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
Seafloor Spreading
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.
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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.
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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.
Magnetic Fields: Evidence of Seafloor Spreading
The oceanic crust that makes up the ocean floor is mainly igneous rock basalt. Basalt forms when molten material cools at or near the earth's surface, and it contains a fair amount of iron. Before it has hardened into rock, iron mineral crystals within basalt can move and align with the magnetic north pole. Once the molten rock has set, the particles become fixed in place like tiny magnets providing a permanent record of the magnetic alignment of the earth at that time. Scientists have discovered that the magnetic poles of the earth have reversed many times throughout its history.
Magnetic reversals are also recorded in the rock making up the oceanic crust, and the pattern they create is evidence of seafloor spreading. The rock samples collected by scientists from various locations show a couple of things. First, the earth's magnetic poles have indeed reversed, and second, on either side of a mid-ocean ridge, the magnetic field pattern reversals are precisely the same. The diagram shows these findings in an over-simplified representation. It is helpful to keep the following in mind to understand the diagram better:
- The diagram shows seafloor spreading over three progressive periods with the interval labeled 'a' being the oldest and 'c' the youngest.
- The newest crust in each interval is at the center
- The center represents a divergent plate boundary through which molten material is pushing through.
- The dark/orange colored stripes show magnetically aligned rock in one direction, and the white stripes show magnetic alignment in the opposite or reversed direction.
- Note that the magnetic reversal pattern on one side of the boundary is exactly the same as the other.
This characteristic is scientific evidence that rock on either side of the center ridge formed simultaneously and got pushed away as new material moved up and in.
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Age of the Seafloor: Evidence for Seafloor Spreading
Scientists collected various samples of crustal rock and the ocean sediments above it and used radiometric dating to determine their ages. Radiometric dating gives an accurate age reading on material that is thousands to millions of years old.
Scientists discovered that as they moved away from the mid-ocean ridge, the samples of both the crustal rock and sediments on the ocean floor increased in age. The newest, youngest rock was closest to the mid-ocean ridge. This evidence supported the theory of seafloor spreading. The diagram showing the magnetic stripes shows how newly formed rock is gradually pushed away from the ridge, making way for even newer rock.
Additionally, they found that although the earth is 4.6 billion years old, the oceanic crust is less than 200 million years old.
Lesson Summary
The theory of seafloor spreading explains that molten material moves in at the boundaries between tectonic plates pushing them apart as new oceanic crust is created. This process is called seafloor spreadingand it occurs at divergent boundaries. Its discovery played a big part in developing the theory of plate tectonics which explains how continents move and how the crust is created and destroyed. The heat from the earth's core is the source of energy that powers the movement of the molten material and tectonic plates above it.
Seafloor spreading was discovered by Harry Hess who used maps created by sonar readings of the ocean floor. He noticed long chains of underwater mountains with the rift valley at their centers (now known as mid-ocean ridges). He proposed that new seafloor was being added at the ridges causing the ocean floor to spread apart on either side of the ridge. Scientific evidence for seafloor spreading supports this theory. For example, the age of the seafloor closest to the ridge is the youngest, and it progressively gets older as the distance from the ridge increases. Analysis of the magnetic alignment of minerals in rock on either side of the ridge showed the exact pattern of reversals. A century ago, Alfred Wegener had proposed that all continents were once one giant landmass that somehow separated and moved, but he couldn't explain how or why. Seafloor spreading explained the how and why and is now a foundational part of the theory of plate tectonics.
The Hess Theory
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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.
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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
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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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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
|
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.
|
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
|
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
To unlock this lesson you must be a Study.com Member.
Create your account
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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