James Dwight Dana's old tectonics; global contraction under divine direction

Abstract

The old global tectonics of James Dwight Dana was one of America's first major contributions to theoretical geology. That theory began with Dana's experiences in the Pacific on the Wilkes Exploring Expedition (1838-1842), which paralleled closely the experiences of Charles Darwin a few years earlier. He refined Darwin's hypothesis of oceanic subsidence in 1843 by adding geomorphic evidence of subsidence, differential crustal responses, and variable island ages, and went on to develop a comprehensive global theory during the remainder of his life. Dana accepted the long-standing assumption that the Earth began molten and had contracted as it cooled. Early in his career, he recognized the fundamental geologic difference between continents and ocean basins, which he believed had arisen early in the history of the planet. He inferred that the northwest and northeast trends of many linear island chains, shorelines, and mountain ranges reflected fundamental cleavage lines, which he thought had originated during Archean thermal contraction and continued to influence subsequent evolution of the crust. Because continents "were first free from eruptive fires," they must have cooled first and, being very old, must also be permanent. With their active volcanoes and depressed topography, ocean basins must be the chief loci of cooling and contraction. Furthermore, their greater subsidence inevitably causes lateral pressure, folding, and uplift of continental margins to form mountains. The geosyncline was a late refinement from 1873 in response to Hall's 1857-1859 "theory of mountains with the mountains left out" (according to Dana). Contractive pressure buckled the continental margin; a downbuckle or geosynclinal received thick sediment derived by erosion of a complementary upbuckle or geanticlinal. Finally, the whole system failed and became stabilized as an addition to the growing continent while a new geosyncline-geanticline couplet formed oceanward. Dana regarded North America as the perfect, simple example of continental evolution, which "revealed God's plan of creation" better than any other continent, therefore it could instruct the rest of the world. Its margins reflect the northwest and northeast cleavage lines with the oldest Azoic rocks representing the "first germinant spot" or nucleus around which the continent had expanded by additions of mountain belts through successive "vibrations of the crust." Thus was born the important concept of continental accession or accretion with "contraction as the power, under Divine direction, for humanizing the earth." Dana's old global tectonics had profound influence even after thermal contraction lost favor around 1910. First Chamberlin's gravitational contraction and later thermal convection extended that influence and helped nurture the American resistance to continental drift until the new tectonics appeared in the 1960s.