additional load build up. The newer extrusive dome then grew in
diameter and merged with the first dome. Excavation of the solidified
model then revealed two distinct intrusion paths that bifurcated at
the bottom of the steel cone (model D-18, Fig. 10). Models D-19 and
D-21 (Fig. 9) together with models D-25, D-26 and D-27 prepared
with plaster of mixing ratio 2.3 formed coulées or lava lakes.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
9
Table 1
Description of experimental conditions and results of the analogue models.
Plaster thickness Experiments
Degree of initial load
Duration
Maximum load Description
2.5
2.5
2.5
2.5
2.5
2.5
D-1, D-2, D-3, D-4
D-15
D-7, D-8
D-5
D-6
D-9, D-12
Large (25–50 bar)
Medium (25 bar)
Low or none (b 10 bar)
Low or none (b 10 bar)
Medium (50 bar) for first 3 s
Low or none (b 10 bar)
5s
150 s
150–180 s
180 s
220 s
150 s
50 bar
60 bar
100 bar
120 bar
130 bar
125–150 bar
2.5
2.5
D-11
D-14
Low or none (b 10 bar)
3s
Medium (50 bar) for first 3 s 300 s
120 bar
125 bar
2.15
D-16
None
12 s
50 bar
2.2
2.2
D-17
D-18
None
None
210 s
190 s
75 bar
75 bar
2.3
D-19, D-21
None
60 s,240 s
50 bar
2.3
2.4
2.5 + 2.3 + 2.2
D-25, D-26, D27
D-22, D-23, D-24
D-31, D-32, D-33
None
None
None
90–210 s
70 bar
100–160 s 75 bar
180–240 s 80 bar
Extrusions of all models prepared with mixing ratio of 2.3 were
“molded” by the clastic talus of uplifted crater filling and all resulted in
similar shapes of extrusive domes; the talus of model phreatomagmatic tephra was pushed aside and was partly overridden by the
rising plaster during growth of these extrusions. Last stages of D-22
and D-24 experiments were marked by emplacement of distinct
portions (lobes) of the analogue material like in our earlier
experiments on growth of complex lava domes (Závada et al.,
2009a). Plaster models that pierced through the crater center had
relatively wider stem below the extrusive part (models D-22 and D24, Fig. 9). Three last experiments (D-31, D-32, and D-33), which
imitated extrusion of magma from a stratified magma chamber,
resulted in extrusive domes similar in shape to model D-24 (D-31 and
D-33) or successive emplacement of plaster lobes into the crater from
a thick stem (D-32, Fig. 9).
Thick stem, asymmetric single extrusion lobe
Thick stem, extrusive dome formed by two extrusive lobes
Narrow stem, symmetrical extrusive dome on crater periphery
Narrow stem, extrusive dome
Thick stem, teardrop shaped intrusion-cryptodome
Thick lid (3 layers of dried kaoline 0.5 cm thick interlayered with sand), rotation
of the rigid lid was followed by limited lateral extrusion of analogue magma
Thin lid (3 layers of moist kaoline 0.2 cm thick interlayered with sand)
Thin lid (3 layers of moist kaoline 0.2 cm thick interlayered with sand),
thick stem, teardrop shaped intrusion
Thin lid (3 layers of moist kaoline 0.2 cm thick interlayered with sand),
narrow stem, lid disrupted into rafts floating on a “lava lake”
Extrusion similar in shape to experiment D-24, stem 7 cm wide
Extrusion of two individual extrusive domes on crater periphery, second dome
extruded 25 s after the first dome, inflated and merged with the first dome
Lava lake fed by a narrow stem that pierced the model diatreme at the
periphery of the maar crater
Asymmetrical “coulées” or “lava lakes” fed by stems about 8 cm wide
Extrusive dome fed by a stem 7–12 cm wide
Experiment imitating extrusion of magma from a stratified magmatic chamber,
with thick (2.5) plaster in the center of the source container, surrounded by
annulus of thinner plaster portions — 2.3 and 2.2 on the periphery; extrusive
domes similar to D-24 or successive extrusion of lobes from a thick stem (D-32)
Only models D-22 and D-24 were selected for thermal modeling
and further detailed comparison of their internal fabrics on the basis
of the previously stated preconditions of similarity with the Bořeň
phonolite body. In contrast, models D-15 and D-11 were discarded
due to their unrealistically rapid emplacement, although even these
satisfy the preconditions.
