Assessing magma fragmentation processes from the characteristics of volcanic ash

上下文。火山是世界范围内主要的自然灾害之一。加拿大有200多座潜在的活火山，并且暴露在美国火山的危险之中。火山产生的颗粒大小不一；直径小于等于2毫米的火山灰通常是在最危险的火山爆发时形成的。岩浆破碎是由于管道内气泡的生长和岩浆的加速作用造成的，而喷发岩浆破碎则是由于岩浆-水的爆炸性相互作用造成的。这些不同类型的碎裂与不同的危险相关。岩浆破碎过程的解释主要基于火山灰颗粒的形状、表面特征和结构特征。然而，不同的研究人员使用了极不相同的方法，这使得不同研究之间的比较非常困难。我和几个国际合作者制定了一套标准的火山灰特征描述方案。

CONTEXT. Volcanoes are one of the major natural hazards worldwide. Canada has more than 200 potentially active volcanoes, and is exposed to hazards from volcanoes in the USA. Volcanoes produce particles from a range of sizes; those 2 mm across or less constitute volcanic ash and typically form during explosive eruptions, the most dangerous ones. Magmatic fragmentation is due to the growth of gas bubbles and acceleration of magma in the conduit, whereas phreatomagmatic fragmentation occurs in response to explosive magma-water interaction. These different types of fragmentation are associated with distinct hazards. Interpretation of magma fragmentation processes is based largely on the shapes, surface features and textural characteristics of ash particles. However extremely variable methodologies have been used by different workers, which makes comparisons between different studies very difficult. I have developed a standardized protocol to characterize volcanic ash with several international collaborators. OBJECTIVES. The long-term objective of the research program is to develop the capability to assess magma fragmentation processes from the characteristics of volcanic ash. The end-result will be a set of “fragmentation diagrams”, where fields will be defined for different types of magma fragmentation, and different eruptive styles. Different diagrams will be produced for different magma compositions. SCIENTIFIC APPROACH. To construct the fragmentation diagrams, the international volcanological community needs to characterize ash from the whole spectrum of magma compositions and eruptive styles. The contribution of my group over the next 5 years will relate to basaltic monogenetic volcanoes (scoria cones and maars) and related eruptive styles. We will collect ash from volcanoes and from laboratory experiments with variably bubbly basaltic magma, and analyze this ash with the standardized protocol. NOVELTY. The research program will produce the following novel outputs: (i) One or several magmatic fragmentation field(s) on fragmentation diagrams, based mostly on tephra blankets around basaltic scoria cones; (ii) One or several phreatomagmatic fields on fragmentation diagrams, based on basaltic maars and tuff cones; (iii) An experimental study on the influence of magma vesicularity (content in gas bubbles) on the magmatic and phreatomagmatic fragmentation of basalt. EXPECTED SIGNIFICANCE. The scientific outcome will be the clear steps taken towards being able to interpret the fragmentation mechanism for unwitnessed volcanic eruptions using the characteristics of volcanic ash. This will allow better prediction of future activity and better hazard mitigation at active volcanoes. Specific benefits for Canada will be a better understanding of what a future scoria cone eruption would potentially look like in BC, and the training of 8 HQP, who will then be well placed to seek employment in the resource sector, geological surveys or universities.