Insights into the October–November 2010 Gunung Merapi eruption (Central Java, Indonesia) from the stratigraphy, volume and characteristics of its pyroclastic deposits

Abstract

The 2010 eruption of Merapi was the second most deadly in the historic record of this volcano, claiming over 380 lives. By relating the observations of this eruption with detailed examination of deposit distribution, stratigraphy and sedimentology, a reconstruction of the properties of the pyroclastic density currents (PDCs) is presented, including the valley controlled block-and-ash flows (BAFs) and widespread, energetic pyroclastic surges. The distribution, volume and mobility characteristics of all types of PDC during the eruption sequence show evidence for levels of intensity unseen since the large-scale 1872 and 1930 eruption phases, especially during the climactic events of October 26 and November 5. Many tephra falls interbedded with PDC units show that most dome-collapse events occurred along with and between explosive vulcanian eruptions. The 2010 eruption produced very long-runout BAFs, reaching 16.1km in the Kali Gendol on November 5. This runout could be explained by its large-volume (20millionm3), around 10 times that of previous Merapi BAFs during the last 130years. Major avulsion of these dense BAFs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive PDC deposits. Spreading avulsed BAFs were a particular hazard downstream of ~10km where the landscape is less dissected. Less clear, however, is why pyroclastic surges extended up to 10km from the vent on November 5 and >6.4km on October 26. These expanded much farther from BAF margins (~2km) than ever seen before at Merapi. In one location they were decoupled from valley-centered BAFs with high momentum, traveling initially laterally across steep valley systems, before draining downslope. At this site, on the western side of the upper Gendol at around 3km from source, surge decoupling was apparently exacerbated by upstream collision and deflection of high-flux, hot and gas-rich BAFs against the cliffs of Gunung Kendil. The 1.4km-long cliff face was impacted directly for the first time in 2010 events, and may have been responsible for the formation of larger than normal turbulent ash-rich clouds above BAFs. These results imply that future eruption events under the present summit and upper flow-path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard, especially now that dense forest has been destroyed on the upper southern slopes of the volcano.