An important class of metamorphic rocks exposed at the surface of the Earth's crust, and inferred to make up a large portion of the deeper crust. Granulites are known from experimental petrologic studies to have formed at higher temperatures, and in many cases, higher pressures, than most other crustal rock assemblages. Thus, they are believed to have formed at considerable depths in the crust. The transport of whole granulite terrains, sometimes hundreds of kilometers in lateral dimension, from depths of 12–18 mi (20–30 km), poses a major problem.