Let's consider this environment as the eruption and deposition occured:
- Sedimentation and erosion were both occurring quickly, as shown by poor sorting, angular grains, rapid changes in dominant grain sizes, and common disconformities.
- Overall, though, the pile was accumulating much faster than consolidation and lithification; it probably wasn't until long after the eruption stopped that much of the material could be considered to be "rock."
- During the episode, the pile of debris would be subject to frequent, likely locally powerful, seismic shock from two sources: earthquakes related to magma moving underground, and frequent phreatic blasts as ground and surface water interacted with hot basaltic magma and lava.
- I picture this situation as a rapidly growing "circular delta," with the central vent as the metaphorical sediment source. It seems likely that with such rapid accumulation, oversteepening and slumping were frequent. This would lead to tensional regimes in the topset beds, normal faulting in the foresets, and compression in the bottomsets. Because sedimentation was so rapid, the stress regime was almost certainly changing rapidly and constantly. This may have been exacerbated by rapid shifts from loading to unloading, and back, as sites of deposition became sites of erosion.
- Add on top of all this that the alteration of basalt to palagonite results in a net increase in volume. I'm not sure by how much, but in a pile like this, that effect probably shouldn't be ignored.