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

The upper part of Merapi Volcano is characterised by a summit crater breach ed to the southwest and occupied by an unstable lava dome. Episodes of dome growth have usually resulted in partial dome collapse events that have gen erated pyroclastic Rows. In recent years, these have inundated the southwes tern 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 d educed from the stratigraphy and chronology of volcaniclastic deposits. Thi s record forms an important basis from which the average periodicity of eru ptions at Merapi, and the severity and the extent of future eruptions can b e assessed. The most complete record of eruptive activity from Merapi Volcano is found in the deeply dissected volcaniclastic fans of the lower flanks. Here, succ essions of numerous coal se ash and lapilli beds are interbedded with andic soil material, thin pyroclastic Row and surge deposits, and a hitherto unr ecognised inter-regional tephra marker bed. Many of these tephra beds can b e reliably correlated around the flanks of Merapi Volcano and can be distin guished on the basis of physical appearance and stratigraphic association. These formations can be reliably used to correlate units around the Ranks o f the volcano. In the deeper parts of this stratigraphic succession, pyrocl astic products originating from the adjacent Merbabu Volcano are also recog nised 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 x 10(8) m(3)) eruptions have been generated at Merap i Volcano. These eruptive deposits are typically produced from pyroclastic flow and/or pyroclastic fall events. Most of the fall beds are characterise d by single to multiple, upward-fining, pumiceous or scoriaeous units, some times 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 vulcani an open-vent styles of activity appear to have been common at Merapi Volcan o. Such eruptive episodes are in contrast with the style of activity that h ad 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 assessmen t and contingency planning programme. (C) 2000 Elsevier Science B.V. All ri ghts reserved.