Pyroclast characteristics of a subaqueous to emergent Surtseyan eruption, Black Point volcano, California

We present the first-ever quantitative analysis of pyroclast textures from subaqueous deposits of a Surtseyan emergent volcano. The volcano, Black Point, formed in Mono Lakeʹs Pleistocene precursor, Lake Russell, at ~ 13,300 ± 500 BP, from an explosive basaltic eruption involving the interaction of shallow lake water (~ 105 m) with vesiculating magma. The bulk of the edifice comprises unconsolidated tephra and represents submerged stages of eruption. Thin summit deposits formed during a final phase of emergent eruption and produced a group of low tuff rings. Lapillus clasts (16 mm to 32 mm) and fine ash grains (4 Φ, 0.0625 mm) from the subaqueous deposits have textures that reveal a conduit history of bubble nucleation, growth, collapse, intense microlite crystallisation, and magma–water interaction. Lapilli vesicularity ranges broadly to high values (up to 89%), while vesicle number densities (5.4 × 102–4.8 × 104 vesicles per mm3) are comparable to those published for Strombolian and Plinian basaltic eruptions (> 1 × 104 vesicles per mm3). Vesicle size and volume distributions suggest continuous bubble nucleation, but the suite of data as a whole indicates that a late-stage nucleation event played a significant role in shaping the eruption. Ash grain morphology and surface features indicative of magma–water interactions attest to waterʹs role in fragmentation. Overall the pyroclast textures are consistent with water modification of a rather intense basaltic eruption, which might have been of a violent Strombolian or Plinian style if the eruption had been “dry” and occurred subaerially.