A Model For Alternating Effusive and Explosive Rhyolite Volcanism: The -1350 A.D. Eruption of Tarawera, New Zealand

公元1350年左右，塔拉维拉火山的活动是新西兰最大的(5-10千米岩浆量)喷发，也是近千年来世界上最大的流纹岩喷发之一。喷发产物在当地被称为卡哈罗亚组。该事件的特点是喷发方式的显着波动，从高爆炸性到热液性，作为主要的火山碎屑坠落沉积，并侵位了几个熔岩穹隆。这项与新西兰火山学家的合作研究将建立在塔拉韦拉火山早期地图的基础上，以产生一幅主要流纹岩喷发过程的综合图片。对Kaharoa熔岩穹顶、块体和火山灰火山碎屑流沉积之间关系的详细野外测绘，以及对秋季沉积物中各层的仔细记录，将确定扩散、碎屑特征和粒度分布的细节。对喷发产物进行采样，进行粒度、颗粒形态、晶体和泡泡大小分布研究，将有助于确定岩浆在喷发和喷发机制下的物理状态。流纹岩火山碎屑岩和穹顶熔岩样品的结构分析，与本研究合作部分将获得的地球化学数据相结合，将揭示有关喷发不同阶段岩浆的物理状态(过冷度、脱气、结晶)及其对喷发方式变化的控制信息。研究结果将成为世界范围内这类火山活动的一般模型，进一步了解岩浆脱气和渗漏速度对熔岩穹隆形成火山喷发方式变化的控制，并记录与热流流纹岩活动一致的广泛分布的脉状矿床。这一结果也将是对新西兰和世界各地未来这种类型火山活动复发的火山危险评估的一个组成部分。

9815042SelfThe c. 1350 AD activity of Mount Tarawera was the largest (5-10 km3 magma volume) eruption in New Zealand, and one of the largest rhyolitic eruptions in the world, during the last millennium. The eruption products are locally known as the Kaharoa formation. This event featured remarkable fluctuations in eruption style from highly explosive to effusive as a major pyroclastic fall deposit and several lava domes were emplaced. This collaborative study with New Zealand volcanologists will build on earlier mapping of Tarawera volcano to produce an integrated picture of the processes of a major rhyolite eruption. Detailed field mapping of the relationships between the Kaharoa lava domes, block and ash pyroclastic flow deposits, and careful logging of the layers in the fall deposit will determine the details of the dispersal, clast characteristics, and grain size distribution. Sampling of the eruption products for granulometric, grain morphologic, and crystal and vesicle size distribution studies will help determine the physical state of the magma upon eruption and eruption mechanisms. Textural analysis of the rhyolite pyroclastics and dome lava samples, when combined with geochemical data to be obtained in the collaborative part of this study, will reveal information about the physical state (degree of undercooling, degassing, crystallization) of the magma during various phases of the eruption and the control that this exerted on the changing eruption styles. Results of the study will be a general model for this type of volcanism worldwide, further understanding of the controls on variations in eruption style in lava dome-forming volcanism by magma degassing and effusion rate, and documentation of widely dispersed plinian deposits coincident with effusive rhyolitic activity. The results will also be an integral part of a volcanic hazard assessment for future recurrences of this type of volcanism, both in New Zealand and worldwide.