Stratigraphy and geochemistry of Merapi Volcano, Central Java, Indonesia: Implication for assessment of volcanic hazards

Abstract

Merapi volcano is one of the most active volcanoes in the world and during the last 3000 yrs. it has been more or less continuously in eruption. Merapi volcano is a stratovolcano with a south-west facing horseshoe-shaped summit crater filled by a lava dome. Its composition ranges from basalt to basaltic andesite (52 to 56% SiO2) and it has high Al2O3 (18.5 to 20.2 % wt) and medium to high K2O (1.1 to 2.4% wt) contents. The volcanic history of Merapi has been established from the study of the tephra deposits exposed on its flanks. Twenty formations have been defined, two of these are the products of Merbabu volcano to the north (Kajor and Patrantephras) and the remainder are sourced from Merapi volcano. The oldest formation recognised as being from Merapi volcano is Sumber (c. 3000 yrs. ago) and the youngest is Pasarbubar tephra (c. 1800 AD ?). The stratigraphic sequence of Merapi tephras can be grouped into pre-1800 A.D (from older than 3200 yrs. B.P to 1800 A.D) and post-1800 A.D (from 1800 to 1996 A.D) products. Based on thickness of deposits, stratigraphic associations, geochemical trends and compositional groupings, at least 3 eruptive epochs can be recognised in the eruption history of Merapi, namely Bakalan (older than 1960 yrs. B.P), Tegalsruni (1960 to 1112 yrs. B.P), and Selokopo (younger than 1112 yrs. B.P) epochs. The post-1800 A.D activity of the past decade is included in the Selokopo epoch. The history of Merapi volcano has revealed the magma composition evolved from medium-K to high-K with time. The appearance of high-K magma compositions occurs at the end of the eruption which produced the Bakalan tephra (c.3100 yrs. ago). The transition period was during the formation of Tegalsruni (1960 yrs. B.P) up to Temusari tephra (c. 1780 yrs. B.P) which is assumed to be a peak of activity during this epoch. The Plalangan tephra (c. 1650 yrs. B.P) which is stratigraphically above the Temusari tephra is the first eruption of entirely high-K composition. From c.3100 yrs to 1650 yrs. ago, five moderate to large eruptions have taken place and three of those are large (Bakalan Tegalsruni and Temusari tephra). The types of eruption at Merapi volcano range from gentle lava dome effusion to violently explosive eruptions and these indicate two eruption styles related to open and closed vent conditions respectively. The closed vent style was dominant in pre-1800 A.D times, where eruptions related to dome collapse were rare and eruptions ranged from sub plinian and plinian to vulcanian. Post-1 800 A.D eruptions, mainly of open vent type, have been related to dome collapse events occasionally leading to vulcanian explosive types. The change in activity from plinian to dome collapse-triggered eruptions is poorly defined but occurred about the same time as the Kepuhharjo tephra (c. 250 yrs. ago). Merapi eruptions range from small to large magnitude events (up to VEI 4). Commonly, large eruptions appear to be interspersed amongst small and moderate eruptions. As a whole, large Merapi eruptions have been triggered by the supply of more basic magma from deeper levels in the system and is evidenced by mixing of two different magma compositions. Small and moderate eruptions tap a range of compositions controlled by fractional crystallisation as illustrated by the trend of SiO2/K2O variation with time. Large eruptions typically have complex geochemical trends, whilst small eruptions show relatively simple trends. Observation of pre-1800 A.D and post-1800 A.D eruptions has revealed that the pre-1800 A.D eruptions are mainly controlled by deeper magmatic processes than the post-1800 A.D eruptions. Hornblende is more abundant in pre-1800 A.D eruptions, whilst in post-1800 A.D eruptions it is rare. The formation of hornblende is assumed to take place at 360 to 453 MPa. In pre-1800 A.D times, a large eruption (up to VEI 4) has occurred once every 400 yrs. (150 to 500 yrs.). In post-1800 A.D times, a large eruption (up to VEI 3) has occurred once every 30 yrs. At present, Continuous growth of the lava dome at Merapi summit has resulted in mainly small dome collapses with occasional vulcanian explosion (less than VEI 3). This evidence suggests that a large eruption followed a larger dome collapse or a plinian eruption may take place in the near future.