4.3. Internal fabrics in model extrusive domes
In lower parts of the solidified models D-22 and D-24, where plaster
intruded into the cone from the steel conduit, the channel that
transferred the plaster to the higher levels is either locally attenuated
(D-22) or doubly curved (D-24). Both models reveal an upward widening
vertical stem with irregular walls. The upper part of the stems for both
bodies flares into an extrusion above the model maar-diatreme crater.
Fig. 9. Shapes of experimental bodies in vertical sections with contours depicting their internal flow planes as indicated by disrupted color banding. Note that some extrusion shapes
are characterized by two similar experiments depicted by different color contours (D-2–D-4; D-6–D-14; D-7–D-8; D-9–D-12; and D19–D-21). For description of experimental
conditions and development of the experiments see Table 1. Thick red contours for some of the models indicate superposed landscape of Bořeň body above the probable diatreme
crater. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
10
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
Fig. 10. Photographs of selected experimental intrusions into maar-diatreme structures before slicing. Scalebars are 10 cm long. Z and X axes indicate the top and front directions of
the models within the experimental apparatus.
For the AMS analysis, model D-24 was drilled in one vertical
section and model D-22 (Fig. 11) was cut along two vertical and
mutually perpendicular sections. All sections were drilled in a
hexagonal grid of 1 cm spacing, but for both sections of model D-22
only every other core in a row and in every other vertical column was
measured. The core samples are 0.8–1 cm long and have 1 cm in
Fig. 11. Photographs of vertical sections and AMS pattern of the selected analogue models. Landscape profiles through the Bořeň body depicted in Fig. 6 are superposed on both
sections of the D-22 model. Magnetic foliations and lineations represent the K1K2 sections of AMS ellipsoids and K1 directions, respectively. Because magnetic foliations regularly dip
at high angle from the sections, their dip angle is not plotted.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
diameter. The eccentricity (P parameter) and shape (T parameter) of
the AMS ellipsoid can be characterized by the following parameters
(Nagata, 1961; Jelínek, 1981).
P = K1 = K3
T = ð2η2 −η1 −η3 Þ = ðη1 −η3 Þ
where K1 N K2 N K3 are the principal susceptibilities, η1 = ln K1, η2 = ln K2,
η3 = ln K3. The P parameter indicates the intensity of preferred
orientation of magnetite particles in plaster models. If 0 b T b + 1, the
AMS ellipsoid is oblate (the magnetic fabric is planar); T = + 1 means
that the AMS ellipsoid is rotationally symmetric (uniaxial oblate).
If − 1 b T b 0 the AMS ellipsoid is prolate (the magnetic fabric is linear);
T = − 1 means that the AMS ellipsoid is uniaxial prolate. If T = 0, the
ellipsoid is neutral.
The internal concave traces of flow planes envisaged by the disrupted
colored plaster correlate with the strike alignment of magnetic foliations
(Fig. 11). Second section of model D22 also reveals lastly emplaced lobe
forming a bulge on the surface of the body. In the extrusive parts of both
models, the pattern of magnetic foliations and lineations resembles that
in our earlier models of plaster extrusions above a sand layer (Závada et
al., 2009a); magnetic lineations dipping to the center of the bodies at the
base of the extrusions and lineations perpendicular to the vertical
sections in upper parts of extrusive lobes. Contour diagrams of P and
T parameters for both models reveal highly anisotropic (P = 1.24− 1.45)
and oblate (T = 0.4 − 1) fabrics in the central columnar domains
extending to the top of the extrusions and fabrics with low anisotropy
in a narrow domain encompassing the highly anisotropic internal part.
These low intensity fabric domains reveal neutral to prolate shapes with
T ranging from −0.55 to 0.1 and are also locally marked by lineations
with high plunge angles from the vertical section.
5. Columnar joint patterns constrained by thermal
mathematical modeling
Finite element thermal mathematical modeling of cooling was
carried out for models D-22, D-24, and also for model D-6, since a
teardrop shape of the Bořeň body and other phonolite intrusions was
suggested in earlier works (Kopecký, 1978, 1991; see the profile in
Fig. 1). For both models D-22 and D-24, thermal models and analysis
of the thermal structure was done for two perpendicular vertical
sections and compared with the columnar jointing pattern on the
NW–SE and SW–NE sections through the Bořeň phonolite body
(profiles depicted in Fig. 6). Similar comparison was done also for a
three-dimensional thermal model using simplified geometry of
experiment D-22.
All thermal models were provided with thermophysical parameters of rocks collected around the Bořeň body; the basement
orthogneiss, Cretaceous marls, Tertiary basanites, phreatomagmatic
lapilli tuff collected at Zlatník Hill (Fig. 2a) and the phonolite (Table 2).
