Final answer:
A rock exposed at Earth's surface after being in equilibrium with conditions at depth is usually metamorphosed to adapt to the new environmental conditions, generally becoming a metamorphic rock.
Step-by-step explanation:
On exposure at Earth's surface, a rock that had been in equilibrium with pressure and temperature conditions at depth is usually metamorphosed. Metamorphism is a process where a pre-existing rock, known as the protolith, undergoes physical and chemical changes due to environmental changes, such as a decrease in pressure or a change in temperature when it reaches the Earth's surface.
These changes can cause a recrystallization of the rock's minerals and alter its texture and composition to create metamorphic rock. The most stable state of a rock is the one it originally formed under, so when these conditions change significantly, the rock adapts to establish a new equilibrium. This can happen due to factors such as the movement of tectonic plates or exposure to different temperatures and pressures at the Earth's surface compared to its interior. Metamorphic rocks are key indicators of the geologic processes that shape the Earth over time.