Explosive rhyodacitic volcanism: The evolution and frequency of pre-1883 eruptions at Krakatau volcano, Indonesia

Jennifer Livesley Beauregard, University of Rhode Island


Major explosive eruptions of Krakatau volcano, Indonesia, during the last 8,000 years have discharged crystal-poor calc-alkaline dacites and rhyodacites. The geochemistry and petrography of these rocks indicate that fractional crystallization of a basaltic parent produced these evolved magmas. Magma mixing is also an important process producing some of the evolved magmas. Pre-eruptive temperatures and possibly magma chamber depth have also varied between eruptive events. ^ Variations in magma composition, petrogenetic processes, and chamber conditions are attributed to a compositionally-stratified magma chamber, with a possible secondary, shallow chamber. Eruptions of homogeneous rhyodacite occur when a thick layer of rhyodacitic magma is present at the top of a stratified chamber. In contrast, eruptions of strongly mixed rhyodacite and andesite occur when the rhyodacite layer has been largely depleted in the main chamber or when andesitic magma intrudes a shallow chamber of rhyodacitic magma. Sequential tapping of a compositionally-zoned chamber may partially explain the proposed cyclic nature of Krakatau's eruptive behavior cited by previous workers. ^ The frequency of explosive eruptions at Krakatau was also investigated. Nine primary tephra layers have been identified in a sediment core recovered west of the Sunda Straits, Indonesia. Based on the mineralogy and compositions of glass shards, Krakatau volcano appears to be the source of six of the tephra layers. Toba caldera appears to be the source of one tephra layer, specifically the eruption which produced the Youngest Toba Tuff (YTT). ^ The presence of YTT ash in this core has several implications. Using this layer as a chronostratigraphic marker, the Krakatau tephra layers range in age from 4,000 to 127,000 years B.P., with an average recurrence interval of about 1 event per 21,000 years. ^ The occurrence of YTT ash in the Sunda Straits region also has implications for the size of the eruption and transport of material. The distribution of ash suggests the co-ignimbrite column injected ash into both hemispheres. The distribution may also have been affected by winds during the summer monsoon. Revised estimates of the size of the YTT are identical to previous estimates (2800 km3 DRE). ^

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Recommended Citation

Jennifer Livesley Beauregard, "Explosive rhyodacitic volcanism: The evolution and frequency of pre-1883 eruptions at Krakatau volcano, Indonesia" (2001). Dissertations and Master's Theses (Campus Access). Paper AAI3025530.