11
Thermal conductivities and capacities of the samples were measured
in laboratories of Institute of Geophysics ASCR using an instrument,
which is based on optical scanning method (Popov et al., 1999).
Thermal models were constructed using software Comsol and
Fracture (Kohl and Hopkirk, 1995). Initial temperature 1100 °C of
the phonolite bodies in the model was calculated from composition
stability in minerals of the Želenický Hill phonolite by Pazdernik
(1997). Latent heat of the phonolite of 300 kJ (Calkins et al., 2008),
corresponding to the heat released during crystallization of lava, was
also implemented in the model. A layer covering the extrusions about
8–16 m thick is included in thermal models for bodies D-22 and D-24
to account for possible presence of glassy and porous pumice that was
locally found in massive phonolite outcrops on the nearby Kaňkov
phonolite body. We assume that the same porous pumice found on
Kaňkov could have formed a continuous carapace on the top of Bořeň
body, where it worked as an effective thermal insulant impeding the
thermal flow during cooling of the phonolite. Similarly, porous scoria
formed capping units on the peperitic lava lake at Ság-hegy in
Hungary (Martin and Németh, 2004). We provide comparison for the
rate of cooling below the roof of phonolite extrusion in the D-22
thermal with and without the surficial pumice layer. These rates of
cooling are also compared with that above the base of the same body.
To determine the temperature at which the columnar joints in the
phonolite formed, the thermal expansion coefficient was measured
from three 0.8 cm wide and 1 cm long samples drilled perpendicular
to the phonolite magmatic fabric and measured for thermal variation
of thermal expansion coefficient α by Marcel Potužák on LudwigMaximilian University in Münich using Netzsch DIL 402C dilatometer.
The measured curves of all three samples revealed almost identical
ascending trend of α value with increasing temperature and two
distinct peaks at 620 and 920 °C (Fig. 12). The first peak at 620 °C is
interpreted to reflect transient expansion of the sample due to the loss
of water from secondary minerals filling the cavities in the phonolite.
Large increase in thermal expansivity coefficient value indicated by
the peak at 920 °C is considered to reflect the temperature
corresponding to formation of columnar joints in cooling phonolite
lava. To trace the trends of columnar joints that would form during
Table 2
Physical parameters measured for all lithologies used in thermal models; k, cp, α and T
correspond to thermal conductivity, capacity, diffusivity and initial temperature of
specific units at the onset of cooling in the model, respectively. For marlstone,
anisotropy of α and k is characterized by two values, measured in direction parallel and
perpendicular to sedimentary layering, respectively. Thermal capacity of phonolite is
indicated by two values for solid and molten state. The higher value corresponds to
molten state. The “pumice” corresponds to a porous pumice found on the Kaňkov Hill.
Phonolite (1)
Phreat. lap. tuff (2)
Marlstone (3)
Basanite (4)
Orthogneiss (5)
Pumice (6)
k
(W/m K)
ρ
(kg/m3)
cp
(J/kg K)
α × 10− 6
(m2/s)
T
(°C)
1.5
1.25
2.2/1.7
2
3.5
0.8
2600
2200
1600
2900
2600
1600
850–1150
1000
1000
810
850
1000
0.7
0.6
1/0.8
0.9
1.5
0.5
1100
20
20
20
20
20
Fig. 12. Variation of thermal expansion coefficient with increasing temperature
measured on the Bořeň phonolite sample. Peak on the curve at 620 °C is interpreted
to reflect the transient expansion of the sample due to loss of water released from
secondary minerals in the cavities. Second peak at 920 °C is interpreted to reflect the
temperature corresponding to formation of columnar joints in cooling phonolite lava.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
12
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
Fig. 13. Thermal models constructed for selected shapes of analogue models with displayed isotherms of 920 °C spaced at 10 year intervals for models D-6 and D-22 and 50 year
intervals for model D-24. Thick red contour and thick dashed black lines indicate the relief and trend of columnar jointing on the Bořeň body. Thin black lines indicate trends of
columnar jointing that are perpendicular to the 920 °C isotherms as resolved from the thermal model. See Table 2 for the lithology codes. Green lines indicate the contact between
the cooling fronts in the models. Spots A and B mark two points where the rate of cooling was evaluated (Fig. 15). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
cooling of the model bodies, we draw lines perpendicular (Jaeger,
1961; Spry, 1962; DeGraff and Aydin, 1987) to the 920 °C isotherms
plotted at 10 (and 50) year time intervals (Fig. 13). Only conductive
cooling was considered, because no precipitates were found on
columnar joint faces, which would indicate circulation of fluids in the
cooling front and contribution of convective cooling (DeGraff and
Aydin, 1993).
