The core of an ancient, deeply eroded mountain range exposes rocks such as gneiss and schist containing high-pressure, high-temperature mineral assemblages. What is the most likely plate tectonic origin of these rocks?
- They formed from the rapid cooling of basaltic lava at a mid-ocean ridge.
- They are remnants of an accretionary wedge scraped off a subducting plate at low temperatures and high pressures.
- They were buried deep within the crust and heated during a continental collision event. (correct answer)
- They formed from contact metamorphism in the shallow crust immediately adjacent to a volcanic caldera.
Explanation: The correct answer is C. Gneiss and schist are high-grade metamorphic rocks that form under conditions of intense heat and pressure. This large-scale process, known as regional metamorphism, occurs deep within the crust. A continental collision is the ideal tectonic setting to create these conditions, as it dramatically thickens the crust, burying rocks to great depths where they are subjected to the necessary high temperatures and pressures. Choice A describes igneous rock formation. Choice B describes the formation of low-temperature, high-pressure rocks like blueschist, not high-grade gneiss. Choice D describes contact metamorphism, which is localized and typically occurs at lower pressures.