What is the primary way that metamorphic rocks form? A. Lava erupts from Earth and cools. B. Water, wind, and gravity move particles from one place to another. C. Rock fragments break apart and collect as sediment. D. Existing rocks join together under high heat and pressure.

Explanation: Metamorphic rocks are formed when a sedimentary or igneous rocks undergoes changes due to the variation in the temperature and the pressure conditions. This are also considered as the secondary rocks as they are directly derived from a pre-existing rock. For example, Slate, Phyllite, Schist, Gneiss and Marble.

It makes up about 10-15 % of the total rocks on the earth's surface. The high grade rocks belongs to the metamorphic rocks.

The rocks undergoes metamorphism in the following types, namely-

Contact metamorphism- When metamorphism occurs because of the intruding magma under the crust. For example- Hornfels.
Local metamorphism- When metamorphism occurs over a large area, including the tectonic plates. For example- Gneiss.

Thus, the correct answer is option (D).