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63-569: The Project FAMOUS study area was located on a section of the Mid-Atlantic Ridge about 700 kilometers (380 nautical miles) west of the Azores ( Sao Miguel ) at 36° 50’ north latitude. It includes a 30–32 km (16–17 nmi)-wide median valley or rift valley on the crest of the Mid-Atlantic Ridge that trends slightly east of north. Within the median valley lies the present boundary between

126-405: A base or reference level h b {\displaystyle h_{b}} , the depth of the ocean d ( t ) {\displaystyle d(t)} is of interest. Because d ( t ) + h ( t ) = h b {\displaystyle d(t)+h(t)=h_{b}} (with h b {\displaystyle h_{b}} measured from

189-471: A few meters wide were found. These mark the transform faults between adjacent spreading centers and rift valleys. Because the fracture zones are up to 10 km wide in places, this observation indicates the shear zone or transform fault, migrates over time within the fracture zone itself. The bounding fracture zones, A and B are not orthogonal or perpendicular to the rift valley as is expected for transform faults and spreading ridge connections. This has led to

252-510: A general rule, fast ridges have spreading (opening) rates of more than 90 mm/year. Intermediate ridges have a spreading rate of 40–90 mm/year while slow spreading ridges have a rate less than 40 mm/year. The highest known rate was over 200 mm/yr during the Miocene on the East Pacific Rise . In the 1960s, the past record of geomagnetic reversals of Earth's magnetic field

315-513: A new experimental approach to sea floor geology and was considered a major technical achievement at the time. The demonstration of the viability of sea floor observations by submersibles made possible the subsequent discoveries of hydrothermal vents at the Galapagos spreading center and on the East Pacific Rise at 21° N. The project succeeded in defining the morphology and structure of the spreading center or median rift valley along with locating

378-489: Is a slow-spreading center, while the East Pacific Rise is an example of fast spreading. 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. 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

441-468: Is constant in time, i.e. T = T ( x , z ) . {\displaystyle T=T(x,z).} By calculating in the frame of reference of the moving lithosphere (velocity v ), which has spatial coordinate x ′ = x − v t , {\displaystyle x'=x-vt,} T = T ( x ′ , z , t ) . {\displaystyle T=T(x',z,t).} and

504-510: Is small compared to L 2 / A {\displaystyle L^{2}/A} , where L is the ocean width (from mid-ocean ridges to continental shelf ) and A is the age of the ocean basin. The effective thermal expansion coefficient α e f f {\displaystyle \alpha _{\mathrm {eff} }} is different from the usual thermal expansion coefficient α {\displaystyle \alpha } due to isostasic effect of

567-496: Is tectonic plate slab pull at subduction zones , rather than magma pressure, although there is typically significant magma activity at spreading ridges. Plates that are not subducting are driven by gravity sliding off the elevated mid-ocean ridges a process called ridge push . 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

630-480: Is the density of water. By substituting the parameters by their rough estimates: gives: 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 ~ Ax / L , where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and A is the ocean basin age. Rather than height of the ocean floor h ( t ) {\displaystyle h(t)} above

693-489: The Afar region , September 2005, 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

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756-570: The Bouvet triple junction in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level, for example in Iceland . The ridge has an average spreading rate of about 2.5 centimetres (1 in) per year. A ridge under the northern Atlantic Ocean was first inferred by Matthew Fontaine Maury in 1853, based on soundings by

819-600: The Mississippi River , Amazon River and Niger River ). The Fundy Basin on the Atlantic coast of North America between New Brunswick and Nova Scotia in Canada is evidence of the ancestral Mid-Atlantic Ridge. Seafloor spreading Seafloor spreading , or seafloor spread , is a process that occurs at mid-ocean ridges , where new oceanic crust is formed through volcanic activity and then gradually moves away from

882-681: The Tjörnes fracture zone connects Iceland to the Kolbeinsey Ridge . The ridge sits atop a geologic feature known as the Mid-Atlantic Rise , which is a progressive bulge that runs the length of the Atlantic Ocean, with the ridge resting on the highest point of this linear bulge. This bulge is thought to be caused by upward convective forces in the asthenosphere pushing the oceanic crust and lithosphere . This divergent boundary first formed in

