The appalachian mountains are approximately 480 million years old, making them the oldest mountains in north america. the appalachian mountains are folded mountains that were once as high as 20,000 feet. now they reach to about 3,000 feet. how did these mountains form? a) folded mountains occur when two plates of the earth's crust separate. b) folded mountains are the result of many volcanoes erupting all at the same time. c) folded mountains form over millions of ears as sediments are deposited and build up. d) folded mountains occur when two plates of the earth's crust push against each other.