The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 115, Issue 12
Displaying 1-6 of 6 articles from this issue
Special Issue   Collapse calderas (I): Structural control and magma plumbing system
Articles
  • Shigekazu Kusumoto, Agust Gudmundsson, Keiji Takemura
    2009 Volume 115 Issue 12 Pages 625-634
    Published: 2009
    Released on J-STAGE: May 29, 2010
    JOURNAL FREE ACCESS
    Based on the elastic theory, we derived 2-D analytical solutions for estimating (1) the depth of a magma chamber giving rise to an initial caldera (ring-fault) of a given radius, and (2) the magma chamber volume change required for the ring-fault formation. To facilitate mathematical treatment, we assume, first, that the magma chamber may be modelled either as a finite sphere or a point source and, second, that the ring-fault formation is due to magma chamber contraction (volume reduction) in the radial directions. Using the Coulomb criterion, we show that the relationship between ring-fault radius and chamber depth can be described by a linear function where the proportionality coefficient consists of elastic constants and the ratio (a/d), where a and d are the radius and depth of the finite-sphere magma chamber, respectively. In the point-source model the volume change required for ring-fault formation is proportional to the cube of the magma chamber depth. In the finite-sphere model, the solution consists of two terms. The first reflects the effects of the magma chamber depth; the second reflects the effects of a sphere radius and contributes to the volume change required for ring-fault formation if the a/d ratio is large. However, the effect of the second term is negligible if the a/d ratio is small. Since we consider only surface stresses in deriving the equations, the present results cannot be used to explain the mechanical details of caldera formation. Nevertheless, because our solutions are similar to relationships derived from observational data, they may provide useful first-order estimates of the magma chamber depth and volume change during caldera formation.
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  • Yasuko Okamoto, Hiroaki Komuro
    2009 Volume 115 Issue 12 Pages 635-642
    Published: 2009
    Released on J-STAGE: May 29, 2010
    JOURNAL FREE ACCESS
    This paper describes the results of analogue experiments of the inflation and deflation of a vertically elongated magma chamber. The model comprised a balloon (analogue magma chamber) buried in starch powder (analogue crust).
    The main results of the experiments are as follows:
    1. Minor funnel collapse (apical-graben type) is caused by inflation of a vertically elongated magma chamber. The diameter of the collapse zone is 0.8-1.6 km in natural scale, possibly too small to be considered a caldera.
    2. The deflation of a vertically elongated magma chamber produces a flat-floored caldera with a small area of central collapse. The natural size of the caldera is estimated to be 0.9-4.4 km in diameter and 200-400 m in depth. This model may provide an analogue of the Kilauea caldera.
    3. Compared with a purely deflation model, a smaller amount of caldera subsidence results from inflation and subsequent deflation of the magma chamber, probably due to crustal dilatancy following pre-caldera doming.
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  • Kenji Yoshida, Genki Takahashi, Teruyoshi Imaoka
    2009 Volume 115 Issue 12 Pages 643-657
    Published: 2009
    Released on J-STAGE: May 29, 2010
    JOURNAL FREE ACCESS
    The Shiratakiyama Formation of the Abu Group, southwest Japan, is part of a dissected caldera within a complex of Cretaceous volcanic and plutonic rocks. The formation contains the products of rhyolitic and andesitic magmas emplaced in a back-arc region. It is important to understand the genetic relationship between the volcaniclastic ejecta and structural constraints on these rocks in order to determine the evolution of caldera volcanism.
    The orientations of bedding planes within the Shiratakiyama Formation suggest the occurrence of a buried asymmetric structure within basement rocks. The depth of the basement surface increases toward the center of the caldera in the northern part of the Shiratakiyama Formation, dipping at 40° to 70°, whereas in the southern half of the caldera the surface dips at 20° or less. This asymmetric basement surface is also discordant with the orientation of basement rocks themselves. In addition, the formation is bound by intersecting high-angle normal faults and/or intrusive rocks. These observations suggest the presence of a small (6×4 km) cauldron, here named the Shiratakiyama cauldron.
