Mount Batur Caldera
Batur caldera is situated in the north eastern part of Bali Island, and about 70 km north of the capital city of Denpasar. There are six main routes of travel to the caldera that can be attained from Denpasar and one route from Singaraja. The most useable of these is the road from Denpasar to Kintamani passing Bangli city.
Batur caldera is one of the finest calderas in the world, about 13.8 by 10 kilometers ( Van Bemmelen, 1949) with another circular composite collapse structure with a diameter of 7.5 km formed in its center. The rim varies in height from 1267 to 2152 meters (the marginal cone of Mount Agung ). Within caldera is the active strato volcano of Batur volcano and a lake. The highest peak of Batur volcano is about 1717 m above sea level and 686 m above the surface of lake Batur, and is located at 8Â°14’ 30’’ S and 115Â°22’30’’ E ( van Padang, 1951 ; Kusumadinata,1979 ).
According to Kemmerling (1918) the Mount Sukawana and Mt. Abang volcanic remnant indicate that the previous stratovolcano of Batur was higher than Agung Volcano (3142 m ), and the present caldera complex was caused by collapse of the old-stratovolcano. The young stratovolcano of Batur has erupted at least 22 times since the 1800’s, generally with strombolian eruptions and lava flows.
Geomorphology of G. Batur can be seen as : caldera, craters, volcanic cones, cinder cones, steep and gentle slope. All these morphology unit situated at least between two morphology complex that have different source; east part by G.Agung morphology complex, and west part by G. Buyan-Bratan morphology complex. Basically morphology of G.Batur can be distinguished into : Inner caldera morphology, consist of G.Batur main cone, cinder cones and low land plain inside the caldera. Caldera slope morphology, consist of southwest part slope, north-northeast slope and G. Abang cone. Stratigraphy of G.Batur can be summarized as follow : Pre-caldera rocks : lava, pyroclastic flow, cinder cones and lahar.
The oldest rocks of the pre-caldera basement are sedimentary and volcanic rocks of Miocene to Pliocene ages. The pre-caldera statovolcano was built up to more than 3,000 m above sea level. It is composed of basaltic andesite thepras and basalt lavas. These older volcanic rocks crop out in the walls of Batur caldera, where they commonly underlie the Batur ignimbrite. The pre-caldera rocks mainly crop out in the east (G.Abang) and northern parts of the caldera. Also underlying the Batur ignimbrite in the west and northwestern walls of the caldera are andesite units (Tanjungbatu lavas) widespread to the north and northwest through out Flores sea.
Pre-caldera rocks also include four basaltic cinder cones (Paleg cinder cones) that are distributed on the northeastern slope of the caldera. They are located on northeast-southwest trending lineaments, and are generally covered by caldera stage and post-caldera rocks. They even may be covered by pyroclastic airfall deposits from G.Agung (a few kilometers southeastward of these cinder cones).
The cinder cones are distributed on the northeast slope of the caldera and consist of four cones which have one or more craters in each cone. The heights of the cones vary slightly, because they were constructed on a slope ranging from 125 to 250 m. The cinder cones make up almost 80 percent of the scoria deposits; the reminder was deposited as a thepra blanket surrounding the cones.
The deposits are exposed within the walls of the inner craters, but some outcrops are exposed in gullies or quarries, and range from lapili to bombs up to 50 cm long. The bombs are highly vesicular and have convoluted, ropey surfaces. Many are welded together to form agglutinate layers. Many cindercones show crater openings in the same direction, they are little eroded but show evidence of incipient gullying in the form of broad shallow radial swales, and burial by pyroclastic airfall deposits. Petrographical and chemical analysis indicate that the bombs are highly vesiculer-olivin basalt and contain subhedral plagioclase phenocrysts (less than 10 %).
The cinder cones occur slightly, apparently at random around the norteast slope of the caldera and generally associated with the most areally extensive thepras and lava flows. Most of the cinder cones have been completely burried either by thick thepras or by succesive sheets of lava flows from pre-caldera eruptions.
