Volcanic geology of Galunggung, West Java, Indonesia

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Theses / Dissertations
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Doctor of Philosophy
University of Canterbury. Geology
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Volume Title
Bronto, Sutikno

Galunggung volcano is located in West Java, Indonesia and covers an area about 275 km2. The volcano is very active and the slopes are highly populated (over 1.5 million people). There is therefore always the threat of volcanic disaster. This study investigates the character of past Galunggung volcanic activity and assesses likely future activity in order to advise on volcanic hazard and risk. The approach involves a study of stratigraphy, mineralogy and petrology of the Galunggung rocks, and the presentation of volcanic hazard zonation maps. Galunggung volcanic rocks are included within the Galunggung Group and can be divided into Old Galunggung Formation, Tasikmalaya Formation and Cibanjaran Formation. The first formation represents rocks of Old Galunggung stratovolcano (50,000 - 10,000 yrs. BP ?), the second formation covers rocks erupted during caldera formation (4200 ± 150 yrs. BP) and the third one comprises rocks erupted in 1822, 1894, 1918 and 1982-83. The Old Galunggung Formation consists mainly of pyroclastic flow, pyroclastic fall and lahar deposits and lava flows which have a total rock volume of about 56.5 km3. This activity ended with the intrusion of a cryptodome under the crater. The cryptodome blocked the existing vent and subsequent activity moved to the weakest part of the old cone to the ESE, resulting in the caldera forming-event. This destructive eruption formed a horseshoe-shaped caldera and ejected more than 20 km3 of material comprising debris avalanche, pyroclastic flow, pyroclastic fall, pyroclastic surge and lahar deposits. Historic eruptions separated by relatively long dormant periods produced less voluminous (< 0.4 km3) volcanic deposits. Galunggung volcanic rocks are basalt (49 - 53 % SiO2) to basaltic andesite (53 - 57 % SiO2) having porphyritic textures with medium sized phenocrysts (15 - 40 %), mainly plagioclase (av. 18 %) and clinopyroxene (1.6 %). Olivine is observed in basic rocks, whereas orthopyroxene and magnetite are present in the most evolved rocks. Amphibole is common in pyroclastic deposits and gabbro clasts ejected during caldera formation. On the basis of Mg contents, Galunggung rocks are divided into: 1. high-Mg basalt (12.5 10 % MgO) , 2. "Transitional" high-Mg basalt (9 - 6.5 % MgO) , 3. low-Mg basalt (< 6 % MgO), 4. high-Mg basaltic andesite (7 - 6 % MgO) and 5. low-Mg basaltic andesite (< 5 % MgO). The high-Mg basalts are subdivided into low-K high-Mg basalt (<0.4 % K2O) and medium-K high-Mg basalt (0.6 % K2O). Alkali and incompatible elements increase whereas Mg, Fe, Ca and compatible trace elements decrease with increasing SiO2. The high-Mg basalts are the most primitive Galunggung rocks with highest Mg# = 75 - 69, Ni (up to 193 ppm), and Cr (711 ppm) but lowest incompatible elements. The "primitiveness" of the basalts is also reflected by their 230Th/232Th ratio (= 0.68) which is one of the lowest ratios yet found. The Galunggung high-Mg basalts are considered to represent liquid compositions which have been derived from upper mantle peridotites. The low-K high-Mg basalt originate from spinel-peridotite by 15 % melting at about 50 km depth, and the medium-K high-Mg basalt from plagioclase-peridotite by 25 - 40 % melting at about 30 km depth. These primitive magmas probably rose rapidly to the surface as mantle "diapirs". During Old Galunggung volcanic activity, low-K high-Mg basalt magma moved upward diapirically and formed a magma chamber in the crust at a depth of about 10 km. Fractionation of this magma formed low-Mg basalts and basaltic andesites. This activity ended when a medium-K high-Mg basalt intruded as a cryptodome. Another low-K high-Mg basalt magma migrated into the crust and fractionated to produce low-Mg basalt basaltic andesite. Gas was trapped and high water pressure was attained; and amphibole gabbro solidified in the roof of the magma body. These rocks were erupted during the Galunggung caldera forming-event. In 1982-83, a new generation of low-K high-Mg basalt magma was erupted. Fractionation in a conduit system changed compositions at the top part but not significantly in the lower part of the magma body. During the eruptive sequence firstly low-Mg basaltic andesite, then high-Mg basaltic andesite, "transitional" high-Mg basalt, and finally the low-K high-Mg basalt were erupted. Rhyolite pumice erupted in September 1982 is considered to be a product of melting of Miocene dacite by the high temperature (1300ºC) Galunggung high-Mg basalt magma. Galunggung eruptions vary from non-violent effusive to destructive explosive events. These create hazards which are divided into four levels. First degree hazards are long-term and require further study. In this thesis hazard maps are presented for second, third and fourth degree hazards. Evacuation routes are suggested away from the volcano as all arrangements must be planned well in advance of an actual event.

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Copyright Sutikno Bronto