Erosion by pyroclastic flows on Lascar Volcano, Chile
Erosion by pyroclastic flows on Lascar Volcano, Chile
复制标题
智利拉斯卡火山火山碎屑流的侵蚀
DOI：
10.1007/s004450050162
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发表时间：
1997
影响因子：
3.5
通讯作者：
S. Matthews
中科院分区：
文献类型：
--
作者：
R. Sparks;M. Gardeweg;E. Calder;S. Matthews
1993年4月19-20日智利拉斯卡火山喷发时产生的 火山碎屑流产生了壮观的侵蚀特征。Scree和Talus被从火山侧翼的渠道和陡峭的斜坡上剥离。裸露的基岩和巨石遭受严重磨损，在粗角砾岩上产生光滑的表面，在基岩和大巨石上产生条纹和撞击痕迹。侵蚀沟发育，波长为0.5~2 m，深度为0.1~0.3 m。沟槽通常在大型巨石或块体下游成核，在上游一侧有条纹，从而形成峭壁和尾部结构。当水流加速通过地形限制或在陡峭的斜坡上移动时，就会产生侵蚀特征。据推测，火山碎屑流在运动过程中被分离成富含岩屑和浮石的部分。富岩屑矿床赋存于高达14°的斜坡上，而富浮石矿床仅产于小于4°的斜坡上，主要分布在1993年扇体的边缘和远端。富含岩屑的矿床含有从基底侵蚀并从喷口运移的大片岩屑(达1 m)，而富含浮石的矿床只含有小片(通常为2 cm)的岩屑。这些观察表明，岩屑分离到水流的底部，是造成大部分侵蚀现象的原因。侵蚀特征、岩屑分布和沉积形态表明，1993年的水流是以颗粒相互作用为主的高度集中的雪崩。在一些地方，水流滑过基岩造成磨损和长条纹，这意味着大型块体被锁定在固定位置约1 S。然而，不同角度的较短条纹、冲击痕迹、沉积物分成富浮石和富含岩屑的部分，以及整个沉积物中基岩来源的岩屑的混合表明，碎屑通常具有一定的运动自由，颗粒的相互推挤使得流体内部发生了混合和密度分离。
Abstract Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows.