Classification of ignimbrites and their eruptions
Classification of ignimbrites and their eruptions
复制标题
DOI：
10.1016/j.earscirev.2021.103697
复制
发表时间：
2021-06-15
影响因子：
12.1
通讯作者：
Cas, Ray A. F.
中科院分区：
文献类型：
--
作者：
Giordano, Guido;Cas, Ray A. F.
术语“褐沸石”可能包括地球上最大范围的矿床类型之一，与提供火山碎屑密度流的爆炸性喷发柱的部分或全部坍塌有关。令人惊讶的是，没有像尘埃沉积物那样对褐沸石类型进行量化分类，这是现代火山学的一个显著缺陷。到目前为止，这阻碍了对褐沸石的标准化描述符的识别和对其特征的记录方法的改进，例如在Walker于1973年提出的分类方案的基础上对尘埃沉积物进行的记录。尽管有一些早期的尝试，但点火类型并不符合喷发式命名法。在这篇文章中，我们探索和讨论了一种分类方案的描述符，该描述符基于圆度、面积范围、宽高比和体积的相关性，该数据库包括92个原生岩，这使得我们能够考虑到目前对火山碎屑流动力学的理解。我们指的是单一的点火沸石流出单元，即在火山口外环境中就位而没有明显的沉积破裂，并形成单独的冷却单元，与内部岩相结构无关。我们的主要发现是，点火沸石的散布面积/当量跳动与体积呈显着的幂函数关系。跳动与增加供给火山碎屑流的质量流量直接相关。体积与流动事件的大小有关。因此，我们认为，通过测量一阶现场观测数据，如体积和扩散面积，可以评估相关火山碎屑流的大小和强度，对于以点火沸石为主的大型喷发，提供了评估喷发的机会。根据已确定的关系，我们认为，起源于单个点源喷发柱坍塌的火成岩，通常小于1km~3，根据它们与喷发的类型，将其命名为“火山喷发火成岩”和“普林尼期火成岩”。沿环形断层裂隙喷口形成的较大的火山喷发应被视为与一种单独的喷发类型有关--相对于常见的夏威夷-普林斯趋势--其中，由于环形裂隙喷口导致的质量流量增加的影响占主导地位，并控制着所产生的坍塌喷泉和火山碎屑流的动力学，无论在火山口坍塌开始之前的喷发类型是什么。这些岩石被命名为“形成火山口的火山岩”，并根据其喷发体积的增加而进一步细分为小型、中型、大型和超大型。
The term "ignimbrite" probably encompasses the one of the largest ranges of deposit types on Earth, associated with the partial to total collapse of explosive eruption columns feeding pyroclastic density currents. Surprisingly, there is no quantified classification scheme for ignimbrite types, as there is for fallout deposits, and this is a remarkable deficiency of modern volcanology. This has so far prevented the identification of standardized descriptors for ignimbrites and the improvement of methods for the documentation of their characteristics, such as happened for fallout deposits, building on the classification scheme proposed by Walker in 1973. Despite some earlier attempts, ignimbrite types do not conform to eruption style nomenclature. In this paper, we explore and discuss descriptors for a classification scheme based on the correlation of runout, areal extent, aspect ratio and volume from a compiled database comprising 92 ignimbrites, which then allows current understanding of pyroclastic flow dynamics to be considered. We refer to single ignimbrite outflow units, i.e. emplaced without significant breaks in their sedimentation, in extra-caldera settings and forming individual cooling units, irrespective of internal lithofacies architecture. Our main finding is that ignimbrites show remarkable power-law relationship between dispersal area/equivalent runout and bulk volume. Runout is directly related to increasing mass flow rate feeding the pyroclastic current. Volume is related to the magnitude of the flow event. We therefore propose that by measuring first order field observables such as bulk volume and dispersal area provides the opportunity to evaluate magnitude and intensity of related pyroclastic currents and, for large eruptions dominated by ignimbrites, of the eruption. Based on the relationships identified we propose that ignimbrites that originated from the collapse of single point-source eruption columns, usually smaller than 1 km3, are named "Vulcanian ignimbrites" and "Plinian ignimbrites" depending on the style of the eruption they are associated with. Larger ignimbrites that originated from caldera-forming eruptions along ring-fault fissure vents should be regarded as related to a separate eruption style - with respect to the common Hawaiian-Plinian trend -, where the effect of increased mass flow rate due to ring-fissure vents is dominant and controls the dynamics of the resulting collapsing fountains and pyroclastic flows, irrespective of the kind of eruption style that preceded the onset of the caldera collapse. These are named "caldera-forming ignimbrites" and are further subdivided into small, intermediate, large and super, based on their increasing erupted volume.