The Central United States, from about Iowa all the way to Colorado, is notable for being a very long plate that rises East to West, to the extent that Denver while sitting in what appears to be a flat plane, is still 5000 ft above sea level, before starting into the foothills of the Rockies. If you think about this, it's actually pretty odd.
My theory is that this escarpment was once a relatively flat and level stretch of land. When the Pacific was forming, there was a trough (long since eroded) that created new material outward The eventually forced the subduction areas around the then North American West Coast to push up, in essence creating a chain of volcanoes that eventually surfaced, even as the increased subduction pushed up the plate itself. These proto-continent island chains in turn were pushed into the West Coast, creating tall mountains as these long island chains essentially crumped against the coast, blocking the subduction regions and forcing new subduction to occur West of the new west coast.
This is why there are clearly defined valleys both within the Rockies, and more recently, between the Rockies and the Cascades, which are still being formed. The Olympic mountains (and the Olympic Peninsula) may be the remnants of a much more recent example of this. with what now forms the Washington and Oregon West Cost having once been islands (with Vancouver Island being a likely verification of this. Within 15-25 million years, the Puget Sound will collapse like an according, creating more new mountains, even as Vancouver slams into the British Columbia mainland.
The Pacific Ocean ridge has stopped spreading, and the pressure is now on the Atlantic side as it continues to expand. This should cause the build-up of new protocontinental islands but may also cause the uplifted valleys to slow sink back down. It's worth noting that these tectonic moves are SLOW. The Atlantic expansion will eventually drive the continents to form a new supercontinent about 250 million years from now, Pangaea Proxima. To put that into perspective, 250 million years ago, the dinosaurs were just beginning to differentiate from reptiles.
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