Fragmentation Mechanisms Associated with Lacustrine Lava–Water Explosions

Abstract:

Rootless cones form when degassed lava interacts explosively with water contained in the near-surface substrate, and represents an end-member system that can elucidate mechanisms of magma–water interactions due to the absence of primary degassing-induced fragmentation. The proportion of finely fragmented ejecta (i.e. ash), generated in rootless explosions, even if the volume is small relative to coarser ejecta, may contribute significantly to the explosion energy release. Explosive melt–water experiments indicate that the degree of melt–water mixing and energy release are proportional to the abundance of blocky grains, fragmented by brittle disintegration, which effectively contribute thermal energy to the explosive lava–water interaction. In order to determine the state of the lava at the point of ash-grade fragmentation in rootless explosions we examined grain morphology over the following size ranges: 0.5–0 ϕ (1.41–1 mm, very coarse ash), 1.5–2 ϕ (0.354–0.250 mm, medium ash), and 3.5–4 ϕ (0.088–0.062 mm, very fine ash). We found that rootless ash is composed of blocky, mossy, and fluidal grains with a minor component of aggregates (≤ 2%) and glassy shards (< 35%). Typically, (1) very coarse ash contains blocky (9–58%), mossy (8–36%) and fluidal (28–67%) grains only, (2) medium ash contains blocky (32-76%), mossy (9–39%) and fluidal (15–43%) grains with a minor abundance of glassy shards (≤ 8%), and (3) very fine ash is dominated by blocky clasts (53-80%), with lower shard (12–34%) and fluidal (4–24%) components. We observe that the abundance of fluidal grains decreases while the abundance of blocky grains increases with decreasing grain size. Also, the abundance of blocky grains decreases with increasing stratigraphic height, indicating that as rootless explosions progressed, brittle fragmentation of lava is less pronounced, suggesting that the efficiency of lava–water mixing dropped, most likely due to reduced availability of external water. However, we do not observe consistent correlation between the abundance of blocky grains and the proportion of ash or mean grain size of individual beds, both of which are a function of explosion energy. Therefore, future work could improve upon this work by taking into account the total bed mass, and provide estimates of the mass of different ash morphologies.