Age of oceanic crust. Youngest crust(red) is along spreading centres.

Plates in the crust of the earth, according to the plate tectonics theory

Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Plate tectonics and hotspots Divergent plate boundaries At the Seafloor spreading helps explain continental drift in the theory of plate tectonics. Continental drift is the movement of the Earth 's Continents relative to each other Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere

Earlier theories (e. g. , by Alfred Wegener) of continental drift were that continents "plowed" through the sea. Alfred Lothar Wegener ( November 1, 1880 – November 2 or 3 1930 was a German Scientist and Meteorologist. A continent is one of several large Landmasses on Earth. They are generally identified by Convention rather than any strict criteria with seven regions The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. Harry Hammond Hess ( May 24, 1906 – August 25, 1969) was a Geologist and United States Navy officer in World War Princeton University is a private Coeducational research university located in Princeton, New Jersey. The 1960s decade refers to the years from the beginning of 1960 to the end of 1969 The theory is well-accepted now, and the phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere. Convection in the most general terms refers to the movement of molecules within Fluids (i The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the

Incipient spreading

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. In Geology, a rift is a place where the Earth 's crust and Lithosphere are being pulled apart and is an example of Extensional tectonics Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere The Great Rift Valley is a name given in the late 19th century by English explorer John Walter Gregory to the continuous geographic trough approximately in length that runs The process starts with 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 more dense objects, the area being heated becomes a broad dome (see isostasy). Isostasy (Greek isos = "equal" stásis = "standstill" is a term used in Geology to refer to the state of gravitational equilibrium between the 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. A triple junction is the point where the boundaries of three Tectonic plates meet The separated margins of the continents evolve to form passive margins. A continent is one of several large Landmasses on Earth. They are generally identified by Convention rather than any strict criteria with seven regions A Passive margin is the transition between oceanic and Continental crust which is not an active plate margin

Early spreading

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 Red Sea is a Salt water Inlet of the Indian Ocean between Africa and Asia. 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, a 60 km fissure opened as wide as eight meters. NOTE This intro is the result of careful NPOV work Please do not make potentially controversial edits to it without first discussing on the talk page Afar is one of the nine ethnic divisions ( Kililoch) of Ethiopia, and is the homeland of the Afar people. 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. Petroleum ( L petroleum, from Greek πετρέλαιον, lit

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 Delta, the delta of the Niger River in Nigeria, is a densely populated region sometimes called the Oil Rivers because it was once a The Niger River has formed in the failed rift arm of the triple junction.

Spreading at a mid-ocean ridge

Continued spreading and subduction

The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. 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. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other 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 (EPR)) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow spreading centre while the EPR 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. The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland Sea

Debate and search for mechanism

At the Mid-Atlantic Ridge (and other places), 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. The Mid-Atlantic Ridge (MAR is a Mid-ocean ridge, a Divergent tectonic plate boundary located along the floor of the Atlantic Ocean. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided In Geology, a crust is the outermost solid shell of a planet or moon Continental drift is the movement of the Earth 's Continents relative to each other When Alfred Wegener first presented a hypothesis of continental drift in 1912, conservative geologists, especially in North America, demanded to know where the motive force could possibly lie. Alfred Lothar Wegener ( November 1, 1880 – November 2 or 3 1930 was a German Scientist and Meteorologist. Wegener suggested that the continents ploughed through the ocean crust. 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. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged 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 East Pacific Rise is a Mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean.

It is still a matter of some debate whether seafloor spreading is driven primarily by the force of rising magma at these locations, or if it is driven by the force of sinking oceanic crust at subduction zones and these upwellings are merely a side effect. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other It is likely however that some seafloor spreading is driven by active upwelling and some by passive upwelling.

See also

• Divergent boundary
• Mid-ocean ridge
• Morley-Vine-Matthews hypothesis

External links

• Animation of a mid-ocean ridge