Geomorphology 110 (2009) 118–127

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Geomorphology

j ourna l homepage: www.e lsev ie r.com/ locate /geomorph

Block-and-ash flow deposition: A conceptual model from a GPR survey onpyroclastic-flow deposits at Merapi Volcano, Indonesia

C. Gomez a,b,⁎, F. Lavigne a, D.S. Hadmoko a,c, N. Lespinasse a, P. Wassmer a

a Paris 1 Université Panthéon Sorbonne, Laboratoire de Géographie Physique CNRS UMR 8591, Meudon, Franceb Paris 7 Université Paris 7 Denis Diderot, Laboratoire Prodig, Paris, Francec Universitas Gadjah Mada, Dept. of Physical Geography, Yogyakarta, Indonesia

⁎ Corresponding author. Paris 1 Université PanthéGéographie Physique CNRS UMR 8591, Meudon, France.

E-mail address: Christopher.Gomez@cnrs-bellevue.fr

0169-555X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.geomorph.2009.03.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 June 2007Received in revised form 12 March 2009Accepted 30 March 2009Available online 7 April 2009

Keywords:GPRPyroclastic-flowBlock-and-ash flowsDepositionConceptual modelVolcanoMerapiJavaIndonesia

In 2006, a series of block-and-ash flows swept the southwestern and southern flanks of Merapi Volcano, Java,Indonesia. In the K. Gendol valley, near the village of Kaliadem, we conducted a GPR survey on the most distallobe of the June 14th second block-and-ash flow deposit. For this 100 m-long transect, we used a commercialGPR RAMAC© mounted with 100 MHz antennas. We measured the topography with a synchronized GPS anda laser rangefinder. Back at the laboratory, we processed the dataset with the software REFLEX®. Data of thesubsurface reveals a series of layers, separated by strong reflective horizons. These horizons are themanifestation of intercalations of fine materials in between more coarse layers. The architecture of theselayers presents progradation, retrogradation and aggradation patterns that we relate to the block-and-ashflow deposition process. Based on these observations we proposed a relative chronology of the depositionand a simple conceptual model of the deposition. The model show that the block-and-ash flow can depositeither long, close to horizontal single layers, or shorter layers that imbricate themselves, following differentpatterns (progradation, retrogradation or aggradation). Nevertheless we remained cautious, since we onlystudied a very short portion of the deposit, and similar experiences need to be repeated. Moreover there arereflections in the radargram that we could not identify, and further studies need to be conducted.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Block-and-ash flows (BAF) are small volume pyroclastic flows(Michol et al., 2008). They are a serious threat for inhabitants ofvolcanoes located over subduction zones, like at Colima Volcano(Saucedo et al., 2004), Unzen Volcano (Miyabuchi, 1999), Montserrat(Smith and Roobol, 1990), or Merapi Volcano (e.g. Abdurachman et al.,2000). At Merapi Volcano, dome collapses trigger BAF (Boudon et al.,1993), which are referred to as “Merapi-type” pyroclastic flows ornuées ardentes (Voight et al., 2000).

BAF deposits are unsorted, ash-matrix supported materials. On flatand open areas, the internal stratification is weak or inexistent, whereasthe deposits are often stratified in channels (Macias et al.,1997; Saucedoet al., 2004). This stratification includes three principal units: a basalavalanche deposit, a ground surge and an ash cloud deposit (Druitt,1998; Schwarzkopf et al., 2005).Up to thepresent, therehavebeenmanystudies of pyroclastic-flowdeposits (e.g. Sparks,1976; Druitt et al.,1989;Suzuki-Kamata and Kamata, 1990; Miyabuchi et al., 2006; Charbonnier

on Sorbonne, Laboratoire de

(C. Gomez).

ll rights reserved.

and Gertisser, 2008), but empirical data is difficult to collect. Therefore,flow and deposition research is essentially based on models.

Scientists commonly agree on pyroclastic surge deposition byaggradation, however models lead to different conclusions for theground surge. At first, the idea of en masse freezing depositionprevailed (Sparks, 1976). Later, Branney and Kokelaar (1992) broughtup the idea of progressive aggradation deposition. More recently,Schwarzkopf et al. (2005) studied the 1994 and 1998 Merapi's block-and-ash flows and concluded on an initial pulse that solidifiedrearward from its front and from bottom to top. While these modelsseparate flow and surge, Burgisser and Bergantz (2002) presented anintegrated approach linking flow and surge. Hence, considerable roomremains for further work.

