Field Observations and Surface Characteristics of Pristine Block-and-Ash Flow Deposits from the 2006 Eruption of Merapi Volcano, Java, Indonesia

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Merapi volcano, block-and-ash flows, deposit characteristics, generation mechanisms, emplacement mechanisms

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A series of pristine block-and-ash flow deposits from the May–June 2006 eruption of Merapi represent an exceptional record of small-volume pyroclastic flows generated by gravitational lava-dome collapses over a period of about two months. The deposits form nine overlapping lobes reaching ~ 7 km from the summit in the Gendol River valley on the volcano's southern flank, which were produced by successive flows generated during and after the major dome-collapse event on June 14. Both, single pulse (post-June 14 events) and multiple-pulse pyroclastic flows generated by sustained dome collapses on June 14 are recognised and three types of deposits, spread over an area of 4.7 km², are distinguished, totalling 13.3 × 106 m3: (1) valley-confined basal avalanche deposits (11.7 × 106 m3) in the Gendol River valley, (2) overbank pyroclastic-flow and associated surge deposits (1.4 × 106 m3), where parts of the basal avalanche spread laterally onto interfluves and were subsequently channeled into the surrounding river valleys and (3) dilute ash-cloud surge deposits (0.2 × 106 m3) along valley margins. Variations in the distribution, surface morphology and lithology of the deposits are related to the source materials involved in individual pyroclastic-flow-forming events and varying modes of transport and deposition of the different flows. Inferred flow velocities of the largest block-and-ash flows generated on June 14 vary from 43.8–13.5 m/s for the basal avalanche and from 62.6–24.2 m/s for the ash-cloud surge. The minimum temperatures range from 400 °C for the basal avalanche to 165 °C for the overlying ash cloud. Due to the potential of being re-channeled into adjacent river valleys and flowing laterally away from the main river channel, the overbank pyroclastic flows are considered the most hazardous part of the block-and-ash flow system. The conditions that lead to their development during flow transport and deposition must be taken into account when assessing future pyroclastic flow hazards at Merapi and similar volcanoes elsewhere.

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Journal of Volcanology and Geothermal Research, v. 177, issue 4, p. 971-982