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Seafloor Spreading

heidi Kent, Charles Spencer
  • Author
    heidi Kent

    Heidi has taught middle school science, health, and English for more than 22 years. She has a Master's Degree in General Science from North Dakota State University. She is a member of the MSTA, has chaired Professional Development and Continuing Education at the Ashby Public School.

  • Instructor
    Charles Spencer

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

Learn what seafloor spreading is, who discovered it, and how it explains the movement of continents and plates. Learn about the evidence for seafloor spreading and analyze diagrams that illustrate it. Updated: 11/19/2021

What is the Seafloor Spreading Theory?

What is seafloor spreading theory? When looking at a map of the Atlantic Ocean, it looks like the continents could fit together like a puzzle. Is it possible that at one time they were fit together? What could have caused these large landmasses to move? The theory of seafloor spreading provides the answer to how continents and the crust beneath them move.

The earth's crust is broken into large slabs called tectonic plates. The plates are in slow continual movement. The seafloor spreading definition is the geologic process that occurs at the boundary between 2 plates where molten material within the earth pushes its way up, causing the plates to move away from each other. At these divergent boundaries molten material cools and hardens, creating new oceanic crust or seafloor. New crust is being made even today, but the earth's size is not increasing because elsewhere at certain convergent boundaries plates collide. When one plate is pushed beneath another, it will melt and become molten material again.

The discovery and evidence of seafloor spreading were utilized by scientists as they developed the theory of plate tectonics. Plate tectonics explains the movement, creation, and destruction of the earth's crust and is one of the central unifying theories of earth science.

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.

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Who Discovered Seafloor Spreading?

The discovery of seafloor spreading is credited to Harry Hess, who first proposed the idea in the early 1960s. During World War II, Hess had been an admiral in the Navy, traveling aboard vessels that used sonar to locate German U-boats. Since he was also a geologist, he was naturally interested in ocean floor topography as well. As he traveled across the oceans, he took constant sonar readings of the ocean floor: these readings and others were taken after the war and were turned into topographic maps. As Hess studied the maps, he noticed many interesting features, particularly the vast flat plains on the ocean floor that were interrupted by long, tall chains of mountains often divided by a deep rift valley in the middle. Hess proposed that new crust was added to the ocean floor at these ridges (now known as mid-ocean ridges), causing it to widen and push apart in what he called seafloor spreading.

The topography map of the Atlantic Ocean shows the mid-ocean ridge running through the middle and following the curve of the continents.

Note that the chain of mountains down the middle which is where Hess proposed that new material was being added to the ocean floor causing seafloor spreading.

Topography of the Atlantic Ocean

Another piece of evidence Hess took into account as he considered seafloor spreading was that ocean floor sediments were far younger than expected based on the age of the earth. His idea that a new crust was being formed in the process of seafloor spreading could explain that as well.

Hess's idea about seafloor spreading also revived interest in Continental Drift, an idea proposed by Alfred Wegener 40 years prior. Wegener believed that at one time, all the continents of earth were joined together in one colossal landmass he called Pangea. He had a great deal of evidence to support this hypothesis, but his ideas were disregarded because he could not explain how or why the continents split apart and moved. The discovery of the process of seafloor spreading provided the explanation Wegener had been missing. Today, his idea that all continents were once joined and then split apart and moved is accepted as scientific fact.

Seafloor Spreading Diagram

The force that causes seafloor spreading and the moving of continents and tectonic plates originates at the earth's extremely hot, dense core. This heat energy transfers out through the layers within the earth. At a certain level within the upper mantle, the heat causes the material to melt and become molten. The hotter material rises, and the cooler material sinks in cycles called convection currents which move the tectonic plates above. If rising molten material occurs at a plate boundary, it can force its way up and push the plates away from each other. As it cools and hardens, it creates a new crust.

At a divergent boundary, molten material pushes up between 2 plates causing them to move away from each other. New crust is added when the molten material cools and hardens.

Seafloor Spreading Diagram

The diagram is a simplified representation showing molten material from the mantle pushing its way up at a divergent boundary creating mountains on either side. The layer labeled oceanic crust is made of cooled magma. The arrows show the movement of the two plates away from each other as the newly formed crust pushes through. As new crust is continually added, it pushes older crust farther and farther from the ridge center. The mid-Atlantic Ridge shown in the Atlantic Ocean topographic map formed this way also.

Evidence of Seafloor Spreading

Scientists have collected and studied a great deal of data since Hess first proposed his idea of seafloor spreading. They have studied the ages of the rock making up the oceanic crust and the sediment lying on top of it; they have examined the magnetic orientation of iron crystals in the rocks at various locations. With new technology, they have acquired photographic evidence of newly forming seafloor crust as it happens. All of their findings are evidence that supports the theory of 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 magnetic pattern

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Video Transcript

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 magnetic pattern

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Frequently Asked Questions

Who proposed the theory of seafloor spreading?

Harry Hess proposed the idea of seafloor spreading in 1960. He used maps created by solar readings of the ocean floor. He explained that seafloor spreading caused these features as new crust was made. Seafloor spreading became a theory as more scientific evidence continued to support it.

How is seafloor spreading explained and proven?

Seafloor spreading is explained and proven by evidence. Under the oceans lie long mountain chains or ridges. The ocean floor crust on either side of a mid-ocean ridge shows the youngest rock closest to the ridge and the oldest rock farther from it. Also, the pattern of magnetic reversals on either side of the ridge is precisely the same as each other.

What are the main points of the seafloor spreading theory?

The main points of seafloor spreading theory include the idea that molten material moves in at divergent plate boundaries. This movement creates a new crust and pushes the plates apart.

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