Dipartimento di Scienze, Università di Roma Tre, Roma, Italy.
Institute of Earth Sciences "Jaume Almera", CSIC, Barcelona, Spain.
Sci Rep. 2017 Jun 30;7(1):4444. doi: 10.1038/s41598-017-04880-6.
The aspect ratio of ignimbrites is a commonly used parameter that has been related to the energy of the parent pyroclastic density currents (PDCs). However this parameter, calculated as the ratio between the average thickness and the average lateral extent of ignimbrites, does not capture fundamental differences in pyroclastic flow mobility nor relates to lithofacies variations of the final deposits. We herein introduce the "topological aspect ratio" (ARt) as the ratio of the local deposit thickness (Ht) to the distance between the local site and the maximum runout distance (Lt), where Ht is a proxy for the PDC tendency to deposit, and Lt a proxy for the PDC mobility or its tendency to further transport the pyroclastic material. The positive versus negative spatial gradient d(ARt)/dx along flow paths discriminate zones where PDCs are forced (i.e. where they transport the total energy under the action of mass discharge rate) from zones where they are inertial (i.e. where they transport the total energy under the action of viscous or turbulent fluidization). Though simple to apply, the topological aspect ratio and its spatial gradient are powerful descriptors of the interplay between sedimentation and mobility of PDCs, and of the resulting lithofacies variations.
火山灰熔岩的纵横比是一个常用的参数，它与母体火山碎屑密度流（PDC）的能量有关。然而，这个参数是通过平均厚度与火山灰熔岩平均侧向延伸的比值来计算的，它没有捕捉到火山碎屑流流动性的基本差异，也与最终沉积物的岩相变化无关。本文引入了“拓扑纵横比”（ARt），它是局部沉积物厚度（Ht）与局部位置与最大流动距离（Lt）之间的距离之比，其中 Ht 是 PDC 沉积倾向的代理，Lt 是 PDC 流动性或进一步输送火山碎屑物质的倾向的代理。在流动路径上，d(ARt)/dx 的正负空间梯度可以区分 PDC 被迫的区域（即在质量排放率作用下输送总能量的区域）和惯性区域（即在粘性或湍流流态化作用下输送总能量的区域）。尽管拓扑纵横比及其空间梯度的应用简单，但它们是描述 PDC 沉降和流动性相互作用以及由此产生的岩相变化的有力描述符。
