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

间歇和/或爆发强度的突然变化是许多大爆发的基本但很少研究的部分。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波动几个数量级的强度。这种转变的原因仍然知之甚少，当我们的稳态喷发过程的模型变得越来越优雅和复杂的时候。如果能够解释甚至预测这种变化，灾害咨询和火山喷发“预测”的质量将大大提高。这项研究的重点是这种“不稳定性”在一个强大的持续爆发的起源，在几个小时的持续时间和突然的，但短暂的变化，风格/intensity.The Novarupta在1912年事件的爆发爆发是世界上最大的20世纪世纪爆发，并被广泛引用为一个很好的例子，一个历史性的大型持续喷发的爆发活动。Novarupta持续的普林尼期/熔结凝灰岩喷发在喷发开始后16和45小时短暂突然停止，60小时后最终让位于熔岩圆顶的渗出（Hildreth和Fierstein，2000年）。在1912年爆发的破火山口倒塌发生在通风口，但10公里远，从而保存非常近的通风口的产品，在100米的通风口，和中间的喷发产品有一个“化学地层学”，使相关的同时代产品的高度不同的字符之间的广泛分布的地方。这使我们能够从一个非常好的约束的角度来解决喷发动力学突然变化的基本问题，并在过去1000年中记录的五个最大的喷发之一。拟议研究的目标是评估以下方面所起的相对影响：a）气泡的增长和破裂、脱气和同生喷发微晶结晶的速率和时间; B）喷口的粗糙度和稳定性; c）岩浆喷发前状态的变化，包括喷发前挥发物浓度，以改变1912年喷发的动力学。我们开始的假设是，在Novarupta爆炸性火山活动的每一幕的关闭是由“上升驱动”的体积占主导地位的英安质岩浆的物理状态的变化，而在喷发强度的短暂波动是由不规则性和粗糙度的通风孔壁和有限的通风孔壁倒塌所产生的羽流不稳定的产品。改变流动行为的影响，在管道中，作为岩浆流变学响应于溶解的挥发物的浓度降低，并增加了在泡沫和晶体的丰度，将推断从仔细选择的浮岩样品中的泡囊和晶体人口的图像分析从任何一方的普林尼存款的休息和突然波动的喷发强度。将通过绘制围岩岩屑碎屑的性质和丰度的变化图，研究外部环境因素（喷口壁粗糙度和不稳定性、喷口迁移和扩大）。

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.