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NERC

New insights into the deposit architecture and emplacement mechanisms of block-and-ash flows using ground penetrating radar

About Merapi

Merapi, a 2911-m-high, steep-sided andesitic volcanic complex in Central Java (Fig. 1) is one of the most frequently erupting volcanoes in Indonesia and best known for its nearly persistent volcanic activity characterized by the extrusion of viscous lava domes (Fig. 2) and collapse of these domes to produce Merapi-type nuées ardentes (Fig. 3) and related block-and-ash flow deposits (Fig. 4).

map showing Merapi

Fig. 1. Map of the Indonesian subduction zone system (after Gertisser & Keller, 2003).

These flows may travel up to 10 km away from the volcano and in the past such flows have affected populated and fertile land around the volcano. In 1930, several villages were destroyed and more than 1300 people were killed by block-and-ash flows. More recently, block-and ash flows killed 66 victims during a small-scale eruption in 1994, whilst the renewed activity of 2006 led to more fatalities.

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The lava dome of Merapi A small nuée ardente during the 2006 eruption of Merapi The 2006 block-and-ash flow deposits in the upper Gendol River valley
Fig. 2 The lava dome of Merapi in September 2006 (photo by Sylvain Charbonnier).
Fig. 3 A small nuée ardente during the 2006 eruption of Merapi (source unknown). Fig. 4.The 2006 block-and-ash flow deposits in the upper Gendol River valley (photo by Sylvain Charbonnier).

This latest eruptive episode resumed with the extrusion of a new lava dome in March 2006 after five years of quiescence.

Lava dome growth increased during April 2006 and was rapidly followed by periods of multiple rockfalls and dome-collapse events during May 2006.

During this first eruption phase, rockfalls and dome-collapse pyroclastic flows were mainly directed towards the southwest flank of Merapi into the Krasak, Bedok and Boyong River valleys, with runout distances of less than 4 km .

Distribution of the 2006 eruption deposits on the south-western and southern flanks of Merapi.
Fig. 5 Distribution of the 2006 eruption deposits on the south-western and southern flanks of Merapi. (after Charbonnier & Gertisser, 2008).

Lava extrusion rates increased during the second eruption phase that followed a devastating M6.3 earthquake near the town of Bantul, 35 km south of Merapi, on May 27, 2006 (Fig. 6).

Partial crater wall collapse in the summit area allowed pyroclastic flows to travel down the densely populated southern and south-eastern flanks of Merapi during the third eruption phase in June 2006 (Fig. 5).

Destroyed building in Bantul
Fig. 6 Destroyed building in Bantul following the M6.3 earthquake on May 27, 2006 (photo by Ralf Gertisser).

Between June 3 and 12, several dome-collapse pyroclastic flows inundated the Gendol River valley with maximum runout distances of 4.5 km. On June 14, the activity peaked with two sustained dome-collapse events that lasted over period of tens of minutes and the occurrence of at least two pyroclastic flows with runout distances of 5 and 7 km in the Gendol River valley (Fig. 7). The largest of these flows spilled over the valley sides of the Gendol River in several places and caused two fatalities and partial burial of the village of Kaliadem (Fig. 8).

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block-and-ash flow deposits Houses in the village of Kaliadem destroyed by pyroclastic flows
Fig. 7. The June 14, 2006 block-and-ash flow deposits in the lower Gendol River valley on Merapi’s south flank (photo by Ralf Gertisser). Fig. 8. Houses in the village of Kaliadem destroyed by pyroclastic flows from June 14, 2006 (photo by Sylvain Charbonnier).

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