Fold mountains form when the edges of two tectonic plates push against each other. This can occur at the boundary of an oceanic plate and a continental plate or at the boundary of two continental plates. Fold mountains are often composed primarily of sedimentary rock.
Fold mountains usually occur along the edges of continents, as this is where the sedimentary deposits tend to accumulate on a geological time scale. When the oceanic plate collides into the edge of the continental plate, the sedimentary material folds up on itself like an accordion.
Examples of fold mountains include the Appalachian Mountains, the Rocky Mountains, the Ural Mountains and the Himalayan Mountains. Both the Rockies and the Himalayas are of relatively recent origin, and are no more than 25 million years old. By contrast, the Appalachians and the Urals are old mountain ranges that were formed at least 200 million years ago. Evidence of their age can be seen in their rounded peaks, which have eroded with the wind and rain over millions of years.
In addition to fold mountains, Earth also holds volcanic mountains, fault-block mountains and erosional mountains. Hawaii is an example of a volcanic mountain, while fault-block mountains and erosional mountains are usually much smaller and include fewer identifiable examples.