Final answer:
Metamorphic rocks are formed when pre-existing rocks undergo changes in temperature, pressure, or exposure to chemically reactive fluids. The primary agents of metamorphism are heat and pressure, although chemically reactive fluids can also play a role.
Step-by-step explanation:
Metamorphic rocks are formed when pre-existing rocks undergo changes in temperature, pressure, or exposure to chemically reactive fluids.
These changes cause the minerals in the rock to recrystallize and alter their size, shape, and composition. The primary agents of metamorphism are heat and pressure, although chemically reactive fluids can also play a role.
Heat softens the minerals, allowing ions to migrate and resulting in the reorientation of minerals in response to pressure. The degree of foliation, or the orientation of minerals, increases with increasing pressure.
Some metamorphic rocks exhibit distinct layers of minerals called foliation, while others may not show any noticeable orientation and are called non-foliated or massive metamorphic rocks. The type of metamorphic rock formed depends on the rock's original composition and the specific conditions of temperature, pressure, and fluid content during metamorphism.