Insights into the October November 2010

Journal of Volcanology and Geothermal Research

journalhomepage:www.elsevier.com/locate/jvolgeores

Insights into the October–November 2010 Gunung Merapi eruption (Central Java, Indonesia) from the stratigraphy, volume and characteristics of its pyroclastic deposits

a b b b Shane J. Cronin c ⁎ , Gert Lube , Devi S. Dayudi , Sri Sumarti , S. Subrandiyo , Surono

b Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North, New Zealand c Center of Volcanology and Geologic Hazard Mitigation (CVGHM), Jalan Diponogero, Bandung 40122, Java, Indonesia Balai Penyelidikan dan Pengembangan Tekhnologi Kegunungapian (BPPTK), Jalan Cendana, Yogyakarta 55166, Java, Indonesia

article info

abstract

Article history: The 2010 eruption of Merapi was the second most deadly in the historic record of this volcano, claiming over 380 Received 16 March 2012

lives. By relating the observations of this eruption with detailed examination of deposit distribution, stratigraphy Accepted 15 January 2013 Available online 24 January 2013

and sedimentology, a reconstruction of the properties of the pyroclastic density currents (PDCs) is presented, including the valley controlled block-and-ash flows (BAFs) and widespread, energetic pyroclastic surges. The distri-

Keywords: bution, volume and mobility characteristics of all types of PDC during the eruption sequence show evidence for Merapi

levels of intensity unseen since the large-scale 1872 and 1930 eruption phases, especially during the climactic Pyroclastic flow

events of October 26 and November 5. Many tephra falls interbedded with PDC units show that most dome- Pyroclastic density current

collapse events occurred along with and between explosive vulcanian eruptions. The 2010 eruption produced Pyroclastic surge

very long-runout BAFs, reaching 16.1 km in the Kali Gendol on November 5. This runout could be explained by Dome

its large-volume (20 million m 3 ), around 10 times that of previous Merapi BAFs during the last 130 years. Major Block-and-ash flows

avulsion of these dense BAFs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive PDC deposits. Spreading avulsed BAFs were a particular hazard downstream of ~10 km where the landscape is less dissected. Less clear, however, is why pyroclastic surges extend-

ed up to 10 km from the vent on November 5 and >6.4 km on October 26. These expanded much farther from BAF margins (~2 km) than ever seen before at Merapi. In one location they were decoupled from valley-centered BAFs with high momentum, traveling initially laterally across steep valley systems, before draining downslope. At this site, on the western side of the upper Gendol at around 3 km from source, surge decoupling was apparently exac- erbated by upstream collision and deflection of high-flux, hot and gas-rich BAFs against the cliffs of Gunung Kendil. The 1.4 km-long cliff face was impacted directly for the first time in 2010 events, and may have been responsible for the formation of larger than normal turbulent ash-rich clouds above BAFs. These results imply that future eruption events under the present summit and upper flow-path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard, especially now that dense forest has been destroyed on the upper southern slopes of the volcano.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction 2011 ). Such currents also occurred in the most lethal event known, on 18–19 December 1930; at least 1369 people were killed by pyroclastic

Many eruptions of Merapi over the last 80 years have generated de- density currents (nuées ardentes) ( Neumann van Padang, 1933 ). The vol- structive and deadly pyroclastic density currents (PDCs) in association

cano has become the type-example for “Merapi-style” block-and-ash with lava dome effusion ( Boudon et al., 1993; Abdurachman et al.,

flows, engendered by the gravitational collapse of growing lava domes, 2000; Andreastuti et al., 2000; Voight et al., 2000; Schwarzkopf et al.,

with the bulk of deposition confined to valleys ( Abdurachman et al., 2005; Charbonnier and Gertisser, 2008; Gertisser et al., 2012 ). The

2000 ). The cases where eruptions have been particularly deadly or de- ~18 most destructive of these eruptions and the six that caused fatali-

structive, were attributed to: (A) avulsion of the dense basal avalanche ties (including those in 2006 and 2010) produced currents that spilled

part of BAFs (or basal avalanches) from channels due to dynamically out of channels ( Bourdier and Abdurachmann, 2001; Lube et al.,

thickening flow fronts ( Schwarzkopf et al., 2005 ), or rapid changes in channel confinement and sinuosity ( Lube et al., 2011 ); (B) decoupling

⁎ Corresponding author. Tel.: +64 63569099; fax: +64 63505632. of low-density pyroclastic surges from basal avalanches due to high E-mail addresses: s.j.cronin@massey.ac.nz (S.J. Cronin), g.lube@massey.ac.nz

ash contents (possibly amplified during collapses of hot gas-rich (G. Lube).

domes), as well as high volumetric fluxes in confined valleys ( Bourdier 0377-0273/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jvolgeores.2013.01.005

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

and Abdurachmann, 2001 ); or (C) surge-generation by sudden Schwarzkopf et al., 2002 ). Other mechanisms for generating highly de- hydraulic-jumps, such as at major breaks in slope, or descent of steep

structive and widespread pyroclastic density currents from similar cliffs (e.g., Kelfoun et al., 2000; Bourdier and Abdurachmann, 2001;

types of eruptions include directed explosive blasts from gas-pressured

Table 1 Eruption chronology summary based upon records of the Balai Penyelidikan dan Pengembangan Tekhnologi Kegunungapian (BPPTK) in Yogyakarta, and the Center of Volcanology

and Geologic Hazard Mitigation (CVGHM) in Bandung. Date

Remarks 20.10–22.10

Dome activity

Eruption columns

Pyroclastic density currents

Summit inflation since increased incandescence

2006 dome showing

Small explosions on 21.10

Rockfalls reported on 20.10

and 21.10

September, increasing HCl and decreasing H 2 O gas levels

23.10–25.10 Sharp increase in inflation

Steam and gas emissions

M 7.7 Mentawai earthquake c. 1200 km WNW; 10 km evacuation radius

26.10 Strong incandescence;

3 BAF lobes in Gendol with explosions destroyed

Multiple explosions reaching

8 seismically detected PDCs in Kali

runouts of 4.95, c. 4.9 and c. old dome

1.5 km height starting 17:02;