Thermal model constructed for the geometry of a large cryptodome that pushed upwards and sideways the entire filling of the
model maar (model D-6, Fig. 13) revealed that the columnar joints
growing from the margins of the body converge into an elliptical and
vertically elongated core that is centered at the basement orthogneiss–marlstone contact. Superposed landscape contour of the SW–
NE profile through the Bořeň body with columnar joint pattern
Fig. 14. A simplified three-dimensional thermal model roughly reflecting the geometry of analogue model D-22 revealing distribution of temperature after 200 years of cooling.
Oblique top view of the finite-element model geometry (A) — lithology codes are indicated in Table 2, vertical profiles of the thermal model correspond to the NW–SE (B) and SW–
NE (C) sections through the Bořeň body. Thick white contour and dotted white lines indicate the relief and trend of columnar jointing on the Bořeň body. Directions and relative
dimensions of arrows correspond to the direction and degree of thermal flux, respectively.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
measured in the field reveals complete mismatch between the results
of thermal model and the reality. The columnar joint trends in the
thermal model are perpendicular to the trends of columns measured
in the field. Thermal model for extrusion D-24 (Fig. 13) indicates a
close match between the columnar jointing on the NW–SE profile of
Bořeň body and that inferred from the thermal model. However, in
perpendicular section compared with SW–NE transect of the elevated
plateau of Bořeň, relative dimensions of the model do not correspond
to the dimensions of exposed phonolite body, although the resolved
columnar jointing pattern roughly matches with the real one.
Comparison of the thermal model results for experiment D-22 in
both perpendicular vertical sections and NW–SE and SW–NE transects
of the elevated plateau on Bořeň indicates a close match between the
model and reality, except the low-angle difference between the
columns implied by the thermal model of the second section and
upper collonade on NE side of the body (Fig. 13). The contact height
between the upper and lower cooling fronts is consistent with the
suture level between the upper and lower colonnades according to
both thermal models D-24 and D-22.
A three-dimensional thermal model corresponding with D-22
experiment (Fig. 14) indicating topology of isotherms after 200 years
of cooling confirms the results of the previous two-dimensional
models (Fig. 13). The hot core is centered at the suture level between
the two collonades on the superimposed profile of Bořeň and the
isotherms are mostly perpendicular to the trends of columns in the
field. The 3D thermal model (Fig. 14) implies that the cooling of the
phonolite monolith to 100 °C and 20 °C lasted 4000 and 10,000 years,
respectively.
The presence of an insulating pumice cover greatly reduces the
degree of cooling of the phonolite extrusions. This is depicted in
Fig. 15. The rate of cooling of a point 5 m below the roof of the
phonolite extrusion with shape of model D-22 is 2.7× faster than in
equivalent extrusion that is covered by 8 m thick layer of pumice. In
contrast, degree of cooling close to the top and bottom of this
extrusion (points A and B, second section of D-22 in Fig. 13) is similar
when covered by the layer of pumice, but slower for the top part
during the initial cooling period.
Fig. 15. The degree of phonolite lava cooling with and without 8 m thick pumice
carapace. Degree of cooling was evaluated in points A and B, located 5 m below the roof
and 5 m above the base of extrusion D-22, respectively. For exact location of the points,
see the 2nd vertical section of model D-22 in Fig. 13. Cooling of point A below the
extrusion roof without the insulating pumice carapace is about 2.7× faster than cooling
with the insulating layer. The rate of cooling of the top and bottom parts (A and B) of the
extrusion is similar, when it is covered with the insulating layer.
13
6. Discussion
Combined techniques of structural analysis of fabric and cooling
joint systems on the Bořeň phonolite body together with systematic
analogue modeling of magma intrusion into phreatomagmatic craters
and thermal mathematical modeling of cooling for different shapes of
bodies obtained by analogue modeling provide a unique possibility to
pinpoint the original shape, the internal flow kinematics and
emplacement dynamics for the Bořeň phonolite body. Our results
bring interesting insight into emplacement mode of crystalline
magma into phreatomagmatic craters in general.
