What is the main cause of regional metamorphism in rocks?

The main cause of regional metamorphism in rocks is heat and pressure from geological events. This type of metamorphism typically occurs over large areas and is associated with tectonic processes, such as the collision of tectonic plates, which can lead to intense pressure and significant increases in temperature.

During these geological events, the existing rocks—known as parent rocks—undergo physical and chemical changes. The heat may come from the deep burial of rock or from nearby magma bodies, while the pressure is usually a result of the weight of overlying materials or tectonic forces. This combination of heat and pressure alters the mineral structures, often resulting in new minerals that are stable under the new conditions.

In contrast, the other options represent processes that influence rock properties but do not primarily drive regional metamorphism. Weathering is a surface process that breaks down rocks into smaller particles and does not lead to metamorphism. Hydration involves the addition of water to minerals, which can cause changes but is more relevant to contact metamorphism rather than regional. Biological activity influences sedimentary processes but has little impact on the broader changes characteristic of metamorphic processes. Thus, heat and pressure from geological events are critical for creating the conditions necessary for regional metamorphism.