Seafloor Spreading: Theory & Definition

Instructor: 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.

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.

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