Pumice: a post-fragmentation product?

普林尼亚火山喷发是最具爆炸性的火山喷发之一，也是火山弧沿线的主要地质灾害之一。在美国，几座火山已经产生了毁灭性的普林尼亚火山喷发，有一天还会再次发生。普林尼亚火山喷发的产物被喷发到大气中，可以传播数千公里。它们的后果可能会严重破坏基础设施和农业，并最终包围全球，扰乱航空并影响气候。通常，超过70%的普林尼安物质是由被称为浮石的新鲜岩浆的轻质碎片组成的。浮石的整体形成似乎大部分人都知道；在它上升到地表的过程中，岩浆随着气泡的成核和生长而起泡。这一过程一直持续到气泡内的气体压力导致岩浆分解成多孔碎片，然后喷出大气中。浮石的结构在岩浆碎屑时被认为是“冻结的”，而碎片(微米到厘米)的大小在管道、喷发柱和/或火山碎屑流中运输过程中的颗粒间碰撞时被减小。过去的研究使用浮石的结构和粒度分布来对管道内发生的喷发过程施加定量限制，特别是对岩浆破碎，尽管最初的碎片粒度分布在很大程度上仍然未知。这项研究是由对来自梅迪辛湖火山(CA)的浮石进行的新的纹理观察推动的，这些观察表明，一些浮石实际上是由许多较小的岩浆碎片组成的，这些岩浆在管道内碰撞、凝聚并部分烧结。该项目旨在(1)评估这些新记录的结构是否随浮石大小以及不同强度和管道形状的喷发之间的不同而变化，(2)根据岩浆破碎的条件及其产物的特征进一步调查这一发现的意义，(3)确定是否可以通过实验室的烧结和减压实验来定量地再现所观察到的结构。这项研究将支持一名早期职业科学家、一名博士生和本科生，他们将获得各种现场方法、实验方法和分析工具的经验。研究与教育之间的联系将包括开发两门新的一年级兴趣小组课程，让学生接触火山学，以及火山学与其他学科(人类学、考古学和新闻学)之间的联系。除此之外，它还与火山危险性评估有关，因为它区分了爆炸性火山喷发和热情洋溢的火山喷发。特别是，从岩浆碎裂中继承下来的火山碎屑的粒度分布的演化是羽流发展和火山灰弥散模型的重要输入。因此，更好地理解碎片化及其产物不仅对基础科学很重要，而且对实际原因也很重要。为了验证上述假设，研究小组将首先量化来自五次流纹岩喷发的数百个多孔火山碎屑的大小、形状和纹理，这些火山碎屑来自五次流纹岩喷发，喷发量超过三个数量级。这些喷发发生在纽伯里、梅迪辛湖、火山口湖和长谷火山，并通过不同形状的管道(亚圆形、堤坝或未知形状)喷发。其次，研究人员将通过将构成浮石碎屑的颗粒个体化，并表征它们的大小和形状分布，来破译新观测到的岩浆碎裂方面的含义。最后，将进行实验，试图重现观察到的纹理。流纹岩玻璃将与不同数量的H2O水合，然后粉碎，最后在减压实验中烧结，这些实验再现了碎裂程度和地球表面之间的岩浆上升条件。初步观察、分析和实验表明，(1)在不同火山的几次普林尼亚喷发的浮石中出现了团聚结构，(2)形成单个浮石碎屑的单个颗粒的尺寸分布与前者来自管道中的初级破碎的尺寸分布一致，以及(3)在某些初始颗粒尺寸、水含量和减压速率的特定条件下，可以实验地再现观察到的结构。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Plinian eruptions are among the most explosive volcanic eruptions and one of the principal geological hazards along volcanic arcs. In the US, several volcanoes have produced, and will again one day, devastating Plinian eruptions. Products from a Plinian eruption are ejected into the atmosphere and can travel up to thousands of kilometers. Their fallout can seriously damage infrastructure, agriculture, and eventually encircle the globe, disrupting aviation and affecting climate. Typically, more than 70% of the ejected Plinian material consist of light weight fragments of fresh magma called pumice. The overall formation of pumice seems mostly understood; during its ascent to the surface, magma foams as gas bubbles nucleate and grow. This process continues until gas pressure inside the bubbles causes the magma to break up into porous fragments that are ejected in the atmosphere. The texture of pumice is thought to be “frozen in” at the time magma fragments, whereas the size of fragments (microns to centimeters) is reduced upon inter-particle collisions during transport in the conduit, the eruptive column and/or the pyroclastic flows. Past research has used the texture and size distribution of pumice to put quantitative constraints on the eruptive processes occurring inside the conduit, especially on magma fragmentation, even though the initial fragment size distribution remains largely unknown. This study is driven by new textural observations made on pumice from Medicine Lake Volcano (CA) that suggest some pumices are in fact made of numerous smaller pieces of magma that collided, agglomerated and partly sintered inside the conduit. This project seeks to (1) assess whether these newly documented textures vary with pumice size and between eruptions of diverse intensity and conduit shape, (2) further investigate the implications of this discovery in terms of the conditions for magma fragmentation and characteristics of its products, (3) determine whether the observed textures can be quantitatively reproduced by sintering and decompression experiments in laboratory. This study will support an early career scientist, a PhD student, and undergraduate students who will gain experience with a wide variety of field methods, experimental approaches, and analytical tools. The link between research and education will include the development of two new First-year Interest Group courses that expose students to volcanology and the links between volcanology and other disciplines (anthropology, archeology and journalism).Magma fragmentation is a process of fundamental importance to volcanology. Beyond that, it has relevance to volcanic hazard assessment, as it discriminates an explosive volcanic eruption from an effusive one. In particular, the evolution of the size distribution of pyroclasts inherited from magma fragmentation is an important input of plume development and tephra dispersion models. Thus, a better understanding of fragmentation and its products is important not only for basic science, but also for practical reasons. To test the aforementioned hypotheses, the research team will first quantify the size, shape and textures, both in 3D and in 2D, of hundreds of porous pyroclasts from five rhyolitic eruptions that spanning more than three orders of magnitude in eruptive volume. These eruptions occurred at Newberry, Medicine Lake, Crater Lake and Long Valley volcanoes and through conduits of different shape (sub-circular, dike, or unknown shape). Second, the researchers will decipher the implications, in terms of magma fragmentation, of the new observations by individualizing the particles that make up pumice clasts and characterizing their size and shape distributions. Finally, experiments will be carried out to attempt to reproduce the observed textures. Rhyolitic glass will be hydrated with different amounts of H2O, then crushed, and finally sintered during decompression experiments that reproduce the conditions of magma ascent between the fragmentation level and the Earth’s surface. Preliminary observations, analyses and experiments show that (1) agglomerate textures occur in pumices from several Plinian eruptions at different volcanoes, (2) the size distribution of individual particles making individual pumice clast is consistent with the former being derived from primary fragmentation in the conduit, and (3) it is possible to reproduce the observed textures experimentally under certain conditions of initial particle size, water content and decompression rate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.