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Geomorphology 136 (2012) 4558
Contents lists available at SciVerse ScienceDirect
Geomorphology
j ourna l homepage: www.e lsev ie r.com/ locate
/geomorphStructural and morphometric irregularities of eroded
Pliocene scoria cones at theBakonyBalaton Highland Volcanic Field,
Hungary
Gbor Kereszturi a,b,c,, Kroly Nmeth a
a Volcanic Risk Solutions, Institute of Natural Resources,
Massey University, PO Box 11 222, Palmerston North, New Zealandb
Department of Geology and Mineral Deposits, University of Miskolc,
Miskolc-Egyetemvros, H-3515, Hungaryc Geological Institute of
Hungary, Stefnia t 14, H-1143, Budapest, Hungary Corresponding
author at: Volcanic Risk Solutions,Massey University, PO Box 11
222, Palmerston North, N
E-mail address: [email protected] (G. Keresz
0169-555X/$ see front matter 2011 Elsevier B.V.
Adoi:10.1016/j.geomorph.2011.08.005a b s t r a c ta r t i c l e i n
f oArticle history:Received 7 January 2010Received in revised form
8 February 2011Accepted 4 August 2011Available online 16 August
2011
Keywords:Morphometric ageLava-spatterScoria coneSlope
angleDigital Elevation ModelSlope decrease rateScoria cones of the
Mio-Pliocene BakonyBalaton Highland Volcanic Field (BBHVF) are
built up by wide rangeof volcanic rocks, including intercalated
lava flows/dykes, pyroclastic breccias and scoriaceous lapilli
withvarious degrees of welding or agglutination. According to KAr
and ArAr dating, ages of the fourteen scoriacones within the field
span between 5.2 and 2.5 Ma. From these fourteen, seven cones were
selected whichare suitable for morphometric analysis, i.e. visible
in the field and have identifiable boundaries. Themorphometric data
were either derived by manual measurement on topographic maps and
by DigitalElevation Model-based calculations. Using the same input
contour line data from 1:10,000 maps, basic coneparameters such as
cone height, basal and crater width have been measured in order to
calculate parameterslike Hco/Wco ratio and average slope angle. The
results of these three-parameter-based manual calculationshave been
compared to the DEM-based results in order to highlight the
controls of degradation, pitfalls inmorphometric parameterization
and the differences between these two calculation methods. Based on
theresults, three main controlling conditions have been identified
that are together responsible for thepreservation and erosion of
the scoria cones located in the BBHVF: (1) age of the cone, (2)
climate during thedegradation and the (3) inner architecture of the
edifice. In terms of morphometric dating, the
traditional,three-parameter-based method tends to give inaccurate
results on (1) scattered and/or truncated cones and(2) on the
edifices characterised by highly effusive behaviour during the
emplacement.Institute of Natural Resources,ew Zealand.turi).
ll rights reserved. 2011 Elsevier B.V. All rights reserved.1.
Introduction
Scoria (or cinder) cones are themost commonvolcanic
landformsonEarth (Wood, 1980a,b) as parasitic cones on larger
polygeneticvolcanoes or as volcanic fields (Settle, 1979). Eruption
styles associatedwith scoria cone formation range from periodic
magmatic fragmenta-tion to minor intermittent phreatomagmatic
phases (Martin andNmeth, 2006). Given dominantly a basaltic magma
composition, themajority of the eruptions are generally governed by
the speed of therising magma, which is primarily determined by
viscosity and gascontent and can produce predominantly
Strombolian-style eruptions(McGetchin et al., 1974; Parfitt and
Wilson, 1995). However, typicalStrombolian-style eruptions at most
of scoria cones are completed byadditional Hawaiian lava
fountaining (Di Traglia et al., 2009), violentStrombolian (Pioli et
al., 2008) or phreatomagmatic eruptions (Gisbertet al., 2009).
Previous work revealed that the main morphometric parameters
ofscoria cones, such as cone height and average cone slope decrease
duringdegradation (Colton, 1967; Porter, 1972; Settle, 1979). Other
morpho-metric parameters have been developed (e.g. cone/crater
elongation,breaching azimuth etc.) to characterise cone erosion
(Dohrenwend et al.,1986; Hooper and Sheridan, 1998), and tectonic
setting (Corazzato andTibaldi, 2006). Morphometric characterization
also aims at providingbasic information about the relative age of
scoria cones (e.g. Sucipta et al.,2006). However, as a consequence
of the aforementioned scoria conediversity as well as of
post-eruptive tectonic influence and erosion, inmany
casesmorphometric age estimation is particularly difficult on
older(N1 Ma) scoria cones.
Our knowledge about the precision of morphometric dating is
alsolimited because most of the morphometric studies have focused
onrelatively young (b1 Ma) scoria cones. Scoria cone dating based
on theirmorphometry benefits from standard morphometric
characteristicssuch as ratio of cone height/basalwidth (Hco/Wco)
and slope angle (Save,Fig. 1). Both characteristics decrease with
time (Dohrenwend et al.,1986; Hooper and Sheridan, 1998). Since
they are determined fromcone height, basal and crater width (if
any), these three parameters areessential to get age-representative
values for the calculations.
According to Favalli et al. (2009), the Hco values of flank
cones ofMt. Etna (Italy) are largely controlled by the inclination
of the substrateas well as the lava burial effect, which reduce the
cone height. Favalli
http://dx.doi.org/10.1016/j.geomorph.2011.08.005mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2011.08.005http://www.sciencedirect.com/science/journal/0169555X
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Fig. 1. Definitions of traditional morphometric parameters of a
scoria cone.
46 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558et al.
(2009) also suggest that a slightly decreasing trendofHco/Wco
ratiowith time can be observed among the flank cones in spite of
therelatively young age of the examined cones (6500 years BP).
Another morphometric parameter for scoria cone datingmethods
isthe slope angle, which has been partly studied by Favalli et al.
(2009). Itraises some unsolved questions: how slope angles vary
with differentmethods of calculation, e.g. formulae-based or
Digital Elevation Model-derived? How the eruption/erosion diversity
of the cones is reflected inthe slope angle? What are the precision
and limitations of slope angle-based dating?
