Wikartadipura, S. (1971) Laporan Pemeriksaan Daerah bahaya/ Waspada
Sementara. Kawah Idjen pada Lereng Utara-Timur, Volcanological Survey of ...
Ijen Volcanic Complex The Ijen complex, situated in East Java near the city of Banyuwangi, is the easternmost volcanic centre in the island of Java. The large caldera complex hosts a large number of volcanic edifices of which Ijen and Raung are the most active. The Ijen crater (Kawah Ijen) contains the world’s largest lakes of highly acid (pH45,000 BP (Jampit), 37,900±1850 (Suket), 29,800±700 (Ringgih), 24,400±460 (Old Pawenen), 21,100±310 (Malang) and 2,590±60 (Ijen).
Figure 3.4. Plot of SiO2 versus K2O for lavas from the Ijen complex (Sitorus, 1990) Lavas from the Ijen Complex show a large variation in SiO2 contents (46-63 wt.%) ranging from basalt to dacite (Fig. 3.4). Basalts are medium to high-K, whereas most andesites plot in the high-K field. Pre-caldera and caldera products show a large scatter. Compositions of lavas from individual post-caldera centres are generally coherent, but collectively do not yield well-defined trends in variation diagrams. Plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides are common phenocrysts, while olivine is restricted to relatively mafic post-caldera rocks. Biotite is only found in the Glaman lava dome.
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Eruptive activity The present activity is restricted to Ijen volcano, which has hosted an acid crater lake for at least 200 years. Documented historic eruptions did not produce juvenile magmatic products but were predominantly phreatic in nature. The following summary is based on Kusumadinata (1979) and Volcanic Activity Reports of the Smithsonian Institution Global Volcanism Program: 1796 1817 1917 1921-1923 1936 1952 1962 1976 1991 1993 1994 1997
1999
Phreatic eruption 15-16 January: phreatic eruption (flooding of mud towards Banyuwangi, fairly large volume of lake water discharged into Banyupahit River) 25 February-14 March: lake seemed to boil; repeated phreatic eruptions, mud thrown up to 8-10 m above the lake surface Increasing lake water temperature; steaming gases above water surface 5-25 November: phreatic eruption producing lahar similar to that of 1796 and 1817 22 April: steam eruption up to 1 km high; mud thrown up to 7 m above the lake surface 13 April: 7 m high eruption; gas bubbles on lake surface, about 10 m in diameter 18 April: bubbling water up to 10 m high, changing of watercolour 30 October: bubbling water at Silenong for 30 minutes 15,21,22 March: bubbling water and changing of water colour, 25-50 m high gas outpouring at high velocity; this activity was recorded as seismic tremor between 16 and 28 March. 3,4,7 July and 1 August: phreatic eruptions, changing of lake water colour, water outpouring, booming noise, clotted steam; all centred in the middle of the lake 3 February: minor phreatic eruption from the south part of the lake. Coincident with the eruption, the lake level rose ~1 m. Late June: period of increased seismic activity; changing of lake water colour; gas bubbles and areas of up welling; strong sulphuric odour; birds were seen falling into the water; one or more sulfur workers near the summit reported dizziness and headaches. 28 June: two phreatic eruptions at the Seating location. An accompanying detonation was heard at the sulfur-mining site 2 km from the summit and volcanic tremor was recorded with an amplitude of 0.5-1 mm. The following week, 6-12 July, yellow-grey sulfur emissions were observed from the crater and a loud "whizz" noise was heard. The crater lake's water was brownish-white and had sulfur agglutinate floating on the surface. Seismicity had increased starting in early April. The number of B-type events remained high (more than 34/week) for most of the period through mid-June. Seismicity then gradually declined through mid-July, after which the weekly number of B-type events remained stable at an average of 9/week. During the period of 18 May through the week ending on 21 June a "white ash plume" rose 50-100 m.
Volcanic hazards VSI has distinguished three types of hazard areas. A danger area that can be struck by lava flows, pyroclastic flows, eruptive lahars, volcanic bombs and exhalation of poisonous gases covers 65000 km2. It includes the low-lying terrains within the caldera and the Banyuputih river valley down to the coast, and has a population of about 12000 (1985). An alert area threatened by ejecta (bombs and air fall) covers an area with a diameter of 8 km around the crater. A second alert area where rain lahar might be expected is defined by river valleys inside the caldera as well as on the
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northeastern outer flanks. The areas cover 68.5 km2 and host a population of 73000 in about 68 villages (1985). Mild phreatic eruptions in the lake that occasionally occur pose threats within the crater area.
Figure 3.5. Hazard map showing the areas threatened by lahars, pyroclastic flows, lavas and air fall deposits.
