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Seafloor Spreading: Theory, Definition & Quiz

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Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Seafloor spreading is a part of plate tectonics. Its discovery provided a mechanism for continental drift that Alfred Wegener could not explain. In this lesson, you will learn about this important geologic process.

We also recommend watching What is Pangaea? - Theory, Definition & Quiz and Sea Floor Spreading and Polar Reversal

A Mystery Solved

In 1912 when Alfred Wegener proposed that the continents had once been joined together and had split apart, the biggest weakness in his hypothesis was the lack of a mechanism that would allow continents to move through ocean basins. At the time, everyone believed the oceans were permanent features, and there was no credible way for the continents to plow through the rocks of the seafloor.

But in 1962, a geologist and U.S. Navy Reserve Rear Admiral named Harry Hess came up with an answer. Rather than plowing through seafloor rocks, Hess proposed that it was the seafloor itself that was pushing the continents apart. He believed that the location and topography of the Mid-Atlantic Ridge was not coincidence. The ridge, he thought, was where new seafloor was being added to the earth's lithosphere, which in turn pushed the continents apart. He called it seafloor spreading.

The Hess Hypothesis

Harry Hess proposed that new seafloor crust was continually formed at mid-ocean ridges. Source: NASA.
mid-ocean ridge

Hess argued that the Mid-Atlantic Ridge was a boundary where two lithospheric plates were rifting (being pulled apart). As that happened, rising magma from the upper part of the mantle filled in the cracks that formed in the earth's crust.

After the magma solidified into the igneous rock basalt, additional rifting pulled those rocks apart, too. In effect, Hess proposed the existence of a magma-driven conveyor belt that continually added new seafloor, very slowly over time, widening the Atlantic Ocean basin and pushing apart the continents to either side.

So, rather than plowing through seafloor rocks, Hess proposed that it was the seafloor itself that was pushing the continents apart. It was an insightful hypothesis, but was there any evidence to confirm Hess's idea? Or would he suffer the same criticisms that Wegener had endured?

Evidence in the Rocks

Seafloor Magnetism

Not long after Hess published his ideas, other scientists published their measurements of the magnetic properties of Atlantic Ocean seafloor basalt. They had discovered an unexpected pattern preserved in the rocks.

As new seafloor basalt is added over time, it records the pattern of reversals in the polarity of the magnetic field. Source: U.S. Geological Survey.
seafloor magnetic pattern

When igneous rocks - like basalt - crystallize, the iron atoms in them align with the magnetic field of the earth. Geologists were aware that the north-south magnetic polarity of the earth's magnetic field had reversed on occasion. But in the seafloor basalt, the researchers found a pattern of repeated magnetic field reversals preserved in bands of basalt running parallel to the axis of the Mid-Atlantic Ridge.

More important to Hess's hypothesis, the pattern repeated in a mirror-image on opposite sides of the ridge. The only possible explanation was that new basalt rocks were constantly forming and moving away from the ridge in opposite directions, preserving in them the polarity changes of the magnetic field.

No Old Seafloor

Seafloor basalt is youngest at mid-ocean ridges (dark red) and oldest adjacent to continents (dark blue). Map source: NOAA.
seafloor age

Additional confirmation of Hess's mechanism came later as radiometric age dating techniques were used to determine ages of seafloor basalt. The seafloor rocks on the Mid-Atlantic Ridge were only a few million years old, while those closest to the continents were about 200 million years old.

Seafloor spreading had been proved. Harry Hess was right. And Alfred Wegener was vindicated

Lesson Summary

Seafloor spreading is the mechanism by which new seafloor lithosphere is constantly being created at mid-ocean ridges. It was proven by patterns of magnetic field polarity preserved in seafloor basalt and by age dating of the rocks. It provided an explanation for how continents on either side of the Atlantic Ocean were drifting apart, thus validating Alfred Wegener's 1912 hypothesis.

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