Here, we explore the complexity of scoria cone
morphologypreserved in eroded, Pliocene scoria cones of western
Hungary withan aim of understanding internal and external factors
that may havebeen involved in the formations and preservation of
the morphologicalfeatures of scoria cones. Scoria cones of the
BakonyBalaton HighlandVolcanic Field (BBHVF) are older, 3.82.5 Ma
(Balogh et al., 1986;Wijbrans et al., 2007), than the cones which
have been analysed bymorphometrically elsewhere (Wood, 1980b). At
least ten scoria coneremnants, with an addition of two deeply
eroded cones (Tihany andKab-hegy) and a further two covered by
thick Quaternary sedimentshave been recognised at BBHVF through
detailed investigations over theFig. 2. Digital Elevation Model
(DEM) of the centrapast decade (Martin and Nmeth, 2004; Auer et
al., 2007). Of thesefourteen scoria cones, this study deals with
the morphometry of sevenselected locations (Fig. 2), which have
identifiable/visible geologicallywell-defined boundaries and
available, reliable KAr and/or ArAr ages.Each scoria cone of the
BBHVF has a unique eruption history recorded intheir primary
pyroclastic successions such as intercalated welded lavaspatters
(Figs. 3AB), interbedded coherent lava units (Fig. 3C), andvarious
types of scoriaceous lapilli-dominated cone-building
pyroclasticsuccessions (Fig. 3DF). These types of deposits are
typical ofStrombolian-style explosive eruptions, Hawaiian-style
lava fountainingand lava effusion (Martin and Nmeth, 2004). The
majority of theseprocesses have been inferred to take place during
the cone-buildingeruptive phases, except for post-eruption
mass-movement, and aretogether responsible for the complex
cone-buildingevents. However, therelative roles of these processes
are not yet fully understood in terms ofmorphometry. A further aim
of this study is to demonstrate the possiblepitfalls in automatic
application of formulae-based morphometricaldating generally used
for younger, b1 Ma, scoria cone fields to establishthe relative
morphological age of the cones.
2. Materials and methods
The input data for both the manual and GIS-based
parameteriza-tion were the Hungarian Military maps with scale of
1:10,000 with5 m contour intervals. After georeferencing, Hungarian
National Grid(EOTR) projection and Hungarian Datum (1972), the
contour lines ofthese topographic maps were digitalized. The
boundaries of lavafields/flow (if any) and the scoria cones derived
from 1:50,000geological map of the BBHVF (Budai et al., 1999)
updated by new fieldobservations.
2.1. Manual parameterization
Hco (Fig. 1) is expressed as the arithmetic mean of the
difference ofthe basal height and maximum (Hco max) and minimum
(Hco min)elevation of the cone measured on topographic maps
(Porter, 1972;Settle, 1979). This parameter only gives valuable
results in the case ofscoria cones located on gentle slopes (2.55),
platform-type volcanicfields (Favalli et al., 2009), and the
BBHVFmeets this requirement. Priorto the volcanism at the BBHVF,
the intra-Carpathian basins (especiallythe later Pannonian Basin)
were filled by a large volume of siliciclasticsediments of Lake
Pannon and associated river systems during the LateMiocene through
the Pliocene (Magyar et al., 1999) and therefore amorphologically
flat landscape was in place before the volcanism.l part of the
BBHVF with the studied locations.
image of Fig.2
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Fig. 3. Textural diversity of pyroclastic successions of scoria
cones of BBHVF. (A) Agr-tet: lava spatter-rich blocks around the
scoria cone remnant; (B) Kopasz-hegy: alternatingerosion-resistant,
lava flow units (LR2) and intercalated tuff breccias (TB1); (C)
Bondor: outcrop at the breaching site (TB1weakly agglutinated,
scoriaceous breccias; LR1dykeunit); (D) Kopcsi-hegy: ballistically
transported, weakly agglutinated scoriaceous lapilli-dominated
unit; (E) Badacsony: contact zone of the maar lake infilling lava
lake (LR1) andthe capping (TB1) scoriaceous pyroclastic breccias;
white arrows represent small-scale tumuli structures; (F)
Boncsos-tet: alternating highly (TB1) and weakly (TB2)
agglutinated,scoriaceous pyroclastic breccia units.
47G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558Wco is
the average of themaximumandminimumdiameter of a cone(Fig. 1).
Based on the pioneering works of Porter (1972) and Wood(1980a) the
worldwide median value of Wco is ~800 m, whereas themean value is
~900 m (based on datameasured from910 scoria cones).The average
terrestrial and still intact scoria cone size is characterised
bythe following equations between the cone height and width
(Porter,1972; Wood, 1980a):
Hco = 0:18Wco 1
Wcr = 0:40Wco 2
Crater diameter or width (Wcr) is calculated from the average
ofthe minimum and maximum diameter of the crater measured
fromtopographic maps.
From the parameters above, further age-representing
parameterssuch as Hco/Wco ratio or average slope angle can be
calculated. Bothparameters decrease gradually with time, thus,
correlate well with theage of the volcanic edifice (Porter, 1972;
Dohrenwend et al., 1986;Hooper, 1995; Hooper and Sheridan, 1998;
Sucipta et al., 2006; Donizet al., 2008; Favalli et al., 2009).
The average slope angle (Save) derived from the three
basicparametersnamely theHco,WcoandWcr. Formallywrittenas
(Hasenakaand Carmichael, 1985):
Save = tan1 2Hco = WcoWcr 3Save = tan1 2Hco =Wco 4
where the Eq. (4) is for those scoria cones with lack of
measurablecrater.
2.2. DEM-based parameterization
As parallel with the manual parameterization, DEM-based
slopeangle calculations were performed for the study areas. The
DEMs wereobtained by linear interpolation method (Gorte and
Koolhoven, 1990).This method rasterizes the input contour lines
into the user-definedhorizontal grid cell size (Gorte and
Koolhoven, 1990), which iscommonly referred to horizontal
resolution. Extremely high resolutionDEMs therefore can be created
from relatively low-scale topographicinput data. However, the
proper DEM resolution is dependent on thenature of the input data,
i.e. contour lines or spot heights,and theproperties of the
terrainmodelled (Hengl, 2006; Jordan, 2007).Thus, theresolutionof
DEMswasdetermined on the basis of input data propertiessuch as
distance between neighbouring contour lines (Hengl, 2006;Hengl and
Evans, 2008). To find the proper horizontal grid cell
size,neighbourhood operator was used to find those rasterized
contour linepixels that lie immediately adjacent to each other on a
33pixelmatrix.This special pixel, called touchingpixels,maybea
sourceof error. Thesemeasurements showed that a 22m grid cell size
was small enough toavoid touching pixels in the area of interest
(outer flanks). In order to
image of Fig.3
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48 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558get a
more generalised picture for slope angles, our final DEMswere
smoothed by an average 33, i.e., 66 m, moving window. Theelevation
values for each pixel have been calculated for 3 decimals.
An additional source of error in digital modelling is that flat
pixelsare mostly the result of an inadequate determination of
horizontalgrid cell size (Garbrecht and Martz, 1997; Jordan, 2007).
These pixelshave zero first derivates, i.e. zero slope angles, thus
can modify theresults of slope angle estimates. No flat pixels have
been found withinthe area of interest. The overall accuracy of
DEMswas characterised byroot mean square error, which is defined as
(Fisher and Tate, 2006):
RMSE = ZDEMZref 2
= n 5
where the ZDEM is the pixel elevations of the DEM, the Zref is
thereference points (in this case spot heights from the
topographicmaps)and the n is the number of reference points. The
RMSE is alwayspositive. In terms of accuracy, the DEM studied in
the present studyare characterised by 0.8 m up to 2.8 m.
