Curiosities: Why are the US’s coastlines so different?

A: The answer resides in the interaction between the giant “tectonic plates” that form Earth?s crust, said Phil Brown, a professor of geoscience at the UW-Madison. “The east coast of North (and South) America are passive plate margins, which have subdued topography on land and broad shallow continental shelves that may extend 200 miles off shore, before diving to the Atlantic abyssal plain.”