Gayatri Indah Marliyani
San Diego State University, USA
Gadjah Mada University, Indonesia
Merapi is a strato-volcano located in Central Java, Indonesia, about 30 km north of Yogyakarta city which has more than one million inhabitants (Figure 1, Figure 2). It is part of the volcanic front of the Sunda-Banda magmatic arc produced by subduction of the Australian plate under the Eurasian plate. The Indonesian archipelago resulted from complex and diverse tectonic processes (Simandjuntak & Barber, 1996, Wilson 1989, Hamilton, 1979). The present phase of orogenic activity in Indonesia commenced in the mid-Miocene and is still in progress (Simandjuntak & Barber, 1996). In westernmost Sumatra, it involves strongly oblique convergence and major strike slip transcurrent fault movement within the magmatic arc (Figure 3). Continuing to the east, in Java and Nusa Tenggara, normal convergence produces an orogenic belt and Andean type subduction zone. This subduction zone is characterized by trench, accretionary complex, forearc basin and Quaternary active volcanoes built on the margin of the Sundaland continent. Merapi is one of these volcanoes. Further to the north east, convergences of oceanic plates set the Sangihe and Halmahera magmatic arc, while in Sulawesi, it involves collision of microcontinental blocks with subduction systems along the eastern margin of Sundaland. Moving to the south of Sumba, it involves collision of the northern margin of the Australian continent with the subduction system along the southern segment of the Banda Arc. A more advanced stage of collision of the northern margin of the Australian continent with a magmatic arc on the Philippine Sea plate forms the easternmost part of Indonesia, Irian Jaya.
Having an average eruption frequency of once every 4-5 years with more explosive and larger episodes every few decades, this volcano is considered one of Indonesia’s most active volcanoes. Despite the danger of living close to a volcano, many people occupy fertile land surrounding Merapi, risking exposure to pyroclastic flow and possible larger explosive eruptions. For this reason, Merapi was selected as one of the focus volcanoes during the International Decade for Natural Disaster Reduction (Newhall et al. 1994).
In this writing, I describe Merapi based on published data and limited personal field observation. This description includes morphological aspects, eruption type and history, rock types, and also includes monitoring for hazard assessment and recommendations.
Merapi forms a bell-shaped topography that has a mean dip angle of 5° up to 1300 m and 15° up to the summit of 2911 m (Berthommier, 1990). The porphyric nature of the lava and alternating deposits of lahars and pyroclastic flows forms an un-compacted and highly porous material that is easily eroded. Merapi’s morphology is characterized by steep erosional valleys of all sizes and radial ridges (Mizutani, 1990).
There are two high temperature fumarolic fields near the Merapi summit, Gendol and Woro, located 150 m and 250 m SE of the centre of the summit crater. The maximum fumarolic temperatures in the Gendol field are greater than 800°C, while those in Woro are higher than 600°C (Zimmer and Erzinger, 2003). The SO2 gas is continuously discharging from the fumaroles and the lava dome.
Merapi has behaved as a classical strato-volcano, with alternating phases of effusion of lava flows and vertical vulcanian explosions that could generate scoria flows (Camus et al, 2000). The major event that interrupted this behavior was a sector collapse, with an inferred associated blast. Later, strong magmatic and probably phreatomagmatic events occurred, preceding the present dome-building phase.
The total Merapi eruption volume is estimated between 100 and 150 km3 (Berthommier, 1990) with present rate of effusions at about 105 m3 per month over the past 100 years (Siswowidjoyo et al., 1995). A strong uncertainty remains concerning the beginning of its activity. If the effusion rate is assumed to be constant since the beginning of its activity, Merapi could be between 8.300 and 125.000 years old (Camus et al, 2000). However the geological evidence, Camus et al (2000) suggests that the rate of flow may have decreased during the evolution of Merapi. Thick and long lava flows were progressively replaced by smaller ones, then by slow dome extrusions. If so, Merapi is much younger. According to Newhall et al (1995), Merapi shows evidence of over 7000 years of explosive eruptions, and Charbonnier & Gertisser (2008) give an age around 40.000 years.
