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  • 17.12.2018
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Historical Geology/Sea floor spreading - Wikibooks, open books for an open world

Plate Tectonics: Seafloor Spreading, Subduction and Megathrust Earthquakes

Seafloor spreading , theory that oceanic crust forms along submarine mountain zones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. This idea played a pivotal role in the development of plate tectonics , a theory that revolutionized geologic thought during the last quarter of the 20th century. Shortly after the conclusion of World War II , sonar -equipped vessels crisscrossed the oceans collecting ocean-depth profiles of the seafloor beneath them. The survey data was used to create three-dimensional relief maps of the ocean floor, and, by , American oceanic cartographer Marie Tharp had created the first of several maps that revealed the presence of an underwater mountain range more than 16, km 10, miles long in the Atlantic—the Mid-Atlantic Ridge. The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in As the magma cools, it is pushed away from the flanks of the ridges.

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 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' or aulacogen.

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. 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. Seafloor 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.

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.

Seafloor spreading and dating methods

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.

Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial high is found within the crustal accretion zone. 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. At the Mid-Atlantic Ridge and in other mid-ocean ridgesmaterial 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.

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 . Seafloor spreading, theory that oceanic crust forms along submarine a variety of methods—including coring, dredging, and deep-sea drilling—date only to the. Scientists can determine the age of the seafloor thanks to the changing The figure shows the spreading ridge about 5 million years ago (a), about 2 to 3 get an absolute age of the seafloor, scientists use the radioactive dating technique.

When Alfred Wegener first presented a hypothesis of continental drift inhe suggested that continents plowed 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.

The Age of the Ocean Floor

At first the driving force for spreading was argued to be convection currents in the mantle. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along, or slab pull.

The magmatism at the ridge is considered to be passive upwelling, which is caused by the plates being pulled apart under the weight of their own slabs.

However, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones. In this case the plate are sliding apart over the mantle upwelling in the process of ridge push.

The depth of the seafloor or the height of a location on a mid-ocean ridge above a base-level is closely correlated with its age age of the lithosphere where depth is measured. The age-depth relation can be modeled by the cooling of a lithosphere plate [18] [19] or mantle half-space in areas without significant subduction.

Seafloor spreading

In the half-space model, [20] the seabed height is determined by the oceanic lithosphere temperature, due to thermal expansion. The simple result is that the ridge height or ocean depth is proportional to the square root of its age.

Finally, we assume the system is at a quasi- steady stateso that the temperature distribution is constant in time, i. Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t i.

In fact, the dating of the ice and seafloor sediment cores is a gigantic .. large role in the development of the idea of seafloor spreading (Daintith ). Despite the popular perception that radioisotope dating methods yield. and specifically catastrophic plate tectonics (CPT).1–3 There has even been the . radioisotope dating methods, and the timescale itself, some of which will be. Paleomagnetism is the study of the record of the Earth's magnetic field in rocks, sediment, Paleomagnetism continues to extend the history of plate tectonics back in from the study of sea floor spreading zones and the dating of volcanic rocks. . Paleomagnetic studies are combined with geochronological methods to .

Both coefficients are related by:. From Wikipedia, the free encyclopedia.

Obviously, the ocean floor had a story to tell, but what? the ocean floor, new techniques for determining the geologic ages of rocks ("dating") one of the clinching arguments in support of the seafloor spreading hypothesis. For the same reason, if the sea floor is spreading out from the rifts, their dating methods to the basalt sea-floor on either side of a rift, the. Seafloor spreading is a geologic process where there is a gradual addition of of the molten material, seafloor drilling, radiometric age dating and fossil ages.

A process at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.

Unsourced material may be challenged and removed. November Because of this correlation between age and subduction potential, very little ocean floor is older than million years and almost none of it is older than million years.

Therefore, seafloor dating isn't that useful for studying plate motions beyond the Cretaceous. For that, geologists date and study continental crust.

The lone outlier the bright splash of purple that you see north of Africa to all of this is the Mediterranean Sea. At million years, it still pales in comparison to the four-billion-year-old rock that can be found on the continental crust.

The ocean floor is a mysterious place that marine geologists and oceanographers have struggled to fully grasp. In fact, scientists have mapped more of the surface of the Moon, Mars, and Venus than the surface of our ocean. You may have heard this fact before, and while true, there is a logical explanation as to why.

Seafloor mapping, in its earliest, most primitive form, consisted of lowering weighted lines and measuring how far the sunk. This was done mostly to determine near-shore hazards for navigation. The development of sonar in the early 20th century allowed scientists to get a clearer picture of seafloor topography.

It didn't provide dates or chemical analyses of the ocean floor, but it did uncover long oceanic ridges, steep canyons and many other landforms that are indicators of plate tectonics. Every so often it has occurred over times over the past million yearsthe poles will suddenly switch.

Paleomagnetism

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