Ijen Volcanic Complex
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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 (pH<0.5) and mineralised volcanic water. A permanent solfatara on the lakeshore continuously produces native sulfur, which is mined by local workers. Occasional outbursts of phreatic activity, centred within the lake, have formed the main threat in recent times. Apart from potential dangers of lahars, it has been known since long that the acid nature of the water also generates environmental problems. In 1921 a dam was built to regulate the water level, but water percolates through the porous wall, and forms the headwaters of a 40 km long acid river. After a first stretch in within the caldera, it breaks through the caldera rim and reaches an inhabited and cultivated alluvial plain before reaching the Java Sea. In this area, virtually all of the acid river water is used for irrigation. Extensive coffee plantations cover much of the highland within the caldera. The crater lake and surroundings are a natural park of great scenic beauty. Together with hot springs and waterfalls in the caldera catchment, it attracts an increasing number of tourists. Geological setting The Ijen caldera has a diameter of about 14-16 km. Its northern margin is clearly visible as a typical caldera escarpment with a steep inner slope and elevations ranging from 850 to 1559 m. The southern and eastern walls are covered by the marginal volcanoes of Suket (2950 m), Jampit/Pendil (2338 m), Rante (2644 m), Merapi (2799 m), Ijen (2386 m), Pawenen (2123 m) and Ringgih (1965 m). The lowest part of the caldera (near Blawan village) has an altitude of 850 m, suggesting that the maximum depth of the caldera is about 700 m. Inside the caldera the topography is dominated by a large number of extinct volcanic cones: Cilik (1872 m), Pendil (2375 m), Anyar (1276 m), Genteng (1712 m), Gelaman (1726 m), Kukusan (1994 m), Papak (2099 m), Widodaren (2100 m), Blau (1774 m), Gendingwaluh (1519 m), Lingker (1630 Figurem) and 3.1 Kunci (1788 m). 25 Figure 3.2 Figure 3.3. Schematic overview of the history of the Ijen caldera after Van Bemmelen (1941) and Sitorius (1990). Pre-caldera activity is supposed to have started prior to 300,000 years ago, probably forming a large single stratovolcano (Old Ijen) with an estimated altitude of 3500 m. Lavas and pyroclastics of these deposits disconformably overlie Miocene limestone. The caldera formation is associated with the eruption of a large volume (~80 km3) of pyroclastic flow deposits, which reach a thickness of 100-150 m and are most widespread on the northern slope of the complex. The event occurred some time before 50,000 years ago, based on a K-Ar date (50±20ka) of a lava flow of Mt. Blau, which is considered to be the oldest post-caldera unit. This age also constrains the formation of lakes on the caldera floor. Lake sediments comprising shales, sand and river channel deposits are exposed in the northern area near Blawan. Post-caldera 26 activity (phreatomagmatic, phreatic, strombolian and plinian) produced the rim cones, which are generally composite edifices, and the inner cones, which are predominantly constructed by cinders. These younger volcanoes produced the ash and scoria cones, lava flows, pyroclastic flow and surge deposits and debris avalanche material that now cover the caldera flow. Radiocarbon dating of the pyroclastic flow deposits (Sitorus, 1990) yielded ages of >45,000 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. 27 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 Phreatic eruption 1817 15-16 January: phreatic eruption (flooding of mud towards Banyuwangi, fairly large volume of lake water discharged into Banyupahit River) 1917 25 February-14 March: lake seemed to boil; repeated phreatic eruptions, mud thrown up to 8-10 m above the lake surface 1921-1923 Increasing lake water temperature; steaming gases above water surface 1936 5-25 November: phreatic eruption producing lahar similar to that of 1796 and 1817 1952 22 April: steam eruption up to 1 km high; mud thrown up to 7 m above the lake surface 1962 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 1976 30 October: bubbling water at Silenong for 30 minutes 1991 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. 1993 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 1994 3 February: minor phreatic eruption from the south part of the lake. Coincident with the eruption, the lake level rose ~1 m. 1997 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. 1999 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 28 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. 29 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.