The analogue modeling results imply that the intrusion and
extrusion style depends primarily on the yield strength (thickness)
of the analogue magma and initial overpressure build-up in the
analogue magma chamber prior to extrusion. Since none of the
phonolite extrusions into phreatomagmatic craters were witnessed, it
is impossible to scale the experiments. Nevertheless, the analogue
modeling approach proved as a very instructive method for understanding the physics of complex mechanical interaction between the
non-Newtonian magma and loose (Coulomb-like) phreatomagmatic
tephra filling the funnel shaped volcanic craters. In our approach, we
present the scaling analysis to constrain possible timescales for
emplacement of the Bořeň phonolite assuming that the dynamic
similarity between model D-22 and original was closely approached
on the basis of their similar relative dimensions and internal fabrics.
Suspensions of plaster powder and water proved as convenient
analogues of magmas with high crystal content such as phonolites or
trachytes (Závada et al., 2009a,b). We attempted to mimic the
geometry of phreatomagmatic craters as revealed by geological and
geophysical surveys (Schulz et al., 2005; Lorenz and Kurszlaukis,
2007), although the slope angle of the upper part of the maardiatreme established in the Cretaceous sediments is hypothetical.
6.1. Shapes of the extrusions
Cooling and solidification of lava can significantly influence shapes
of extrusions by formation of solid crust which inhibits lateral
spreading and rise of new lava (Griffiths and Fink, 1993; Sakimoto
and Zuber, 1995). For intermediate lava compositions and typically
andesite lavas, the effect of cooling on lava solidification is negligible,
because the cooling is controlled by degassing induced crystallization
(Sparks et al., 2000). Hale (2008) numerically modeled lava dome
extrusion simplifying the rheology of an andesite lava dome to
Newtonian viscous fluid, which was confined laterally by deformable
talus growing by rockfalls and disintegration of the solid surface. For
crystal-rich phonolites investigated in this study, degassing induced
crystallization and solidification could also have accompanied their
emplacement. Degassing and accumulation of the gasses in the
surficial parts of the phonolite extrusions is reflected by the presence
of pumice on the top of Kaňkov hill, similar to the surficial vesicular
pumice layers in obsidian lava domes (Fink, 1983). Since the original
thickness and the mechanical properties of the pumiceous layer are
not known, it is difficult to consider its influence on lava dome
extrusion dynamics. Nevertheless, utilization of isothermal material
like plaster of Paris for modeling lava domes of intermediate
composition is justified with the process of degassing induced
crystallization and solidification (Sparks et al., 2000). The volume of
aggrading talus from the solidified phonolite blocks around the
extrusions (Hale, 2008) would be probably negligible in contrast to
the volume of phreatomagmatic tephra talus which is pushed aside
behind the expanding extrusions. In summary, we can consider that
the shapes and emplacement dynamics of the isothermal experimental extrusions into maar-diatreme structures can closely represent
their natural phonolite lava equivalents.
Most of the extrusions prepared with suspensions of mixing ratio
2.5 are considered as unrealistic equivalents to natural phonolite
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
14
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
extrusions, because these were emplaced rapidly within only few
seconds (D-2, D-4, and D-11) or were confined by too competent “lid”
of maar sediments (D-9 and D-12, Fig. 9). In the experiments with
thick plaster and medium to large initial overpressure (25–50 bar),
the plaster pushed upwards and sideways large portion or the entire
volume of the crater filling, while in experiments with low or no initial
overpressure, the plaster formed narrow conduit below an extrusive
dome. Although quite large magma overpressures would be required
to form the cryptodomes D-6 and D-14 or extrusive domes D-7 and D8, their geometries could correspond to high aspect ratio remnants of
phonolite bodies in the České středohoří Mountains or other Tertiary
volcanic provinces in the foreland of Alpine orogeny (Cloos and Cloos,
1927; Camus, 1975; Arbaret et al., 1993). In contrast, extrusion of the
same material on flat surface would result in low aspect ratio lava
domes or flows (Závada et al., 2009a) like the phonolite coulée
forming the Ryzelský Hill (Fig. 1).
