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Organic block coatings in block-and-ash flow deposits at Merapi Volcano, central Java

Published online by Cambridge University Press:  17 September 2008

E. DONOGHUE ,
V. R. TROLL ,
L. M. SCHWARZKOPF ,
G. CLAYTON  and
R. GOODHUE
E. DONOGHUE*
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
V. R. TROLL
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
L. M. SCHWARZKOPF
Affiliation:
GeoDocCon, Unterpferdt 8, 95176, Konradsreuth, Germany
G. CLAYTON
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
R. GOODHUE
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
*
†Author for correspondence: donoghue@tcd.ie
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Abstract

The 1954, 1994, 1998 and 2006 block-and-ash flow deposits from Merapi volcano, Indonesia, contain andesite blocks that display localized, dark surface coatings. These coated blocks are typically found in clusters, but may also occur isolated within the block-and-ash flow deposits. The coatings form irregular patches (tens of centimetres across) on block surfaces, and are characterized by a silver, metallic appearance. The origin and composition of the coatings have not previously been determined, and no comparable deposits have been described in detail from any other volcanically active region of the world. Carbon isotope analyses of coated and uncoated bulk rock material, and of a range of charred and uncharred wood samples, suggest that the dark block coatings at Merapi volcano are organic in origin. Scanning electron microscopy and reflected light microscopy indicate that such coatings may form due to the migration and subsequent deposition of organic carbon in cavities within the andesite blocks. The most recent field evidence from the 2006 block-and-ash flow deposits at Merapi indicates that in situ charcoalification of plant material after block deposition and/or during the final stages of flow, is the major mechanism that forms localized concentrations of block coatings. Isolated coated blocks may result either from recycling of older deposits, or, more likely, from charcoalification and redistribution of plant material caught between colliding lava blocks during flow.

Keywords

coatingandesitecharcoalblock-and-ash flowMerapi volcano
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 1 , January 2009 , pp. 113 - 120
Copyright
Copyright © Cambridge University Press 2008

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Footnotes

‡

Present address: Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden

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