Stratigraphy and Radiocarbon Dating of Pyroclastic Deposits at Merapi Volcano

Previous section : Merapi, History and Future Changes

Stratigraphy and radiocarbon dating of pyroclastic deposits at Merapi Volcano, Central Java, reveals ,10,000 years of explosive eruptions. Highlights include:

(1) Construction of an Old Merapi stratovolcano to the height of the present cone or slightly higher. Our oldest age for an explosive eruption is 9630 ± 60¹⁴ C 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 slope and sent one or more debris avalanche(s) down its southern and western flanks. Impoundment of Kali Progo to form an early Lake Borobudur at ~3400 ¹⁴C y B.P. hints at a possible early collapse of Merapi. The latest somma-forming collapse occurred ~1900 ¹⁴C y B.P. The current cone, New Merapi, began to grow soon thereafter.

(3) Several large and many small Buddhist and Hindu temples were constructed in Central Java between 732 and ~900 A.D. (roughly, 1400–1000 ^c y B.P.). Explosive Merapi eruptions occurred before, during and after temple 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 ± 50¹⁴ C y B.P. Eruptions ~700–800 v y B.P. (12–14th century A.D.) deposited ash on the floors of (still-occupied?) Candi Sambisari and Candi Kedulan. We speculate but cannot prove that these eruptions were triggered by (the same?) partial collapse of New Merapi, and that the eruptions, in turn, ended “caretaker” occupation at Candi Sambisari and Candi Kedulan. A new or raised Lake Borobudur also existed during part or all of the 12–14th centuries, probably impounded by deposits from Merapi.

(5) Relatively benign lava-dome extrusion and dome-collapse pyroclastic flows have dominated activity of the 20th century, but explosive eruptions much larger than any of this century have occurred many times during Merapi’s history, most recently during the 19th century.

Will the relatively small eruptions of the 20th century as a new style of open-vent, less hazardous activity persist for the foreseeable future? Or, alternatively, are they merely low-level “background” activity that could be interrupted upon relatively short notice by much larger explosive eruptions? 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-level activity. This conclusion has important implications for hazard evaluation.

Edifice deformations are reported for the period 1988–1995 at Merapi volcano. Cross-crater strain rates accelerated from less than 3 x 10^-6/day between 1988 and 1990 to more than 11 x 10^-6/day just prior to the January 1992 activity, representing a general, asymmetric extension of the summit during high-level conduit pressurization. After the vent opened and effusion of lava resumed, strain occurred at a much-reduced rate of less than 2 x 10^-6/day. EDM measurements between lower flank benchmarks and the upper edifice indicate displacements as great as 1 m per year over the four years before the 1992 eruption. The Gendol breach, a pronounced depression formed by the juxtaposition of old lava coule ´es on the southeast flank, functioned as a major displacement discontinuity. Since 1993, movements have generally not exceeded the 95% confidence limits of the summit network.

A history of over two centuries of eruptive activity at Merapi has been reconstructed from various sources, with the information organized in chronological order so that it may serve as a concise, reasonably comprehensive information source and a guide to the literature. A major difference in eruption style exists between the activity of the twentieth century and that of the previous centuries, although the frequency of larger events seems about the same. Twentieth century activity mainly comprises effusive growth of viscous lava domes and lava tongues, oversteepened parts of which gravitationally collapse to produce the nue ´es ardentes style commonly defined as “Merapi-type”. In the 1800s, however, larger explosive eruptions occurred, and the associated fountain-collapse nue ´es ardentes were larger and travelled farther than any produced in the twentieth century. These events, too, may be regarded as typical eruptions for Merapi.

Conclusion

Although the eruptive style of Merapi seems to have changed, the considerable hazards associated with the previous, more dangerous eruption style must not be ignored. The nineteenth century activity is consistent with the pattern of one relatively large event every century or so, based on the long-term eruptive record deduced from stratigraphic, mapping, and age-dating studies (Newhall et al., 2000; Andreastuti et al.,). Relatively large events have occurred many times in the past and will certainly happen again. Activity in the twentieth Century has been anomalously mild. The occurrence of such a large event, with only modest (or inadequately appreciated) precursors, could lead to a disaster unprecedented in Merapi’s history—given the large and increasing population living on the volcano flanks. The conclusion is that a recurrence of the kind of large explosive events typical of the 1800s is likely in the future, and that current hazard evaluations should not play down the possibility of these larger eruptive events, despite the dominance of smaller events in the twentieth century record.

Acknowledgements

*Geoinformation for Spatial Planning and Risk Management - Batch 6/2010 - Gadjah Mada University*

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