Merapi (Central Java, Indonesia): An outline of the structural and magmatological evolution, with a special emphasis to the major pyroclastic events

Abstract

Merapi Volcano, in the central part of Java, is regarded as the most active and most dangerous volcano in Indonesia. On the basis of field studies and geochronological data, its history is divided into four Periods: Ancient, Middle, Recent and Modern Merapi. According to our preliminary age data, the Ancient Period may have begun around 40 000 y BP and lasted until 14 000 y BP when the Middle Period begun. The Recent Period begun around 2200 y BP and was replaced by the Modern Period after the eruption of 1786. During the Middle Merapi stage, a Mount St. Helens-type edifice collapse occurred. Our data suggest that his event is younger than 6700 y BP and older than 2200 y BP. During the Recent Merapi stage, violent magmatic to phreatomagmatic eruptions twice interrupted the growth of the volcano. The older Gumuk phreatoplinian deposits cover the entire cone; charcoal found within these deposits gave 14C ages of 2200 and 1470 y BP. The overlying Sambisari ash deposits were emplaced by violent pyroclastic surges directed towards the south, i.e. to the present location of the town of Yogyakarta. They buried the Shivaitic temple of Sambisari at the start of the 15th century. Modern Merapi is characterised by the persistent growth of a summit dome, periodically interrupted by partial or total collapse of the dome to generate frequent Merapi-type nuées ardentes (blocks-and-ash flows and associated surges), or more exceptionally, fall-back St. Vincent type nuées ardentes (scoria flows). In contrast, previous stages were characterised by effusion of long lava flows, alternating with violent explosive phases, generating essentially St. Vincent-type nuées ardentes. Merapi lavas are calc-alkaline, with a compositional range between 49.5 and 60.5 wt% SiO 2. Moreover, high-K basaltic andesites represent about 90% of Merapi lavas. Disequilibrium features are common in the mineralogical assemblage (plagioclase, orthopyroxene, clinopyroxene, olivine, amphibole, titanomagnetite, scarce alkali feldspar). Macroscopic and microscopic textural heterogeneities are related to magma mixing, that may have buffered the compositions of lavas and caused the restricted range of compositions. Our studies of past activity at Merapi provide some insights into future activity, and to related volcanic hazards. It is clear that Merapi has the potential of eruptions much more powerful than observed during the historic period.