Geological History, Chronology and Magmatic Evolution of Merapi

This chapter provides a synthesis of the geological history, chronology and magmatic evolution of Merapi. Stratigraphic field and geochronological data are used to divide Merapi into three main evolutionary stages and associated volcanic edifices (Proto-, Old and New Merapi) and eight broad volcano-stratigraphic units to characterise the eruptive activity and structural evolution of the volcano through time. Complementary petrological, geochemical and isotopic data are used to characterise the eruptive products of Merapi and shed light on the geochemical evolution and petrogenetic processes. The data indicate that the eruptive products of Merapi are mainly basaltic andesite of both medium-K and high-K type and support a two-stage petrogenetic model, where primary magmas are derived from a heterogenous, Indian Ocean MORB-like mantle source metasomatised by slab-derived components. Subsequently, these magmas are modified during transfer through the crust by complex magmatic differentiation processes, including contamination by carbonate rocks of the local upper crust. The available data indicate that, since similar to 1900 C-14 y BP, the lavas and pyroclastic rocks of Merapi are essentially of the high-K type and that regular geochemical variations with systematic shifts in whole rock SiO2 content occurred since at least the Late Holocene, although erupted magma compositions have remained broadly uniform since the mid-twentieth century.