Gendol at:

18 km plume spreading W

17:02 (c. 9 min)

4.8 km seen on photographs

(VAAC Darwin)

17:18 (c. 4 min)

taken on 28.10; BAF avulsion

17:23 (c. 5 min)

into Tlogo valley; mobile surges

17:30 (c. 2 min)

into Kinahrejo and Kendil areas;

17:37 (c. 33 min)

at least 35 deaths

18:16 (c. 5 min) 18:21 (c. 33 min)

28.10 New dome extrusion

Multiple small explosions

3 seismically detected PDCs

Garuda Airline accident; ash

starting at c. 16:30

in Gendol with estimated

damage to left jet engine

runouts of 3.5 km

33 seismically detected PDCs in Gendol with estimated runouts of 2–4 km

30.10 Multiple explosions; 18 km

2 seismically detected PDCs at:

Ash fall in Yogyakarta

plume spreading S

00:16 (c. 7 min into Lamat, Senowo and Krasak drainages) 00:35 (c. 22 min into Gendol, Kuning, Krasak and Boyong drainages)

31.10 New dome growth

4 seismically detected PDCs

1.11 Dome growth

6.1 km plume

7 seismically detected PDCs in Kali

Aster Image captures thermal

(VAAC Darwin)

Gendol with estimated runouts of

signature of new dome, and

c.4 km

6.5–7 km long BAF deposit in Gendol

2.11 Dome growth

6.1 km plume

26 seismically detected PDCs;

(VAAC Darwin)

estimated runouts to 3.5 km

Reported runout for 14:04–14:27 dome destruction; very

3.11 Dome growth and

38 seismically detected PDCs:

PDCs was c. 10 km in Gendol; strong seismicity and explosion

11:11–13:19 (c. 2 min)

Runout of 14:44 PDCs in Gendol signals throughto 5.11

14:00–14:03 (4 c. 1 min

PDC/rockfall signals)

to c. 9 km reported at c. 17:30;

14:04–14:27 (series of

19 other visually confirmed PDCs

c. 5 min PDC signals)

with reported runouts of c. 4 km

14:44 (1.5 h lasting PDC signals)

4.11 Dome growth and

PDCs into Gendol to c. 9.5 km,

dome destruction

and into Bebeng, Putih, Boyong, Senowo and Apu drainages

5.11 Dome growth and

BAFs in Gendol to 16.1 km; dome destruction

>15 km plume spreading

Largest explosion (related to main

WSW–WNW (NASA Earth

event) starting on 5.11 at 0:05

1st PDC arrival at c. 15.5 km

Observatory)

on 5.11 at c. 0:25 (from eye-witness accounts); surges into Kinahrejo and Gendol areas

6.11–7.11 New dome growth

Multiple incandescent explosions

Explosions and PDCs into multiple

Main ash dispersal to WNW on

and ash plumes reaching heights

drainages;

6.11 and to WSW on 7.11

of c. 7 km on 7.11

PDCs increasing in runout on 7.11 and reported to reach c. 5–8 km runouts in Gendol

Multiple ash plumes reaching

Few PDCs gradually decreasing

Lahars occurring in drainages

0.8 to 7.6 km heights

in runout

from NNW to SE

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Fig. 1. Overview map showing the distribution of the 2010 pyroclastic density current (PDC) deposits and its main depositional facies: channel-con fined BAF units, avulsed massive overbank BAF deposits, bordered by a sharp singe zone, and pyroclastic surge deposits. The white dashed lines along the central axis of the Kinahrejo and Kendil surge areas demark the transect of the stratigraphic profiles presented in Fig. 7 .

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

domes (e.g., Woods et al., 2002 ) and collapse of dense vertical eruption longest flows occurring from 1702 h (c. 9 min), 1737 h (c. 33 min), columns associated with vulcanian explosions from domes (e.g. Clarke

and 1821 h (c. 33 min) ( Table 1 ). Throughout this two-hour phase, et al., 2002 ).

many tens of vertical explosions from the old 2006 dome were The 2010 eruption of Merapi was the second-most deadly event

observed, producing ash-rich plumes up to 1.5 km above the volcano. known from the volcano, with 386 deaths and 131 injuries reported

Upon arrival in the village of Kinahrejo (350 m west of Kali Gendol; by reliefweb ( www.reliefweb.int ) in January 2011. Observations of

Fig. 1 ), rescue teams noticed that PDCs had spilled out from Kali the events, along with initial assessments of the damage and deposi-

Gendol into Kali Opak and Kali Kuning to cause at least 35 casualties tion areas, revealed that this loss of life was in part due to basal ava-

in the Kinahrejo area. ASTER imagery, along with photographs taken lanches having extremely long runouts; they reached up to 16 km

on October 27 and 28 in the Kinahrejo and Kali Adem areas, showed from the volcano, whereas past basal avalanches causing fatalities

three 2–5 m high, bouldery fronts of PDC deposit lobes. These units mostly reached 8–13 km ( Bourdier and Abdurachmann, 2001 ). The

completely filled the box-shaped canyon stretch of Gendol valley be- only historic account suggesting similarly large Merapi BAFs is that

tween 3.9 and 5 km from source ( Fig. 1 ). In addition, 0.5 to 1 m-thick, of the 1872 eruption reporting runout distances of just over 17 km

overbanking deposits (poorly sorted unbedded breccias of coarse ash from the summit dome ( Hartmann, 1934 ). In addition to the long

to fine blocks) covered the eastern interfluves of Gendol valley be- runout of basal avalanches, the wide lateral spread of PDCs outside

tween 3.9 and c. 5 km, reaching the adjacent Tlogo tributary. the main channels was responsible for fatalities. This eruption event

