This is a really long, albeit to me totally fascinating, video about the mantle tomography evidence for major terrane and plate movements on the margin of North America mostly between about 100 Ma* and 50 Ma. But I'm not proposing my farflung correspondents watch the whole thing necessarily. But, please pull it up and watch between 56:00 and 60:00 to see how, much like Australia ramming New Guinea, several large and small microcontinents (comparable to Borneo, Sumatra, or Japan, say) were accreted to North America to make the Western Mobile belt, which is all of the three West Coast states, and some adjacent areas (not including the Rockies, a different story), much of Mexico, all of Brit. Columbia, and all of Alaska. From 170 Ma to the present. If this doesn't pique your interest just a little bit, you're hopeless when it comes to "lay interest in science." No offense.
(*means "million years ago," usually pronounced "M-a"... notice that Karin sometimes just says "Years," since it's obvious she means "million years").
These ideas are somewhat controversial in detail, but in general the fact that something very like this happened is by now indisputable.
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