Located inCentral Java, Indonesia,Merapi Volcano (2965m) is oneofthemost active volcanoes in theworld,withmore than eighty eruptionssince the 16th century (Voight et al., 2000), of which a dozen causedcasualties (Simkin and Siebert, 1994; SEAN, 1989). Its latest eruptionstarted in July 2005 with an increase seismic activity and thedeformation of the summit, eventually resulting in a new domeextrusion in March 2006. From May 5th to 27th, rockfalls (guguran)and BAF ran down the southwest flanks into the K. Krasak, K. Bedok andK. Boyong valleys, traveling a maximum of 4 km. OnMay 27th, a violentearthquake in Bantul, 35 km to the south of Yogyakarta, is believed to

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have reactivated the volcanic activity. On June 4th, the dome reached116 m above the previous summit. The following block-and-ash flowstravelled to the south in the K. Gendol valley, reaching 7.5 and 4.5 kmonJune 14th. The activity slowly decreased thereafter.

A complete day-by-day chronology of the event is archived on thewebsite of the Volcanological Survey of Indonesia (http://portql.

Fig. 1. Study location on the South flank of Merapi Volcano. (1) Pyroclastic-flow deposits of 1valleys; (4) SABO dams built by the Ministry of Public Work in collaborationwith the Balai Suupstream towards the summit of Merapi Volcano. The grey arrow indicates the lobe we studieof June 14th. (b) This second picture was taken a little downstream from the previous one anarrows indicate the location of the lobe of the 2nd block-and-ash flow deposit, where we c

vsi.esdm.go.id/joomla), and Charbonnier and Gertisser (2008) gavean interesting description of the surface characteristics of the surfacefeatures of the entire deposit.

For our survey, we worked on the most distal lobe of the secondBAF deposits of June 14th 2006, 4.5 km from the summit, near thevillage of Kaliadem (Fig. 1). We investigated a 100 m long transect

994 and 2006; (2) Villages and populated areas on the volcanic cones; (3) streams andngai dan Sabo and the Japanese Ministry of Public Work PWRI. (a) This picture is lookingd, and the black arrow indicates the superelevation bend of the first block-and-ash flowd shows the superelevation bend on the left side of the valley (black arrows). The greyonducted this study.

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with GPR (Ground Penetrating Radar), with the two following aims:(1) assessing the architecture of block-and-ash flows deposits, witha non-destructive survey of the sub-surface before erosion; and(2) relating this architecture to a probable deposition processes.

2. GPR on volcanic materials

GPR technology is based on electromagnetic waves characteristicsthat respond to dielectric properties and discontinuities of thematerial they travel in.

The tool is a portable device that consists of a radio controller, auser interface and two synchronized antennas. One of the antennasproduces the electromagnetic signal, and the other receives it.Antenna frequencies typically range from 10 MHz to 1200 MHz. Lowfrequency antennas can penetrate to ≈50 m depth with a precisionless than a meter, while high frequency antennas can offer millimeterprecision but only penetrate a few centimeters into the subsurface.Hence, GPR is particularly attuned to shallow surface exploration. Inearth sciences, the use of GPR surveys increased from the 1990s.Recent developments concern hydrostratigraphy (e.g., Kostic et al.,2005), landslide assessments (e.g., Grandjean et al., 2006), faultssystems in geology (e.g., Rashed et al., 2003), archeology (e.g., Bakeret al., 1997), and tsunami deposits (Gomez et al., in press). GPRexploration also developed on volcanic terrains. Themethod is usuallycalibrated against outcrops (Russell and Stasiuk, 1997; Gomez-Ortizet al., 2007) or against other geophysical methods, like ERI — ElectricResistivity Imagery (Gomez-Ortiz et al., 2006). Investigated materialsinclude lava, airfall pumice, channeled and non-channeled pyroclasticflow deposits (Russell and Stasiuk, 1997; Gomez-Ortiz et al., 2007;Gomez et al., 2008), and mud volcanoes (Chow et al., 2006). Despitethose developments, volcanic terrains present strong challenges toGPR users because of the significant changes of dielectric propertieswith the mineralogy, the state of oxidization, etc. (Olhoeft, 1998;Heggy et al., 2003). Therefore, it is important to conduct appropriatedata processing, in order to retrieve reliable data.