From October 28 to 30, multiple explosions occurred and were was remarkable for several other reasons, including its rapid onset,

associated with at least 38 seismically detected PDCs that flowed up its sudden acceleration in magma extrusion rates ( Pallister et al.,

to 4 km along the Kali Gendol. The longest PDC signals (including 2013 ) and the degree of thermal and mechanical damage to a broad

an event of 22 min) and an ~ 18 km high ash-laden eruption column sector of the southern part of the volcano.

were produced on October 30, with ash falling in the city of In this paper we present an overview of the observational record

Yogyakarta to the south. This phase ended with the apparent com- and chronology of the 2010 Merapi eruption, along with a detailed

plete destruction of the 2006 dome as revealed by photographs of stratigraphy of the deposits emplaced during it. These data are used

the summit area.

to provide insights into the eruption dynamics and help to explain its extreme impacts on people and infrastructure. Field sedimentolog-

2.3. Phase 3 — October 31 to November 5 (first five days of the magmatic ical observations are augmented by image-based analysis to establish

phase of Surono et al., 2012 )

a detailed event framework, along with maps of the spatial distribu- tion of deposits produced during key phases of the eruption and cal-

This phase was associated with the rapid growth (at an average culations of unit volumes. These data are used to illustrate the large

rate of c. 25 m 3 s −1 between November 1 and 4; Pallister et al., range in dynamic properties and modes of PDC processes exhibited

2013 ) of a new, initially hemispherical dome (becoming more elon- during this eruption, which was considerably more complex than

gate after November 1) emplaced within the summit crater open to many other recent historic events at Merapi.

the SSE. From October 31 to November 2, dome growth was accompa- nied with increasing numbers of rockfalls and small PDCs, reaching

2. The 2010 eruption chronology of Gunung Merapi

2 to 4 km in the Kali Gendol. On November 3 the eruption intensified. PDCs reached estimated runouts of c. 10 and c. 9 km at 1437 and

An overview of the eruption between October 26 to November 7 1730 h, respectively, based on visual and seismic records. This trig- was compiled from daily reports of the Balai Penyelidikan dan

gered an immediate extension of the evacuation zone out to a Pengembangan Tekhnologi Kegunungapian (BPPTK) in Yogyakarta,

15 km radius. From the evening of November 3 and up to November and the Center of Volcanology and Geologic Hazard Mitigation

5, visual observations were fragmentary. Seismic intensity peaked (CVGHM) in Bandung ( Table 1 ; for a more general chronology

on November 5 at 0005 h, associated with the climactic explosion of please also see Surono et al., 2012 ). Additional information on

the 2010 episode that destroyed the new dome. Witnesses at plume heights and dispersal was derived from reports of the Volcanic

Desa Plumbontrasutan, 15.5 km south of Merapi and 160 m east of Ash Advisory Centre of Darwin and the NASA Earth Observatory.

Kali Gendol, reported strong rumbling noises beginning at around Overall, the brief 2010 eruption episode, which was essentially semi-

2340 h on November 4. They first noticed heat and a red glow from continuous, can be subdivided into four main phases of dome growth

within the adjacent Kali Gendol at c. 0025 h on November 5, as well and associated PDC and tephra fall generation.

as fires in the vegetation flanking the valley. PDCs reached 16.1 km in Kali Gendol, the longest runout flows known from the historical re-

2.1. Phase 1 — October 20 to 25 (last five days of the intrusion phase of cord at Merapi since 1872. The PDCs spilled onto western interfluves Surono et al., 2012 )

between 3.9 and 13.7 km and extended into the adjacent Tlogo, Gorido and Opak tributaries ( Fig. 1 ). Surges from this event expanded

After four years of quiescence that followed the May–June 2006 beyond the limits of the October 26 events in both the Kinahrejo and eruptions ( Charbonnier and Gertisser, 2008; Lube et al., 2011 ),

Kendil areas. This event resulted in more than 200 additional fatali- intrusion of fresh magma into the 2006 dome was indicated by its

ties, although an extension of the evacuation zone to 20 km radius increasing incandescence, together with a dramatic increase in CO 2 by CVGHM and resulting evacuations shortly before the November 5

emissions and temperatures of fumaroles ( Surono et al., 2012 ). This eruption saved many thousands of lives ( Surono et al., 2012 ). Tephra was accompanied by emissions of steam-rich plumes through the

from the November 5 eruption plume spread from the WSW to dome, increasing rockfall activity and a sharp increase in summit

WNW, falling in Yogyakarta and Magelan. inflation.

2.4. Phase 4 — November 6 to 7 (last two days of the magmatic phase of

2.2. Phase 2 — October 26 to 30 (initial explosion phase of Surono et al.,

Surono et al., 2012 )

2012 ) The climactic November 5 eruption destroyed the new dome, but On October 26 at 1702 h (all times given as western Indonesian

lava extrusion continued at high rates on November 6 ( Pallister et al., time, GMT+5) a sudden onset to the explosive phase of the eruption

2013 ). Multiple explosions occurred during November 6–7, as lava episode began. Seismic signals indicate eight individual PDC events

extrusion continued at high rates on November 6, generating PDCs flowed into the Gendol valley/Kali Gendol ( Fig. 1 ), with the three

that reached at least 5 km in the Gendol Valley. Dome growth that reached at least 5 km in the Gendol Valley. Dome growth