The first derivates, i.e. slope angle, of a DEM is formally
written as:
SLOPE = arctanfx2 + fy2 6
where the fx=z/x and fy=z/y, for bivariate function,
z=f(x,y),then the absolute value of the gradient vector. In a
grid-basedenvironment, first derivates can be calculated by various
filters forexample three-, four- or eight-point methods (Sharpnack
andAkin, 1969; Zevenbergen and Thorne, 1987; Jones, 1998). In
thesefilters above, the numbers correspond to the number of pixels
evolvedin the gradient vector calculation on a 33 pixel matrix. In
the case ofmorphometric parameterization for scoria cones, the user
usuallyFig. 4. Agr-tet: (A) The location of the scoria cone and the
measured morphometric paramecross-section of the present cone and
lava field with the applied base height surface whichcurve) slope
angles of the outer cone's flank.intends to approximate the average
slope angle regardless thesmall, local-scaled variance of the
terrain modelled. In other words,the linear, eight-point,
unweighted (called Prewitt operator) filtergives the best results
because this filter (1) calculates a moregeneralised slope
estimates as compared to three- or four-pointmethods (Jones, 1998),
(2) shows high resistance for interpolationerrors (Raaflaub and
Collins, 2006) because of its high smoothingeffect due to the
first-order trend surface fitted to the 33 pixelmatrix (Sharpnack
and Akin, 1969). In this filter, the derivates definedas:
fx = Z3 + Z6 + Z9Z1Z4Z7 = 6X 7
fy = Z1 + Z2 + Z3Z7Z8Z9 = 6Y 8
where the Z1Z9 correspond to the pixel elevation reads from the
topleft cornel to the bottom right position in a 33 pixel matrix.
The Xand Y refer to the grid cell size along the two main
directions.
In this study, four types of slope angle values such as Smean,
Smed,Smode and Smax were calculated only on the outer flanks of the
volcaniccones. The Smean is the weighted average of the slope
angles by thetotal number of pixels related to a slope value, while
the Smed, Smodeand Smax refer to the median, mode and maximum
values of the slopeangles, respectively. The inner crater slopes,
the breached side of thecones, slopes dissected by large valleys as
well as local flat areas, e.g.local maxima/peaks or local
minima/depressions, due to interpolationerror were eliminated from
the delimited areas (e.g. Figs. 4 and 5).Nonetheless, the delimited
cone slopes still contained 3686191,677individual pixel values,
which bring relevant information on thepresent slope angle pattern
(Table 1).ters such as maximum andminimumWco as well as the
measured outer slopes. (B) Thewere used in the volume calculations.
(C) Slope angle histogram and cumulative (red
image of Fig.4
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Fig. 5. Kopasz-hegy. For detailed information see Fig. 4.
49G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558Besides
the slope angle estimates, the volume of the preservededifices
(Vcone) and related lava flow field (Vlava) were calculated fromthe
high resolution DEMs by applying various dipping base heights(see
details in Figs. 410). The source of lava flows have
beenestablished on the basis of (1) stratigraphical and
topographicalrelationship, (2) existence of the low, radial
alignment of lava rockcovered ridges around the base of the cone,
(3) existence of outcropsas well as (4) present cone
morphology.
3. Studied locations
In this section, seven scoria cones from the BBHVF have
beendescribed,whichhave various geomorphic shapes and
ages.Wepresenttheir locations, eruption ages, main volcanological
and geomorpholog-Table 1Morphometric parameters of the seven scoria
cones from the BBHVF Estimated by bothmanualweighted arithmeticmean
(Smean),median (Smed),mode (Smode) andmaximum(Smax) slope ang
Cone Measured from DEM
Elevationa.s.l.
Hco min Hco max Hco Wco min Wco max Wco Wcr
In metre
Agr-tet 511 56 81 68.5 620 788 704.0 250Kopasz-hegy 303 8 22
15.0 699 732 715.5 Bondor 378 40 60 50.0 916 1614 1265.0
650Kopcsi-hegy 303 20 43 31.5 529 742 635.5 350Badacsony 437 17 29
23.0 710 876 793.0 400Boncsos-tet 448 52 67 59.5 665 1207 936.0
Gajos-tet 373 20 22 21.0 786 1226 1006.0 ical features as well as
the main morphometric values, and addresswhether or not they are
suitable for traditional morphometric dating.Furthermore, we also
select which slopes represent the best the roughage of the
cone-formation (i.e. not disturbed by subsequent volcano-logical
and/or erosional processes) and are therefore suitable for
testing(e.g. compare with the existing KAr and ArAr ages).
3.1. Agr-tet
Agr-tet Volcanic Complex is located at the northern, elevated
partof BBHVF (Figs. 2 and 4). The volcanic cone is surrounded by a
lavaplateau erupted on Mesozoic carbonates (Budai et al., 1999).
Based onArArgeochronology the scoria coneerupted about ~3.3
Maagoand theextended lava field was emplaced around ~3.0 Ma ago
(Wijbrans et al.,andDEM-basedmethods. Notes: 1Calculated by Eq.
(3); 2Calculated by Eq. (4); 3Thele values. Values inboldwereused
to evaluate the cones' age basedon theirmorphometry.
Calculated manually Calculated from DEM
Dcr Hco/Wco Eco Save 1 Save2 Smean3 Smed3 Smode3 Smax3 Std. Dev.
Vcone
Ratio In degree in km3
bre 0.097 0.787 16.9 11.0 14.5 13.3 12.7 41.9 6.0 0.01632bre
0.021 0.955 2.4 11.9 10.8 10.1 36.4 4.3 0.00065bre 0.040 0.568 9.2
4.5 10.8 10.8 9.4 35.9 3.5 0.0318515.0 0.050 0.713 12.4 5.6 11.7
11.0 8.3 35.6 5.7 0.00592bre 0.029 0.811 6.6 3.3 8.0 7.1 8.3 40.9
4.1 0.00592 0.064 0.551 7.2 9.2 8.7 5.4 36.1 3.5 0.01277 0.021
0.641 2.3 4.0 3.8 1.7 13.9 2.1 0.00796
image of Fig.5
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Fig. 6. Bondor. For detailed information see Fig. 4.
50 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 45582007).
The relatively long-lasting eruption (~0.3 Ma) indicates a
stablemagma supply beneath Agr-tet (Martin and Nmeth, 2004)
resultingin the accumulation of 0.365 km3 lava (Table 2).
Explosive volcanic activity of Agr-tet is characterised by
Strombo-lian- and Hawaiian-styles that produced scoriaceous fall
beds inter-bedded with agglutinated lava spatter (Martin and Nmeth,
2004;Csillag et al., 2008). The inner structure of the eroded cone
consists ofstrongly to moderately agglutinated, lava-spatter
dominated deposits(Fig. 3A).
The original crater of Agr-tet is still recognisable; however, a
largebreach has occurred at the NW side of the cone (Fig. 4). The
preservedgeomorphology of the cone is characterised by the
existence of low,radial ridges around the eastern foot of the
Agr-tet, which can beinterpreted as the remnants of individual
lavaflows (Csillag et al., 2008).
The size of scoria cone of Agr-tet is slightly smaller than
anaverage scoria cone after Wood (1980a), with ~704 m (Wco) and~68
m (Hco) and a ~250 m wide crater as well as small cone volumeabout
0.016 km3 (Table 1). The Save calculated by traditional
methodbetween 16.9 and 11 (Table 1) and with the DEM-based
method~14.5 (Smean). In addition, the Hco/Wco ratio is around
0.097.
