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It's an interesting tale, and one that was discounted as a work of fiction at the time. While there is no evidence of a volcanic eruption at that time in our mountains, what Edwards said he saw can be explained by other phenomenom that was very much real.
When most folks think of earthquakes in the United States, their first thoughts are usually towards the west coast, specifically California or perhaps Alaska, both places that have had extreme seismic events in our lifetimes that have caused great damage and vast suffering.
Most do not think of Missouri and the midwestern part of the country, but it is there that some of the nation's most extreme earthquakes took place, roughly 200 years ago. From 1811-1812, the same time as Edwards' letter, a series of earthquakes rattled the center of our country, culminating with the New Madrid, Missouri earthquake, a ~8 magnitude quake which remains one of the strongest recorded earthquake in our nation's history.
In this earthquake, New Madrid was destroyed. Many houses were severely damaged in St. Louis, and their chimneys were toppled. The seismic area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.
So what does this have to do with NC? The time of Edwards' letter and the New Madrid earthquakes is probably more than coincidental.
The area around Hot Springs is geologically active, hence the warm springs there, and the chemistry of the rocks are very interesting, with high salt levels and other features that make it one of the more unique places in the mountains of NC.
Considered an old wive's tale until the past several years, earthquake lights happen around the time of earth tremors:
Were Edwards' "volcanic eruption" included in his earthquake reported actually EQLs? That's probably the best explanation. The Hot Springs area has the right chemistry in the rocks, and Edwards' observations were at about the right time. Knowing that and keeping in mind that his letter was written and published around the time of the New Madrid events to the west, it's probably safe to assume that what he saw and reported wasn't a volcanic eruption, but instead, Earthquake Lights.
What's left is one of the better but lesser told pieces of folklore in our state's rich history.