The Appalachian Mountains were formed when colliding tectonic plates folded and upthrusted, mainly during the Permian Period and again in the Cretaceous Period. The folds and thrusts were then eroded and carved by wind, streams and glaciers. These erosive processes are ongoing, and the topography of the Appalachian Mountains continue to change.
The early Appalachians were born during the middle Ordovician Period, about 440-480 million years ago, when an oceanic plate called the Iapetus plate collided with a neighboring plate called the North American craton, creating a subduction zone and associated volcanoes, which thrust, warped and uplifted the mostly sedimentary rock of the Iapetus plate. Over the ensuing 200 million years, as the supercontinent Pangaea formed, collisions of microplates continued to sweep in, folding and uplifting the region.
As Pangaea began to break up around 220 million years ago, geologic pressures declined, as erosion wore down the upthrusts, folds and peaks that had formed. By the end of the Mesozoic Era, about 75 million years ago, the Appalachian Mountains had been eroded to almost a plain. A new period of volcanic and tectonic activity that began in the Cenozoic Era, about 65 million years ago, created new folds and peaks that ongoing erosive processes have carved into the Appalachian Mountains we know today.