Morphometric assessment: Agr-tet and its lava field are not a
classicexample of a scoria cone because the present cone mostly
compriseserosion resistant, lava spatter-dominated deposits as a
result of Hawaiianand Strombolian-type eruptions. Furthermore, the
long-lasting eruptionwith multiple lava effusion stages, according
to Wijbrans et al. (2007),likely influenced the freshly deposited
scoriaceous lapilli and ash bedscausing significant welding and
agglutination. However, the cappingscoria cone edifice is suitable
for all methods of morphometry-baseddating because its shape is
preserved, except for the crater opening fromNNW. Consequently,
both the traditional (including Eqs. (3) and (4))and
DEM-basedmethods yield valid results, which are in the same
range(1214; Table 1).3.2. Kopasz-hegy
Kopasz-hegy Volcanic Complex (KVC; Figs. 2 and 5) consists of
twoindividual volcanic centres in the western boundary of the Kl
Basin(Fig. 2). According to KAr dating, the age of Kopasz-hegy
spansbetween 2.82 and 2.59 Ma (Balogh, K. pers. comm.). Based
ondistribution, type and bedding characteristics of pyroclastic
deposits,initial phreatomagmatic activity at Kopasz-hegy was within
a NSaligned paleovalley (Kereszturi and Nmeth, in press). The
northerneruption centre is capped by at least four preserved, lava
spatter andscoriaceous breccia-dominated (Fig. 3B) mound-shaped
hills, whichare ~515 m high and few hundreds of metres in diameter
today.These mounds consist of layers (usually a few dm thick) of
greyish toblackish, densely to non-welded, scoriaceous pyroclastic
breccias(Fig. 3B). Rootless (clastogenic) lava flows 1015 m long
with avolume of a few tens of m3 were emplaced during the late
stageevolution of Kopasz-hegy. These erosion resistant rocks must
havehad control over the erosion of the cone.
The distribution of the preserved scoria cone flanks is limited
dueto the quarrying and the volcanic/erosional processes. The
craterwidth is doubtful and its location is only inferred between
the small,preserved scoria mounds (Fig. 5). The direction of the
breaching issouth. This has an obvious effect on morphometry; for
example theWco is 715 m and the Hco is only 15 m and the volume is
relatively low0.0006 km3 (Table 1). This results in a very low
Hco/Wco ratio 0.021.The slope angles strongly vary between 2.4 and
11.9 (Table 1).
image of Fig.6
-
Fig. 7. Kopcsi-hegy. For detailed information see Fig. 4.
51G. Kereszturi, K. Nmeth / Geomorphology 136 (2012)
4558Morphometric assessment: The present geomorphic shape, in spite
ofits young age, indicates that theKopasz-hegywasprobably
truncatedbyvolcanic (e.g. vent migration) or erosional processes,
e.g. gully erosion.The existence of a double cone systemwith a
phreatomagmatic tuff ringbase, one partly destroying the other and
the central crater, is supportedby the contrasting KAr ages
measured (Balogh, K. pers. comm.). Thus,the cone is not suitable
for traditional, morphometric dating because itscrater has almost
been removed: only the Save is calculatedby the Eq. (4)(Table 1).
However, the DEM-basedmethod for Smean gave a reasonablevalue of
slope angle (~11.8; Table 1).3.3. Bondor
Bondor is a remnant of a complex volcano situated at the
northernpart of the BBHVF (Fig. 2). Several KAr radiometric datings
from thecoherent lava body of Bondor suggest an age range of 3.8
and 2.29 Ma(Balogh et al., 1986; Balogh and Pcskay, 2001). Thewide
range of KArages may result from long-lasting and intermittent
volcanic activitywith individual eruption stages (Kereszturi et
al., 2010). Based on thedepositional setting, the age of the upper
scoria cone is inferred to bebetween 2.9 and 2.29 Ma by KAr dating
methods (Balogh et al., 1986;Balogh and Pcskay, 2001; Kereszturi et
al., 2010). This age intervalhas been confirmed with the last
unpublished dating of 2.52 Ma(Balogh, K. pers. comm.).
Initial stages of the evolution of Bondor comprise a
basalphreatomagmatic tuff ring unit, which is ~1.5 km in diameter.
Thisbasal tuff ring hosts a few lava flow units and the capping
scoria cone(Martin and Nmeth, 2004). A few outcrops at the eastern
sectorexpose the inner structure of the Bondor scoria cone. The
scoria coneis characterised by chaotically bedded, reddish,
moderately vesicu-lated, partly welded scoriaceous lapilli and
block layers and cross-cutting dykes (Fig. 3C). The effusive
activity, which is directly linked tothe scoria cone, produced lava
flows that are only preserved at theeastern part of Bondor (Fig.
6). Small topographic ridges at the SEfoot of the cone indicate
that the lava flow came from the scoria cone(Kereszturi et al.,
2010). The rest of the lava plateau may be related toother effusive
activity during the evolution of Bondor, probably afterthe initial
maar-forming eruptions.
The Bondor is one of the largest scoria cones within the BBHVF
inWco (~1265 m), Wcr (~650 m) and volume (V=0.031 km3; Table 1).The
cone height is small (~50 m). The Hco/Wco ratio is 0.04 due to
thelarge basal diameter of the cone (Table 1). The slope angle
variesbetween 4.5 (Save calculated by Eq. (4)) and 10.8 (Smean and
Smed).The Smode is 9.4.
Morphometric assessment: Bondor is one of the largest
erosionalremnants of BBHVF built up by typical scoria cone
deposits. Small-scale breaching occurs at the eastern slopes, but
this morphologicalirregularity does not significantly disturb the
morphometric param-eterization process. As a result of this, the
Bondor is suitable for bothdating methods (Fig. 6).3.4.
Kopcsi-hegy
The Kopcsi-hegy scoria cone (Figs. 2 and 7) has a
well-preservedcone with a still enclosed crater, located near the
largest nested maar-system of Fekete-hegy Maar Volcanic Complex in
the central part ofBBHVF (Martin and Nmeth, 2004; Auer et al.,
2007). The age ofKopcsi-hegy is about 2.61 Ma (Wijbrans et al.,
2007).
The basal part of the cone consists of massive lapilli tuff
bedsinterpreted as products of initial phreatomagmatic explosive
erup-tions (Nmeth and Martin, 1999). A NS elongated ridge can
beidentified south of the present foot of the volcanic edifice
(Fig. 7),which may have been formed by valley-filling deposits of
the initialphreatomagmatic eruption (Nmeth and Martin, 1999). The
scoria
image of Fig.7
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Fig. 8. Badacsony. For detailed information see Fig. 4.
52 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558cone
grew over these basal phreatomagmatic rock units and
ischaracterised by chaotically bedded, reddish to brownish, partly
toweakly welded, highly vesicular lapilli and block-sized
scoriaceousdeposits with abundant ballistically transported spindle
bombshosting mantle-derived peridotite lherzolite nodules (Fig.
3D). Nolava flows are known to be associated with Kopcsi-hegy (Fig.
7).
