How do metamorphic rocks return to the surface?

Metamorphic rocks can return to the surface through a process called exhumation. The overlying rock is eroded away, exposing the metamorphic rocks. Some of the oldest rocks on our planet are metamorphic rocks and form the 'core' of continents called PreCambrian Shields.

By USGS - https://earthquake.usgs.gov/data/crust/maps.php, Public Domain, https://commons.wikimedia.org/w/index.php?curid=306268