3. Field methodology and data processing

We conducted a field survey with a commercial GPR Ramac© onthe distal lobe of the June 14th second BAF deposit (Fig. 2). We

Fig. 2. Photograph of the block-and-ash flow deposit lobe studied. On this picture P. Wassm

simultaneously measured the topography with a GPS and a laserrangefinder, starting upstream, on the lobe towards the lobe's mostdistal reach.

We then processed the data with Reflex® software as follows.Firstly, we eliminated the open-air signal and we lowered the surfaceecho. Secondly, we enhanced hyperboles on the radargram in order tocalculate the real velocity of the signal from the hyperbola slopeangles (hyperboles are traces on the radargram produced by specificobjects). Based on these calculations, we could compute the real depthof the subsurface. Thirdly, we modified the gain and energy decay inorder to compensate the signal intensity loss with depth andaccentuate the horizontal structures. In the end, we performed aKirshov migration and inserted the topography, for retrievingstructure angles and orientations that conform to reality. Some partsof the radargram with significant signal losses and/or disturbanceswere interpolated from their nearest neighbors. Wemarked the signaldisturbances and losses on Fig. 3 with grey vertical stripes.

To complete this data processing, we proceeded through a graphicdifferentiation of the various units we distinguished (Fig. 3) and wenumbered them from bottom to top (Fig. 4).

4. Results

4.1. The unit architecture, from radargram to reality

We countedmore than 260 units on the radargram (more units arehidden by the surface echo, and 260 is certainly an underestimation).The length of each unit varies between 0.6 m and 34.7 m (Supple-mentary Table 1), with an average of 7 m (49% less than 5 m and 82%less than 10 m). These units are inclined at −10° (10° belowhorizontal), with angles ranging from −45° to +23°. Hence, alllayers do not follow the valley inclination, since they can be inclinedupstream. Based on the different angles we distinguished 8 differentgroups of units (Fig. 5).

Located at the bottom of the deposit, group A displays units withangles ranging between −10° and +20°. However they are close tothe horizontal on average and they are bound to the “previoustopography” below the deposit. Units inclined from −10° to −15°dominate group B. All units of group B display negative angles.Group C is dominated by horizontal units and a few layers with

er is the scale (about 1.80 m tall). The photograph is looking upstream to the North.

Fig. 3. GPR data at different stages. The upper left part of the figure presents the GPR data before we proceeded to data processing. In the upper right, the limits between every unitwere extracted. Finally, the succession of units was retrieved. At this stage the units are unknown, but the horizons that separate the units are observed to be either of low reflectivity(majority) or of high reflectivity.

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positive angles. Units with positive angles are preferentially located atthe top and in the upstream part of the group. The fourth group (D)can be linked to group B, since most units have the same inclination,however the difference lies in the intercalation of lower angle units.Groups E and F are characterized by units that are close to horizontal,with a few intercalations of units inclined between 0 and 10°. Group Gis located at the front of the deposit and corresponds to a series ofunits that are overlapping each other at the most distal reach of thedeposit. Therefore they exhibit a strong inclination, over−30°. Finally,we identified two groups of units we named groups H. Groups H arecharacterized by positive angles, showing units with angles rangingfrom +10° to over +20°.

The architecture — as on the radargram — is dominated byhorizontal stratification of a series of units. The inclination of lowerunits is mostly governed by the underlying topography, whereashigher units progressively break free of this constraint, with groups Hdisplaying inclinations opposite to the slope orientation (Fig. 5).

Each unit detected with the GPR is bounded by reflective horizonsof undetermined nature. These horizons can have either a highreflectivity (bold lines on Fig. 6A) or low reflectivity (thin lines onFig. 6A).

According to GPR experimentation and past surveys, reflectivehorizons arise from a clear discontinuity in dielectric properties of thematerial. This can be due to either a thin horizon of fine material, or a

Fig. 4. Explanation of the measures performed on the data set. Measures are performed from the final radargram, once it was totally processed. The distance is calculated on the flatand does not take into account the topography.

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thin horizon ofmore reflectivematerial, or a change in thematerial (e.g.from one unit to another, from dry material to humid material, etc.). Inour case study, visual observations at the study site in 2007 offered anoutcrop showing a few thin layers in between more coarse material(visual observations by S.R. Hadmoko and C. Gomez). Although thisobservation accounts for a few thin layers that we can reasonably relateto the horizons with strong reflectivity, we did not find any evidence ofthe low reflective horizons that we saw on the radargram.