The morphometry of the small Kopcsi-hegy scoria cone is
simplewith Wco 635 m and Hco 31 m, as consequences of the partly
closedcrater that has a diameter of 350 m and depth ~15 m (Table
1). Thecone volume is about 0.005 km3 (Table 1). The lowest slope
angle is5.6 (Eq. (4)) and the highest is 12.4 (Eq. (3)). The
Hco/Wco is around0.05.
Morphometric assessment: The Kopcsi-hegy scoria cone, the
onlycone of the BBHVF with an almost intact crater, was formed
byStrombolian activity following initial phreatomagmatic eruptions.
Nolate-stage lava flow is known to be derived from the
Kopcsi-hegyscoria cone. This cone is one of the most suitable for
morphometricparameterization both with the traditional and the
DEM-basedmethods. However, in the case of the latter method, we
calculatedthe slope angle only from the undisturbed slopes (Fig.
7). Here, Smeangave an 11.7 relatively steep slope whereas
formulae-based methodsgave 12.4 (Table 1).
3.5. Badacsony
Badacsony (Figs. 2 and 8) forms a butte in the southern part of
theTapolca Basin (Fig. 2). The Badacsony volcano is about 3.45 Ma
old onthe basis of KAr radiometric datings (Borsy et al., 1987).
Badacsony isan eroded tuff ring filled with thick capping lava and
associated scoriacone (Martin and Nmeth, 2004). The contact zone of
the cappingscoria cone and the basal lava lake units are exposed
due to quarrying(Fig. 3E). The Badacsony volcanic complex consists
of a present day23 m high, intra-maar scoria cone with ~793 m basal
width and~400 m crater width, which correlates well with average
worldexamples (Wood, 1980a). The cone has been largely removed by
theerosional processes, thus the present volume of the cone is
only0.005 km3 and the Hco/Wco is very low (0.029).
As a consequence of erosion, the crater of the scoria cone is
notclearly visible in the field and it seems to have been breached
from thenorth. In addition, the original edifice has been incised
by a small gullyfrom the east. The present boundary of the scoria
cone is slightlyundermined by 20th century quarrying. As a
consequence of slopeangle pixels in rim position are higher due to
the quarrying (Fig. 8).Thus, wemeasured DEM-based slope angle
parameter (Smean) only forthe S and SW slopes of the Badacsony
(Fig. 8). The Save of the wholevolcanic edifice is 6.6 and 3.3
calculated using the Eqs. 3 and 4 and aslightly higher values
(Smean=8.0) is accounted for the southernslopes of Badacsony (Fig.
8).
Morphometric assessment: The morphometry of the present
scoriacone has been modified by the quarrying as well as the
marginalsituation of certain (e.g. eastern and western ones) slopes
of the cone(Fig. 8). This destruction highly disturbs the
traditional, formulae-based results of slope angle. However, for
the DEM-based slope anglecalculation, the southern slopes which are
less eroded have beenfound suitable (Fig. 8).
3.6. Boncsos-tet
Boncsos-tet (Figs. 2 and 9) is a large asymmetrical scoria
coneremnant in the central part of BBHVF and it lies at the margin
of theFekete-hegy Maar Volcanic Complex (Auer et al., 2007). There
are noavailable KAr and ArAr radiometric ages directly from the
cone, but
image of Fig.8
-
Fig. 9. Boncsos-tet. For detailed information see Fig. 4.
53G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 4558there
is a KAr age date obtained (3.3 Ma) from the nearby lavaplateau of
Fekete-hegy (Auer et al., 2007). In addition,
morphometricestimation and comparison to the nearby Gajos-tet
scoria coneconfirmed partly a similar age range (Kereszturi,
2010).
The inner structure of Boncsos-tet scoria cone is exposed in a
small,abandoned quarry at the northern side of the remnant (Fig.
3F). In thisquarry, thickly bedded, reddish to greyish, alternating
densely (TB1 inFig. 3F) and partly welded (TB2 in Fig. 3F)
scoriaceous pyroclasticbreccias and lapilli tuffs with steeply
dipping beds (3545) are inagreement with its proximal location
relative to the presumed originalcrater (Csillag, 2004).
The position of Boncsos-tet (located on the rim of the
lavaplateau of Fekete-hegy) favours the formation of large
landslides dueto slope instability (Csillag, 2004). There is
nomorphological evidenceof any type of breaching because the
location of the crater is alsodoubtful.
The morphometry of Boncsos-tet is still youthful because Hco
isabout 59 m and the Wco is 936 m. The volume is 0.012 km3.
Theaverage slope angle parameters vary from 7.2 up to 9.2 (Table
1).
Morphometric assessment: The present erosion remnant of
Boncsos-tet is largely asymmetric due to degradation during the
post-eruptiveperiod. As a result of landslides and the crater
removal, Save is onlycalculated by the Eq. (4) (Save=7.2) due to
the missing Wcr value.Although only the eastern slopes of
Boncsos-tet have been foundsuitable for the slopes-angle
calculation by DEM, it gives a similar resultSmean ~9.2
(Smean).
3.7. Gajos-tet
Gajos-tet (Figs. 2 and 10) is a poorly preserved, eroded
scoriacone from the Fekete-hegy Maar Volcanic Complex in the
central partof BBHVF (Fig. 2). The lava flow of Gajos-tet was
penetrated by theKapolcs-1 drill hole (Jmbor, 1980; Kereszturi,
2009) and its age of3.82 Ma is constrained by the upper lava flow
unit (Balogh et al.,1986).
The scoria cone and related lava flow are inferred to have
filled apreviously formed maar crater and spilt over the crater rim
(Auer et al.,2007). TheGajos-tet scoria cone ismade up of black to
reddish-basalticscoriaceous pyroclastic rocks, but the exposure of
the cone deposits isvery limited due to the vegetation cover and
the gentle slope angle.However, small, WE aligned lava ridges can
be identified in the DEMand confirmed in the field, which are
similar in size and location to theridges in the cases of Bondor
and Agr-tet.
The advanced erosional stage is reflected by the morphometry
ofthe volcanic cone (Fig. 10). Hco is 21 m, Wco is 1006 m, hence
Hco/Wcoratio is only 0.021 (Table 1). The preserved volume of the
deeplyeroded scoria cone is as low as 0.007 km3. The related lava
flowvolume has also been calculated as Vlava=0.017 km3. Because
thecrater is fully degraded, the Save calculation is only carried
out by theEq. (4), giving a result of 2.3. At the same time, for
DEM-basedcalculation, we sampled the southern slopes, which are
situated in asheltered position on the lava mesa of Fekete-hegy,
and the resultshows a slightly higher slope angle value ~4.0 (Table
1).
Morphometric assessment: The Gajos-tet is a normal-sized
scoriacone, when compared with the world average of 800 m in
width,sitting on relatively undisturbed position upon the basalt
mesa ofFekete-hegy. The small-scale topographic ridges demonstrate
thateffusive stage(s) led to the emplacement of a 1.5 km long lava
flow(Fig. 10). The preserved volcanic edifice is suitable for the
morpho-metric measurements (with the DEM-based method), but due to
themissing crater the calculation of Save values by the
traditional,formulae-based methods is impossible (Table 1).
image of Fig.9
-
Fig. 10. Gajos-tet. For detailed information see Fig. 4.
