The Geodynamic Setting and Geological Context of Merapi Volcano in Central Java, Indonesia

Mount (Mt.) Merapi is a Quaternary stratovolcano situated on the active volcanic front in Central Java. It is a part of the active volcanic arc chain of Java Island formed by the Sunda-Banda subduction of the Indo-Australian oceanic plate beneath the Eurasian continental plate. The volcanic activity of Merapi shows a recurring pattern of eruptive events, with a background of frequent Volcanic Explosivity Index (VEI) 1-2 eruptions approximately every 4-5 years, interrupted by more violent (VEI 3-4) eruption episodes every few decades. As one of Indonesia's most active volcanoes, Mt. Merapi draws admiration and fear from many Indonesians and the world. The volcano plays a significant role in understanding the Quaternary volcanism in Java, as it has been continuously erupting since the historical records began. Located within the Central Java depression zone, Mt. Merapi is bound by the Southern Mountains Zone to the south and the Kendeng and Rembang zones to the north. The basement of Mt. Merapi is composed of limestones of the Sentolo Formation and Tertiary volcanic and volcaniclastic rocks of the Nglanggran and Semilir formations. The region is dissected by several larger fault systems, namely the Progo Muria Fault and the Opak River Fault. Mt. Merapi and the Merbabu-Telomoyo-Ungaran volcanoes (located to the north of Merapi) follow a NW-SE-trend, similar in orientation to the regional fault systems, suggesting that geological structures control the emplacement of magma and its ascent to the surface. Tomographic studies underneath Mt. Merapi also indicate the presence of a NW-SE trending gravity anomaly that may represent a wider, more regional extent of the magma reservoir beneath the Merapi-Merbabu-Telomoyo-Ungaran volcanic chain.