The Theory of Seafloor Spreading
Seafloor spreading is a geologic process where there is a gradual addition of new oceanic crust in the ocean floor through a volcanic activity while moving the older rocks away from the mid-oceanic ridge. The mid-ocean ridge is where the seafloor spreading occurs, in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.
Seafloor spreading was proposed by an American geophysicist, Harry H. Hess in 1960. By the use of the sonar, Hess was able to map the ocean floor and discovered the mid-Atlantic ridge (mid-ocean ridge). He also found out that the temperature near to the mid-Atlantic ridge was warmer than the surface away from it. He believed that the high temperature was due to the magma that leaked out from the ridge. The Continental Drift Theory of Alfred Wegener in 1912 is supported by this hypothesis on the shift position of the earth’s surface.
The Process of Sea Floor Spreading
The mid-ocean ridge is the region where new oceanic crust is created. The oceanic crust is composed of rocks that move away from the ridge as new crust is being formed. The formation of the new crust is due to the rising of the molten material (magma) from the mantle by convection current. When the molten magma reaches the oceanic crust, it cools and pushes away the existing rocks from the ridge equally in both directions.
A younger oceanic crust is then formed, causing the spread of the ocean floor. The new rock is dense but not as dense as the old rock that moves away from the ridge. As the rock moves, further, it becomes colder and denser until it reaches an ocean trench or continues spreading.
It is believed that the successive movement of the rocks from the ridge progressively increases the ocean depth and have greater depths in the ocean trenches. Seafloor spreading leads to the renewal of the ocean floor in every 200 million years, a period of time for building a mid-ocean ridge, moving away across the ocean and subduction into a trench.
The Subduction Process
The highly dense oceanic crust that is formed after a progressive spreading is destined to two possible occurrences. It can either be subducted into the ocean deep trench or continue to spread across the ocean until it reaches a coast.
Subduction is the slanting and downward movement of the edge of a crustal plate into the mantle beneath another plate. It occurs when an incredibly dense ocean crust meets a deep ocean trench. On the other hand, if the ocean crusts continuous to move along the ocean and not found a trench, no subduction will occur. It will continue to spread until a coast is found and literally pushing it away towards its direction.
Two possible things could happen in the subduction of ocean crust. Once the subduction occurs, a melting happens due to a tremendous friction. The ocean crust is then melted into magma. The magma could either go back into the mantle for another convection currents leading again to another sea floor spreading or it could burst through a crack in a continental crust and creates a volcano.
Subduction and sea-floor spreading are processes that could alter the size and form of the ocean. For instance, the Atlantic Ocean is believed to be expanding because of its few trenches. Due to this, continuous Seafloor spreading occurs and makes Atlantic Ocean floor to be connected to other continental crust making the ocean gets wider over the time.
On the other hand, the Pacific Ocean has more trenches that lead to more subduction of ocean crusts rather than the formation of the mid-ocean ridge. The Pacific Ocean is believed to be continuing to shrink.
Evidence of Sea Floor Spreading
Harry Hess’s hypothesis about seafloor spreading had collected several pieces of evidence to support the theory. This evidence was from the investigations of the molten material, seafloor drilling, radiometric age dating and fossil ages, and the magnetic stripes. This evidence however was also used to support the Theory of Continental drift.
1. Molten material
Hess’s discovery on the warmer temperature near the mid-Atlantic ridge when he began the ocean mapping, led to his evidence about the molten material underneath the ocean. The condition on the mid-oceanic ridge was substantially different from other surfaces away from the region because of the warmer temperature. He described that the molten magma from the mantle arose due to the convection currents in the interior of the earth.
The convection current was due to the radioactive energy from the earth’s core that makes the materials in the lower mantle to become warm, less dense and rise. The flow of the materials goes through the upper mantle and leaks through the plates of the crust. This makes the temperature near the mid-oceanic ridge becomes warm and the other surface to become cold because as the molten magma continues to push upward, it moves the rocks away from the ridge.
2. Seafloor drill
The seafloor drilling system led to the evidence that supports the seafloor-spreading hypothesis. The samples obtained from the seafloor drill reveals that the rocks away from the mid-oceanic ridge were relatively older than the rocks near to it. The old rocks were also denser and thicker compared to the thinner and less dense rocks in the mid-oceanic ridge.
This means that the magma that leaks from the ridge pushes the old rocks away and as they increasingly become distant, they more likely become older, denser, and thicker. On the other hand, the newest, thinnest crust is located near the center of the mid-ocean ridge, the actual site of seafloor spreading.
3. Radiometric age dating and fossil ages
By the use of radiometric age dating and studying fossil ages, it was also found out the rocks of the sea floor age is younger than the continental rocks. It is believed that continental rocks formed 3 billion years ago, however the sediments samples from the ocean floor are found to be not exceeding 200 million years old. It is a clear evidence that the formation of rocks in the sea floor is due to reabsorption of materials.
4. Magnetic stripes
In the 20th century, the magnetic survey was conducted in the Mid-ocean ridge in order to investigate evidence of sea-floor spreading. By using the magnetometer, the magnetic polarity will be shown through a timescale that contains the normal and a reverse polarity. The minerals contained in the rocks are oriented opposite to the magnetic field. The patterns of the magnetic field will then be compared to the rocks to determine its approximate ages.
The investigation of the mid-ocean-ridge, using the magnetic stripes resulted in the three discoveries. First, stripes of normal and reversed polarity were alternate across the bottom of the ocean. Second, the alternate stripes of normal and reversed polarity formed a mirror image to the other side of the ridge. The third is the abrupt ending of stripes when it reached the edge of the continent or an ocean trench. It was concluded that the sea floor is composed of different rocks according to ages and that they are positioned equally in opposite directions. This records that there is a constant movement and spreading of rocks on the ocean floor.
References: britannica , National Geographic
Photo by: flickr
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