Some 250 million years ago, what's now the northeastern coast of the U.S. was attached to northwestern Africa when the landmasses were basically all together as a supercontinent called Pangaea.
Starting in the Late Triassic Period, around 225 million years ago, Pangaea began rifting apart.
Breakup of North America & Africa from Triassic into Jurassic Periods
245, 210, 180, & 150 million years ago (Ron Blakey, NAU Geology)
Continents rift apart due to the formation of tensile stresses in the lithosphere, typically due to the formation of mantle plumes, which then stretch the crust. The crust accomodates by thinning and faulting, dropping down linear rift valleys. The thinner crust results in partial melting of the mantle which forms magma which rises to erupt on the floor of the valley.
The next step in the stage is linking up of some of the rift valleys to fracture the continental crust. The area sinks low enough that seawater floods in forming a linear sea. Look at the Red Sea at left. That was once a rift valley between the Arabian Peninsula and Africa that completely broke through into a linear sea. This will someday happen on the African mainland separating the Horn of Africa.
Here in the Northeast, there are a number of failed rift valleys that got left behind when Pangaea finally broke apart (similarly, when the Horn of Africa breaks off, it will be along either the eastern or western arm of the East African Rift and the other will be left as a failed rift).
In the lower Hudson Valley of New York, we have a geologic province called the Newark Rift which extends up from New Jersey into Rockland County (south of the Hudson Highlands and west of the Hudson River).
The Newark Rift was one of those failed rift valleys but during the Triassic Period, when small meat-eating bipedal dinosaurs like Coelophysis were running around, it was an active place of earthquakes and eruptions of lava flows. Just like eastern Africa today (sans dinosaurs).