Physical Geology 01C

J Bret Bennington

The Plate Tectonic Revolution II (seafloor spreading)

Updated 12/99

Seafloor Spreading

In 1962 an American geologist named Harry Hess published a paper in which he proposed a theory to explain all of the puzzling features being discovered about the seafloor. Hess’s proposal was admittedly speculative, Hess himself called in ‘geopoetry’, but it did suggest a single explanation for all of the features of the deep ocean, as well as for continental drift.

Hess suggested that instead of plowing through the seafloor, the continents were riding along with the seafloor which itself was moving. The key components of Hess’s theory of seafloor spreading:

1. New ocean floor is welling up as molten rock from the mantle below along the center of the mid-ocean ridges. As the ocean floor moves away from either side of the ridge, creating the central fissure, molten rock moves up from the mantle to replace it. This is why the ridges show high heat flow, and it is why they stand up as a ridge - the new, warm seafloor is less dense and floats high on the underlying mantle. As the seafloor moves away from the ridge system it cools and sinks.
2. Volcanoes that form along the ridge are originally close to the surface of the ocean where they can be eroded flat on top. Over millions of years they ride along with the seafloor and are carried into deeper and deeper water.
3. If the ocean floor is created at the mid-ocean ridges and moves along like a giant conveyer belt away from the ridges then it must go somewhere where it is eventually destroyed. Hess proposed that the deep ocean trenches are where the seafloor finally descends back into the mantle. The trenches themselves are a result of the downward bending of the seafloor as it begins its descent.
4. Finally, Hess reasoned that if the ocean floor is continually being remelted into the mantle and created anew at the ridges then it shouldn’t be surprising that the ocean floor is not very ancient. Furthermore, the reason that the mud and sediments deposited on the ocean floor thin toward the mid-ocean ridges is because the age of the seafloor gets younger and it has had less and less time to accumulate sediments.

When it came out in print, Hess’s work of geopoetry generated little widespread interest. However, for a few people it was just the explanation they were looking for the help them make sense of their own work.

One year later, the evidence that would eventually prove Hess right began coming in.

Magnetic polarity stripes on the ocean floor

Geologists know that when a rock cools from a molten state, the tiny particles of iron mineral in the rock align with the direction and polarity (N-S) of the Earth’s magnetic field. Geologists had also discovered that rocks of different ages have different polarities, apparently because the Earth’s magnetic field occasionally changes polarity from N-S to S-N and then eventually back again.

Two british geophysicists named Fred Vine and Drummond Matthews had been making magnetic maps of the seafloor along the Indian Ocean ridge. Vine and Matthews discovered that the volcanic rock of the seafloor is magnetized in stripes of opposite polarity. Furthermore, the pattern of stripes - their width and polarity - is symmetrical on opposite sides of the mid-ocean ridge.

Vine and Matthews realized that the best explanation for the pattern of magnetic stripes in the seafloor was Hess’s theory of seafloor spreading. As the seafloor was created at the ridge it picked up the current magnetic polarity of the Earth. As the seafloor spread, half of each stripe would be carried in one direction while half went in the other direction. If the magnetic polarity of the earth changed then a new stripe would be created between the previous stripes.

Slide: magnetic polarity transect

Slide: magnetization of new seafloor along spreading ridge (3 slide sequence)

Slide: symetrical pattern of magnetic polarity along mid-ocean ridge

Seafloor spreading appeared to be the only way to explain the pattern of magnetic polarity stripes observed by Vine and Matthews. However, there was an important test that would prove the connection between the stripes and seafloor spreading. Magnetic polarity reversals were well-studied and dated from continental rocks. The pattern of magnetic reversals going back in time forms a sort of bar code that can be correlated (matched-up) from place to place. The pattern of reversals from the sea floor appeared to match the pattern observed on the continents. If rock of the sea floor could be dated and if the age of the reversals matched the ages seen on the continents, then sea floor spreading would be confirmed. [slide]

DSDP

In 1968, a program was started to drill into the seafloor at many places around the world and retrieve samples of the sediments lying on top of the ocean basement as well as samples of the basement itself. A special ship was built and outfitted by the the National Science Foundation to be able to send drilling pipe thousands of feet down to and into the seafloor. The DSDP (Deep Sea Drilling Project) provided the evidence that clinched Hess’s theory.

Slide: Joides Resolution drilling ship

Slide: Drill pipe on the Glomar Challenger

Slide: Sediment drill core

Samples of the sediments from just above the basement of the seafloor could be age dated based on their fossils of tiny sea creatures. The igneous rock of the ocean floor itself was dated by radiometric methods. The DSDP proved beyond a doubt that the ocean floor did get younger toward the mid-ocean ridges and that the seafloor ages matched the predictions from the pattern of magnetic anomolies, confirming the Vine and Matthews hypothesis. Furthermore, as Hess predicted, nowhere is the seafloor extremely ancient. As of 1993, the oldest region of seafloor discovered, a portion near one of the Pacific trenches, dates back only about 180 million years. Considering that the Earth is 4.6 billion years old, this is not very old [slide].

Benioff Zones

Seismologists such as William Benioff also contributed to proving Hess’s theory by showing that the distribution of earthquakes around the world made sense if what Hess said was true. Earthquakes are concentrated along the mid-ocean ridges and trenches. The mid-ocean ridge earthquakes are all shallow, less than 60 km deep. Earthquakes along trenches are both shallow and deep. Furthermore, the earthquakes associated with trenches occur in bands, called Benioff zones, with the shallow earthquakes near the mouth of the trench and bands of deeper earthquakes occuring successively toward the interior of the continent near the trench. [slide]

Earthquakes at ridges are shallow because they are caused by the eruption of molten rock near the surface of the rift. The pattern of earthquakes at trenches makes sense if there is a slab of lithosphere sinking into the mantle at an angle. The shallow earthquakes occur near the top of the sinking slab as it moves, while the deeper earthquakes occur as the deeper parts of the slab sink into the mantle.

Epilogue

Hess rescued the idea of continental drift by giving it a plausible mechanism. Instead of the continents plowing through the ocean floor they are carried along with the seafloor as it moves from mid-ocean ridge to ocean trench.