[pierrec]

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.

[dredmorbius]

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.