On the basis of field studies and geochronological data, Camus et al (2000) divided its history into four periods: Ancient, Middle, Recent and Modern Merapi. The Ancient Period may have begun around 40.000 y BP and lasted until 14.000 y BP as the Middle Period begun. The Recent Period begun around 2200 y BP and was replaced by the Modern Period after the eruption of 1786. During the Middle Merapi stage, a St. Helens-type edifice collapse occurred. During the Recent Merapi stage, two violent magmatic to phreatomagmatic eruptions interrupted the growth of the volcano.
The older phreatoplinian deposits cover the entire cone; charcoal found within these deposits gave 14C ages of 2200 and 1470 y BP (Camus et al, 2000). The overlying ash deposits, referred to as Sambisari ash deposits, were emplaced by violent pyroclastic surges directed towards the south, i.e. to the present location of the town of Yogyakarta, burying the Shivaitic temple of Sambisari at the start of the 15th century. Many other Shivaitic temples, such as Prambanan, Kadisoko, Kedulan, (Figure 7) were found buried under thick volcaniclastic deposits south of Merapi, indicating that the pyroclastic flow and surge deposits can reach as far as 25-35 km. It is believed that there are still many temples and other remains of ancient civilizations still undiscovered under the deposits. This distance makes it clear that Merapi produces not only dome-collapse pyroclastic flows, but also pyroclastic flows related to moderate to large explosive eruptions (Camus et al, 2000).
Modern Merapi is characterized by the persistent growth of a summit dome, known as Merapi-type activity, which is described as a semi-continuous outpouring of viscous lava producing a summit dome, interrupted by periodic gravitational dome collapse or total destruction triggering violent block-ash flows and associated surges. The ash produce is referred to as Merapi-type nuées ardentes (Figure 4) (Escher, 1933, Voight et al., 2000). Sometimes, a more exceptionally, fall-back St. Vincent type nuées ardentes (scoria flows) occurs.
Since the mid-1500s, eighty eruptions have been recorded and almost half have been accompanied by the dome-collapse pyroclastic flow (Simkin and Siebert, 1994). About sixteen of Merapi’s past eruptions, including the latest eruption episode in 2006, have caused fatalities (Charbonnier and Gertisser, 2008). Most pyroclastic flows events in the 20th Century that were produced by collapse domes produced limited amount of lava and traveled relatively short distances. Occasionally, as in 1930, unusually large collapse related flows traveled 10 km from the summit into populated areas. However, several studies on the older deposits revealed that many eruptions during the 7–19th centuries A.D. were substantially more violent and swept broad sectors of the volcano with explosion type pyroclastic flows (Kemmerling, 1931; Neumann van Padang, 1931, 1933, 1936/1937; Escher, 1933; Hartmann, 1934, 1935). These eruptions, much larger and more explosive and violent than any of the 20th Century, have occurred at irregular intervals of several decades as identified in 1768, 1822, 1849, 1872, and 1930–1931. The 1872 eruption is the only one of a St. Vincent-type during the Modern Period (Hartmann, 1934), but many deposits attest that it was a very frequent type during the preceding periods. In 1872, all the flanks of the volcano were covered by ash-and-scoria pyroclastic flows. An interesting fact quoted by Hartmann (1934) is that the building up of the volcanic column was progressive, and preceded by two days of spectacular events describes as roarings sounds, intensive volcanic tremor and smaller explosions, which explains the small amount of causalities recorded, since many inhabitants had left the danger zone before the climax of the eruption.
In contrast, the 1930–1931 eruption, in spite of an exceptionally high lava output, the eruption was quiet, without pyroclastic flows that last for 23 days. So, the cataclysmal explosive phase was unexpected, explaining the great number of fatalities. This unusual behavior was related to the opening of a vent at a place lower than usual, which is at the foot of the older summit domes. This opening can be caused by hydraulic fracturing, or by utilization of a pre-existing weak zone, or both (Camus et al, 2000). It is important to recollect that the eruption began after 11 months of increasing seismic activity. This type of sub terminal eruption seems to be exceptional at Merapi. If this type of eruption would occurs again, it could be on the same flank of the volcano, or on its south flank, where there is a fractured zone with fumaroles and small solfataras, at about 300 m below the summit (Camus et al, 2000).