945-510: The Triassic period, when a series of three-armed grabens coalesced on the supercontinent Pangaea to form the ridge. Usually, only two arms of any given three-armed graben become part of a divergent plate boundary. The failed arms are called aulacogens , and the aulacogens of the Mid-Atlantic Ridge eventually became many of the large river valleys seen along the Americas and Africa (including

1008-476: The USS Dolphin . The existence of the ridge and its extension into the South Atlantic was confirmed during the expedition of HMS Challenger in 1872. A team of scientists on board, led by Charles Wyville Thomson , discovered a large rise in the middle of the Atlantic while investigating the future location for a transatlantic telegraph cable . The existence of such a ridge was confirmed by sonar in 1925 and

1071-711: The equator , the Mid-Atlantic Ridge is divided into the North Atlantic Ridge and the South Atlantic Ridge by the Romanche Trench , a narrow submarine trench with a maximum depth of 7,758 m (25,453 ft), one of the deepest locations of the Atlantic Ocean. This trench, however, is not regarded as the boundary between the North and South American plates, nor the Eurasian and African plates. The islands on or near

1134-424: The heat equation is: where κ {\displaystyle \kappa } is the thermal diffusivity of the mantle lithosphere. Since T depends on x' and t only through the combination x = x ′ + v t , {\displaystyle x=x'+vt,} : Thus: It is assumed that v {\displaystyle v} is large compared to other scales in

1197-503: The 1960s, Canadian scientists had begun a detailed study of the Mid-Atlantic Ridge at a latitude of 45° N that included multiple expeditions by surface ships. With Project FAMOUS located on the ridge in more clement latitudes around 37° N, a coordinated multi-national, multi-ship series of more than twenty expeditions took place over four years, between 1971 and 1974. Bilateral briefing meetings were held as new expeditions were completed. The unique operational features of Project FAMOUS included

1260-427: The 1960s. The phenomenon is known today as plate tectonics . In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading. 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

1323-660: 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 centers (the East Pacific Rise) with spreading rates of up to 145 ± 4 mm/yr between the Pacific and Nazca plates . The Mid-Atlantic Ridge

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1386-543: The Mid-Atlantic Ridge, from north to south, with their respective highest peaks and location, are: Northern Hemisphere (North Atlantic Ridge) : Southern Hemisphere (South Atlantic Ridge) : The submarine section of the Mid-Atlantic Ridge close to southwest Iceland is known as the Reykjanes Ridge . The Mid-Atlantic Ridge runs through Iceland where the ridge is also known as the Neovolcanic Zone . In northern Iceland

1449-523: The Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones, except the minor subduction in the Lesser Antilles and Scotia Arc . In this case the plates 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

1512-486: The North American and African tectonic plates. The floor of the rift valley is 2,400–2,500 m (7,900–8,200 ft) deep and 1 to 3 km (0.54 to 1.62 nmi) wide and the bounding rift mountains are at a depth of about 1,300 m (4,300 ft), or about 1 km (3,300 ft) above the floor. The rift valley is 40 km (22 nmi) long and it is offset to the eastward in the north at Fracture Zone A; in

1575-487: The North Pacific): Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages as young as 20 million years: Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to

1638-456: The United States. At first the driving force for spreading was argued to be convection currents in the mantle. Since then, it has been shown that the motion of the continents is linked to seafloor spreading by the theory of plate tectonics, which is driven by convection that includes the crust itself as well. The driver for seafloor spreading in plates with active margins is the weight of

1701-539: The approach using instruments with increasing detailed resolution, the submersible divers were trained to recognize the volcanic terrain they could encounter through prior field exercises undertaken in Iceland and Hawaii . The pressure hull of the ALVIN submersible was also specially upgraded to allow it to reach the great depths of the rift valley. In total, forty-four dives with the three submersibles were completed over dive seasons in 1973 and 1974. Project FAMOUS represented