Fig. 5. Groups of units per class of angle. The different groups exhibit a series of horizontal gbottom of the deposit (−15°), whereas angles exhibit in the upper part of the deposit can

Based on this observation and our knowledge of reflections onradargrams, we grouped units into 20 layers that correspond to layersof coarse materials separated by layers of thin material (Fig. 6B).Layers display different lengths, from ten of meters to almost ahundred meters, with thicknesses varying from a meter to more thanthree meters. Unlike the units we previously depicted, all the layersfollow the general slope of the deposit, although they exhibitcomplications in the detail.

roups of units. These units exhibit angles that are close to that of the topography at thebe over +20°.

Fig. 6. Layers bounded by strong reflective horizons. The strong reflective horizons are thin layers of ash that produce a stronger reflection, and that allow delineation of layerscomposed of coarse materials.

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4.2. Deposit architecture and deposition implications

After the examination of the layers' architecture we identified threedifferent patterns that provide evidences on different depositionprocesses (Fig. 7). (1) Progradation is characterized by layers thatoverlap each other, so that the upper layer reaches a greater distancedownstream (Fig. 7A). This type of architecture is present in the depositfor layers 10, 11 and 15 or 9, 12 and 13, or 3, 6 and 8. (2) Retrogradation(Fig. 7B) is the exactopposite of progradation, the top layers donot reachthe distal extents of the previous layers. This type of architecture isexemplified at our study location by layers 17,15, 20 and 18 or 1,2 and 3.(3) Finally, we defined an architecture as vertical aggradation (Fig. 7C),when the architecture of the layer seems random.

The deposit's architecture obviously implies different depositionalprocess that generated the present architecture, with progradation,retrogradation and aggradation phases. Although we recognizedifferent elements that were deposited in sequence, it is impossibleto know if the deposit is the result of a single phase of deposition orseveral phases. We cannot shed light on the origin of the differentparts of the deposition, but we can propose a relative chronology ofthe deposition process.

The deposition process of this lobe of BAF started with the layer 1that covered the previous topography, and extend almost along theentire 100 mwe studied. Above this first layer, layers 2 and 3 exhibit aretrogradation patternwith shorter extend. It cannot be determined iflayers 2 and 3 were deposited before or after the two layers 4 and 5located at the front of the lobe. The order is either 1, 2, 3, 4 and 5 or 1, 4,5, 2 and 3. Then layers 6, 7 and 8 were deposited in an unclearaggradation pattern. Considering the short extension of layer 7 andthe way layer 8 imbricates at the back of layer 7, layer 7 was certainlypartly eroded by the further deposits. This may also be true of layers 4and 5 that have short extensions. Then, there is a long layer that seemhomogeneous and that covers all the previous units, the same wayunit 1 does (layer 9). On top of this layer there is a vertical aggradationseries in the most distal part of the lobe (layers 12, 13 and 7) and aprogradation in the center of the transect (layers 10, 11 and 15). Overthose deposits, layers 14, 16, 20, 18 and 19 were deposited.

Hence, we can describe a general deposition process based on theposition of each layer and the way they overlap each other, however itleaves room for uncertainty since we cannot always determine whichlayer was deposited first. Moreover our data do not account forerosion processes that may have accompanied deposition process.

Fig. 7. Graphic explanation of the three main deposition patterns, named after Branney and Kokelaar (2002). The three patterns are present at the front of the BAF deposit.

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Then, this result needs to be treated with care, and we can only expecta general view of the phenomenon.

4.3. The June 14th 2006 BAF deposition process: a conceptual model

Based on the previously described series of layers, we tried toconceptualize the deposition process of the BAF of June 14th 2006 at itslobe. The model we propose here explains the apparent phases ofdeposition, but we need to extend this work in order to determinepattern that are common to all channeleddeposits of BAF, like thatwhichBranney and Kokelaar (2002) conducted for ignimbrite sedimentation.

Our conceptual model captured two different deposition patternsthat repeated twice at the study location (Fig. 8). The first pattern weidentified is the deposition of a long single layer that covered all theprevious layers (Fig. 8A, C). The second pattern concerns prograda-tion, retrogradation or vertical aggradation, where a series of layers,

Fig. 8. Conceptual model of the June 14th deposit at its most distal lobe. The deposition procewith the deposition of a long layer close to the underlying topography (A and C). During tpatterns: aggradation, progradation, retrogradation (terminology after Branney and Kokela

imbricate themselves in one way or another (Fig. 8 B, D, E). In the 14thJune deposit, these two patterns alternate, both appearing twice.

