IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4P1_1D-O17

Room B1

Date/Time: July 24 15:30-15:45

Effect of volatiles, crystallization, and lava dome effusion rate on pyroclastic surge
dispersal at Merapi volcano, Java, Indonesia
Kimberly Genareau1 , Shane J. Cronin2 , Gert Lube2
1

Lehigh University, U.S.A., 2 Massey University, New ZealandE-mail: kig210@lehigh.edu

The behavior of volatile elements in an erupting magma will have direct effects on the rheology of effused lavas
and the dynamic properties of pyroclastic density currents resulting from dome collapse and explosive activity.
Pyroclastic density current deposits from the October/November 2010 surge-producing events at Merapi volcano,Java, Indonesia were examined using a combination of scanning electron microscopy (SEM), stereo-scanningelectron microscopy (SSEM), secondary ion mass spectrometry (SIMS), and electron microprobe (EMP) analyses.
Samples from the two climactic and lethal surge events on 26 October and 5 November were examined, in addition
to several intermediate surges, taken from a series of transects between 3 and 8 km from source. Microtextural andgeochemical properties of the tephras reveal variations in volatile behavior that influenced the physical propertiesof the resulting pyroclastic surges. The number density of plagioclase feldspar microlites is ten times higher inthe 5 November event, while the number of pyroxene/Fe-oxide microlites is fifteen times higher compared to the26 October event. SSEM analyses of the intermediate surges reveal variations in vesicle morphology during thetransition between the two main surge-producing events. EMP analyses indicate synchronous growth of feldspar
microlites and phenocryst rims in both phases of dome effusion. Degassing of the 26 October dome at relatively
greater depths in the plumbing system lead to a significant shift of the feldspar liquidus, resulting in alkali-richcompositions compared to the dominantly plagioclase compositions found in the 5 November lava crystals. Despite
efficient degassing of the earlier dome, enough volatiles remained in the interstitial melt to cause sudden, significant
vesiculation of the residual melt following decompression. In addition, CO2 contents of groundmass glasses in the26 October dome material is significantly higher (2-3 times) than that of the 5 November lavas. Textures within lapilliof the 5 November surge deposits revealed the development of permeable vesicle pathways, enabling efficientdegassing and decompression-induced crystallization of phenocryst rims and microlite phases in the shallow
conduit in a few days prior to collapse. Trends in volatile elements during the final stage of feldspar phenocryst
growth will be presented. Variations in lava textural and chemical characteristics are a function of lava effusion rateand decompression path between the different effusive stages. The behavior of volatile elements, mainly H2 O, CO2 ,and Li, between the main surge-producing events of 26 October and 5 November, 2010, affected magma rheology
over several days during an increased rate of lava dome effusion, altering the composition of the products, thegeneration of fine ash, and the transport properties (total volume, run-out distance) of resulting pyroclastic density
currents.

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