54 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012) 45584.
Discussion
4.1. Controls on scoria cone degradation in the BBHVF
According to early papers (Breed, 1964; Colton, 1967;
Dohrenwendet al., 1986), two parameters (Save and Hco/Wco ratio)
are the mostsuitable for the morphometry-based dating of
monogenetic scoriacones. However, neither slope angle nor Hco/Wco
ratio decreases at aTable 2Morphometric age vs. radiometric age.
Note: 1DEM-based mean slope angle values of the scone related lava
fields; 4Geological ages based on KAr and ArAr radiometric
determina2007; Kereszturi, 2010).
Cone Smean1 Hco/Wco2 Vlava3
degree Ratio km3
Agr-tet 14.7 0.097 0.365
Smean based morphometrical age estimateKopasz-hegy 11.9
0.0001Kopcsi-hegy 11.7 0.000Bondor 10.8 0.056Boncsos-tet 9.2
0.084Badacsony 8.0 0.102Gajos-tet 4.0 0.017
Hco/Wco based morphometrical age estimateBoncsos-tet 0.064
0.084Kopcsi-hegy 0.050 0.000Bondor 0.040 0.056Badacsony 0.029
0.102Kopasz-hegy 0.021 0.000Gajos-tet 0.021 0.017constant rate,
thus the morphometric modifications do not show alinear trend
(Dohrenwend et al., 1986). Because, the slopes of youngcones are
more susceptible to rapid erosion, whereas degradationespecially
where vegetation developsslows down with time (Wood,1980b). Thus,
the precision of the relative dating also decreases withincreasing
geological age. Consequently, whereas general rules areapplicable
for young and fresh cones, the older cones may follow thesetrends
to a lesser extent having larger errors in relative age
estimation.tudied cones; 2Hco/Wco ratio calculated from the Table
1; 3Volumetric parameters oftions (Balogh et al., 1986; Balogh and
Pcskay, 2001; Auer et al., 2007; Wijbrans et al.,
Effusion scale Expected age basedon morphometry
Real geological agebased on KAr andArAr dating (Ma)4
Large-scale Absolute youngest 3.33.0
Small-scale Youngest 2.52.8Non 2.6Medium-scale
2.32.9Medium-scale 3.3 and (2.83?)Medium-scale 3.4Medium-scale
Oldest 3.8
Medium-scale Youngest 3.3 and (3.02.8?)Small-scale
2.6Medium-scale 2.92.3Medium-scale 3.4Small-scale
2.52.8Medium-scale Oldest 3.8
image of Fig.10
-
55G. Kereszturi, K. Nmeth / Geomorphology 136 (2012)
4558Previously, two main factors have been identified that
determinethe major changes on the shape and morphometry of the
scoria cones:age of the edifice and the climatic setting (Wood,
1980b). Based onmorphometry and volcanological settings of the
scoria cones of BBHVF,the effects of these main factors on
degradation can be recognised.However, we also found a third
factor, namely, the textural character-istics of the cones (e.g.
erosion-resistant components), which has alsoplayed an important
role on slope processes and thus morphometry.
4.1.1. Age control on degradationPrevious attempts that were
targeted to date scoria cone fields used
various parameters to approximate edifice ages (Sucipta et al.,
2006). Inorder to detect the variations of these age indicator
parameters amongolder cones, we systematically examined the
behaviour of Smean andHco/Wco ratio (Table 2). The results are
varied because Hco/Wco ratios donot show a clear trend (for the
detailed explanation see later; Table 2).Only the DEM-based Smean
values show a systematic decreasing trendwith time similar to other
studied scoria conesworldwide (Dohrenwendet al., 1986; Favalli et
al., 2009). However, there is one exception, Agr-tet, which does
not fit to the overall decreasing trend, because of itshaving an
old age but the highest slope angle values (Table 2). Based onthis
trend, the eruptive age control on morphometric parameters
isconfirmed in the case of older scoria cones as well.
4.1.2. Climatic control on degradationThe progressively more
gentle slopes, i.e. the decreasing slopes
values with time due to degradation were calculated from
theobtained Smean and Save parameters (Fig. 11). The rates show a
widerange because of the differences in morphometric methods. The
slopeangle rates derived from the traditional, formulae-based
methods(Eqs. (3) and (4)) are 3.94/Ma and 0.79/Ma, respectively. A
higherrate (5.03/Ma) was calculated using the DEM-based Smean
values(Fig. 11). In the case of formulae-based method (Eq. (3)),
the lowerFig. 11. Diagram of slope angle values and degradation
rates of scoria cones of BBHVF calcunewly developed DEM-based
(Smean) methods. Note the poor line fitting for the slope
angletruncated cones.value of Save degradation rates are the result
of the limited number ofsuitable cones (only three cones were
suitable for parameterizationdue to the lack of crater), whereas
the DEM-based estimation is basedon six cones. The Agr-tet scoria
cone, which evolved in a differentway (e.g. dominant effusive
activity, different deposits preserved inthe scoria cone
successions), was systematically discarded from theslope angle rate
calculations in order to get more representative slopedecrease rate
for the classical scoria cone examples. From the abovementioned
rates, we accepted the result of DEM-based methodbecause these
result are in agreement with KAr and ArAr ages (seedetails
later).
The calculated rates of slope decrease at BBHVF (~5/Ma)
arecomparable with other published rates, for example in the
Cimavolcanic field, Mojave Desert, California, where the average
rate ofslope angle decrease (Save) is around 6/Ma (Dohrenwend et
al.,1986). In addition, the San Francisco Volcanic Field (Arizona)
hasshown similar slope decrease over the last 2 Ma period of time,
forinstance the Woodhouse age cones; ~8/Ma (Wood, 1980b).However,
we have to keep in mind the fact that these cones aremostly
characterised by erosion resistant inner structure, such aswelded
or agglutinated deposits, thus the calculated Smean decreaserate is
at least minimum estimate. Interestingly, both of the
analoguefields for BBHVF are characterised by mostly low annual
precipitation(100500 mm/year) and dry climates which favour a slow
degrada-tion and relatively good preservation of the volcanic
edifice (Wood,1980b). This slow rate of slope degradation may
indicate a similarsemi-arid climate in the Carpathian Basin during
the Pleistocene,which is in accordance to other climate studies
(Fbin et al., 2000;Kovcs et al., 2011; Sebe et al., 2011).
4.1.3. Architectural control on degradationInternal settings,
e.g. age of the edifice, and external factors, e.g.
climate, both control scoria cone degradation as we have
seenlated by various methods including traditional, formulae-based
(Eqs. (3) and (4)) ands calculated by traditional techniques and
the limitation of this method on older and/or
-
56 G. Kereszturi, K. Nmeth / Geomorphology 136 (2012)
4558previously. In the BBHVF, however, an additional third
maincontrolling parameter such as volcanic architectural diversity
can berecognised in the BBHVF that affect the cone morphometry.
In general, the architecture of the scoria cones has two
end-member types based on their explosive and effusive activity.
