For a history paper, my argument is that the American Frontier acted as a sort of pressure relief valve for Colonial dissidents who did not agree with Puritan theology. What is a more appropriate word or term for pressure relief valve?
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How about diversion (since the pressure was diverted by the Frontier) or distraction, or respite (if it was only a temporary relief)?
You could say that the dissidents sought "solace" from the Puritans in the American Frontier.
If these dissidents felt oppressed by religious dogma, solace works well.
To use a verb, you could say that westward expansion into the American Frontier "alleviated" the pressure colonial dissidents felt in a largely puritanical society.
To be perfectly honest, though, I think a "pressure release valve" is a perfectly acceptable metaphor to explain Westward Expansion--it's not overly casual and requires no great leaps in logic.