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
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges. At a spreading center basaltic magma rises up the fractures and cools on the ocean floor to form new seabed. Hydrothermal vents are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Additionally spreading rates determine if the ridge is a fast, intermediate, or slow. As a general rule, fast ridges see spreading rate of more than 9 cm/year. Intermediate ridges have a spreading rate of 4–9 cm/year while slow spreading ridges have a rate less than 4 cm/year.
Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift postulated that continents "ploughed" through the sea. The idea that the seafloor itself moves (and also carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is well accepted now, and the phenomenon is known to be caused by convection currents in the asthenosphere, which is ductile, or plastic, and the brittle lithosphere (crust and upper mantle).[clarification needed]
In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named triple junctions and can be found in several places across the world today. The separated margins of the continents evolve to form passive margins. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.
If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region last September,[when?] a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and eustasy. As a result, the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists.
Sea floor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.
Continued spreading and subduction
As new seafloor forms and spreads apart from the mid-ocean ridge it slowly cools over time. Older seafloor is therefore colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to isostasy. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centres (the East Pacific Rise) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow-spreading centre, while the East Pacific Rise is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.
Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.
Debate and search for mechanism
At the Mid-Atlantic Ridge (and in other areas), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented a hypothesis of continental drift in 1912, he suggested that continents ploughed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States.
Since then, it has been shown that the motion of the continents is linked to seafloor spreading. In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the Earth's great fiery welts, like the Mid-Atlantic Ridge and the East Pacific Rise.
The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along. The magmatism at the ridge is considered to be "passive upswelling", which is caused by the plates being pulled apart under the weight of their own slabs. This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it.
Sea floor global topography: half-space model
To first approximation, sea floor global topography in areas without significant subduction can be estimated by the half-space model. In this model, the seabed height is determined by the oceanic lithosphere temperature, due to thermal expansion. Oceanic lithosphere is continuously formed at a constant rate at the mid-ocean ridges. The source of the lithosphere has a half-plane shape (x = 0, z < 0) and a constant temperature T1. Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v, which is assumed large compared to other typical scales in the problem. The temperature at the upper boundary of the lithosphere (z=0) is a constant T0 = 0. Thus at x = 0 the temperature is the Heaviside step function. Finally, we assume the system is at a quasi-steady state, so that the temperature distribution is constant in time, i.e. T=T(x,z).
By calculating in the frame of reference of the moving lithosphere (velocity v), which have spatial coordinate x' = x-vt, we may write T = T(x',z,t) and use the heat equation: where is the thermal diffusivity of the mantle lithosphere.
Since T depends on x' and t only through the combination , we have:
We now use the assumption that is large compared to other scales in the problem; we therefore neglect the last term in the equation, and get a 1-dimensional diffusion equation: with the initial conditions .
The solution for is given by the error function:
Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t) can be calculated by integrating the thermal expansion over z:
where is the effective volumetric thermal expansion coefficient, and h0 is the mid-ocean ridge height (compared to some reference).
Note that the assumption the v is relatively large is equivalently to the assumption that the thermal diffusivity is small compared to , where L is the ocean width (from mid-ocean ridges to continental shelf) and T is its age.
The effective thermal expansion coefficient is different from the usual thermal expansion coefficient due to isostasic effect of the change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by:
where is the rock density and is the density of water.
By substituting the parameters by their rough estimates: m2/s, °C−1 and T1 ~1220 °C (for the Atlantic and Indian oceans) or ~1120 °C (for the eastern Pacific), we have:
for the eastern Pacific Ocean, and:
for the Atlantic and the Indian Ocean, where the height is in meters and time is in millions of years. To get the dependence on x, one must substitute t = x/v ~ Tx/L, where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and T is the ocean age.
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