3. Methods Immediately after the alert level was reduced to level 2 in

mid-January 2011, fieldwork began to examine deposits in relation to the eruption chronology reported above. Rain-induced reworking of deposits had already taken place, and the quality of exposure was already rapidly decreasing during the first 3-week visit. The spatial distribution and internal variability of fall and surge units were deter- mined through detailed excavations and descriptions of surface features along N–S and E–W transects in the southern (hereafter referred to as the Kinahrejo area) and SSE sectors (the Kendil area). Excavated sections were described on an mm-scale with the objective of delimiting individual PDC and fall units by concentrating on bed- ding, sorting, and contact relationships. Site description included geo- metric measurements of the number density and internal properties of dunes, bedding geometry, granulometry and componentry, as well as the relationship of deposit geometry to the substrate geomor- phology. Damage to structures and vegetation was also documented. Further sedimentological analysis on 234 surge and 150 fall samples, along with geochemical investigations, is ongoing. For the October 26 and November 5 PDCs, detailed measurements of flow directionality were made from aligned vegetation fragments and debris, erosional furrows, and tree fall directions. Special attention was given to estab- lish accurate volume estimates by collecting a dense grid of thickness data points for each of the PDC units. After identifying the 2010 PDC stratigraphy, the areal footprints of the individual sedimentary facies (valley-confined BAF facies, avulsed BAF overbank facies, and surge facies) of each PDC unit were mapped in the field using a combination of hand-held GPS and real-time kinetic GPS data in January/February and June 2011. For the October 26 and November 5 PDC deposits, high-resolution Ikonos and Geoeye1 satellite images (as well as photographs taken on October 27–28) were used to refine the foot- print maps for these two eruptions down to a few meters resolution. Different approaches were taken for the three sedimentary facies to integrate thickness data with the area measurements. The areal foot- print of the BAF channel facies was split into 500 m longitudinal segments. Incremental volumes for each of these segments were computed as trapezoids using an average thickness and an average channel wall inclination as arithmetic means of these parameters de- termined at roughly 100 m intervals in the field. At the same 500 m longitudinal intervals, incremental volumes for the bordering avulsed BAF facies on the left and right interfluves were computed by multi- plying the segment areas with the local intermediate thickness of the interfluve deposits. Volumes for areas where avulsed BAFs had drained and thickened into secondary tributaries (e.g. the Opak valley

on the true right interfluve) were computed as separate segments to account for the different lateral thinning pattern of avulsed flows on flat interfluves and within these tributaries. Volumes for the surge units were computed from isopachs drawn at 5 cm intervals for bed thicknesses up to 10 cm (zones of no deposition that occurred locally in the proximal surge areas were cut out of the 0–5 cm isopach) and at 10 cm intervals for beds thicker than 10 cm. The spacing of thick- ness data points behind the isopachs (along the systematic N–S and E–W transects) varied from less than 20 m in proximal areas with strong lateral thickness variation to around 300 m in distal areas. In that way, both the large-scale thinning pattern (over several hundreds of meters) and the medium-scale variation in thickness (over several tens of meters) due to varying topography (across ridges, valleys, large terraces, etc.) could be accounted for in the isopachs. Small-scale variation in surge bed thickness (over less than 10 m and thus at similar length scale as that of surge bed forms) due to interaction with small terraces and buildings was doc- umented locally to quantify the sedimentary pattern.

The 5 to 45 m-thick sequence of basal avalanche, BAF and scoria- and-ash flow deposits in the Kali Gendol valley were still hot during the first field campaign, generating frequent secondary phreatic ex- plosions especially during rain-storms and rain-triggered lahars. In June 2011 these areas were cooled and well exposed by box-canyon ero- sion. Boundaries and dimensions of depositional units were documented at approximately 100–500 m intervals (depending on variability) be- tween 3.6 and 16.1 km from source and the two largest BAF units (both November 5) were sampled for grain-size analysis at ~1.5 km intervals. At each sampling location, 300–450 kg of deposit was sieved down to 8 mm at one phi intervals in the field, with the finer fraction analyzed by laboratory sieving and Laser-Particle-Diffraction.

4. Results

4.1. Deposit classification and distribution The 2010 Merapi pyroclastic deposits included: tephra fall, BAF

deposits, scoria-and-ash flow deposits and pyroclastic surge deposits. Proximal tephra fall deposits covered many sectors around the sum- mit, but beyond 2.5 km, medial to distal tephra fall was distributed in several overlapping lobes between WNW to SSE sectors.

The main sequence of PDCs affected an area of 24.5 km 2 , on SSW to SSE sectors of the cone, focussed on the Kali Gendol ( Fig. 1 ). Minor PDCs were also emplaced within the upper Senowo, Krasak and Boyong drainages. BAF deposits form the thickest and most voluminous PDC units that reach a maximum runout of 16.1 km in Kali Gendol. This valley was also the main flow path for BAFs in the June 2006 eruption ( Lube et al., 2011 ) when failure of the 1911 Gegerbuaya lava ridge created an opening in the summit crater of Merapi to the south. This opening has progressively deepened to form a V-shaped notch oriented to the SSE. The upper 1.8 km of the flow path down Kali Gendol ( Fig. 2 ) is a SE-trending chute with an initial gradient of 45° that shallows to 25°. At 1.8 km BAFs were diverted by Gunung Kendil, which forms a 1.4 km-long, sub-vertical cliff face, oriented SSW ( Fig. 1 ). The 170 m-wide valley in this area was an open and sparsely unvegetated terrain of variably incised 2006 BAF units, lahar terraces and rockfall surfaces, with a gradient decreasing from 25° to 10°. At 3.2 km, a southward oriented ridge forms the sub-vertical western valley wall. From 4 to 5 km, Kali Gendol hosts a narrow (b15 m), 10–30 m deep, S-trending stretch of box-shaped canyon. Prior to the 2010 eruptions, this was bounded by a > 40 m-high ridge to the west and a lower margin to the east. This area was the main egress for massive overbank flows avulsing from BAFs in the Kali Gendol during the June 2006 eruption ( Charbonnier and Gertisser, 2008; Lube et al., 2011 ). Near-complete infill of this 1 km canyon stretch occurred with BAF deposits on 26 October 2010. Later BAFs spilled out more broadly on the western

Fig. 2. Topographic profile along Kali Gendol illustrating the mapped extent of valley- confined BAF units I to IX and scoria-and-ash flow units X and XI. Marked runouts for BAFs

II to V are minima because the lower part of the valley-confined sequence is largely buried by laharic terraces.

