ABSTRACT
The formation of the earth sciences was one of the most important intellectual and institutional transformations of twentieth-century science, comparable to the unification of physiology, bacteriology, and medical chemistry within a new field of 'biology' in the late nineteenth century. This chapter will trace two sets of developments. First, it examines major breakthroughs and conceptual developments in the earth sciences, focusing on the plate tectonics revolution and the recognition of catastrophic processes, including celestial impact, as geological forces. Second, it addresses significant social, professional, and disciplinary developments, such as military funding for geophysics after 1945 and growing concern with human impact on Earth's climate and related environmental issues, which helped to create a distinct entity called the earth sciences. Because geophysics expanded rapidly in the second half of the twentieth century, and several key concepts were primarily developed after 1950, this period receives particular emphasis.
Scientific views of the Earth, and the physical processes which governed it, changed considerably during the twentieth century. Although many new ideas were debated by earth scientists in this time, two fundamental conceptual shifts came to over-
