Assessing magma fragmentation processes from the characteristics of volcanic ash

点击翻译按钮获取中文摘要

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.