Lahar deposit consists of breccias and conglomerater components, accompanied by minor amounts of better sorted tuffaceous sandstone; all were formed by erosion of the pre-caldera rocks. Caldera stage rocks : ignimbrites, pyroclastic flow, lava. Batur caldera is assumed to have subsided in response to the eruption of the Ubud and Gunungkawi ignimbrites. The contrast between relatively thick outflow ignimbrite and generally thin intracaldera ignimbrite indicates that subsidence began later in the eruption and continued as the deepening caldera filled with the ignimbrite. At the most conservative estimate, about 108 cu.km of outflow sheets and intracaldera ignuerupted.
The paroxysmal eruption occurred about 29,300 years ago, and formed a steep-walled depression more than 1 km deep. The subsidence was probably differential during collapse, being much deeper in the southeastern parts than in the northwestern end. This is deduced from the present features of the caldera wall which remains very steep in the southeastern part and the removal of more than a half of the old startovolcano of Mt.Abang.
Batur caldera I, as defined by its topographic rim, is elliptical in shape and about 13.8 by 8 km in size with its longest axis trending northwest-southeast, parallel to the interpreted graben. A half of the wall is spectacularly exposed and can be traced almost continuously, and another half is buried by dacitic-phreatomagmatic deposits from the post-caldera activities. The second ring-collapse caldera (caldera II) occurred within the first caldera, and is about 7.5 km in diameter.
Post caldera rocks : surge deposit, ignimbrites, phreatomagmatic deposits, air falls, maars, cindercones, lavas, surficial deposits.
Post caldera volcanism took place within the collapsed caldera II. The volcanic activity is marked by the formation of maars and cindercones. Among such events, the phreatic and phreatomagmatic deposits cover most of the caldera area. More than ten maars and cindercones were formed, scattered on the caldera floor, but some of them were buried by Batur eruptive products. Some crater remnants and cones can be recognized, although they are not perfectly circular.
The major post caldera volcanic event is the eruption of glassy lava of olivine basalt to basaltic andesite, which was erupted from the center of the caldera. These eruptions continue to the present day, and have built up a group cones of Batur volcano. At least three large cones have been built during its formation, and many craters lie on an east-west trend.
A big lake has formed in the eastern part of the caldera. The deepest part is about 81 m. The lake is ellipsoid in shape, elongated nearly north-south, mostly bounded by a steep wall along the east side, but the west side is bounded by lava fllows from Batur volcano. The lake water has fluctuated in temperature from 20Â° to 22Â°C, and pH ranges from 7 to 8.5. It is commonly blue or greenish blue in color.
The volcanic rocks of Batur caldera are predominantly composed of dacites and andesites (58-68% SiO2) with only subordinate amounts of precaldera basalts to basaltic andesites, all of which have geochemical characteristics typical of Island arc calc-alkaline suites. There is overall trend of decreasing proportions of silicic rock-types with decreasing age.
The general petrologic progression of the Batur caldera suggests the the progressive rise, differentiation, and crystallization of the magma beneath the caldera. The rocks become more mafic as age decreases from voluminous dacitic composition (63-68% SiO2) to the post caldera lavas and tephras of basalt to basaltic andesite composition (51-54% SiO2). This may indicate compositional zonation in the magma chamber in which more silicic differentiated magma overlay more mafic magma.
The evolution of Batur caldera can be generally related to seven stages. Premonitory activity (stage 1) would be represented by the pre caldera stratovolcano which was about 3000 m above sea level, and consisted of basalt and basaltic andesite compositions. The eruption of voluminous dacitic ignimbrites (Ubud and Gretek ignimbrites) about 29,300 years ago represents stage 2, accompanied by collapse to produce an ellipsoidal caldera depression as caldera I (stage 3). The depression was deeper in the northwestern part and caused leaking from the evolving andesitic magma reservoir and eruption of high silica andesite lavas (stage 4). Stage 5 is the second large eruption of dacitic ignimbrites (Gunung Kawi and Batur ignimbrites) about 20,150 years ago, accompanied by rising andesitic magma through the cracks or ring fracture to produce the domes and dykes. The second caldera collapse (stage 6) formed a circular shape in the center of the first caldera, as caldera II, followed by the second collapse of caldera I, and formation of the lake, maars and cindercones in the caldera moat. The terminal stage (stage 7) is post caldera volcanism, represented by a stratovolcano constructed in the center of the caldera II, comprising basalt to basaltic andesite compositions.