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Time gap during VEI>6 caldera-forming eruptions: constraint from paleomagnetic directions for preceding airfall and following ignimbrite

Takeshi Hasegawa 1, Nobutatsu Mochizuki2, Gravley Darren3, Shohei Shibata1, Madison Myers4, Chie Kusu1

  • Affiliations: 1Department of Earth Sciences, Ibaraki University, Ibaraki, Japan 2Department of Earth and Environmental Science, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto, Japan 3School of Earth and Environment, University of Canterbury, Christchurch, New Zealand 4Department of Earth Sciences, Montana State University, Bosman, United States

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 53

  • Programme No: 3.11.13

  • Theme 3 > Session 11

Abstract

Recent paleomagnetic studies have proposed durations of some VEI>6 caldera-forming eruptions were longer than previously thought. These estimates are based on angular difference of paleomagnetic directions from the deposits and changing rate of geomagnetic secular variation (GSV). Here, we review these results with focusing on a pair of preceding airfall and overlying flow. Mean paleomagnetic directions (MPDs) of these deposits were well-determined (α-95<3°) by improved sampling techniques. Paleomagnetism from marine cores and present observations indicate the GSV change rate as 0.01°~0.1°/year. Therefore, time resolution of this method is "decades". Datasets were compiled for nine caldera-forming eruptions. In three cases (Taupo/Mashu/Aira), the MPDs between fall and flow are indistinguishable (α-95 overlap) suggesting no time gap more than decades. In six cases (Kikai/Kp1/Shikotsu/Mamaku/HRT/FCT), the angular difference of MDPs for the fall/flow is 7° to 15°, and applying the fastest GSV (0.1°/year), the time difference estimates as 70 to 150 years. Recent (Holocene) and smaller (VEI=6) eruptions (Taupo/Mashu) show no significant time gap between the fall/flow. The historical records and observations indicate a stable Plinian column provides a fall, and eventually the column collapses for generating a flow. In this case, the two MPDs will be indistinguishable. While, Druitt&Sparks (1985) proposed "1st overpressure stage (providing fall+small flow)" and "2nd caldera-collapse stage (generating ignimbrite)". Applying our data to this model, catastrophic caldera-forming eruptions could require a considerable time break after preceding Plinian eruptions. Our next challenge is petrologically detecting decades difference of magma residence time between the fall and flow.