Which process involves chemical changes in minerals and rocks resulting from introduction of external sources such as fluids or melts?

• A. Metasomatism
• B. Metamorphism
• C. Mineral
• D. Mantle Keels

Answer: (A)

Metasomatism is the chemical alteration of minerals and rocks driven by the introduction of external sources such as fluids or melts. When these fluids or melts move through rock, they transport dissolved elements, remove some components, and deposit new minerals, changing the rock’s composition and mineral assemblage. This mass transfer and new mineral formation is the hallmark of metasomatism, setting it apart from other processes that mainly rearrange minerals under high temperature or pressure without adding new material. For example, hydrothermal fluids can alter wall rocks to form minerals like chlorite, epidote, or calcite, reflecting the chemical input from outside. The other terms aren’t processes that describe this specific chemical exchange: a mineral is a substance, not a process; metamorphism is broader and centers on changes due to heat and pressure (often with recrystallization rather than external material addition); mantle keels are geological features, not a rock-altering process.