Isostacy and the longevity of the Appalachian Mountains
The Appalachian Mountains are around 480 million years old, literally older than trees. So how do they still exist, given the fact that the Earth loves to erode away every surface that even thinks of protruding from up from the ground. This can be explained through one simple principle of plate tectonics: isostacy.
Essentially, the continental plates that form every large landmass on Earth float--on top of the semi-plastic asthenosphere. This means that as mass is removed from the top of a continental plate, that part of the plate floats higher on the asthenosphere. For every one meter of erosion, the landmass rises around 89 centimeters to compensate. This gives plenty explanation for why a formation as old as the Appalachians can still be present on Earth's surface after enduring nearly 500 million years of erosion.