Final answer:
Minerals formed at depth may be unstable at the surface due to differences in temperature and pressure, leading to metamorphic changes.
Step-by-step explanation:
Minerals formed at depth within Earth may be unstable at the surface due to very different temperature and pressure conditions. When minerals that formed deep within Earth, where temperatures and pressures are substantially higher, are exposed to the Earth's surface, they may become unstable because the surface conditions are much cooler and the pressure is significantly lower. This instability can lead to physical or chemical changes in the minerals, such as a change in crystal structure or chemical composition, often resulting in the formation of metamorphic rocks.
These rocks started off as one kind of protolith, or parent rock, and metamorphosed into a new form due to external changes in their environment.