248

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S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Fig. 3. NNE–SSW-trending type location for the Merapi 2010 sequence of valley-confined BAF and scoria-and-ash flow deposits (units I to X) in Kali Gendol near the Kali Adem Sabo dam at 5 km (B depicting a close-up of the southern part of the section shown in A). The black-green scoria-and-flow unit XI, which had been (locally) emplaced close to the former centerline of the Gendol valley has already been eroded by lahars, but was exposed further upstream and downstream.

interfluves, such that lateral flow lobes also entered the adjacent trib- parallel to the valley margin. These Maridjan temple surges inundat- utaries of the Tlogo and Gorido valleys, which rejoin the Gendol at

ed one of the volcano monitoring stations and reached the upper two

6.7 km. From 4.9 to 10.7 km, and particularly after a c. 40 m waterfall prayer sites of the former mountain guardian Mbah Maridjan. at 5.4 km, the 4–5° sloping Gendol valley widens (out to 460 m) and

The second and largest area of surge deposition, the Kinahrejo deepens (to 45–60 m). This large channel was in parts almost

surges (8.2 km 2 ) occurred to the west of Kali Gendol from 3.1 to completely filled by post-26 October BAF deposits. From 10.7 km

9.2 km ( Fig. 1 ). They comprised a maximum N–S extension of 6.5 km onwards, the main river channel is narrow (b50 m wide) and shallower

and a maximum E–W extension of 1.6 km. The surges reached the (b13 m deep) cutting into a broad ring-plain landscape.

north-eastern parts of Kaliurang village in the west and the Merapi Scoria-and-ash-flow deposits capped the thick BAF sequence in Kali

Golf course in the south.

Gendol and ran out to at least 6.7 km. From 3.9 to 5.5 km, where the The third area of surge deposition with a footprint of 7.1 km 2 channel was filled, the scoria-and-ash-flows form thin (0.3–1.5 m

occurred S and SE of Gunung Kendil ridge from 1.8 to 7.3 km runout thick) sheets with multiple, overlapping fronts spreading laterally out-

distance ( Fig. 1 ). These Kendil surges bordered the massive overbank side the former valley margins.

deposits of Kali Gendol, Kali Tlogo and Kali Gorido in the west and Pyroclastic surge deposits (hereafter referred to as surge deposits)

Kali Woro in the east.

constitute the fourth and most wide-spread PDC units of the 2010 se- quence. There were three zones of surge deposition. Most proximally,

4.2. Valley-confined and avulsed BAFs and scoria-and-ash flows

a small (0.2 km 2 ) area of surge inundation occurred alongside the eastern margin of the upper Gendol valley at 650 m runout length

Between 4.9 and 5.2 km from the summit dome ( Fig. 3 ), ( Fig. 1 ). Alignments of felled and broken trees are approximately

15–25 m high cliff sections extended to almost intersect the buried

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Kali Adem Sabo dam (Fig. 4D in Lube et al., 2011 ). The 2010 channel deposits comprise 10 major BAF and scoria-and-ash flow units with

well-defined lower and upper boundaries and thin ash and surge beds between. Along with thin mantling fine-ash fall deposits, thin surge units comprise b30 cm-thick stratified, strongly pinching and swelling fine-ash to medium lapilli beds, with abrupt changes in grain-size and variably oxidized reddish to gray matrix. At the base of the 2010 deposit sequence, dark-orange and light red overbank units BAF 2 and BAF 3 of the 14 June 2006 sequence occur ( Lube et al., 2011 ).

The basal 2010 unit I is a pyroclastic surge deposit, comprising pinching and swelling, 5–30 cm-thick stratified and cross-stratified medium to coarse ash that contains superficially charred twigs. This unit is characteristically rich in plagioclase crystals, fresh light-gray, blocky to irregular-shaped andesitic lava clasts, and vesiculated fresh gray and transparent glass shards, as well as a wide spectrum of sub-angular yellow, red and brownish altered lithics. In places, up to 3 cm pockets of light gray to light reddish gray, medium- to fine-ash fall cap unit I.

Unit I is covered by four BAF units (II to V), totaling 5–6.5 m thick- ness inside the Gendol channel. Boundaries between the massive, unstructured deposits are marked by abrupt grain-size changes. All units are dominated by fresh, dense, light gray andesitic lava fragments up to 1.5 m in diameter within a medium-ash matrix com- prising dominantly plagioclase crystals with minor vesiculated glass and scoria. Unit IV is distinctive in being slightly darker in overall color and richer in ash than the others, while Unit V is light-gray and typically boulder-rich with coarse-tail reverse grading. Unit V could be distinguished within Kali Gendol to almost 8 km from source ( Fig. 2 ). Comparison of their position in pre- and post-2010 channel cross-section profiles implies that the combined thickness of units II to V between 5.4 and at least 6.5 km must have exceeded 20 m. In many locations these units condense to b0.6 m in overbank sequence, with the units becoming finer-grained and stratified with lateral distance from the channels and unit boundaries disappearing.

In contrast to the strongly topographically controlled units II–V, the 6–7 m-thick unit VI and the 2–3 m-thick unit VII BAF deposits show little thickness variation across the Gendol valley and spread

Fig. 4. (A) Scoria-and-ash flow units X and XI on the western interfluves of Gendol at for at least 300 m laterally onto its western interfluves. Clasts within

5.2 km from the summit dome. On-lapping contact of the b0.5 m-thick and (scoria) units VI and VII are noticeably darker gray and more oxidized, and block-rich lobate margin of unit XI to the ~1 m thick unit X. (B) 1.2 m section through

the proximal veneer deposits of units IX, X and XI on the western interfluves of Kali colors are more reddish than in the earlier BAF units. Unit VI is the

Gendol at 5.3 km. Unit X has a fine-ash rich matrix, is locally normal graded and contains thickest and coarsest (clast- to matrix-supported) deposit in most

abundant degassing structures. At base of the sequence, an up to 3 cm thick light gray fine channel exposures down to 9.4 km from source ( Fig. 2 ) and it con-

ash fall (F) occurs in patches. Blown-down trees in the background of the photo are tains many blocks of fresh, dense gray andesite up to 5 m in diameter. contained in surge S4 (stratigraphically below units X and XI) and are locally aligned N–S, while the local flow direction of scoria-and-ash flows was NE–SW. Unit VII is matrix-supported and exhibits faint stratification and occa- sional clast-trains of coarse lapilli to blocks of dense andesitic lava. It

also shows a pronounced coarse-tail inverse grading. Both units are multiple overlapping lobes ( Fig. 4 ). Unit X has a light-gray to white, underlain by 3–25 cm of strongly stratified medium- to coarse ash

glass-rich matrix, shows abundant elutriation pipes in cross-sectional surge beds.

