Seafloor mirror-image on opposite sides of the

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.

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Seafloor Spreading: 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, at the time of Wegener, there was no credible explanation for a way the continents could have plowed 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 Mid-Atlantic ridge is an ocean ridge found along the Atlantic Ocean floor. The ridge, he thought, was where new seafloor was being added to the earth’s lithosphere, which in turn pushed the continents apart. Hess called it seafloor spreading.

The Hess Theory

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 basalt and igneous rock, 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?

Seafloor Spreading: Evidence in the Rocks

Not long after Hess published his ideas, other scientists published their measurements of the magnetic properties of Atlantic Ocean seafloor basalt or the seafloor magnetism. 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 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 was used to determine ages of seafloor basalt. Radiometric age dating is a technique scientists use to determine how long ago materials such as rock were formed. 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.It was determined that the seafloor basalt is youngest at mid-ocean ridges and oldest adjacent to continents.

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. This theory, introduced by Harry Hess, was proven as patterns of magnetic field polarity preserved in seafloor basalt and by age dating of the rocks. This, along with the dating of the rocks through the use of radiometric age dating, provided an explanation for how continents on either side of the Atlantic Ocean were drifting apart, thus validating Alfred Wegener‘s 1912 hypothesis that the continents were once joined together.

Learning Outcomes

Since you’ve finished the lesson, try and do the following:

  • State what the major weakness in Alfred Wegener’s hypothesis was initially
  • Recall Hess’s theory of seafloor spreading
  • Discuss the use of radiometric age dating
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