LAVAS AND TEPHRAS OF MERAPI VOLCANO, JAVA, INDONESIA:INSIGHTS FROM TEXTURAL ANALYSES AND GEOCHEMISTRY

Open Access
Author:
Innocenti, Sabrina
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 22, 2006
Committee Members:
  • Tanya Furman, Committee Chair
  • Derek Elsworth, Committee Member
  • Peter J Heaney, Committee Member
  • Maureen Feineman, Committee Member
  • Barry Voight, Committee Member
Keywords:
  • Merapi Volcano
  • Crystal Size Distributions
  • K-affinity
  • Sunda arc
  • eruptive styles
Abstract:
The semi-continuous activity of Merapi volcano threatens the residents of the villages on its slopes as well as the residents of Yogyakarta, the second largest city in Java. Although Merapi behavior is scarcely predictable, its current eruptive episode lasting since the end of the nineteenth century is dominated by dome growth and collapse. Besides the current activity, deposits from historical and prehistoric Merapi illustrate the occurrence of explosive eruptions of VEI 3-5 in the past, remarking on Merapi’s high destructive potential. Throughout its history, Merapi exhibited effusive activity producing lava flows and viscous lava domes and several explosive events. Over time, Merapi’s geochemistry displayed repeated shifts between medium- and high-K affinity, a characteristic that is also displayed by other volcanoes in Java (e.g. Lamongan and Lasem). This thesis explores Merapi’s dynamic system, and in particular, the changes inherent to lava types (flows versus dome lavas), geochemical affinity (medium- to high-K affinity) and eruptive style (effusive to explosive). Data from textural quantitative analyses, mainly obtained through crystal size distribution techniques, were employed side by side to more traditional geochemical analyses to understand the evolution of Merapi volcano, including magma chamber processes and ascent to the surface. This thesis covers aspects of magma generation at Merapi, demonstrating that medium- to high-K affinity changes are related to events occurring at the subduction zone scale, and are linked to geodynamics and mass transfer. Further, this thesis links the realm of rock physical descriptions to that of geochemistry by exploring the relations between magma chamber processes and resulting textures. Finally, as the shifts in eruptive behavior are fundamental for the development of a hazard mitigation program at Merapi, this thesis compares textures of Merapi pyroclastic falls, bombs and lavas to explore the changes between explosive and effusive eruptions. The results show evidence in support of multiple distinct plumbing systems consistent with the current geophysical models of magma reservoirs. It is also concluded that varying proportions of subducted sediment and subducted crustal fluids can explain shifts between medium- and high-K affinity and that these changes may have been occurring repeatedly over Merapi’s history. Lastly, the examination of textures in tephras and lavas suggests the formation of a viscous plug as the mechanism responsible for the shift between effusive and explosive eruptions.