profiles and contains a spectrum of juvenile clast lithologies ranging The next BAF units, VIII and IX, are more laterally widespread

from very dense dark andesite, light-gray scoria to greenish and and show a distinctive dark gray to black and purplish-gray

white, amphibole-bearing pumice. Unit XI, in contrast, has lower con- matrix color. The matrix-supported unit VIII (4–5 m thick) overlies

tents of dense and pumiceous juvenile components (b30%) and is

a 0.1–0.2 m-thick dark-gray surge deposit enriched in black dense dominated by dark-gray to dark-green scoria. lapilli and ash. The base of unit VIII is faintly stratified medium- to

At the time of fieldwork, fluvial erosion had only partially exposed coarse ash and its upper third comprises five clast-train horizons

the upper Gendol sequence (units VIII to XI). Secondary explosions suggesting deposition from multiple pulses of flow.

and hot deposits hindered working upstream of 3.1 km. On top of Unit IX (5–6.5 m thick) is matrix-supported, contains the same type

the sequence described so far, lobes of late-stage PDC deposits of dark gray to black dense andesite as unit VIII, and can be split over a

occurred with runouts of 2 to 3.95 km. Between 5.2 and 6.5 km units gradational contact between a lower dark-gray and an upper steel-gray

II to IX occur, but from 7 to ~9 km, thick sequences (up to 15 m) of sub-unit. In some places a crystal-rich surge bed occurs between the

lahar terraces dominate, with only rare exposures of units VI to IX. two sub-units. It is capped by a 0.5–3 cm fine- to very fine-ash fall.

Hence, it is uncertain whether units II to V extended this far. Units VI The uppermost units X and XI total 0.3 to 1.5 m and consist of

and VII can be correlated down to 8.7 km and 9.4 km, respectively. scoria-and-ash flow deposits (c.f. Lube et al., 2007 ). Both units contain

From here onwards until 16.1 km, only units VIII and IX were found. high fine- and very fine-ash matrix contents (c. 5 to 10%) and are thin

The sequence of related overbank deposits (massive breccias with and extremely widespread in comparison to the earlier-emplaced BAF

finer clast sizes than in channels) that could be traced onto the inter- units. They typically exhibit steep, boulder-rich lateral margins and

fluve terraces to either side of Kali Gendol was relatively simple. On

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Fig. 5. Type location for the sequence of surge (S1 to S4) and fall deposits (F) of the Kinahrejo surge area in northern Kinahrejo at a runout distance of 5 km from the summit dome. See the legend in Fig. 7 for specifications on the section sketch shown in (A). Section photograph is oriented roughly W–E, and perpendicular to local flow direction of surges S1 to S4. The white dashed lines highlight the lower boundary of the surge beds.

the eastern interfluves between 3.9 and 6.7 km, all units I to XI occur. 1.5–3 cm gray medium-ash; and an upper b1 cm, red/gray laminated Whereas, on the western interfluves from 4.9 to 6.2 km and in the

(on sub-mm scale) medium- to fine-ash. The fall beds contain vesic- upper Opak valley, avulsed overbank deposits of units VIII to XI

ulated and dense, microlite-poor gray glass, abundant plagioclase, were found. Beyond these reaches, only units VIII and IX formed mas-

sub-angular brown to yellow lithics and pyroxene. sive breccias of overbank deposits on either side of the main valley.

Unit S2 is thin (6.5 cm) and covers an erosional basal wavy contact onto the tripartite fall sequence. In places, the fall sequence was found

4.3. Pyroclastic surge deposit stratigraphy as rip-up bodies plastically folded into the surge beds. S2 shows two sub-units, separated by a b1 cm dark gray to reddish, medium to fine

4.3.1. Kinahrejo surge deposits laminated ash. The lower sub-unit is gray stratified coarse ash, while The complete sequence of Kinahrejo surge deposits could be traced

the upper unit is massive pale-gray normally graded coarse- to continuously from the western cliffs that border Kali Gendol at 3.1 km

medium-ash. S2 beds are covered by a very thin (0.2–0.5 cm) red/pink from the summit dome to the lower end of Kinahrejo Village at

fine-ash containing accretionary lapilli, superficially-charred twigs and

5.2 km ( Fig. 1 ). The thickest sequences (20–50 cm), formed stoss and leaves, up to a 3–7.5 cm thick package of sub-millimeter laminated, lee-side wedges around topographic obstacles such as ridge tops and

reddish medium to very-fine ash fall.

agricultural terraces (much of this landscape is anthropogenically ter- Surge unit S3 is very thin (1–2 cm) medium to coarse ash that is raced for cropping, with variable treads between 1 and 3 m in height).

massive and slightly normally graded with few vegetation fragments The deposits formed dunes, up to 26 m-long and up to 1.5 m-high,

and a gently undulating lower contact. S3 is covered by a 1.5 cm thick and as much as 1.5 m-thick sequences occur in channel floors in the

package of light-gray to light-red, thinly laminated, very-fine and headwaters of Kali Opak and Kali Kuning.

fine-ash fall layers. Surge beds S2 and S3 are very similar in composi- The complete sequence of Kinahrejo surge beds is best described

tion: light-gray, dense, blocky to irregular-shaped, microlite-rich dome in northern Kinahrejo at an elevation of 1129 m ( Figs. 5 and 7 ). The

lava fragments>plagioclase crystals>pyroxene crystals>minor altered

49 cm profile contains four surge units (S1 to S4) separated by fine lithics and slightly vesicular opaque and transparent glass fragments. to medium-ash fall beds. The base of the sequence is formed by a