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The crater lake The lake (2200 m a.s.l) has a regular oval shape (600 x 1000 m), a surface area of 41 x 106 m2 a volume estimated between 32 and 36 x 106 m3. In 1921 a dam was built by the Dutch to regulate the water level and prevent catastrophic overflows during the rainy season. Originally sluices were used but these constructions are not operational anymore. Similarity between the topographic maps of 1920 (Kemmerling, 1921) and 1994 (VSI) suggests that the morphology of the crater has not changed much in recent history despite the repeated phreatic eruption events. In contrast, the morphology of the crater-lake bottom has undergone significant changes. Depth soundings in 1925 recorded a maximum depth of 198 m at the deepest point, which was then located east of the centre. In 1938 the deepest point had moved westward with the result that the lake was deeper in the centre (~200 m) and in several points in the western half. Recent depth measurements carried out in 1996 (Takano, unpublished data) suggest that maximum depths are slightly less (Fig. 3.6).
Figure 3.6. Depth contours from a survey in 1996 (Takano, unpublished data).
Detailed monitoring by Dutch volcanologists (e.g., Stehn, 1930) revealed a clear relation between rainfall, lake water level and lake temperature. Yearly precipitation in the Ijen area is variable with maximum amounts op to 2.5 meters. There are significant fluctuations between a dry (May-October) and a wet season (NovemberApril) when the lake level may rise by up to 4 meters. Surface lake temperatures are always higher than air temperature and generally decrease in the rainy season. Lake level increases and accompanying temperature drops are also observed after shortterm periods of heavy rainfall. Between 1980 and 1993 temperatures roughly averaged at about 40oC with a maximum of 46 and a minimum of 32oC (see Delmelle, 1995). Earlier records show lower temperatures, e.g., seasonal fluctuations between 20 and 40oC in 1930 (Stehn, 1930). Temperatures of 34-37oC were measured in the dry seasons of 1996-97. In general, temperatures are fairly homogeneous under normal activity and do not
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change significantly with depth. A transect across the lake made in August 1997 showed temperatures being ~1o lower in the centre than near the shores (Sumarti, 1998). Increases in water level and temperature have been observed during the shortlived periods of higher hydrothermal activity (bubbling and phreatic eruption events, cf., BGVN reports). Water chemistry The Ijen lake is an extreme example of acid sulfate-chloride type crater-lake waters carrying a high load of dissolved elements. A very low pH value of ~0.2 is accompanied by high loads of TDS (~100 g/kg). The high acidity and high concentrations of SO4, Cl and F can be attributed to magmatic volatiles at the lake bottom. Repeated sampling in the 1990s (Delmelle and Bernard, 1994; Delmelle et al., 2000; Sumarti, 1998; Sumarti and Van Bergen, unpublished data), in combination with earlier data, showed that bulk compositions have remained fairly constant over the last 60 years (Table 3.1). A survey in 1997 also revealed that concentrations do not vary markedly with depth (Sumarti, 1998; Takano et al., unpublished data). These observations indicate that the lake is well mixed and that most chemistry-controlling processes are generally stable. However, monthly monitoring in 1997-1998 (Sumarti, 1998) showed compositional changes in response to seasonal influences (Fig 3.8). From September 1997 till January 1998 element concentrations remained fairly constant despite a decrease in water level of almost 4 meters. A subsequent increase in water level, apparently induced by rainfall, was accompanied by decreasing concentrations of most elements. An anomalous increase in Cl and F observed in March 1998 was accompanied by a deviating high temperature, which might reflect an increase in magmatic fluid input through the vents that feed the lake. Figure 3.7. Concentrations of cations in the lake water are close to calculated concentrations (solid dots) assuming isochemical dissolution of 60gr of basaltic andesite in 1 litre of water. (A. Bernard, unpublished).
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Crater lake
Banyupahit
Locality
Banyuputih
upper creek
Elevation (m)
bridge
bridge
Blawan
Asembagus
tributaries .hot spring
2350
2350
2350
1990
1750
1500
1200
950
160
Date
?-Sep-62
10-Dec-93
10-Aug-96
10-Dec-93
13-Aug-96
14-Dec-93
13-Aug-96
13-Aug-96
13-Aug-96
24-Dec-93
Label
ZEL62-2*
IJ-93
IJ-96
BP93-3
BP96-6
BP93-5
BP96-7
BP96-8
BP96-11
BLW-1
35.6
T (°C)
n.r.
41.6
pH
0.02
0-0.2
TDS (g/kg)
91.4
104.1
Na
1100
734
K
1051
Mg
near Kalisat
24-Dec-93
Blawan Kalisengon
24-Dec-93
25.8
20.1
17.8
21.6
24.4
24.2
24.4
22.1
24.4
0-0.2
0.74
0.5
0.76
1.87
4.29
6.41
7.73
7.86
107.0
111.9
49.9
54.0
53.2
6.4
0.9
0.9
0.3
0.5
1160
825
589
517
583
150
45
200
16
32
1306
1473
1419
828
947
859
69
18
46
8.1
15
699
729
630
821
327
527
359
151
37
74
20
52
Ca
888
1197
968
815
696
725
718
351
90
87
42
101
Sr
n.r.
10.9
6.3
10
6.3
8
6.3
2.3
0.5
n.a.
n.a.
n.a.
Ba
n.r.
0.11
0.16
0.13
