Seafloor Spreading
Year of Discovery: 1957
What Is It? The ocean floors slowly move, spreading from central rifts, and
carry the continents on their backs as they do.
Who Discovered It? Harry Hess
Why Is This One of the 100 Greatest?
We now know that Earth’s continents move. Over hundreds of millions of years, they
drift across Earth’s surface. You have likely seen pictures of what Earth looked like 500
million years ago. But just 60 years ago, no one believed that it was possible for massive
continents to move. There was no force great enough to move vast continents weighing trillions of tons.
Then Harry Hess discovered the theory of ocean-floor spreading. That discovery suddenly not only made continental movement plausible, but made drifting continents a fact.
Hess’s discovery was the key evidence that confirmed early theories on continental drift by
Wegener. Hess’s work launched the study of plate tectonics and created new understanding
of the history and mechanics of Earth’s crust and started the serious study of the past motion
of Earth’s continents.
How Was It Discovered?
Standing on the bridge of a mammoth deep-ocean drilling ship in the mid-Atlantic in
1957, Navy Commander Harry Hess watched as a crane operator maneuvered the drilling
pipe sections from atop the drilling derrick mounted high above the deck. This was the first
time a ship had been able to drill and collect core samples from the ocean floor 13,000 feet
below. Hess had designed and managed the operation. He should have been pleased and
proud. But test after test showed the ocean bottom below them was less than 50 million
years old—disproving every theory about the ocean floor that Harry Hess had created and
promoted.
A geology professor before he joined the navy, Hess had been given command of the
transport U.S.S. Cape Johnson operating in the Pacific in 1945. Using Navy sonar systems,
Hess made the first systematic echo-sounding surveys of the Pacific Ocean floor over a
two-year period as he steamed back and forth on navy assignments. He discovered over 100
submerged, flat-topped seamounts 3,000 to 6,000 feet under water between the Hawaiian
and Mariana islands. Hess described these seamounts as “drow ?VB?6?V?B?6??G>( ??@???VBF?V?wW??G2?F?????"&???BwW??B?vV???w?&?fW76?"B&??6WF?????????