The uppermost unit, S4, is thicker (17 cm) and coarser. It com- pinching and swelling (0–17 cm-thick), light-gray, crystal-rich surge

prises a lower ~ 10 cm dark-gray to black, massive coarse-ash to me- deposit (S1) with an irregular and erosion-shaped lower contact

dium lapilli. This unit is distinctive in containing abundant black, to the brown, loamy soil. Vegetation is stripped off the soil and up

microlite-poor, dense, blocky to irregular shaped, juvenile lava clasts, to 15 cm-long clods of ripped-up soil are incorporated, along with

and low contents of exotic colored lithics. The upper portion of S4 is abundant superficially charred wood fragments. Unit S1 comprises

a reddish gray, coarse- to medium-ash, with dune-beds and cross- two sub-units, which are separated in places by an up to 0.5 cm

stratification. Lapilli-sized juvenile components are dominated by thick reddish-gray fine-ash. The lower sub-unit fines upwards overall,

fresh, dense and blocky andesite lava clasts, and rare dense black but comprises alternating, 0.5–3 cm-thick beds of coarse lapilli to fine

lava clasts. S4 is capped by up to 1.5 cm of gray to light-gray fine-ash. ash. The upper sub-unit of S1 exhibits a broad normal grading from

Units S1 to 4 vary considerably in thickness and character laterally coarse- to medium-ash as well as strong pinching and swelling with

and longitudinally. Along the central axis of deposition ( Fig. 1 ) changes an erosive basal contact. All of S1 contains, in order of abundance:

are shown in six stratigraphic sections ( Fig. 7 ). S1 can be traced out opaque, light-gray, blocky to irregular-shaped, dense to poorly

to a runout of 6.4 km. Over the first 1.8 km of this axial profile vesicular juvenile dome lava > plagioclase crystals > reddish altered

S1 surmounted five 20–40 m high ridges, leaving massive to faintly lithics > transparent, grayish to white, moderately vesiculated glass

stratified deposits at high-points, units up to 30 cm-thick in the upper and pumice > clinopyroxene crystals. S1 is covered by a slightly

Kali Opak valley ( Fig. 7 , sites 526 and 608). Farther downslope is a pinching and up to 3.5 cm thick sequence of fall, comprising a lower

slope-break coinciding with the uppermost agricultural terraces. Here

b 0.5 cm, reddish fine-ash containing accretionary lapilli; a middle the surge beds are strongly cross-stratified and form dunes (locations

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Fig. 6. Type location for the sequence of surge (K1 to K3) and fall deposits (F) of the Kendil surge area at the ridge top of Gunung Kendil at a runout distance of 2.25 km from the summit dome. The white dashed lines highlight the lower boundary of the surge beds. See text for description.

537 and 603). The two sub-units in S1 can be distinguished down to The uppermost surge unit K3 is a 6 cm dark gray, faintly stratified

6.1 km travel distance, but by 6.4 km they had thinned down to a loose medium to coarse ash and it contains the same fresh, dense and

0.5 cm thick coarse- to medium-ash rich in leaf and plant fragments. blocky andesite lava clasts as Kinahrejo surge S4. The entire sequence Units S2 and S3 are less widespread, disappearing by 5.3 km from

is capped by 3 cm of light gray vesiculated fine-ash fall. source. They occurred as wedge-shaped deposits flanking ridge crests

A longitudinal profile from the upper Gendol valley, over the Kendil and thicken slightly in valley-floors (location 608). Both units are

ridge along the approximate central axis of the Kendil surge area is preserved only as b1 cm thick pockets of ash beyond 5 km.

described in five stratigraphic sections ( Fig. 7 ). Down to 1250 m and The most widespread surge unit S4 forms the thickest and coars-

3.5 km from the dome, surge beds across the southern flanks of Gunung est beds anywhere in this area. Along the central axis, deposits are

Kendil are less than a few cm-thick. Thicker sequences were emplaced massive to stratified wedges flanking topographic obstacles out to at

behind topographic obstacles and in the floors of south-trending val- least 6.8 km. In valley floors they form up to 65 cm-thick massive

leys. Here, K1 and K2 comprise generally massive to weakly stratified breccias. From 4.9 km, S4 formed up to 1.5 m high and 26 m long

and planar bedded coarse ash deposits. K1 could be traced to a runout longitudinal dunes. The transition from antidunes (with upstream

distance of 3.4 km and K2 only to 3 km. Unit K3 is coarser-grained migrating bed-sets) to normal dunes occurred at ~ 7 km. Between

and showed strongly laterally-variable bed forms ranging from massive

7.5 and 9.2 km, bed forms are planar weakly stratified and finer-grained. to strongly stratified bedding above 3 km, dune-bedded and cross- stratified forms in medial reaches (3.5–5.5 km) out to weakly stratified,

4.3.2. Kendil surge deposits planar and normally graded beds that pinch out at 7.3 km from the The most complete sequence of the Kendil surge deposits is found on

summit.

the lee side of the Gunung Kendil ridge at 1713 m elevation and 50 m above the top of the channel-fill deposits in the upper Gendol valley

4.4. Tephra fall sequence

(2.25 km from source, Figs. 6 and 7 ). A 58 cm profile contains three thin surge units separated by many fine-ash to fine-lapilli fall beds.