Experiments with plaster suspensions of mixing ratio 2.15–2.3
resulted in different extrusion shapes ranging from asymmetric
extrusions fed by stems of variable width to lava lakes and branched
intrusions. These different final geometries likely reflects light differences in the packing density of the loose diatreme filling and
inhomogeneities in the plaster suspension. The “intrusion branching”
encountered in one of the experiments (D-18) could explain the
merging phonolite cupolas of Hněvín and Široký Hills (Fig. 1) or
centrically distributed phonolite bodies of Missouri Buttes (WY, USA)
(Halvorson, 1980). In nature, the intrusion branching could be induced
by subsequent rise of magma batches (e.g. from a stratified magma
chamber) each with relatively lower yield strength owing to decreasing
crystal content. However, this explanation is not fully supported by the
last three experiments. In composite experiment D-32, the flow
diverged in the upper part of the diatreme, while in earlier experiment
D-18 (homogeneous suspension, mixing ratio 2.2), intrusion branched
in the lower part of the diatreme. Alternatively, branching of magma
emplaced in volcanic craters could be also explained by sluggish or
unsteady ascent rate of magma allowing the short-lived conduits to
solidify.
6.2. Fabric development in the extrusions
Interpolation of magmatic fabric manifested by alignment of
sanidine and nepheline (Fig. 6) throughout the investigated Bořeň
body evinces a concave cupola with flat fabrics in the SE-central part
and steep fabrics on the margins. Similar cupola shaped fabric pattern
resulting from the upward plug-flow is typical for most of the
analogue models (Fig. 9). However, the internal fabric pattern, relative
dimensions of the body and the thermal structure best corresponds
with model D-22. For detailed comparison of fabric throughout the
Bořeň body and the analogue model D-22, we inspected sections cut
parallel to the mirror symmetry of the exposed phonolite body (note
the indicated profiles in Fig. 6) and the analogue model and also
sections perpendicular to the first section and transecting the elevated
plateau for the Bořeň body. A number of samples from the central and
SE-central part of Bořeň measured for CPO revealed magmatic
lineations with shallow plunges and N–S to NW–SE trends (Fig. 6,
samples B-24, B-65, B-41, B-43, B-82, and B-94). Same fabric pattern is
characterized by the AMS in analogue model D-22 (note lineations
subparallel with the 1st section and at high angle to the 2nd section in
upper parts of the model in Fig. 11). Fabric in both analogue models
measured for AMS (Fig. 11) revealed highly anisotropic domain
associated with flat and oblate fabrics in the central columnar part of
the stems below the extrusive parts of the models surrounded by
domains with low anisotropy, neutral to prolate shaped fabric and
subhorizontal lineations encircling the central part. This fabric could
be explained by divergent flow on the periphery of the plug flow
domain during ascent of the viscously flowing material within an
upward widening stem (Fig. 11). In contrast, fabric in the margins of
the models, which are vertical and parallel with their boundaries
(Figs. 11 and 16) can be explained by shearing and mechanical
coupling of the vertically ascending material with the surrounding
loose substrate. The flow kinematics in the extrusive parts of the
models is similar to previous analogue and numerical modeling
results (Buisson and Merle, 2004; Závada et al., 2009a) and originate
due to circumferential stretching in the upper lateral parts and radial
stretching at the base of the extrusions (Buisson and Merle, 2002). The
outlined pattern of flow kinematics throughout the extrusions
emplaced into maar-diatremes (Fig. 16) will be distorted for
asymmetric extrusive bodies like both models D22 and D24 (Fig. 11).
Second measured microstructural elements are the lenticular to
sigmoidal cavities in the phonolite lava. Detailed structural mapping
of these cavities throughout a single volcanic body was not carried out
before. The origin of mostly vertical cavities throughout the Bořeň
cupola and sigmoidal cavities dipping at low angle toward cupola
periphery is best illustrated using analogue model D-11. Although this
model was previously discarded for further analysis because of
unrealistically rapid emplacement, it reveals macroscopic vertical
cavities increasing in abundance and dimensions from the bottom of
the stem towards the apical part of the extrusion and local sigmoidal
cavities in the central part (Fig. 17). Similarly oriented, but less
distinct and fewer cavities are also present in model D-22. Large
cavities in model D-11 formed due to fast lateral diffluence and
divergent flow of plaster during extrusion from the narrow conduit.
Since the general kinematic framework of emplacement for both
models is virtually identical and independent of the emplacement
velocity, we can use the cavities in model D-11 for the kinematic
interpretation of cavities throughout the Bořeň cupola.
