What is the primary factor in regional metamorphism?

The primary factor in regional metamorphism is the combination of pressure and heat. This type of metamorphism typically occurs over large areas, often in mountain ranges where tectonic plates collide. The intense pressure from the overlying rock and the heat generated from the Earth's interior or tectonic activity cause the minerals in pre-existing rocks to recrystallize and form new metamorphic rocks.

High temperature alone does not account for the extensive changes that occur during regional metamorphism since the added pressure plays a crucial role in altering the mineral structure. While contact with magma does influence metamorphic processes, it specifically pertains to contact metamorphism rather than regional metamorphism. Geothermal gradients are certainly important in understanding temperature changes over depth but do not encompass the comprehensive effects of pressure synergistically combined with heat, which is the defining characteristic of regional metamorphism.