200 Seafloor Spreading
Hess theorized that guyots had originally been islands dating back to 800 million years
ago, a period before coral existed. His argument rested, in part, on his hypothesis that continual deposits of sediment on the seafloor had made the sea level rise.
When, in 1956, fossils only 100 million years old were found in guyots, Hess changed
his theory to say that guyots had originally been volcanoes that had eroded to flat tops by
wave action. He abandoned this theory when erosion rate calculations showed that the
guyots couldn’t have eroded enough to reach their current depth.
Then his 1957 oceanic core samples showed that the Atlantic Ocean floor was much
younger than the continents and that oceanic sedimentation rates were slower than previously thought. Hess—again—had to search for a new theory.
Luckily, his 1957 survey allowed him to collect core samples from more than 20 sites
across the Atlantic. These tests showed that the age of the ocean bottom grew progressively
older as it moved away from the mid-oceanic ridge and toward either continent.
The seafloor wasn’t fixed and motionless as everyone had thought. It had to be spreading, moving as if on a giant conveyor belt, inching year by year away from the mid-oceanic
ridge. Hess argued that magma rose from the earth’s mantle up through oceanic rifts and
spread out laterally across the ocean floor. As the magma cooled, it formed new oceanic
crust. He estimated the oceanic crust to be spreading apart along the mid-oceanic ridge by
one to two inches a year.
Hess’s discovery became known as seafloor spreading and was the foundation of the
plate tectonics revolution in the late 1960s and early 1970s.
Fun Facts: The Pacific Ocean is slowly shrinking as the Americas slide
west. Two hundred million years ago, the Atlantic Ocean didn’t exist.
South America and Africa were joined, as were North America and Europe. The Atlantic is still spreading and growing. So is the Red Sea. In
150 million years, that currently skinny sea will be as wide as the Atlantic
is now.
More to Explore
Bermen, Howard, ed. The National Cyclopedia of American Biography, Volume N-63.
Clifton, NJ: James T. White & Co., 1984.
Daintith, John, ed. Biographical Encyclopedia of Science. 2d ed. Philadelphia: Norton
Scientific, 1994.
Gillispie, Charles Coulston, ed. Dictionary of Scientific Biography. New York:
Charles Scribner’s Sons, 1998.
Hess, Harry. “History of the Ocean Basins.” In Petrological Studies, edited by A.
Engle and H. James. New York: Harper, 1992.
Rubey, William. “Harry Hammond Hess.” In Yearbook of the American Philosophical
Society (1995). New York: American Philosophical Society, 1996.