THE 2006 ERUPTION OF MERAPI, PETROGRAPHY AND GEOCHEMISTRY OF THE JUNE 14, 2006 PYROCLASTIC FLOWS

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

THE 2006 ERUPTION OF MERAPI, PETROGRAPHY AND
GEOCHEMISTRY OF THE JUNE 14, 2006 PYROCLASTIC FLOWS

Supriyati Andreastuti¹, John Pallister², Chris Newhall³

¹Center for Volcanology and Geological Hazard Mitigation
²USGS Cascades Volcano Observatory
³Chris Newhall, U.S. Geological Survey, Santo Domingo, Albay, Philippines

ABSTRACT

Pyroclastic flows from the 2006 Merapi lava dome began when the degassed outer margin of the dome reached the steeply dipping southwest margin of the summit on 12 May, 2006 and collapsed. Subsequent collapses eroded into the more gently-dipping core of the dome as it was fed by fresh magma from a central vent. This suggests that later-erupted lava was less degassed (more pressurized) and consequently, more prone to collapse. High lava dome growth rate was balanced by collapse, which reduced the possibility of a larger summit collapse. However, the collapse in a new direction (to the south) resulted in erosion and failure of part of older dome (Gegerbuaya), which contributed abundant older lithic blocks to the pyroclastic flows.

Determination of juvenile material in pyroclastic deposits of 14 June, 2006 was not immediately apparent from un-eroded high temperature outcrops. Sampling and initial analyses revealed two different textures and compositions, with SiO2 of 52.4 wt % and 55.7 wt %. Subsequently, we used a flux-gate magnetometer to identify juvenile components, and additional analyses of these juvenile samples show a range of 55.5 -55.9 wt % SiO2, overlapping with a sample collected from the 2006 lava dome at the Merapi summit (55.5 wt % SiO2).

The 2006 lava is similar in major-element composition to other lavas of the 3^rd eruptive epoch of Merapi (Andreastuti and others, 2000). Although erupted at a new location on the summit, this similarity suggests that it tapped the same magma reservoir as other eruptions of the past decade. In addition, plots of major-element compositions of samples from Merapi lava since 1782 suggest that the lower-SiO2 samples were derived from older lava, likely that erupted between 1872 and 1913, which previously covered the upper flank of south-southeast of Merapi, including Gegerbuaya.