Why, how and when heat and and huge amounts of polluting gases emanate from the core of the earth is simply not scientifically understood. At best instruments seem to tell when really big eruptions are about to happen. The amount of heat and gasses released has more to do with climate change, arctic, antarctic ice thickness and the fate of polar bears than any other single factor.
A Super volcano eruption anywhere, would darken the sky for months, all over the globe, scatter ash all over the globe and within a thousand miles of the crater have ash build up to several feet thick.
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It happened in 535 ad, maybe twice, once in Indonesia and once in Mexico. The following years without summers killed off a large portion of the Holy Roman Empire. It replaced grain-eating Europeans with goat and cattle eating people from the asian steppes.