Final answer:
Mountains formed by crustal extension are frequently formed, with examples like the Rocky Mountains resulting from mechanisms different from the classic model of continent-continent convergence.
Step-by-step explanation:
The frequency of mountains formed by crustal extension can be described as frequently formed. Although numerous mountains on Earth, like the Alps have formed due to the collision of continental plates, there are also significant occurrences where mountain ranges are shaped by processes like crustal extension. The Rocky Mountains for example were formed partly due to low-angle subduction, a different mechanism than continent-continent convergence.
Further evidence of mountain formation due to crustal movement can be found through the examination of tectonic events that have shaped other ranges such as the Appalachian/Caledonide Mountains. Over millions of years these mountains experienced different tectonic orogenies leading to their present state. It is also worth mentioning that different terrestrial planets have witnessed mountain formation through varied processes not solely through the commonly understood collision events.