The absence of large ignimbrite eruptions suggests both the absence of a large-deep reservoir and of a long stage of volcanic rest (Camus et al, 2000). Inferred from seismic observation, location of the magma reservoir which feeds the eruptions is estimated to be at 1.5 km below the summit (Ratdomopurbo, 2000).
By observing the long recorded behavior of Merapi, the occurrence of explosive eruptions during periods of less explosive dome growth and dome collapse is more likely than the occurrence new open-vent eruptions. An average low level activity of Merapi can be interrupted by a much larger explosive eruption and there is no reliable evidence to assume that the future activity will be as benign as that of the 20th century (Newhall, 2000).
CHRONOLOGY OF 2006 ERUPTION
To give a better understanding of the behavior of modern Merapi, I present here the sequence of the latest eruption of 2006 includes its chronology and deposits.
The 2006 eruption of Merapi consisted of three eruption phases that produced a complex sequence of block-and-ash flows directed mainly towards the south-western (May 2006) and southern flanks (June 2006) of the volcano (Charbonnier and Gertisser, 2008). After a dormant period of nearly five years, volcanic activity at Merapi resumed in July 2005 with an increase in the number of volcanic tremor and deformation of the summit area. This renewed episode of activity ended with the extrusion of a new lava dome in March 2006. In contrast to summit lava domes predating the 2001 eruption, which were mainly located inside the 1961 crater, the 2006 lava dome of Merapi was emplaced near the eastern rim of the 1931 crater, locally known as Gegerbuaya (Figure 6). The period of lava-dome growth that started in March 2006 increased during April and was rapidly followed by periods of multiple rockfalls and dome-collapse pyroclastic flows during May and June 2006.
Due to the presence of a topographic barrier in the south-eastern sector of the 1931 crater (Gegerbuaya ridge; Figure 5), the rockfalls and dome-collapse pyroclastic flows of the first eruption phase from May 5–27, were mainly directed towards the southwestern flank of Merapi into the Krasak, Bedok and Boyong River valleys, with runout distances of <4 km from the summit area.
The new lava dome deformation during the first phase dominated by rock avalanche. Multi-phase earthquakes indicated that the dome was not yet stabilized and that pressure accumulation inside was immediately followed by rock avalanches as the conduit pressure ceased and deflation began (Merapi Volcano Observatory BPPTK, 2006). The volume of the 2006 lava dome on May 22 was estimated at ~2.3×106 m3.
The second eruption phase was associated with a magnitude 6.3 earthquake on May 27, whose epicenter was located 35 km south of Merapi (Fig. 1). Immediately after this event, lava extrusion rates at Merapi increased to 0.1×106 m3/day. On June 4, the summit lava dome reached a volume of >4.0×106 m3 and a height of 116 m above the summit peak (BGVN 32:02). Following partial collapse of the eastern part of the Gegerbuaya ridge (Fig. 5), an increase in the volume of successive pyroclastic flows and associated collapsed material was observed. This succession of events allowed flows to take a different path and travel down the southern and south-eastern flanks of Merapi, which were not affected by pyroclastic flows for more than a century.
During the third eruption phase in June, the activity occurred in two distinct periods. Between June 3 and 12, several dome-collapse pyroclastic flows affected the southern and south-eastern flanks towards the Gendol River valley with runout distances <4.5 km. On June 14, the activity peaked with two sustained dome-collapse events that lasted over periods of tens of minutes produced at least two pyroclastic flows with maximum runout distances in the Gendol River valley of 5 and 7 km. These flows caused two fatalities and partial burial of the village of Kaliadem (Figure 6). This event was preceded by a high lava extrusion rate and oversteepening, creeping and deflation of the lava dome (Merapi Volcano Observatory BPPTK, 2006). After June 14, the number and frequency of pyroclastic flows decreased until the end of the eruption in early July.
