Processes controlling the shape of ash particles: Results of statistical IPA

• Comparison of natural and experimental ash particles using statistical Image Parameter Analysis (IPA).
• The shape of the ash particles is predominantly influenced by the fragmentation mechanism.
• Pre-stresses within the magma body before fragmentation cause small variations in the particle shape.
• Despite evidence for changes in the eruption style, all natural ash particles were formed by phreatomagmatic fragmentation.

Ash particles have unique morphologies and shapes that are characteristic of certain fracture mechanisms and can be used to identify the type of fragmentation such as magmatic brittle or ductile fragmentation and phreatomagmatic molten fuel coolant interaction type fragmentation. Identifying these two different fragmentation processes is especially important in complex volcanic systems where both fragmentation processes can occur. In this study, image parameter analysis and statistical parameter analysis are used to compare ash particles from standardised magma fragmentation experiments with natural ash particles from the Pleistocene Lake Purrumbete maar, southeastern Australia. The pyroclastic Lake Purrumbete maar sequence containsvarious deposit types that show evidence for changing eruption conditions, therefore it is of a main interest to determine the main fragmentation mechanism that formed these deposits. A comparison with experimental ash particles revealed that Lake Purrumbete ash particles show significant differences from experimental samples of magmatic brittle type fragmentation, whereas they show no significant differences from phreatomagmatic molten fuel coolant interaction type fragmentation samples, indicating a predominance of phreatomagmatic fragmentation during the eruption of Lake Purrumbete. The experiments further show that pre-existing stresses also influence the particle shape and may be the reason for the absence of a significant similarity between most of the particle populations.