Harry Hess and Sea Floor Spreading

the mid-ocean ridge is in between the Americas and Europe/Africa During World War II, Captain Harry Hess commanded the Navy supply vessel USS Cape Johnson as it made transport runs back and forth across the Pacific Ocean. Armed with a new technology called SONAR, the USS Cape Johnson could send out sound waves and measure the time it took for the waves to reflect off of the seafloor and return to the ship. These signals could then be interpreted to determine the depth of the ocean floor below the ship.

During peacetime, Hess was a geologist who taught at Princeton University. The consensus among geologists of the time was that the ocean floor was flat and smooth (like a swimming pool) between the continents, but Hess was able to make profiles of the Pacific basin as he traveled back and forth to The Marianas, the Philippines, and Iwo Jima. What he discovered below the surface was not a flat bottom, but a floor disrupted by mountain ranges, underwater plateaus, and crevasses.

After the war, Hess returned to Princeton and began puzzling over his discoveries. After scientists mapped the mid-ocean ridge in the Atlantic in 1953, Hess was able to put together a theory that explained one of the biggest gaps in Wegener's theory of continental drift - how the continents were able to move across the surface of the Earth.

Hess proposed that the mid-ocean ridges were actually tremendous cracks in the Earth's surface where magma was rising from below. As this magma cooled, it expanded, slowly pushing sections of the crust apart. The continents weren't pushing through the crust, as Wegener had thought, but were in fact resting on top of these large sections of crust, like a person sitting on a raft. As the sections of crust were pushed apart, the continents on top were passively moved along with them. His theory, published in 1962, provided a scientifically sound explanation as to how the continents were able to move across the surface of the Earth.

Hess's theory also explained why the fossils found in ocean sedimentary rock were never more than 200 million years old. He postulated that as the crust was pushed away from the cracks, it cooled, became denser, and sank back to the mantle. The oldest crust would have subducted and melted long ago, destroying any fossils that were contained in it.

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