The deposition started with a single long layer; then was followedby a series of layer retrograding and then prograding; then, a longlayer, like the first one covered almost all the previous deposits; andfinally a series of smaller layers were deposited again.

5. Discussion

The 14th June 2006 block-and-ash flow exhibited a complexstructure, with a series of horizontal units bounded by reflectivehorizons on the radargram. The units are mainly inclined like theslope angle on which the deposit lies, even though a few units on thetop of the deposit are oriented in the opposite direction. The reflectivehorizons are of two types, low reflectivity and high reflectivity.According to our field visual observations, strong reflective horizonsseem to correspond with thin layers of fine materials that separate

sses are expressed in two sequences that are repeated twice. The first sequence proceedshe second sequence, a series of shorter layers is deposited (B, D and E) with differentar, 2002).

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layers of coarser materials. However, we could not identify the originof low reflectivity horizons that might be attributed to changes ingrain-size, amongst other factors, but this study cannot provide anyevidence to support one theory or another. We retrieved 20 layersfrom the radargram that exhibit a complex architecture. Thisarchitecture is the signature of different deposition processes (byanalogy with Branney and Kokelaar, 2002). Those processes are eitherprogradation, retrogradation or aggradation. We could also identifythe repetition of two different sequences: (1) deposition of a longlinear layer; and (2) deposition of a series of shorter layers (at leastshorter on the sectionwe studied), which imbricate in different ways:progradation, retrogradation or aggradation. Both sequences arepresent twice in the deposit. Hence, we defined a depositionalmodel for the front of this BAF, based on the identified sequences.

These results and observations are consistent with previousobservations (Sulpizio et al., 2008) in Italy on the Malfa terraces,where a stepwise aggradation deposition of pyroclastic densitycurrents was depicted. The deposition processes are divided accordingto the topography and the stepwise aggradation deposition is relatedto topographies with low slope angles. This corresponds to thecondition at Kaliadem, where our lobe was deposited on the lowerslopes of the volcano. Indeed, the internal structure of the lobesuggests the aggradation mentioned, but there is no trace of it in thetopography. We can combine this explanation with the fact thatchanneled BAF deposits seem to preferentially show a structure,whereas deposits in open areas tend to be massive and structureless,like the deposits that Abdurachman et al. (2000) observed previouslyat Merapi Volcano. These two elements may also explain why Russelland Stasiuk (1997) did not identify any architecture with the GPRwhen they surveyed pyroclastic-flow deposits.

The architecture we observed is certainly linked to the depositionprocess, and can be compared with those interpreted for ignimbritedeposits by Branney and Kokelaar (2002). Indeed, they also identifiedprogradation and retrogradation, similar to the observationswemade.Conclusions on the deposition process are also concordant with theidea of deposition from front to rear (Schwarzkopf et al., 2005) andprogradation deposition processes (Cordoba, 2005), developed inearlier conceptual models. Although, we can relate generally to thosemodels, they do not depict the complex interactions between layersthat we observed.

Unfortunately, we cannot understand from the radargram alonethe origin of the layers. Do they originate from pulses that are due tophases in the dome collapse, or do they form during the emplacementof the block-and-ash flow? To answer these questions we would needto study longer transects that would allow us to compare the data ofthe flow and the deposit.

Finally, we could not identify the cause of low reflectivity horizons.We suggested that they might be related to grain-size, but we did nothave any visual confirmation of this hypothesis. A hint may come fromSulpizio et al. (2008) who identified stratification in pyroclasticdensity currents according to the density of the material.

6. Conclusion

This GPR survey at Merapi volcano revealed the presence ofarchitectures inside block-and-ash flow deposits, and it links them tocorresponding deposition processes. This survey was carried on the“still hot” deposit, before any form of erosion occurred, bringing aninteresting insight to the formation of this lobe deposit. Thecontinuous volcanic activity (Charbonnier and Gertisser, 2008)unfortunately stopped us from conducting further studies on otherparts of the deposit. Indeed, similar studies have to be extended inorder to confirm the present results and try to bring into light thecomplex relations between deposit architecture and processes ofdeposition.

Acknowledgment

We would like to thank K.-J. Sandmeier for his help on the GPRtransects. We also thank Andrew Plater for the careful editorialhandling, and the numerous improvements he brought to this paper.We are also in debt to two anonymous reviewers.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.geomorph.2009.03.024.

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