10,000 Years of explosive eruptions of Merapi Volcano, Central Java: archaeological and modern implications

Stratigraphy and radiocarbon dating of pyroclastic deposits at Merapi Volca no, Central Java, reveals similar to 10,000 years of explosive eruptions, H ighlights include: (1) Construction of an Old Merapi stratovolcano to the height of the presen t cone or slightly higher. Our oldest age for an explosive eruption is 9630 +/- 60 C-14 y B.P.; construction of Old Merapi certainly began earlier. (2) Collapse(s) of Old Merapi that left a somma rim high on its eastern slo pe and sent one or more debris avalanche(s) down its southern and western f lanks. Impoundment of Kali Progo to form an early Lake Borobudur at similar to 3400 C-14 y B.P. hints at a possible early collapse of Merapi. The late st somma-forming collapse occurred similar to 1900 C-14 Y B.P. The current cone, New Merapi, began to grow soon thereafter. (3) Several large and many small Buddhist and Hindu temples were constructe d in Central Java between 732 and similar to 900 A.D. (roughly, 1400-1000 C -14 y B.P.). Explosive Merapi eruptions occurred before, during and after t emple construction. Some temples were destroyed and (or) buried soon after their construction, and we suspect that this destruction contributed to an abrupt shift of power and organized society to East Java in 928 A.D. Other temples sites, though, were occupied by "caretakers" for several centuries longer. (4) A partial collapse of New Merapi occurred <1130 +/- 50 C-14 y B.P. Erup tions similar to 700-800 C-14 y B.P. (12-14th century A.D,) deposited ash o n the floors of (still-occupied?) Candi Sambisari and Candi Kedulan. We spe culate but cannot prove that these eruptions were triggered by (the same?) partial collapse of New Merapi, and that the eruptions, in turn, ended "car etaker" occupation at Candi Sambisari and Candi Kedulan, A new or raised La ke Borobudur also existed during part or all of the 12-14th centuries, prob ably impounded by deposits from Merapi. (5) Relatively benign lava-dome extrusion and dome-collapse pyroclastic how s have dominated activity of the 20th century, but explosive eruptions much larger than any of this century have occurred many times during Merapi's h istory, most recently during the 19th century. Are the relatively small eruptions of the 20th century a new style of open- vent, less hazardous activity that will persist for thp foreseeable future? Or, alternatively, are they merely low-level "background" activity that co uld be interrupted upon relatively short notice by much larger explosive er uptions? The geologic record suggests the latter, which would place several hundred thousand people at risk. We know of no reliable method to forecast when an explosive eruption will interrupt the present interval of low-leve l activity. This conclusion has important implications for hazard evaluatio n. (C) 2000 Elsevier Science B.V. All rights reserved.