1973

Seafloor Spreading

Tanya Atwater (b. 1942)

As more information, maps and topographic data, and fossil evidence became available over the course of the twentieth century, support continued to grow for Alfred Wegener’s once-ridiculed idea of continental drift. The problem had been that there was no obvious explanation or physical process that could explain how a continental mass could plow through an oceanic plate; but this became a moot point in the 1960s with the discovery of seafloor magnetic stripes of reversing polarities and the realization that the mid-ocean ridges were spreading centers where new oceanic crust was being formed. The uppermost crust of the Earth appeared to be operating like a kind of global-scale conveyor belt, with new crust appearing at the ridges and old crust being subducted and destroyed at the trenches and collisional plate boundaries.

But how was it all connected? And did the math all work out: was new crust being created at the same rate that old crust was being destroyed? And what is driving it all? Among the leading geologists working to find those answers is American oceanographer Tanya Atwater, who in a key research paper and several book chapters published in 1973 set out to stitch the world together, calculating and synthesizing information that showed that the seafloor was spreading apart along the world’s mid-ocean ridges (at a typical rate from 5 to 9 centimeters per year), that the seafloor was being subducted at plate boundaries, and that plates were sliding past each other (rather than colliding) along faults such as the famous San Andreas in California.

Atwater applied her and her colleagues’ observations of the topography and geology of the seafloor to make discoveries about how Earth’s dozen or so tectonic plates interact with one another, how continents like North America were built up over time by collisions of ancient, long-gone tectonic plates, and how the active geology and volcanology of much of the ocean floor could provide interesting environments, such as hydrothermal vents, for life to potentially thrive. Indeed, she found, Earth’s plates are all connected in one way or another, and a rich history of those past connections and interactions is still preserved in the geologic record today.

Global maps of the ages of the seafloor crust. Red is the youngest (including new crust being created today); blue is the oldest oceanic crust remaining on the planet, formed between about 150 and 180 million years ago.