It's always worth raising this possibility, though it looks really unlikely in this case. Volcanic and geothermal activity has never released toxic phenols as far as I can tell. Instead you get simpler compounds like carbon dioxide, sulfate, etc. which can also be deadly but only at higher concentrations. This appears to keep killing after dilution.

Fair enough. I saw yellow and though "hrm, sulfur?"

Unless a geothermal event were cutting through and releasing a natural hydrocarbon field (it's happened: Permian–Triassic mass extinction event https://en.m.wikipedia.org/wiki/Permian–Triassic_extinction_...), organic chemicals would be unlikely.

My understanding is that Kamkatchka is too young geologically, as well as otherwise unsuitable, for there to be large petrochemical reserves.