    The Shiratakiyama Formation is divided into two members, here named the Futanoigawa rhyolite ash-flow tuff and the overlying Tenjougatake andesite lava. The formation also contains many associated intrusive rocks, such as porphyrites, felsites, granite porphyry, and intrusive breccias. Thick and voluminous ash-flow tuff is the dominant rock within the cauldron interior. The total volume of ash-flow tuff is ≥ 9.6 km3, and it is locally intercalated with lacustrine rocks, andesite lavas, and volcaniclastic rocks, which represent cooling units. Caldera-collapse meso-breccias occur in the lower part of the ash-flow tuff sequence.
    These findings suggest that the deeper structure of the Shiratakiyama cauldron was formed by asymmetric piecemeal collapse rather than by coherent trapdoor subsidence.
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  • Kuniyuki Furukawa, Masaya Miyoshi, Taro Shinmura, Tomoyuki Shibata, Yo ...
    2009 Volume 115 Issue 12 Pages 658-671
    Published: 2009
    Released on J-STAGE: May 29, 2010
    JOURNAL FREE ACCESS
    Volcanic activity in the Aso area of central Kyushu, SW Japan, is divided into three stages based on caldera formation: a pre-caldera stage, a caldera-forming stage, and a post-caldera stage. We established the stratigraphy, and analyzed the whole-rock Sr and Nd isotopic compositions of volcanic products distributed in the NW wall of Aso Caldera to investigate the eruption style and magma-plumbing system of pre-caldera volcanism. Multiple andesitic lavas with minor dacitic lava make up a large part of the caldera wall. This eruption style of pre-caldera volcanism is difference from that of the caldera-forming stage, which is characterized by gigantic pyroclastic eruptions. The cause of this transition in eruption style from the pre-caldera to caldera-forming stages is probably a decrease in the rate of extensional crustal strain in the area. The whole-rock isotopic compositions of the pre-caldera volcanic products are largely similar to those of the caldera-forming volcanic products, indicating that the magmas of these volcanic products had a common origin. Previous petrological studies have suggested that the silicic magma of the caldera-forming stage originated from crustal melting; consequently, it is also considered that the magma of the Pre-caldera stage formed via this process. The pre-caldera volcanic products show a wide range of 87Sr/86Sr values (0.7042-0.7045), whereas the products of the caldera-forming stage show a lower and narrower range of values (0.7040-0.7042). This observation indicates isotopic heterogeneity of the crust and differences in the composition of the crust that melted during the pre-caldera and caldera-forming stages.
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  • Masaya Miyoshi, Kuniyuki Furukawa, Taro Shinmura, Madoka Shimono, Tosh ...
    2009 Volume 115 Issue 12 Pages 672-687
    Published: 2009
    Released on J-STAGE: May 29, 2010
    JOURNAL FREE ACCESS
    Pre-caldera volcanism in the Aso area (2.2–0.43 Ma), central Kyushu, is characterized by effusive eruptions of multiple lava flows. To clarify the chemical evolution of the magma chamber beneath the Aso area, we investigated the petrological characteristics of these lavas where exposed in the caldera wall.
    The pre-caldera lavas are divided into eight types with distinct petrographic and compositional characteristics: A. cpx-ol basalt, B. ol-2px andesite, C. ol-hb-2px andesite, D. 2px andesite, E. hb-2px andesite, F. hb andesite, G. 2px-hb dacite, and H. bt-hb rhyolite. Incompatible trace element modeling demonstrated that these eight types did not originate via simple fractional crystallization.
    The phase assemblages and abundances of phenocrysts of the pre-caldera andesite-rhyolite differ from those of the caldera-forming and post-caldera andesite-rhyolite. In addition, the pre-caldera andesite-rhyolite contain relatively low concentrations of incompatible trace elements compared to the caldera-forming and post-caldera andesite-rhyolite. These observations may indicate that the physical conditions and/or chemical compositions of the source materials that gave rise to the andesite-rhyolite magmas differed between the pre-caldera and caldera-forming stages.
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