HR: 1330h
AN: V12A-0560    [PDF]
TI: Crystallinity, Petrological, Geochemical Evolution of Explosive Eruptions from Merapi, Java, and Soufriere Hill, Montserrat: A Comparative Investigation
AU: * Andreastuti, S D
EM: sandreas@geosc.psu.edu
AF: Penn State University, University Park, State College, PA 16802
AU: Innocenti, S
EM: sinnocen@geosc.psu.edu
AF: Penn State University, University Park, State College, PA 16802
AU: Voight, B
EM: voight@ems.psu.edu
AF: Penn State University, University Park, State College, PA 16802
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Penn State University, University Park, State College, PA 16802
AU: Cashman, K
EM: cashman@oregon.uoregon.edu
AF: Oregon University, 1272 University of Oregon, Eugene, OR 97403
AB: The types of eruption at Merapi volcano range from gentle lava dome effusions to energetic explosions, with these styles commonly related to open and closed vent conditions, respectively. The closed vent style was dominant in pre-1800 AD times, when eruptions related to dome collapse were infrequent, and explosive eruptions ranged from sub-plinian and plinian to vulcanian. Post-1800 A.D eruptions, mainly of open vent type, have been primarily related to dome collapse events, with occasional vulcanian explosions. The change in activity from plinian events to dome collapses and collapse-triggered explosions is imprecisely defined but occurred about the same time as the Kepuhharjo tephra (c. 250 yrs. ago). Geochemical characteristics of tephra units from pre-1800 AD Merapi eruptions show a range of medium to high K$_{2}$O (1.1 to 2.5 wt%), high Al$_{2}$O$_{3}$ (19 to 21 wt%) and low MgO (1.9 to 3.6 wt%). Silica compositions varied from basalt to basaltic andesite (51-57 wt%). This range in composition may reflect periodic influxes of basaltic magma into crustal reservoirs. Chemically, Merapi tephras show two clear trends in composition: (1) medium-K basalt to basaltic-andesite; and (2) high-K basalt to basaltic-andesite. Through time, the K$_{2}$O content evolves from medium-K to consistently high-K, with the transition between 3000 and 1650 yrs BP. Here we examine the petrographic characteristics of the Tegalsruni and Temusari tephras which erupted about 2000 years BP and 1800 years BP, representing the transition period, and the Kepuhharjo tephra (250 years BP) and 1872 AD eruption, which represent vulcanian eruptions related to dome collapses. The pyroclastic fall deposits of Merapi are characterised by pumiceous fragments with subordinate (\~30%) lithics, low to highly vesiculated pumice, brown to dark brown color, and textures that range from porphyritic, phaneric to aphanitic. The pumices contain up to 50 vol% phenocrysts of plagioclase, clinopyroxene, hornblende and titanomagnetite. In general the Merapi tephras show an increase in crystallinity from the onset to the end of each eruption, reflecting the zonation tapped in the magma reservoir; in the Temusari tephra, this crystallinity change accompanies an increase in SiO$_{2}$. Hornblende is more abundant in larger (sub plinian to plinian) eruptions than in dome collapse (vulcanian) eruptions. Volumetric percentage of plagioclase is variable throughout the Tegalsruni eruption but increases through the Temusari eruption, during which time average plagioclase number densities decrease. A preliminary comparison with the products of recent andesite eruptions of Soufriere Hills volcano (SHV), Montserrat, shows similar trends in phase proportions and CSD although the total phenocryst content of SHV exceeds that of Merapi. These trends may reflect variations in the rate of magma ascent and degassing which will affect the microlite crystallinity. The Montserrat magma was reheated by mafic injection and liberation of heat and volatiles, with slower ascent during effusive phases, and residence within the lava dome, permitting more extensive degassing-induced plagioclase microlite crystallization. Viscosities in general were lower at Merapi because of lower SiO$_{2}$ content. Similar trends were seen in the explosive and effusive phases of the 1980-1986 eruptions of Mount St. Helens.
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting