The 1984 nuée-ardente deposits of Merapi volcano, Central Java, Indonesia: stratigraphy, textural characteristics, and transport mechanisms

Abstract

For many centuries Merapi volcano has generated hot avalanches of blocks, lapilli and ashes, derived from the destruction of partially solidified, viscous lava domes (Merapi-type nuées ardentes). On 15 June 1984, at least four nuées ardentes came down the southwest slope of the Merapi, the first and the last being responsible for more than 99% of the deposits which are now exposed. The first nuée ardente, a Merapi-type nuée ardente, was produced by the destruction of the dome, travelled 7 km from the crater, leaving a measured deposit, 2.7 m thick, 4 km from the crater, near its upper depositional limit, regularly increasing to a maximum measured thickness of 12 m at the front of the deposit. The lower contact is sharp, non-erosive, with pines still rooted in the underlying paleosol. The deposit consists of 50% ash, 33% lapilli, and 17% blocks, with two subpopulations (one Rosin and one normal), and is finespoor, with less than 4% of fine ash (d finer than 4 φ). The deposit displays reverse population grading of both vesiculated and massive clasts, and of the maximum grain size. The maximum size significantly increases regularly down-current over most of the exposed length of unit 1, and bed thickness increases for the entire length of the deposit. The deposit of the second nuée ardente is only 6–21 cm thick, and of very limited lateral extent. It is a normally graded, coarse to fine ash, with a finespoor base. The third unit consists of fines-poor, normally graded coarse ash, exposed in low-amplitude (20–40 cm), 12-m-wavelength dunes. The deposit of the fourth nuée ardente rests in sharp erosive contact on the underlying unit, increasing in thickness down-flow. It consists of transitional coarse and fine-grained strata, 6–130 c cm thick, dipping 5–10° down-flow. The deposit, made up of two subpopulations (one Rosin and one normal), is normally graded over the entire bed, but coarsegrained strata are reversely graded. The relative content of vesiculated clasts increases up-bed in both strata types, from 12% at the base to 40% at the top. The characteristics of unit 1 suggest that it accumulated from a concentrated suspension of cohesionless solids exhibiting non-Newtonian behavior, where dispersive pressure played an important role in the suspension of the clasts. Units 2 and 3 were probably deposited from dilute turbulent suspensions, whereas the upper unit (4) is a classic example of deposition from a high-density turbulent suspension leading to the formation of multiple traction carpets driven by the overlying, lower-density, surge. The horizontal distance travelled by a hot rock avalanche may be influenced by its transport mechanism. Debris flows are mobile on very low slopes-as low as 1°-whereas grain flows, even density-modified grain flows, require relatively high slopes-more than 6° at Merapi-to maintain their mobility. If the present Merapi dome were to collapse and produce a debris flow, its present volume coupled with the minimal 1.5 km vertical drop could travel a distance ranging between 15 and 30 km. However, if transport were by grain flow mechanisms, the mass could come to rest as it reaches a 5–10° slope.