Onthe one hand, most explosive cones are built up mostly by loose
lapilliand ash as well as minor, e.g. locally distributed, lava
spatter,favouring rapid erosion. Alternatively, the more
ash-dominatedcones can be generated by violent Strombolian type
eruptions (Martinand Nmeth, 2006; Valentine and Gregg, 2008). On
the other hand,more effusive scoria cones are made up of the lava
spatter-dominatedbeds, which in turn are more resistant than the
lapilli-dominatedones. Both types above are usually formed by
Strombolian-styleeruptions. Based on field observations, the scoria
cones of BBHVF arecloser to the last end-member, i.e. lava
spatter-dominated cones.
In order to understand the role of effusive activity on
degradation,we categorised the seven studied scoria cones of BBHVF
by the totalvolume of the emitted lava flow/field (Table 2). The
groups are thefollowings: (A) large-scale (0.1 km3); (B)
medium-scale (~0.10.01 km3) and (C) non-effusive or small-scale
effusive activity (~00.0001 km3).
(A) Large-scale effusive activity means a large volume (0.1 km3)
oflava emplaced during the course of the eruption, forming
anextended lava field, e.g. in the case of Agr-tet (Fig.
4).Nevertheless, at Agr-tet, the large volume of lava may
beinterpreted as the effect of possible additional fissure
vent(s)during the emplacement of the lava field. However, the
existenceof small ridges around the cone seems to confirm that
Agr-tetemitted at least a certain portion of the measured lava
plateau.Theeruptionhistory ofAgr-tet is theonly example
forwhichnoevidence exists for phreatomagmatic activity, theentire
evolutionseems to have been governed by magmatic processes and
notphreatic and/or phreatomagmatic fragmentation (Kereszturiet al.,
2011). The large-scale effusive activity of Agr-tet makesthewhole
inner part of the volcanic edificemore erosion resistant(Fig. 3A).
This resistance largely controls the morphometricparameters as well
as slopes of the present volcanic cone andhence the result of
morphometric dating and relative age of thecone.
(B) Medium-scale effusive activity (Table 4) ranges between 0.1
and0.01 km3 of emitted lava, for instance Bondor,
Boncsos-tet,Badacsony and Gajos-tet. These cones are characterised
by mildStrombolianeruptions. In individual cases (e.g. Bondor,
Boncsos-tet), we documented weakly to moderately agglutinate
orwelded pyroclastics; however, their distribution is limited
andgenerally associated with the central crater area and along
cross-cutting dykes. These types of architectural irregularity have
notdisturbed the morphometric parameters to any great extent northe
age estimates.
(C) Non-effusive or small-scale effusive activity means that the
totalvolume of the lava is relatively small or not evident, e.g.
Kopcsi-hegy, Kopasz-hegy. Although they have been formed around2.6
Ma, these cones have not shown as young morphometricparameters as
the Agr-tet. This anomaly can be explained by arapid degradation
period. As a consequence, the minor effusiveactivity is favourable
to rapid erosion of the cones.
Based on this classification and the preserved Smean values,
theimpact of this architectural diversity may correlate with the
totalvolume of the lava flow field related to the monogenetic
edifice. Inother words, the larger effusive activity is favourable
to a higherdegree of welding or agglutination leading the formation
of anerosion-resistant inner core of the edifice. Due to erosion,
these innerirregularities may be exposed and are able to distort
the morpho-metric parameters of the cones, like in the case of
Agr-tet.4.2. Morphometric dating of the Pliocene scoria cones of
BBHVF
4.2.1. Pitfalls of traditional morphometric dating method on
older scoriacones
The traditional morphometric dating (based on either Save or
Hco/Wcoratio) has a wide range of pitfalls, especially for older
scoria cones or forcomplex cones that underwent various
post-eruptive tectonic/erosiveprocesses. Scoria cones are obviously
more diverse volcanic landformsthan previously considered, and it
is likely that the classical loose coarseash and lapilli dominated
edifices rather represent a small fraction of thefull spectrum.
The eruption diversity, both internally (e.g. eruption
stylevariations, magma flux changes) and externally (e.g.
gravitationalcollapse and/or cone over-growth) strongly affect the
initial geometryof the cone on which the syn and subsequent erosion
takes place(Nmeth et al., 2011). Thus, understanding and
identifying any sign ofthese processes could play a key role in
interpreting morphometricdata. The initial effects of eruption
diversity can be enhanced overlong-term (millions of years) erosion
history, and likely createunexpected morphological scenarios of the
eroded cones such asyouthful morphology on an otherwise old
cone.
However, not only are the traditional methods, i.e. Hco/Wco
orSave, affected by syn- and post-eruptive processes. The
evidencefor these modifications can clearly be seen in the large
diversity ofslope angle histogram and the standard deviation of
slope angles(Figs. 410). As a result of various eruptive mechanisms
and thedistribution of pyroclastics preserved in the cone edifice,
the scoriacones in the BBHVF can be subdivided into two groups
based ontheir shapes.
The Type 1 cones have either a scattered/truncated shape,
e.g.Boncsos-tet and Kopasz-hegy and/or a multi-peaked slope
anglehistogram such as the Agr-tet. The standard deviation of slope
anglesis generally large in this case4 that Type 1 cones (Figs. 4
and 9). Theseasymmetric cones resulted from syn- and post-eruptive
modificationand truncationof theedifice, e.g. landslide,
ventmigration, gully erosion.These two cones show the largest
morphometric irregularity anddisturbing effects on traditionally
measured Save and Hco/Wco (Table 1and 2). In these cases, the Save
is only calculated using the Eq. (3) due tothe removed crater.
In the case of Agr-tet, the multi-peaked and slightly
asymmet-ric slope histogram can be the result of physical
properties onpreserved pyroclastics on the western and eastern
flanks of thevolcanic edifice (Fig. 4). In terms of morphometry,
the Agr-tetscoria cone also differs significantly from the other
cones studied dueto its different eruption mechanism (magmatic
processes withoutphreatomagmatic eruptions), architecture (lava
spatter-dominance)as well as its large-scale lava effusive activity
(lava field volume0.365 km3). The absolute age of Agr-tet is 3.3 Ma
(Wijbranset al., 2007) which is similar to Boncsos-tet, 3.33.0 Ma
(Auer et al.,2007), but the primary morphometric age-indicators,
Save (14.7 vs.9.2) and the Hco/Wco ratio (0.097 vs. 0.064), differ
considerably.Consequently, the aforementioned differences together
may beresponsible for deviance of Agr-tet, i.e. the very
youthfulappearance in spite of its old age.
The Type 2 cones are mostly symmetrical in shape that
favourbetter the results of parameterization both by tradition and
DEM-basedmethods. The cones, e.g. Kopcsi-hegy, Badacsony, Bondor
andGajos-tet, show generally compact, one peaked slope histograms
andrelatively narrow, i.e. under 4, standard deviation of slope
angles(Figs. 6 and 8). The only exception is the Kopcsi-hegy, which
ischaracterised by three mounds and saddles between each other(Fig.
