Evolution of Batur caldera, Bali, Indonesia

Abstract

The Batur caldera, in the northeastern part of Bali island, is an elliptical collapse structure 13.8 by 10 km in diameter with another circular composite collapse structure with a diameter of 7.5 km in its centre. Two stages of collapse were interrupted by silicic andesite lavas and domes. The first collapse was initiated by the eruption of about 84 km3 of dacitic ignimbrite, the "Ubud ignimbrite", at about 29,300 years B.P., and caused a steep-walled depression about 1 km deep. The subsidence was probably differential during collapse, being much deeper in the northwestern part than at the southeastern end, and was followed by a graben with northwest-southeast trend. This is deduced from the present features of the caldera wall which exposes the silicic andesite, which was erupted from the northeastern caldera rim, and flowed out along the interpreted graben reaching the north coast. The second ignimbrite was erupted from a large crater about the present lake, and produced about 19 km3 of a similar voluminous dacitic ignimbrite, called "Gunungkawi ignimbrite" about 20,150 years B.P.. This second eruption triggered a second collapse, which created the central circular caldera, and formed a basin structure. Both the Ubud ignimbrite and Gunungkawi ignimbrite consist of uniform dacitic composition, containing glassy, phenocryst-poor (less than 10 percent) pumice with plagioclase, clinopyroxene, sparse hypersthene and olivine, and abundant titano-magnetite. Small apatite crystals are sparse, mostly as inclusions. Most of the outcrops of ignimbrite have white to red (the most abundant nearly 90 percent), and dark grey to black dacitic pumice clasts. The large clasts, up to 20 cm in diameter are in the nonwelded ignimbrite, particularly in the upper part of the Gunungkawi ignimbrite. The intracaldera ignimbrite (called "Batur ignimbrite") of about 5 km3 is densely welded ignimbrite and generally shows typical welded features comprising at least five different flow units, separated by thin (15 - 40 cm) welded pumiceous airfall deposits, with flattened pumice clasts. Another large eruption occurred about 5,500 years B.P., producing about 0.09 km3 of andesitic ignimbrite. This was initiated by phreatomagmatic eruptions, indicated by thick phreatomagmatic and surge deposits, underlying the ignimbrite. The caldera and its vicinity are partly filled, and variably mantled by later eruptive products of dacitic and andesitic phreatomagmatic and airfall deposits. Maars and cindercones characterise the postcaldera stage. There are more than ten craters scattered on the second caldera moat; and followed by continuous eruptions of basalt to basaltic andesite lavas and pyroclastic airfall of mostly strombolian eruptions. Recent volcanism is represented by a large stratovolcano constructed in the center of the caldera, about 686 m high from the lake level, and about 1717 m above sea level. The young stratovolcano of Batur has erupted at least 22 times since the 1800's. The last eruption was in March, 1974; and large eruption in 1926 and 1963 were mostly lava flows. Most rocks are strongly porphyritic (plagioclase particularly is abundant), and they do not show any obvious cumulate compositional features. Mafic and feldspathic minerals are similar in composition throughout the precaldera, ignimbrite and postcaldera rocks. Overall the Batur volcanic rocks do not show any systematic chemical and mineralogical variations with the age of eruptions, all of which have geochemical characteristics typical of island arc calc-alkaline suites. The postcaldera basalts have markedly less potassium, a little more sodium, and markedly less magnesium, calcium, aluminium, iron and titanium the precaldera basalts. In general, both precaldera and postcaldera basalts are quite similar in silica contents, although the precaldera lavas contain more olivine. The petrologic variation within the Ubud, Gunungkawi and Batur ignimbrites is the gradation from relatively phenocryst-poor dacitic ignimbrite at the distal end to relatively phenocryst-rich at the proximal parts. Variations in mineral and chemical composition of the ignimbrites studied from the Batur caldera also show upward increases in phenocryst content and decreases in silica content. This sequence of petrologic variations together with the welding-zonation is evidence of the rapidity of eruption and emplacement of each ignimbrite. aThis is explainable by progressive emptying of a single source chamber containing a compositionally layered magma. The volume of erupted material is roughly proportional to the size of related collapse. The evolution of the Batur caldera suggests the progressive rise, differentiation, and crystallization of a magma chamber beneath the caldera. The general petrologic progression of igneous activity of the Batur caldera complex indicates that the rock types are more mafic as age decreases from voluminous dacitic composition (64 - 68 percent SiO2) to the postcaldera lavas and tephras of basalt to basaltic andesite (51 - 54 percent SiO2). This composition change may indicate compositional layering in the magma chamber, in which more silicic differentiated magma overlies more mafic magma.