Aside from the tephra fall sequences that occur as part of the PDC At the base of the profile, surge unit K1 forms an up to 8 cm thick,

sequence on the southern flanks, considerable tephra fall deposits pale-gray, massive to faintly stratified, loose coarse-ash that overlies

occurred on the western slopes of Merapi. Our type locality of the me- an eroded contact to dark-brown soil. K1 contains abundant plagioclase

dial western fall deposits is situated 7.7 km WSW of the summit crystals along with charcoalized and superficially charred twigs and

dome at the northern interfluves of Kali Putih. It consists of five leaves. Its componentry is identical to that of S1 in the Kinahrejo area.

main fall units F1 to F5 ( Fig. 8 ). At the base of the sequence, unit An up to 7 cm-thick fall sequence covers K1, comprising a basal, sub-

F1 is a 1.5 cm, brown to red, laminated fine ash. It contains (in the millimeter, red fine-ash, a 5–6.5 cm gray medium-ash, and an upper

herein examined full phi size-fractions of 250, 500 and 1000 μm), in

0.5 cm, light-gray to reddish fine-ash. order of abundance, gray-black, dense to poorly vesicular, blocky Surge unit K2 is 4–7 cm thick, gray, crystal-rich coarse ash dominated

and irregular shaped glass > plagioclase > reddish (some sulphur deposit and sits over an eroded contact on the ash fall package. K2 is

stained) lithics > pyroxenes. Rare vesiculated glass occurs in the broadly inversely graded unit and weakly stratified on a centimeter-

medium ash sizes. F2 is a 0.5 cm, gray, medium ash with a similar scale with subordinate medium-ash lamina. It also contains superficially

componentry as F1, but fewer moderately vesiculated andesite glass charred twigs, and is capped by a 0.5 cm-thick red, very fine-ash fall,

shards and lithics, and abundant platy, low-vesicular black juvenile containing accretionary lapilli.

glass. The next 2 cm is a complex set of fall units (F3), from bottom The next 30 cm of section is a complex set of fall units, from bottom

up consisting of a 1.2 cm gray, vesiculated fine ash; grading into a up consisting of: three 3 cm-beds of dark-gray, crystal-rich coarse ash;

0.5 cm gray-brown fine to very fine ash; and capped by red very

4 cm of light-gray medium ash; 1 cm of reddish fine ash; overlain by fine ash. F3 contains rare altered (red, yellow and pale) lithics, and

8.5 cm inversely graded coarse-ash to fine-lapilli, composed of abun- is dominated by plagioclase crystals and rare light brown irregular dant fresh, dense, blocky, dark-gray and microlite-poor andesite;

glass. There is a sharp contact with a 1 cm thick layer of medium to

6 cm, weakly normally graded, reddish-gray medium-ash; and capped coarse ash (F4). F4 is the coarsest bed of the sequence and comprises by 2 cm light gray fine-ash.

fresh gray and blocky, poorly vesicular glass fragments often coated in

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Fig. 7. Spatial distribution of surge units S1–S4 and K1–K3 along the central axis of the surge path in the Kinahrejo and Kendil surge areas (see Fig. 1 for orientation of the axis). Top: Topographic profiles for the Kinahrejo surges (red symbols and section numbers) and Kendil surges (blue symbols and section numbers). Middle: Six stratigraphic sections (526 to 598) characterizing the Kinahrejo surge and fall sequence from 3.5 to 8 km. Bottom: Five stratigraphic sections (574 to 582) characterizing the Kendil surge and fall sequence from

2.25 to 6.5 km runout distance. Major ticks on vertical axis of profiles are 10 cm apart.

a fine ash dusting. The uppermost fall unit F5, with a gradational con- well as using a medium-resolution ASTER image taken on October 28. tact to unit F4, is a 1 cm reddish to light gray, normally graded fine

From this we correlate surge units S1 and K1 and the Gendol channel ash. It is mainly composed of gray blocky glass fragments, plagioclase

unit I to this initial eruptive event. Both surge units are rich in super- and pyroxene and very rare yellowish to light red lithics.

ficially charred vegetation fragments, relating to destruction of the dense forest north of Kali Adem and Kinahrejo and partial destruction

5. Correlating stratigraphy to eruption chronology of forest on Gunung Kendil. Unit I in the Gendol valley sits directly on the 2006 overbank deposits and was photographed at the Kali Adem

The spatial extent of the October 26 PDC deposits was determined Sabo dam on October 28 as ~ 0.5 m thick, with surface dunes. The gra- via photographs taken during the rescue efforts on October 27–28, as

dational contact of units S1 and K1 to overlying, thin and accretionary

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 261 (2013) 244–259

Table 2

Volumes (V × 10 6 m 3 ) and inundated areas (A × km 2 ) of the main phases of PDC de- posits from the Oct–Nov. 2010 Merapi eruption (subscripts: CF=channel facies BAF;

V= overbank facies BAF; S = surge facies).

Date a Total volume 6 3 Total area 2 V CF V V V S A CF A V A S

Unit I S1, K1

3–4 Nov

24.4 b 4.75 b 22.3 b 1.96 0.08 1.33 b 0.87 2.55

Unit II–VII S2–3, K2

5 Nov

Unit VIII–IX S4, K3

b Stratigraphic units as described in the text and figures. Estimates for the Nov. 3–4 channel facies are minima due to lack of exposure in medial reaches.

Fig. 8. Type locality for the western sequence of medial fall deposits (F1 to F5) in Jerong likely events that generated these surge deposits were the: 1404 h Jero 7.7 km WSW of the summit dome. The section has been carved at a shallow angle

November 3 event (producing BAF runouts in Kali Gendol to ~10 km); through the loose ash beds. See text for true thicknesses and deposit description.

the 1444 h November 3 event (with BAF runouts in Gendol to ~9 km); and the November 4 event with an estimated BAF runout of 9.5 km.

There is no direct evidence to tie the Gendol BAF units II to VII to exact emplacement times. However, their runouts (>6.5 km) together with the observational records ( Table 1 ) suggest that they were all emplaced between the afternoon of November 3 (post 1444 h) and on November 4. Unit VII with 9.4 km runout, probably correlates to the latest BAF reported for this phase of eruption on November 4.

The most widespread surge units S4 and K3, along with the BAF units VIII and IX (and associated massive overbank deposits) can be correlated clearly on the basis of their runout and eyewitness reports to the earliest November 5 PDC-forming eruptions ( Fig. 9 ). At the vil- lage of Petong ( Fig. 1 ), massive overbank deposits of unit VIII overlie a gradational contact to surge unit S4, showing that the surge traveled faster than the co-genetic BAF VIII in Kali Gendol. The observational records after Nov. 5 also indicate that the scoria-and-ash flow units