Volcanic Debris Avalanche and accompanying shear zone slip surface formed by a perched scoria cone collapse on Ascension Island, South Atlantic

Volcanic debris avalanches occur when volcanic edifices collapse and flow as landslides. They are preserved in the geological record as volcanic debris avalanche deposits (VDADs). Analysis of these deposits can provide insight into the flow characteristics of the avalanche and its possible triggers.

Here we provide preliminary textural data on the shear zone layer at the base of a small-volume VDAD on Ascension Island, South Atlantic. The deposit has a volume of ~4 x 10⁶ m³, covers 2 km² and originated from the partial collapse of the northern flank of the 300ka Green Mountain scoria cone, which sits at 550 metres above sea level. The avalanche flowed 2 km down a ~10° slope, before stopping at in a small basin against a lava dome at 190 m above sea level.

Over most of its length the VDAD overlies an in-situ Green Mountain scoria fall deposit that was dispersed north during the eruption. The base of the deposit is marked by a fine-grained, ~2 cm-thick shear zone with slickensides. The shear zone is distinguishable in hand specimen from the rest of the deposit by being finer grained and indurated. The bulk of the VDAD is composed of semi-coherent, metre scale blocks of scoria with a poorly sorted volcaniclastic matrix composed of a hetereolithic clast population including randomly orientated clasts of basaltic scoria, pumice and lavas. The toe of the deposit is fractured and flame structures are abundant.

Preliminary Back-scattered Scanning Electron Microscope imaging of the shear zone reveal that porosity and pore interconnectivity decrease markedly towards the centre of the shear zone, and clasts become finer-grained, better sorted and more rounded. Experiments will be conducted on samples of Green Mountain Scoria using Rotary Shear Equipment to place constraints on slip rates and shear parameters. Ultimately, we hope to understand potential triggers of the failure and explore the hazards and potential for similar events on the island in the future.