Explosive Volcanism: Understanding Pauses and Abrupt Shifts in Intensity During the1912 Novarupta Eruption

在许多大型爆炸性喷发中，喷发强度的暂停和/或突然变化是一个基本但很少被研究的部分。稳定的普林尼亚羽流可能会在几个小时的时间尺度上暂停或强度波动几个数量级。在我们对稳态喷发过程的模型变得越来越优雅和复杂的时候，这种转变的原因仍然知之甚少。如果这种变化能够得到解释，甚至可以预见，那么危险建议和喷发‘预测’的质量将会显著提高。这项研究的重点是在一次强大的持续爆发的喷发中，这种不稳定的起源，因为持续几个小时的喷发活动的中断和风格/强度的突然但短暂的变化。1912年的诺瓦鲁塔喷发是20世纪世界上最大的喷发，被广泛引用为历史上大型持续喷发的极好例子。在火山喷发开始16小时和45小时后，诺瓦鲁塔的普林尼安/褐煤持续喷发短暂而突然地停止，最后在60小时后让位于熔岩穹顶的喷出(Hildreth和Fierstein 2000)。1912年喷发期间的火山口坍塌不是发生在喷口，而是10公里远的地方，因此保存了喷发的非常接近喷口的产物，直到喷口100米以内，中间的喷发产物有一种化学地层学，使得能够在广泛分布的地点之间对比具有高度不同特征的同时代产物。这使我们能够从一个非常受约束的角度来解决喷发动力学突然变化这一根本问题，并处理过去1000年中有记录的五次最大喷发之一。拟议研究的目标是评估以下因素所起的相对影响：a)气泡的生长和破裂、脱气和协同微晶结晶的速度和时间；b)喷口的粗糙度和稳定性；c)岩浆喷发前状态的变化，包括喷发前挥发分浓度的变化，以改变1912年喷发的动力学。我们最初的假设是，诺瓦鲁塔火山爆发的每一幕的关闭是由体积占主导地位的英安质岩浆物理状态的“上升驱动”变化造成的，而喷发强度的短暂波动是喷口壁的不规则性和粗糙性以及有限的喷口壁坍塌所产生的羽流不稳定的产物。随着岩浆流变学对溶解挥发物浓度的降低和气泡和晶体丰度的增加做出反应，管道中流动行为的变化的影响将通过对普林尼亚矿床裂隙两侧精选浮石样品中泡囊和晶体种群的图像分析以及喷发强度的突然波动来推断。将通过绘制围岩岩屑性质和丰度的变化来研究外部环境因素(喷口壁粗糙度和不稳定性、喷口运移和加宽)。

HoughtonEAR-0106700Pauses and/or abrupt shifts in eruptive intensity are a fundamental yet little studied part of many large explosive eruptions. Stable Plinian plumes may pause or fluctuate in intensity by several orders of magnitude on times scales of hours. The causes of such shifts remain poorly understood, at a time when our models for steady state eruptive processes become increasingly elegant and sophisticated. The quality of hazard advice and eruption 'forecasts' would be significantly improved if such changes could be explained and even anticipated. This study focuses on the origin of such 'unsteadiness' in a powerful sustained explosive eruption, for both breaks in eruptive activity of several hours duration and abrupt but short-lived changes in style/intensity.The eruption of Novarupta in 1912 event was the world's largest 20th century eruption and is widely cited as an excellent example of a historical large sustained eruption. Sustained Plinian/ignimbrite eruption at Novarupta stopped briefly and abruptly 16 and 45 hours after commencement of the eruption before finally giving way after 60 hours to effusion of lava domes (Hildreth and Fierstein 2000). Caldera collapse during the 1912 eruption occurred not at vent, but 10 km distant, thus preserving the very near-vent products of the eruption to within 100m of vent, and medial eruption products have a 'chemical stratigraphy' that enables correlation of coeval products of highly varying character between widely spaced localities. This permits us to address this fundamental problem of abrupt changes in eruption dynamics from a very well constrained perspective and for one of the five largest eruptions recorded in the last 1000 years. Goals of the proposed study are to assess the relative influences played by: a) the rates and timing of growth and collapse of gas bubbles, degassing and syneruptive microlite crystallization; b) vent roughness and stability; and c) changes in the pre-eruptive state of the magma, including pre-eruptive volatile concentrations, in altering the dynamics of the 1912 eruption. Our starting hypothesis is that the close of each episode of explosive volcanism at Novarupta was caused by 'ascent-driven' changes in the physical state of the volumetrically dominant dacitic magma whereas short-lived fluctuations in eruption intensity were the products of plume instability generated by irregularity and roughness of the vent walls and limited vent wall collapse. The effects of changing flow behavior in the conduit, as the magma rheology responds to decreases in the concentrations of dissolved volatiles, and increases in the abundances of bubbles and crystals, will be inferred from image analysis of the vesicle and crystal populations in carefully selected pumice samples from either side of the breaks in Plinian deposit and abrupt fluctuations in eruptive intensity. External environmental factors, (vent wall roughness and instability, vent migration and widening) will be studied by mapping changes in the nature and abundance of wall rock lithic clasts.