A detailed tephrostratigraphic framework at Merapi Volcano, Central Java, Indonesia: implications for eruption predictions and hazard assessment

Abstract

The upper part of Merapi Volcano is characterised by a summit crater breached to the southwest and occupied by an unstable lava dome. Episodes of dome growth have usually resulted in partial dome collapse events that have generated pyroclastic flows. In recent years, these have inundated the southwestern lower flanks, and in the past, other sectors of the volcano have been affected. This recent pattern of volcanic activity since the 1880s is well known from direct observations. In this study the eruptive record extending back to mid-Holocene times is deduced from the stratigraphy and chronology of volcaniclastic deposits. This record forms an important basis from which the average periodicity of eruptions at Merapi, and the severity and the extent of future eruptions can be assessed. The most complete record of eruptive activity from Merapi Volcano is found in the deeply dissected volcaniclastic fans of the lower flanks. Here, successions of numerous coarse ash and lapilli beds are interbedded with andic soil material, thin pyroclastic flow and surge deposits, and a hitherto unrecognised inter-regional tephra marker bed. Many of these tephra beds can be reliably correlated around the flanks of Merapi Volcano and can be distinguished on the basis of physical appearance and stratigraphic association. These formations can be reliably used to correlate units around the flanks of the volcano. In the deeper parts of this stratigraphic succession, pyroclastic products originating from the adjacent Merbabu Volcano are also recognised and provide evidence of concurrent activity at both volcanoes. It is clear from the stratigraphic record that relatively large magnitude (eruptive volume to 3.7×10 8 m 3) eruptions have been generated at Merapi Volcano. These eruptive deposits are typically produced from pyroclastic flow and/or pyroclastic fall events. Most of the fall beds are characterised by single to multiple, upward-fining, pumiceous or scoriaeous units, sometimes overlain by ash-cloud surge beds. Isopach maps have been produced for several of these tephra beds and most of them show either west–southwest, south and southeast dispersion, except for the Ngrangkah tephra which shows no systematic direction or distribution. Sub-plinian or plinian to vulcanian open-vent styles of activity appear to have been common at Merapi Volcano. Such eruptive episodes are in contrast with the style of activity that had been occurring at Merapi Volcano in the last two centuries. Clearly, the range of eruptive events likely to be experienced at Merapi Volcano needs to be more critically considered in any existing or future hazard assessment and contingency planning programme.