Figure 6. Ikonos satellite image of the summit area of Merapi on May 10, 2006. Lava domes and viscous flows are labeled with the year of extrusion. The white dotted line corresponds to the 1931 crater rim. The Gegerbuaya ridge is formed by 1911 lavas (Charbonnier and Gertisser, 2008).
Most of the lavas of Merapi are calc-alkaline, high-K basaltic andesites, with a restricted compositional range from 52–57% SiO2 (Camus et al, 2000). Some basalts and andesites occurred, but scarce, extending the compositional range to 49.5–60.5% SiO2 (Figure 8). The lavas are highly porphyritic, with phenocryst and microphenocryst contents ranging from 22–62% (Camus et al, 2000).
The mineralogy throughout Merapi’s history is generally very similar, and the characteristic assemblage is plagioclase, clinopyroxene (augite–salite), brown hornblende, olivine, titanomagnetite, and hypersthene (only in basaltic andesites), embed in a clear to brown glassy matrix (Camus et al, 2000). The main accessory minerals are apatite, which occur as microphenocrysts, alkali-feldspar and tridymite as interstitial phases in the groundmass. The groundmass is partly crystalline, with mainly microlites of plagioclase and pyroxenes.
Figure 8. Total alkalies versus silica diagram for Merapi volcanic rocks (Camus et al, 2000). Fields represent the volcanic rock classifications of LeBas et al (1986).
The complex zoning of plagioclase, the wide compositional range of plagioclase for a single sample and disequilibrium textures for amphibole and pyroxene, suggest thermal and chemical disequilibrium (Camus et al, 2000). The additional macroscopic and microscopic evidence for mixed glass indicates the occurrence of a magma mixing process. Magma mixing may have buffered the compositions of lavas at Merapi, resulting in the restricted range of whole-rock composition (52–57% SiO2). Typical phenocryst assemblage is plagioclase > clinopyroxene > amphibole, orthopyroxene, olivine, titanomagnetite. A general trend toward more evolved magmas, from Recent to Modern Merapi is recognized (Camus et al, 2000).
Deposits from the explosive phases of the Modern Merapi period can be classified into three types (Kemmerling, 1832; Escher, 1933, Camus et al, 2000):
(1) Block and ash flow deposits of the Merapi type, commonly produced by dome collapses (Figure 9). These deposits are characteristic of the Modern Merapi Period and, to a certain extent, also occur in the Middle Period. They have not been recognized as the products of the Ancient Period eruptions.
(2) Block and scoria flow deposits, produced by fall-back nuees ardentes of the Saint Vincent type.
(3) Surge-like “pelean” deposits. Grandjean (1931) suggested that the 1930–1931 eruption could have generated violent “pelean” surges; deposits related to such eruptive processes have not been described before 1994 at Merapi.
Surface particle assemblage analyses on 2006 block-and-ash flow deposits were performed by Charbonnier and Gertisser (2008) as presented in Table 1. They grouped the assemblages lithologically into six main types, which are representative of the rock types found within the different flows generated during this eruption period (Figure 10). The juvenile component is a porphyritic basaltic andesite that can be divided into four main lithologies: (1) light grey scoria (2) dark grey scoria, (3) light grey dense clasts, and (4) dense, prismatically jointed clasts. These juvenile components range in density from 1.7 to 2.6 g/cm3 (mean 2.2 g/cm3, n=25). The two other components identified as hydrothermally altered and oxidized clasts are lithologically distinct from the juvenile material and represent accidental lithics which were incorporated into the flows. Their density ranges from 1.8 to 2.4 g/cm3 (mean 2.1 g/cm3, n=20). These clasts represent the old dome fragments and/or lava flows which constitute the south-eastern part of the summit area as it has similarity with the material taken from Gendol solfatara field and the 1931 crater wall.
The institution that is responsible for monitoring the activity of Merapi is Volcanological Survey of Indonesia (VSI). The monitoring encompasses these parameters: seismicity, volcanomagnetism, deformation, geochemistry, visual monitoring of summit morphology and dome evolution, and also lahar detection during episodes of eruption.
