What is the primary cause of contact metamorphism?

The primary cause of contact metamorphism is the interaction between existing rock and molten or solidifying magma. When magma intrudes into cooler surrounding rock formations, it raises the temperature of the surrounding rocks without subjecting them to significant pressure changes. This heat causes the minerals within the existing rock to become unstable and recrystallize, forming new metamorphic rock types that are typically more crystalline than the original material. This process occurs over relatively localized areas around the intrusion, hence the term "contact" metamorphism, as it describes the changes taking place at the point of contact between the magma and the surrounding rock.

The other options do not correctly represent the primary driving force of contact metamorphism. Pressure from tectonic plates is more associated with regional metamorphism, which occurs over larger areas and involves both pressure and temperature changes. Exposure to sunlight does not play a role in metamorphism, as it primarily affects surface weathering and sedimentation processes. Water erosion pertains to the physical wearing away of rock and soil due to water movement, rather than the heat-induced changes that characterize contact metamorphism.