The cavities form due to strain incompatibilities in the vicinity of
inhomogeneities (like larger phenocrysts in the phonolite) and reflect
the shear thickening rheology of the extruding material (Smith, 2000;
Smith et al., 2001). In other words, cavities form in places, where local
shearing of the material cannot be fully accommodated by sliding
along the groundmass microphenocrysts, which are mostly represented by lath-like sanidine crystals in phonolite magma, or platy
hemihydrate particles in plaster suspensions. Incipiently opening
cavities drain the groundmass from residual interstitial liquids, which
is reflected by minute aegirine–augite crystals growing into the
cavities from the Želenický Hill phonolite (Fig. 5c). The result of this
liquid transfer is hardening and transition from ductile to brittle
deformation of the solid–liquid system at low melt volumes and is
analogous to cavitation controlling creep failure of metamorphic rocks
deformed in grain boundary sliding regime (Závada et al., 2007;
Rybacki et al., 2008). Progressing failure of the cavitated lava should
be manifested by increased abundance of cavities, faulting and
dismemberment of the homogeneous texture by ductile tearing and
Fig. 16. Schematic block diagram depicting the interpreted flow kinematics throughout
an extrusive dome emplaced into a phreatomagmatic crater as revealed by the
analogue models.
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
15
Fig. 17. Vertical section photograph (B) and traced flow fabric trends and cavities in model D-11 (A). This model was emplaced rapidly in 3 s after sudden release of highly
overpressured plaster from the container. Note mostly vertical cavities throughout the model and locally sigmoidal cavities in central part of the extrusion (A). The latter reflect
distortion of lenticular cavities formed at high shear stresses (C) into sigmoidal shaped cavities by resumed ductile flow at decreased shear stresses.
break up of the homogeneous fabric into individual domains that are
differentially rotated and bend, fold or extend separately, which is
evident from the phonolite texture in the margins of the cupola,
specifically on locality B65.
Lenticular cavities form parallel to local maximum compressive
stress direction during shearing of the crystal-rich lava (Fig. 17c).
Sigmoidal cavities that dip at shallow angles to the periphery of the
cupola (samples B43, B92, and B94 on Bořeň, Fig. 7; upper part of
model D-11, Fig. 17) are interpreted as distorted lenticular cavities
that formed due to non-coaxial diffluence of lava. Increasing rate of
layer parallel stretching from the core to the apical part of the
extruding cupola, indicated by upwardly diverging serrated layers of
colored plaster (Fig. 17b), generates non-coaxial flow with top to the
periphery sense of shear. Fast non-coaxial flow, associated with high
shear stresses (Smith, 1997), results in dilation hardening and
produces cavities inclined at low angle to the sanidine crystal fabric
(Fig. 17c). Resumed, but limited ductile flow at decreased shear stress
then deforms the originally lenticular cavities into sigmoidal shape.
Sigmoidal cavities with different strike of central and tail parts suggest
that the sense of shear changed during the lava extrusion (Fig. 7).
6.3. Thermal structure of the models and natural phonolite bodies
Application of finite element thermal modeling on shapes of bodies
obtained using analogue modeling is a powerful tool for structural
analysis of fabric and fracture patterns of irregular volcanic bodies.
Fracture patterns that develop during cooling of irregular volcanic
bodies were previously solved theoretically and only for simplified
geometries (Jaeger, 1961; Spry, 1962). Our modeling results imply that
if columnar joints would form in the teardrop shaped cryptodomes,
they would converge to their cores. In contrast, thermal models for the
extrusive domes predict that cooling joints growing from the upper
and lower boundaries of the bodies will confine columns of upper and
lower collonades divided by a subhorizontal suture (Fig. 13). Columns
of the upper collonade are typically wider than the outward flaring
columns of the lower collonade on the phonolite “towers” like Bořeň or
Devils Tower (WY, USA). This contrasts with thick basal columns and
thinner columns in the upper parts of basalt lava flows (DeGraff and
Aydin, 1987; Lyle, 2000) that develop by slow conductive cooling at
the base and faster conductive and convective cooling progressing
from the upper surface and master joints in the upper collonade,
respectively (DeGraff and Aydin, 1993; Lyle, 2000). Thicker columns of
the upper collonade on Bořeň body are attributed to relatively slower
cooling of the top part due to presence of an insulating layer of pumice
and blocky lava. Insulating properties of blocky covers derived from
the lava are well known from acidic lava flows (Harris et al., 2002). The
reconstruction of the original thickness for the insulating carapace
layer is impossible, because, (1) thermal properties of this layer are
probably not well represented by thermal properties measured from a
hand-specimen of phonolite pumice, and (2) the relationship between
column width and thermal gradient is known only qualitatively
(DeGraff and Aydin, 1993). Nevertheless, our evaluation of the cooling
with the 8 m thick insulating layer (Fig. 15) revealed that for the initial
period of cooling (8 years in point A) the cooling is slower than at the
bottom of the extrusion, which is compatible with thicker columns at
the top of Bořeň body.
