Merapi volcano, Java, Indonesia : a comparison of 1984 block & ash deposit and lahar deposit

Abstract

Merapi is a basaltic-andesitic volcano with frequent eruptions (once in 3 years on average) located in Java island, Indonesia. This volcano is one of the numerous island arc volcanoes in West Indonesia which result from the subduction of the oceanic part of the India-Australia plate under the Eurasian plate. The volcano is located on the structural boundary between the east and central Java structural provinces as well as in a structural depression trending along the length of the island. Its activity started in the Pleistocene, the first historical record of its eruptions dates back to 1548. At least since 1786, Merapi activity has been characterised by successive extrusions of lava domes accompanied by block & ash flows; in the last 2.5 decades the domes have been slightly to the west-southwest of the peak. As an active volcano situated in a densely populated area, Merapi is a high risk volcano. Its "degree of danger" value is 75.2 which is "very dangerous". Most of the risk comes from lahar. Every time a lava dome crumbles into a single or a series of block & ash flow, a block & ash deposit is emplaced. The deposit is then remobilised by a heavy rainfall as a lahar. Heavy rainfalls and high annual precipitation are common phenomena of this tropical region. Block & ash deposits and lahars in Merapi have very similar characteristics. They are both matrix supported breccias with volcanic material. Their fragments are up to boulder size, angular and subangular, and are distributed unsystematically. The matrix materials are angular and are practically entirely mechanical impact products of larger volcanic rocks. They are free of clay minerals. The percentage of clay-size material is insignificant. There is no distinction in grain size distributions between lahars and block & ash deposits. The peaks of the distributions are between 1Φ and 2Φ. Their means are between 0.69Φ and 1.63Φ, with high standard deviations (>1) indicating poor sorting. Few crystals are present in the matrix of either kind of deposit. Vesicular particles and glass shards are extremely rare. Matrix materials of block & ash and lahar deposits show no systematic differences in their angle of shearing resistance; their calculated critical slopes are practically the same (about 20°). However, in the field, block & ash deposits generally show convex morphology which is expressed as irregular and broad undulations, while lahars tend to fill in negative morphology or flatten and spread out in lower reaches. Block & ash flows can reach down to 900m elevation on slopes between 30° and 6°, and lahars can be deposited from 900m down to about 150m on slopes between 5° and 1°. Block & ash deposits rarely have associated fluvial sediments, while lahars, particularly those of lower slopes, often are associated with deposits from subsequent fluvial flows.