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.

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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. 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. Geological Survey. 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.

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

What Is Sea Floor Spreading? Have you ever watched a cake baking in the oven? You know that moment when the top starts to cook and split when you can see that that gooey batter in the crack? It eventually cooks, too. Well, it turns out that the surface of the earth is also a little like that. The crusty surface of the cake is similar to the earth's tectonic plates, which are gigantic, slowly-moving pieces of the earth's crust.

The uncooked 'batter' between the plates would be the magma. When the magma cools, it becomes part of the earth's crust. Where the plates are moving away from each other, they are called divergent plates. The shifting divergent plates under the ocean are the reason for sea floor spreading. Spreading: How and Where? Just like the cracks on a baking cake, long ridges called mid-ocean ridges form at the point of sea floor spreading. Think of them as underwater mountain chains.

One of the more well-known mid-ocean ridges is the Mid-Atlantic Ridge. It splits the North American and Eurasian plates apart in the North, and the South American and African plates in the South. But there is a global mid-ocean ridge system which looks like a series of global zippers that the Mid-Atlantic Ridge is part of.

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Wherever the sea floor is spreading, the edges move apart while the magma in the middle hardens. That means the rock in the center is the youngest. As the continents move apart, whole new oceans can form. Scientists believe that's how the Red Sea came into existence. So you may be wondering: if the sea floor is spreading is some places, wouldn't the planet be getting bigger? It seems logical, but the answer is that other parts the planet are shrinking at the same time! There are also convergent plates, where one plate moves underneath another plate.

At those points in the ocean, called oceanic trenches, the earth is literally losing ground. Oceanic trenches tend to be along the margins of the continents, and most are on the margins of the Pacific Ocean. It's these areas that are most prone to earthquakes.

How Did People Learn This? Noticing that the shape of the continents would fit together like puzzle pieces is one thing.

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Figuring out how the continents would have moved around is another. First came the discovery of mid-ocean ridges in the 1950s. Before that, people assumed the land below the ocean was flat and unchanging, but it was the discovery of strange magnetic anomalies along the ridges that required an explanation. The magnetism in the rocks was not lining up as expected. Scientists found out that the earth's magnetic field actually shifts occasionally. Rocks that form at different times may have different magnetism. When it was discovered that the rock near mid-ocean ridges wasn't aligning in one direction, they theorized that the rock was forming at different times.

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As magma cools during sea floor spreading, it would take on the magnetism of the moment. Knowing the new rock was forming at different times helped explain sea floor spreading and the slow movement of continents.

Lesson Summary The earth's crust is not stagnant. Large plates, called tectonic plates, are on the move. Where they are pulling apart, they are called divergent plates. Where they are moving together, and one is going underneath the other, they are called convergent plates. Divergent plates underneath the ocean form mid-ocean ridges. As they split and magma fills the gap, it leads to sea floor spreading.

The largest mid-ocean ridges are in the Atlantic. Convergent plates that form oceanic trenches tend to be along the edges of continents in the Pacific.

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It was the discovery of mid-ocean ridges and magnetic anomalies in the rocks around them that helped explain sea floor spreading and the slow movement of the continents.