Seafloor Spreading, Part 1

The oceans and seas cover three-quarters of the Earth's surface, hiding much of our world from our view. If we could remove all the water and look at the topography of the ocean floor we would behold an extraordinary sight. The floor of the Atlantic Ocean is typical of the amazing topography that is typical of the Earth's seafloor. Down the middle of the Atlantic, starting in the north at Iceland and roughly paralleling the coasts of North America and Europe and South America and Africa is a long scar that resembles a sinuous zipper. This scar is part of the world's longest mountain range, nearly 50,000 km in length, 800 km in width in most places and averaging 4,500 meters in height. This is the Mid-Atlantic Ridge, one part of a globe-spanning mountain range. From the Mid-Atlantic Ridge new land is formed on a daily basis, slowly oozing out from the depths of the Earth and pushing the Old World and the New Worlds further apart by 1 to 6 centimeters every year.

The processes that take place along the Mid-Atlantic Ridge are well studied and understood by today's geologists. The ridge is one part of the dynamic system on the Earth that geologists refer to as plate tectonics. Today we have a very good idea of how the plates move along the surface of the Earth, slipping past each other in some places, growing in others, and colliding together to form the tallest peaks and deadly volcanoes and earthquakes. However, as little as 41 years ago the mechanism that the mid-oceanic ridges are a part of was only understood by a few geologists who were denounced by the majority of the geologists in the world. The concept that the Mid-Atlantic Ridge represents is referred to as seafloor spreading and was championed by a Princeton University geologist, Harry Hess (1906-1969).

Admiral Hess (he served in the Pacific in World War 2 and reached the rank of Rear Admiral in the Naval Reserve) met resistance with his hypothesis of seafloor spreading. Hess's idea was presented in the 1962 book History of Ocean Basins, describing how magma oozes from the Earth's surface along oceanic ridges. The new seafloor spreads away from the ridges eventually sinking into the deep ocean trenches found across the globe. Most geologists at the time resisted Hess's hypothesis, mostly due to the lack of strong evidence. Hess was undaunted and in the next few years he and other geologists were able to prove nearly all the points in his hypothesis.

The Keys to Seafloor Spreading
There are four key pieces of evidence that helped to prove the theory of seafloor spreading. Each of these keys was documented and studied by independent researchers, some prior to Hess's book being published, and each was critical in helping to prove the theory.

Age of the Ocean Floors
Throughout most of the science of geology geologists thought the Earth was static, the continents having remained as they are seen today from the beginning of time. In this view, the ocean floor is just as old as the rest of the rock on the planet. In the years after World War 2 geologists used piston-coring devices to sample areas of the ocean floor. Many geologists were puzzled since the oldest rocks found in the cores were Cretaceous in age. It was thought that the ocean sediments were very thick, having accumulated over millions of years, and that the piston corers were unable to sample the deeper sediments. In the late 1960s the research vessel Glomar Challenger began to take deep core samples across the ocean basins. Cores taken from the ocean floors by Glomar Challenger have shown that the oldest rocks recovered in either the Atlantic or Pacific Oceans is Late Jurassic in age. Another piece of evidence was that rocks nearer the mid-oceanic ridges are younger than rocks found further from the ridges. The first cornerstone for the seafloor spreading theory had been laid.

Magnetic Anomalies
Research vessels based around the magnetometer, a device designed to locate submarines by the deflection of the magnetic field the submarine causes, began to ply the waters of the Atlantic and Pacific in the 1950's. These ships noticed the presence of north-south trending magnetic anomalies in the ocean. The anomalies were areas with a higher magnetic intensity than the average Earth magnetic field that alternated with areas with a lower magnetic intensity. It was found that these magnetic anomalies were symmetrical, centered along the mid-oceanic ridges. For years these magnetic anomalies puzzled scientists when two researchers proposed a reason for their presence.

In 1963 Fred Vine and DH Matthews proposed that the magnetic anomalies were the result of reversals in the Earth's magnetic field. Studies of magnetic reversal had been shown for continental rocks, now Vine and Matthews proposed that the positive anomalies showed time when the Earth had a normal polarity and the negative anomalies were the result of time when the Earth's magnetic field was reversed. Vine and Matthews said the alternating anomalies were the result of volcanism and the symmetry seen would only result if the volcanism occurred at the mid-oceanic ridges and that the seafloor was moving when the Earth's magnetic field switched between normal and reversed polarity and back again. When the magnetic stripes of the ocean floor are compared with the time of the Earth's period of normal and reversed polarity the two show a great degree of similarity. Today, the Vine-Matthews explanation for the magnetic anomalies is one of the cornerstones of the seafloor spreading theory.

The age of the ocean floors and the discovery and interpretation of the magnetic anomalies around the mid-oceanic ridges were the stepping-stones for other discoveries that together became the theory of seafloor spreading. Next time we'll explore the remaining two keys that led to Hess's theory of seafloor spreading.

References and Sources:
Kious, W. Jacquelyne and Tilling, Robert I.; This Dynamic Earth: The Story of Plate Tectonics, 1996.

Seyfert, Carl and Sirkin, Leslie; Earth History and Plate Tectonics: an Introduction to Historical Geology; 1973, Harper & Row Publishers.

Winchester, Simon; Krakatoa: The Day the World Exploded: August 27, 1883; 2003 Harper Collins.

Atlantic seafloor map courtesy of National Geographic Maps at www.nationalgeographic.com.

Seafloor spreading image is reprinted from the online edition of This Dynamic Earth