Marine caldera-forming eruptions
Caldera-forming eruptions, which may erupt up to a few thousands of km3 of magma, are catastrophic volcanic events that pose one of the greatest natural hazards on Earth (Smith, 1979; Lipman, 1984; 1997). Even relatively small caldera-forming eruptions may result in several thousand deaths, and alter the global climate (Rampino and Self, 1982; Newhall and Punongbayan, 1996).
During the Quaternary period, most of the caldera-forming eruptions are thought to have occurred in areas of shallow seas or lakes, with the production of voluminous pyroclastic flows (Watanabe, 1978; Sigurdsson and Carey, 1989; Cas and Wright, 1991; Fisher et al., 1993; Allen and Cas, 2001). In the 3.5 ka Santorini eruption in Greece, large volumes of tephra, pyroclastic flows, and tsunamis accompanied the collapse of volcanic edifice, which devastatingly influenced human activities in the east Mediterranean (Sullivan, 1988; McCoy and Heiken, 2000). Following the 1883 eruption of the Krakatau in Indonesia, a large pyroclastic flow and tsunamis arrived at numerous coastal villages in Java and Sumatra, killing 36,000 people (Simkin and Fiske, 1983; Carey et al., 1996; 2000). Other Quaternary marine silicic calderas have been also discovered on subduction zones and near ocean islands; the Shichito-Iwojima Ridge, Izu-Ogasawara (or Izu-Bonin) Arc (Yuasa et al., 1991; Yuasa and Kano, 2002), the Kermadec Ridge north of the Taupo volcanic zone, New Zealand (Wright and Gamble, 1999; Wright et al., 2003) and its northern extension (Worthington et al., 1999), and the Hellenic Island Arc in Greece (McCoy and Heiken, 1984; Allen and Cas, 1998; Allen and Stewart, 2002).
Although these marine caldera-forming eruptions must have significantly and devastatingly affected the development of coastal human activities and environments around the volcanoes (or even the entire earth), they still remain speculative and controversial, because the rare occurrence and violent nature of this type of eruption make it difficult and almost impossible to directly view all processes involved with such events.
Major controversial topics of caldera-forming eruptions during a past few decades include questions as to how such large explosive eruptions evolve temporally, and how eruption dynamics are characterized by physical parameters, as ejecta volume, mass flux, or duration. Such topics have been mainly studied in subaerial fields of large silicic magmatic systems (Wilson et al., 1980; Bacon, 1983; Heiken and McCoy, 1984; Suzuki-Kamata et al., 1993; Rosi et al., 1996; Wilson and Hildreth, 1997). On the other hand, detailed studies of marine silicic calderas have not been fully completed (e.g. Allen and Cas, 1998; Allen, 2001), especially with respect to the effects of seawater on the dynamics and evolution of such large-scale silicic explosive eruptions.
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Figure 1. Idealized processes during a marine caldera-forming eruption. (a) Pyroclastic density currents are generated from column collapsing, and they enter or travel over sea. Tsunamis may be generated during these processes and reach coastal areas. (b) Resultant deposits are distributed on subaerial and submarine fields near the volcano. These are clues for understanding of the evolution and dynamics of the eruption.
In my study, problematic topics (dynamics and evolution of a marine caldera-forming eruption) are investigated in Kikai volcano, Japan, and the eruption parameters are limited to two views; (1) pyroclastic deposits and their sedimentation characteristics, accompanied by hydrovolcanic processes, and (2) traces of water waves (tsunamis), which are attributed to conversion processes of kinetic and/or thermal energy with material transport during the eruption (Fig.1). These views are remarkably effective for understanding marine eruptions, and it is also noticeable that these views are different from subaerial cases, but are important parts of ongoing studies of the hazards of caldera-forming eruptions. Furthermore, such studies can provide constraints on predicted patterns for future explosive activities of marine volcanoes, and implications for the evolution of large silicic magmatic systems in subduction zones. [Maeno, F. (2006) Dynamics and evolution of a marine caldera-forming eruption at Kikai volcano, Japan. PhD Thesis, Tohoku University]
Key words: Caldera-forming eruption, caldera collapse, Kikai caldera, pyroclastic flow, tsunami