Phreatomagmatic deposits in Batur caldera Bali-Indonesia

Abstract

Batur caldera is situated at 115° 19' to 115° 26' E and 8° 11' to 8° 18' S, on the island of Bali, Indonesia. It lies on the Sunda-Banda volcanic island arc (Hamilton, 1978), and includes calc-alcaline volcanoes (Whitford and Nichols, 1973). It consists of two calderas, the inner is 7 km diameter and the outer is 11 to 15 km diameter. Between the two calderas sits the Kintamani terrace (Kemerling, 1917), situated on the western to northern part. In the centre of the inner caldera, grows Batur volcano (1717 m). Its last eruption in 1974 flowed basaltic lava to the west. The Batur lake sits on the east side of the caldera and is 7 to 8 km length, 2.5 km at the widest point, about 22 km in circumference, and up to 88 m deep. Basaltic andesite to andesitic phreatomagmatic deposits are found inside Batur caldera and Kintamani terrace. They were deposited after the inner caldera formed. The phreatomagmatic deposits on Kintamani terrace were produced by big phreatomagmatic eruptions, and by small phreatomagmatic eruptions on the caldera floor. There are three types of phreatomagmatic deposits : 1. Flow unit, commonly in the lower part, massive, poorly sorted, matrix supported, from very fine ash to block size (dominantly lapilli to block size), angular, and composed various lithics derived from older deposits. 2. Fall unit, commonly overlying the flow unit, bedded, from a few to several centimetres thick, poorly to moderately sorted, from fine ash to lapilli, but sometimes blocks are present, and rich of accretionary lapilli. 3. Base surge unit, planar and cross bedding, chanelling, poorly sorted but better than flow unit, commonly ash to lapilli, and rich in accretionary lapilli. The typical pyroclast morphology of the ash as seen by SEM is mostly blocky, poorly vesicular, and vesicle filling. Some pyroclasts show lamellae of palagonitized. The fine grained clasts are predominantly composed of glass and small amounts of crystals. Most deposits have similar mineralogy: plagioclase and pyroxene and opaque. Olivine is present associted with basic plagioclase and apatite associated with acidic plagioclase. The parent magma is basaltic and has undergone fractional crystallization and mixing with the fractionated part during evolution. The type of magma did not influence the type of phreatomagmatic eruption. The phreatomagmatic eruption will still likely happen again. Its probably produce potential hazards of earthquakes, base surge, mud rain, mud flow, phreatomagmatic flow, maar formation, and tsunami.