Geochemistry is the science that applies the principles of chemistry to understand the composition, structure, and evolution of the Earth and its major systems, including the crust, mantle, oceans, and atmosphere. It investigates the distribution, abundance, and migration of chemical elements to explain the processes that form rocks and minerals, govern the water cycle, and drive large-scale planetary dynamics. As a foundational discipline, geochemistry provides the chemical context for both geological phenomena and the intricate interplay between the Earth and its biosphere, which is the specific focus of biogeochemistry.