Sedimentology of deposits from the pyroclastic density current of 26 December 1997 at Soufrière Hills Volcano, Montserrat
Sedimentology of deposits from the pyroclastic density current of 26 December 1997 at Soufrière Hills Volcano, Montserrat
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1997 年 12 月 26 日蒙特塞拉特苏弗里耶尔山火山火山碎屑密度流沉积物的沉积学
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发表时间：
2002
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通讯作者：
R. Sparks
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作者：
L. Ritchie;P. Cole;R. Sparks
1997年12月26日03:01（当地时间），蒙特塞拉特岛soufri<e:1>山火山发生了主要板块崩塌，随后安山岩熔岩穹窿崩塌。圆顶的坍塌涉及爆炸崩解和高能火山碎屑密度流（PDC）的形成，该电流主要向西南方向分散，并破坏了10平方公里的区域。PDC沉积可分为谷约束相和非约束相。后者的特征是由两个二部分单元（单元I和单元II）组成，它们都由细粒度较差的层（层1）组成，通常由细粒度较细的富细层（层2）覆盖。该序列被解释为记录了强烈的脉动（不稳定）流动，并被第三单元覆盖，这是一个富含青钻的吸积沉降层。与当地地形有关的岩相、厚度、粒度和沉积构造变化明显。PDC具有很强的侵蚀性：它会雕刻出孤立的沉积物丘，并严重冲刷原有的基底。岩相粒度明显，中值直径（mdol）随分选系数（Ãø）的减小而增大。岩相特征在很大程度上取决于70°扇形的方位角，主要的横向（横流）变化发生在与圆顶相似的径向距离上。这些沉积物与1902年佩尔萨梅火山和1980年圣海伦斯火山爆发时产生的沉积物相似。我们推断，崩落熔岩穹丘在爆炸膨胀过程中发生了粒度分选，由此产生的PDC在粒度和密度上都发生了初始分层。沉积物粒度在横向和纵向上的显著变化表明，由于空气夹带和沉积作用，在搬运过程中，密度分层和粒度分选有效地进一步发展。
abstract At 03:01 (local time) on 26 December 1997, major sector collapse followed by collapse of the andesitic lava dome occurred at Soufrière Hills Volcano, Montserrat. The collapse of the dome involved explosive disintegration and formation of a highly energetic pyroclastic density current (PDC), which was dispersed principally to the SW and devastated an area of 10 km2. The deposits of the PDC are divisible into valley-confined and unconfined facies. The latter is characterized by two bipartite units (Units I and II), both of which are composed of a fines-poor layer (layer 1) typically overlain by a finer-grained, fines-rich layer (layer 2). The sequence is interpreted as recording strongly pulsatory (unsteady) flow and is capped by Unit III, an accretionary lapilli-rich fallout layer. There are pronounced variations of lithofacies, thickness, grain size and sedimentary structures related to local topography. The PDC was highly erosive: it sculpted isolated mounds of deposit and heavily scoured the pre-existing substrate. Lithofacies are granulometrically distinct, with median diameter (Mdø) increasing as sorting coefficient (Ãø) decreases. Lithofacies characteristics depend strongly on azimuth over a 70° sector, with major lateral (cross-flow) changes at similar radial distances from the dome. The deposits are similar to those produced in the blast eruptions of Mont Pelée in 1902 and Mount St Helens in 1980. We infer that particle size sorting occurred during explosive expansion of the collapsing lava dome, such that the resulting PDC was initially stratified in both grain size and density. The marked lateral and vertical variations in grain size of the deposits indicate efficient further development of density stratification and grain-size sorting during transport, due to air entrainment and sedimentation.