6.4. Emplacement dynamics of the Bořeň body constrained by analogue
and thermal modeling
Considering that the original shape of the Bořeň phonolite body is
best reflected by the analogue model D-22 on the basis of their relative
dimensions within the probable maar-diatreme crater, similar fabric
pattern and thermal structure, we attempt to estimate the emplacement time of the Bořeň body using the scaling principles of analogue
modeling (Hubbert, 1937; Ramberg, 1981). Assuming that the impact
of cooling on the emplacement dynamics of Bořeň phonolite was
negligible, that the dynamic similarity between the model and original
was attained and that the inertial forces are negligible for slowly
emplaced magma and analogue material, time of Bořeň extrusion to
can be calculated by the following equation: to = ðδ⋅λ = ψÞ⋅tm , where
δ, λ, ψ are the ratios between model and original for densities, sizes and
viscosities (Table 3) and tm = 160 s is extrusion time for the model.
Considering viscosities of the residual melt in the phonolite of 104–105
measured for crystal free phonolite glasses (Giordano et al., 2001),
extrusion time for the Bořeň body ranges between 6 and 66 days. We
can also indirectly calculate the yield strength of phonolite magma
from the shape of Ryzelský Hill coulée (height 100 m, radius 1250 m)
using Nye's analytical solution for axisymmetric dome in static balance
Table 3
Physical properties of analogue model D-22, the Bořeň phonolite body and the scaling
ratios. Properties of analogue material are taken from Závada et al. (2009a), viscosity of
phonolite melt from Giordano et al. (2001). Viscosity of the model material refers to
viscosity of water in the plaster suspension.
Model
Original
Model ratios
Size (m)
0.22
500
λ = 3.9 × 10− 4
Magma properties
Viscosity (Pa s)
Density (kg/m3)
Yield strength (Pa)
0.001
2300
195
10− 4
2530
99,277
ψ = 10− 7
δ = 0.91
σ = 3.6 × 10− 4
Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012
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P. Závada et al. / Journal of Volcanology and Geothermal Research xxx (2010) xxx–xxx
(Nye, 1952; Závada et al., 2009a), which gives a value of 99,277 Pa.
Because the model ratio of yield strength calculated from equation
σ = δ ⋅ λ (Hubbert, 1937) is σ = 3.6 × 10 − 4, value of yield strength for
the model material should be 35 Pa. This value is only about six times
less than 195 Pa measured for plaster suspensions with mixing ratios
of 2.4 (Závada et al., 2009a), which further justifies the dynamic
similarity between model and original and our scaling approach.
6.5. Conclusions
Combined techniques of structural analysis of fractures and fabrics
on natural phonolite bodies together with analogue and thermal
mathematical modeling revealed that the Bořeň phonolite body
probably represents a remnant of an asymmetric extrusive dome
emplaced into a phreatomagmatic maar-diatreme crater within time
period of 6–66 days and cooled down to a background temperature in
10,000 years. Series of analogue experiments revealed that intrusion
of magma into maar-diatremes can result in emplacement of
cryptodomes, extrusive domes or branched intrusions producing
extrusive domes on crater periphery. Analysis of internal fabric of
analogue models corresponding to Bořeň body revealed a domain of
circumferential flow around highly anisotropic fabric associated with
upward plug-flow in the vertical stems underlying the extrusive
domes. Mostly vertical microscopic cavities throughout the exposed
body resulted from rapid lateral diffluence of the magma above the
maar crater. Comparison of thermal models with columnar jointing
pattern on the Bořeň phonolite body suggests that the extrusive
domes or lava lakes emplaced into maar-diatreme craters are marked
by subhorizontal suture between the colonnades of columns. The
vertical level of the suture between the collonades, dimensions of the
erosive remnant and the host maar, together with internal fabrics and
orientation of the columns in the solidified lava allow to reconstruct
original dimensions and shape of the bodies emplaced into maardiatremes.
Acknowledgements
This work was funded in the frame of project ME09011 of the
program KONTAKT of the Ministry of Education of the Czech Republic
and from the “Junior grant” of the Grant Agency of the Czech Academy
of Sciences (GAAV), No. B301110703. Fieldwork with V. Rapprich on
the Kaňkov hill providing the glassy phonolite pumice is also
acknowledged.
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Please cite this article as: Závada, P., et al., Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field,
thermal modeling and AMS analogue modeling data..., J. Volcanol. Geotherm. Res. (2010), doi:10.1016/j.jvolgeores.2010.07.012