7). Around these saddles, the slope angles are lower than on
theflanks, leading to multi-peaked slope histograms and a wider
range ofstandard deviations, i.e. 5.7. In terms of morphometry,
however, theKopcsi-hegy is one of the most suitable cones for
morphometricdating because of the intact shape with a relatively
closed crater. A
-
57G. Kereszturi, K. Nmeth / Geomorphology 136 (2012)
4558possible explanation for the symmetrical shape may be the
presenceof a homogenous distribution and properties of the
cone-buildingmaterials such as lapilli and breccias-dominated tuffs
derived from aseries of Strombolian-type eruptions. Only locally do
these conescontain pyroclastic beds that are more resistance to
erosion, i.e. lava-spatter beds, welded or agglutinated
pyroclastics. The presence ofthese erosion-resistant pyroclastic
beds, however, has little apparentinfluence on the morphometric
parameterizations of the conesregardless the type of method that
has been used.
To sum up, the possible pitfalls in the traditional,
formulae-basedscoria cone age-estimation method are (1) highly
scattered cones, e.g.Type 1 cones: Kopasz-hegy and Boncsos-tet; (2)
truncated cones and(3) cones associated with significant effusive
activity, e.g. Agr-tet.Any of these conditions may modify the
results of relative cone dating(Table 2).
4.2.2. Validity of DEM-based slope angle estimation and its
implicationsto older scoria cones
Our most significant result is that the DEM-based
morphometricdating results has been found appropriate with regard
to the KAr andArAr radiometric ages. In order to assess which of
the morphometricparameters (Smean and Hco/Wco) are more suitable to
characterise theage of the eroded cones of BBHVF, we carried out
morphometricestimation using both methods (Table 2). The Agr-tet
was notincluded in this test because of its different large-scale
effusive activity,which is suspected of causing the apparently
young, morphometric age.
In the case of the Smean-based age estimation, the cones
haveprogressively,more gentle slopes in accordancewith their age
spectrumfrom 2.4 to 3.8 Ma (Table 2). Effusive activity does not
significantlydistort the age obtained, because all the cones
examined emitted0.1 km3 lava (medium-, small-scale or non-effusive
activity).
At the same time, the Hco/Wco ratio shows a slightly
distortedtrend. According to this ratio, the youngest cone is the
Boncsos-tetand one of the oldest is the Kopasz-hegy (Table 2) which
significantlyconflicts with the KAr ages. In addition, these two
examples from theBBHVF have been truncated by various eruptive
and/or erosional-related processes. In these cases, the DEM-based
method provides amore precise age estimate.
5. Conclusions
(1). Scoria cones are built up of wide range of eruption
material,such as lava rocks, densely to non-welded breccias and
lapilli,rarely ash. This textural and structural diversity of
simplescoria cones is common. At BBHVF, the present shapes
ofstudied scoria cones are influenced by diverse
primaryeruption-related (e.g. Hawaiian-type eruption, large to
small-scale effusive activity) as well as secondary syn- and/or
post-eruptive erosion-related (e.g., scoria cone breaching, large
scaleslope failure and anthropogenic activity) processes.
(2). Comparison of DEM-based and traditional methods shows that
anumber of examples, e.g. Kopasz-hegy, Boncsos-tet and Gajos-tet,
are unsuitable for the traditional, formulae-based morpho-metric
parameterization (Eq. (4)) due to the erosionof the
crater.Specifically, Hco/Wco ratio does not show a strong
correlationwith the radiometric age of the scoria cones (Table 2)
owing tothe diverse nature of cone materials as well as the
highlydegraded and breached present morphology.
(3). The DEM-based method gave more reliable and
acceptableresults compared to the radiometric ages (Table 2)
determiningundisturbed outer slope angle values for the studied
cones.The DEM-based slope angle parameterization method is
alsouseful for highly truncated/asymmetric scoria cones, whichhave
only scattered remnants (e.g. Type 1 cones). This isbecause certain
parts of the outer cone slopes are still agerepresentatives after
nearly 4 Ma of degradation. At the sametime, the Hco/Wco ratio does
not show strong correlation withthe real, geological age of the
older scoria cones (Table 2).
(4). Assessing the precision of the measurements, the
DEM-basedslope angle estimation, based on thousands of pixels
repre-senting the cone surfaces, is more precise and robust.
Thetraditional formulae-based method is dependent on
threeparameters (Hco, Wco and Wcr), hence imprecise
parametermeasuring or taking highly disturbed, eroded zones of the
conecould yield unreliable results (see Kopasz-hegy or
Boncsos-tet). A similar tendency between data obtaining methods
hasbeen documented for the flank cones of Mt. Etna by Favalli et
al.(2009). However, the DEM-based slope angle can also
showscattered ranges of the slope histogram leading
multi-peakedhistograms and high standard deviations. If the
interpolationnoise is low, the individual peaks on the slope angle
histogramrepresent either differences in the architecture, i.e.
eruptiondiversity, and/or in the behaviour of erosional processes
thatshaped the flanks of the scoria cones.
(5). The accuracy of relative dating decreases with
increasingedifice age because the inner structure of the cone
(probablymore erosion resistant) gradually becomes more exposed
withtime. The properties and the distribution of these
exposedresistant layers govern the precision of morphometric
dating.
(6). In the BBHVF, three difference controlling factors have
beenidentified namely the age of the edifice, the climate during
thedegradation and the inner architecture of the edifice,
i.e.erosion resistance of pyroclastic rocks. The last factor is
morelikely to have played a major role in the formation of
erosion-resistant collars of scoria cones in the
BBHVF.Acknowledgements
This research was supported by Department of Geology and
MineralDeposits, University of Miskolc, Hungary and PhD Research
Fellowship ofthe Volcanic Risk Solutions, Massey University, New
Zealand (GK). Theauthors would like to thank to K. Balogh
(Institute of Nuclear Research,Debrecen) for the help in
interpreting the existing KAr radiometric data,to G. Jordn
(Geological Institute of Hungary, Budapest) for the
helpfuldiscussions about digital modelling and to G. Csillag
(Geological Instituteof Hungary, Budapest). Constructive comments
by J. Procter, K. Arentsen,D. Kartson, F. J. Dniz-Pez, J.-T.
Thouret and A. Harvey significantlyelevated the quality of the
manuscript.
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Structural and morphometric irregularities of eroded Pliocene
scoria cones at theBakonyBalaton Highland Volcanic Field, Hungary1.
Introduction2. Materials and methods2.1. Manual
parameterization2.2. DEM-based parameterization
3. Studied locations3.1. Agr-tet3.2. Kopasz-hegy3.3. Bondor3.4.
Kopcsi-hegy3.5. Badacsony3.6. Boncsos-tet3.7. Gajos-tet
4. Discussion4.1. Controls on scoria cone degradation in the
BBHVF4.1.1. Age control on degradation4.1.2. Climatic control on
degradation4.1.3. Architectural control on degradation
4.2. Morphometric dating of the Pliocene scoria cones of
BBHVF4.2.1. Pitfalls of traditional morphometric dating method on
older scoria cones4.2.2. Validity of DEM-based slope angle
estimation and its implications to older scoria cones
5. ConclusionsAcknowledgementsReferences
