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Revisión del 18:09 6 may 2010

La expansión de los fondos oceánicos o expansión del suelo marino ocurre en las dorsales oceánicas, donde se forma nueva corteza oceánica mediante actividad volcánica y el movimiento gradual del fondo alejándose de la dorsal. Este hecho ayuda a entender la deriva continental explicada por la teoría de la tectónica de placas.

Teorías anteriores (por ejemplo, la de Alfred Wegener) sobre la deriva continental suponían que los continentes eran transportados a través del mar. La idea de que el propio suelo marino se mueve (y arrastra a los continentes con él) mientras se expande desde un eje central fue propuesta por Harry Hess de la Universidad de Princeton en los 1960s. La teoría se acepta ampliamente en la actualidad, y se cree que el fenómeno es causado por corrientes de convección en la parte débil y plástica de la capa superior del manto (denominada astenosfera en la definición clásica). En la corteza oceánica y en la corteza continental existen diferentes formas de relieve.En los continentes destacan por su altitud,las principales cadenas montañosas o cordilleras

Expansión incipiente

Por lo general, la expansión del suelo marino comienza como un rift en una placa continental, similar al Valle del Rift existente en la actualidad en África Oriental, entre el Mar Rojo y los Grandes Lagos Africanos. El proceso comienza con un calentamiento en la base de la placa continental que la convierte en un material más plástico y menos denso. Ya que los objetos densos "flotan", levantándose sobre los más densos, como prevé la isostasia, el área que se calienta se abomba, transformándose en una amplia bóveda ("domo" o cúpula). A medida que se alza, se producen fracturas que gradualmente se convierten en rifts. El sistema de rifts típico consiste en tres brazos divergentes, separados cada uno por 120º de circunferencia. Estas áreas se denominan "encrucijada triple" (triple junction) y pueden hallarse en varios lugares del mundo en la actualidad, siendo fácilmente identificables en los mapas; por ejemplo: Mar Rojo-Golfo de Adén-Valle del Rift, o Valle del Níger-costas africanas del Golfo de Guinea (donde se abrió el Atlántico, separándose América del Sur).

Enlaces externos

El nivel de los océanos en el Cretácico era 170 metros superior al actual Artículo publicado en El Mundo, 07/03/2008, sobre una investigación publicada en la revista Science. Incluye una simulación del envejecimiento progresivo de las cuencas oceánicas y los cambios en la distribución de océanos y continentes desde hace 140 millones de años.
Otras simulaciones (en inglés).

@@ Línea 7: / Línea 7: @@
 == Expansión incipiente  ==
+Por lo general, la expansión del suelo marino comienza como un [[rift]] en una placa continental, similar al [[Valle del Rift]] existente en la actualidad en [[África Oriental]], entre el [[Mar Rojo]] y los [[Grandes Lagos Africanos]]. El proceso comienza con un calentamiento en la base de la placa continental que la convierte en un material más plástico y menos denso. Ya que los objetos densos "flotan", levantándose sobre los más densos, como prevé la [[isostasia]], el área que se calienta se abomba, transformándose en una amplia bóveda ("domo" o cúpula). A medida que se alza, se producen fracturas que gradualmente se convierten en rifts. El sistema de rifts típico consiste en tres brazos divergentes, separados cada uno por 120º de circunferencia. Estas áreas se denominan "[[encrucijada triple]]" (''triple junction'') y pueden hallarse en varios lugares del mundo en la actualidad, siendo fácilmente identificables en los mapas; por ejemplo: [[Mar Rojo]]-[[Golfo de Adén]]-Valle del Rift, o [[Río Níger|Valle del Níger]]-costas africanas del [[Golfo de Guinea]] (donde se abrió el [[Atlántico]], separándose [[América del Sur]]).
-Por lo to 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 Africa rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the
+<!--
+==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 East Africa 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)|Afar region]] last September, 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 at any time in the process, but if it continues far enough 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==
 The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins.  Since 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 (EPR)) with spreading rates of up to 13 cm/yr.  The Mid-Atlantic Ridge is a "[[wiktionary:textbook|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  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.  It is likely however that some seafloor spreading is driven by active upwelling and some by passive upwelling.
+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 other places), material from the upper [[Mantle (geology)|mantle]] rises through the faults between oceanic plates to form new [[Crust (geology)|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, conservative  geologists, especially in North America, demanded to know where the motive force could possibly lie. Historically, the theory of seafloor spreading won nearly complete acceptance in the absence of evidence for the mechanism. Instead, the past record of [[geomagnetic reversal]]s 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]].
+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.  It is likely however that some seafloor spreading is driven by active upwelling and some by passive upwelling.
 == See also ==