The Godean Debris Avalanche Deposit From a Sector Collapse of Merapi Volcano

Abstract

AbstractDebris avalanche deposits are products of large sector collapses of composite volcanic cones. Horseshoe-shaped craters open downslope to a hummocky topography of the deposits. In the Godean area, Sleman District, Special Province of Yogyakarta, the deposit of a long-suspected large volcanic sector collapse of Merapi has been discovered, forming a hummocky topography north of the Godean palaeovolcanic hills. The exposed remnants of the Godean debris avalanche deposit cover an area of 2 × 2 km and the height of hummocks is < 30 m above the surrounding plain. The exposures are loose block facies composed of pyroclastics, lava flows and reworked deposits, highly fractured into jigsaw cracks and small-scale fault displacements. Andesitic megablocks representing clasts of the matrix facies are widely distributed on the Sedayu plain south of Godean, from where the avalanche propagated southward until the confluence of the Bedog River and the Progo River. In its original form, the Godean debris avalanche flowed 50 km away from Merapi and covered an area of ~390 km2. The gigantic landslide destroyed areas of the Magelang District, Central Java Province, as well as the Sleman, Bantul and the eastern part of the West Progo districts, Yogyakarta Special Province. The total volume of the deposit is estimated at 4.9–8.6 km3. Remnants of the Godean debris avalanche are preserved in the Godean and Sedayu areas because the avalanche hit, and was trapped by the north side of the Tertiary Godean palaeovolcanic hills. Southward, the debris avalanche was stranded on the north side of the wavy hills of the Tertiary Sentolo Formation. We suspect that the Godean debris avalanche was probably caused by the first stage of Merapi sector collapse (CE 1). However, this interpretation and the age of the event are still poorly constrained. To improve our understanding of past Merapi collapses, numerical dating studies, including tephra intercalations in black clay deposits of the ancient Borobudur and Gantiwarno lakes, are necessary. Merapi sector collapses range from small scales to large events like the Godean debris avalanche. In order to reduce volcanic risk from future sector collapses, a serious mitigation effort, focussing particularly on precursor phenomena, is suggested.KeywordsMerapiGodeanDebris avalancheSector collapseGigantic landslideYogyakarta