Regional metamorphism occurs primarily due to high temperature and pressure conditions that are typically associated with tectonic processes, such as mountain building or the collision of tectonic plates. During regional metamorphism, large areas of the Earth's crust are subjected to these conditions over extensive periods, leading to the transformation of pre-existing rocks into metamorphic rocks.
The significance of high temperature lies in its ability to facilitate the recrystallization of minerals, enhancing the formation of new mineral structures. Meanwhile, increased pressure can cause foliation, where minerals align in response to stress, resulting in distinctive banding patterns seen in many metamorphic rocks.
In contrast, low temperature and pressure are not conducive to regional metamorphism, as they do not provide the necessary conditions for the physical and chemical changes to occur. High or low humidity does not directly influence metamorphism to the same extent as temperature and pressure, as metamorphic processes are primarily thermodynamic in nature. Thus, the conditions of high temperature and pressure are essential for the formation of metamorphic rocks in a regional context.