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Keywords:

  • Sedimentology;
  • block-and-ash flows;
  • Merapi volcano;
  • flow parameters;
  • emplacement mechanisms

Abstract

The internal architecture of the 2006 block-and-ash flow deposits of Merapi volcano (Java, Indonesia) was investigated using data collected from 27 stratigraphic sections measured immediately after flow emplacement, and after one and two rainy seasons of erosion. Identification of different depositional units and their longitudinal and lateral facies variations provide detailed information about: (i) the distribution, volumes and sedimentological characteristics of the different units; (ii) flow types and mobility as inferred from associated deposits; and (iii) changes in the dynamics of the different flows and their material during emplacement. Two main types of block-and-ash flows (short-runout to medium-runout block-and-ash flows and long-runout block-and-ash flows) are defined based on flow generation mechanism, flow volume, travel distance, deposit morphology, distribution, lithology and grain-size distribution. Conceptual models for the transport and depositional mechanisms of these two types of block-and-ash flows are presented. Variations in the runout distances observed for short-runout to medium-runout block-and-ash flows are linked directly to different initial flow volumes, degree of fragmentation and material properties of the moving mass during transport, with the largest and finer grained flows having the greatest mobility. Deposition occurs only over a narrow range of basal inclinations close to the angle of repose for pyroclastic material, indicating that such flows behave in a similar way to granular-free surface flows on unconfined planes. The flow mechanisms of long-runout block-and-ash flows at Merapi are interpreted to be similar, in many respects, to unsteady, cohesionless grain flows with an inertial flow regime where collisional forces largely overcome frictional forces. Flow unsteadiness causes the main body to be segmented into different pulses that run closer to each other as the flow moves downslope. Deposition occurs stepwise, with rapid aggradation of stacked sub-units from different parts of the major flow pulses. In such a model, the arrival of each flow pulse front at selected sites in the main river valley controls the generation and development of highly mobile, unconfined pyroclastic flows outside valley regions and their associated overbank deposits.