Seismicity is considered the most important parameter for estimating the probability of an eruption. The seismic signals observed at Merapi volcano are classified as A-type, B-type, multiple phase events, long-period events, tremor and rock fall (Ratdomopurbo, 1995). Currently, the network of eight seismographs established around the volcano, allows volcanologists to accurately pinpoint the hypocenters of tremors and quakes.
Geomagnetic monitoring in Merapi has been carried out since 1977 with a total of four sensor stations established. Those sensors continuously measure the total geomagnetic intensity with a sampling rate of one data sample/minute.
Geochemical monitoring of Merapi has been carried out since 1984. Several fixed points are located at two main solfatara fields of Gendol and Woro for a continuous sampling.
Since 1961, the only change in the morphology of the summit has taking place inside the crater. Alternation of dome formation and dome collapse occurs frequently. As the direction of pyroclastic flows depends strongly on summit morphology and the condition and position of the dome, visual observation of the summit and the dome is necessary. Detailed observations of the crater were conducted by a team sent to the summit to take photographs of the crater and the dome. From the successive photographs, the evolution of the dome can be reconstructed.
The VSI also established six observation posts: Kaliurang, Ngepos, Babadan, Jrakah, Krinjing and Selo. Every observation post is equipped with a telescope to observe changes in the upper part of the volcano, including rock fall activity; source, direction and distance traveled by avalanches, location of dome build up and height of the volcanic smoke.
Lahar is one of the important secondary hazards in Merapi. In 1975, lahar in the Krasak River destroyed the bridge on the main road connecting the provinces of Yogyakarta and Central Java. On December 5, 1996 at Boyong River, 14 mining trucks were buried under the lahar flow. The measurement of the lahar volume based on estimates of the volume of loose material at the slope. Some detectors also placed near some river channels as an early warning system. A lahar event is usually triggered by heavy rainfall, and so the system must be more alert under conditions in which the volume of material reaches a threshold and there has been rainfall of around 50-mm/hour.
All of the data is sent directly to the base station of Merapi Volcano Observatory, Volcanological Survey of Indonesia (MVO-VSI) which is located in Yogyakarta City. The data is processed to maintain the alert level of the volcano on a daily basis. The MVO-VSI is also responsible for producing a volcano hazard map of Merapi and for revising it when necessary.
There are more than one million inhabitants endangered by Merapi. The prominent hazards of this volcano come from the direct and secondary effects of the eruptions. The direct effect is related to block and ash pyroclastic flow and associated surges that are produced by gravitational dome collapses. Secondary effects include laharic flow produced by mixing of its loose material with water and by aerial and water pollution. The areas expected to experience the effect of eruption are mapped by the MVO-VSI. This map divides the area surround the volcano into several zones based on susceptibility to danger from future eruptions. This prediction is largely based on the present morphological condition of Merapi.
In addition, predicting the eruption hazards of Merapi must be estimated not only based on the eruptions observed during the Modern Period, which are relatively small, but also from the much larger eruptions that preceded it. There is a broad spectrum of scenarios describing the large eruptions of the past. The prediction of future eruptions must take this into account, bearing in mind that the scenario likely will differ. For example, the location of the vent could change, or a new dome be created with associated pyroclastic flows going in different direction from eruptions in the past.
Many villages and towns around Merapi are built on deposits of Merapi’s large explosive eruptions. At least 80,000 and perhaps as many as 100,000 people live inside the so-called Forbidden Zone defined by Pardyanto et al (1978). This area lies roughly within a 10 km radius of the summit, mainly on the west and south sides of the volcano. Several hundred thousands more live just a few kilometers outside that zone. The residents are familiar with small dome-collapses but not many realize that their homes and schools are built on deposits of much larger, relatively young, lethal explosive eruptions.
There is no assurance from the geologic record that Merapi will remain as quiet in the next century as it was during the 20th Century. Rather, it is suspected that a major explosive eruption will occur within the coming decades. Large numbers of people, both within and beyond the Forbidden Zone will be at serious risk. Public education and discussion of the intent of the Forbidden Zone, a willingness among all parties to accept some false alarms, and an ongoing search for precursors of a larger explosive eruption are needed to limit the risk.
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