Spatter stability: constraining accumulation rates and temperature conditions with experimental bomb morphology

Abstract

We have developed the first experimental methodology to create a volcanic spatter pile using molten basalt. This method permits reproduction of thermal conditions that yield the wide variety of spatter morphologies observed in nature. The morphology of the clasts is most strongly controlled by the time the clast spends above the glass transition temperature, which is in turn affected by the rate of accumulation and cooling of the deposit. Also, spatter piles that remain hotter over longer durations experience increased fusion between clasts, less void space between clasts, and generally larger aspect ratios. Our experimental method successfully replicated natural microcrystal textures, rheology, and clast size. Work is still therefore required to achieve realistic vesicle distribution and deposit void space. Based on presented experimental work, we estimate emplacement conditions of Southern Idaho spatter vents to have been ~ 850–900 °C, with eruption temperatures closer to 1000–1100 °C. The rapid decrease from eruption temperature to effective emplacement temperature is the result of clast flight as well as equilibrating with the cooler surrounding material. The morphology of the natural clasts matches experiments that have accumulation rates of 2.5–4.5 m/h, which also is consistent with the few measurements made at active eruptions. Finally, we provide a constraint on the temperatures and accumulation rates that can lead to the construction of fused spatter features, as well as provide the steps for future experiments to investigate other aspects (such as compression, impact, and larger sizes) of spatter formation by adapting our methodology.