1764-411: 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

1827-533: The change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by: where ρ ∼ 3.3   g ⋅ c m − 3 {\displaystyle \rho \sim 3.3\ \mathrm {g} \cdot \mathrm {cm} ^{-3}} is the rock density and ρ 0 = 1   g ⋅ c m − 3 {\displaystyle \rho _{0}=1\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}

1890-403: 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. 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. However,

1953-411: The crests of mid-ocean ridges. Spreading centers end in transform faults or in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone a few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary – a line within the crustal accretion zone demarcating

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2016-417: 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

2079-433: The distance of that reversal from the spreading center, the spreading half-rate could be computed. In some locations spreading rates have been found to be asymmetric; the half rates differ on each side of the ridge crest by about five percent. This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading center. Seafloor spreading occurs at spreading centers, distributed along

2142-452: 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 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

2205-769: The floor of the Atlantic Ocean , and part of the longest mountain range in the world . In the North Atlantic, the ridge separates the North American from the Eurasian plate and the African plate , north and south of the Azores triple junction . In the South Atlantic, it separates the African and South American plates. The ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to

2268-411: The hypothetical supercontinent of Pangaea that began some 180 million years ago. The Mid-Atlantic Ridge includes a deep rift valley that runs along the axis of the ridge for nearly its entire length. This rift marks the actual boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava and producing new crustal material for the plates. Near

2331-428: The inner valley floor away from these hills indicating accretion is not taking place beyond the hills. Conceptual models suggest that volcanism within the valley floor is cyclic or episodic, with volcanic activity recurring every 5,000 to 10,000 years. The observed continuous background seismicity infers that faulting is continual and ongoing. In the fracture zones explored by dives and deep towed instruments, shear zones

2394-409: The lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate or mantle half-space in areas without significant subduction . In the mantle half-space model, the seabed height is determined by the oceanic lithosphere and mantle temperature, due to thermal expansion. The simple result is that the ridge height or ocean depth is proportional to

2457-426: The notion that spreading here is oblique to the trend of the rift valley. However. the near-bottom and on-bottom observations find that the narrow shear zones are in fact at right angles to the rift valley trend as would be required by plate tectonics. The observation of pervasive faulting and fracturing of the crust indicated the spreading center was under tension; thus revealing that the driving force for plate motion

2520-454: The ocean surface): The depth predicted by the square root of seafloor age derived above is too deep for seafloor older than 80 million years. Depth is better explained by a cooling lithosphere plate model rather than the cooling mantle half-space. The plate has a constant temperature at its base and spreading edge. Analysis of depth versus age and depth versus square root of age data allowed Parsons and Sclater to estimate model parameters (for

2583-510: 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 . At the Mid-Atlantic Ridge (and in other mid-ocean ridges), material from the upper mantle rises through

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2646-472: 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

2709-450: The problem. The temperature at the upper boundary of the lithosphere ( z = 0) is a constant T 0 = 0. Thus at x = 0 the temperature is the Heaviside step function T 1 ⋅ Θ ( − z ) {\displaystyle T_{1}\cdot \Theta (-z)} . The system is assumed to be at a quasi- steady state , so that the temperature distribution

2772-452: The problem; therefore the last term in the equation is neglected, giving a 1-dimensional diffusion equation: with the initial conditions The solution for z ≤ 0 {\displaystyle z\leq 0} 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

2835-451: The ridge is part of a 40,000-km (25,000 mile) long essentially continuous system of mid-ocean ridges on the floors of all the Earth's oceans. The discovery of this worldwide ridge system led to the theory of seafloor spreading and general acceptance of Alfred Wegener 's theory of continental drift and expansion in the modified form of plate tectonics . The ridge is central to the breakup of

2898-500: The ridge. Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "plowed" through the fixed and immovable seafloor. The idea that the seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S. Naval Electronics Laboratory in San Diego in

2961-415: The rift mountains; a mostly level terrace of varied width below these walls; inner walls to the valley floor that are also normal faults, and the relatively narrow median valley or rift valley floor at the deepest point. The heights of the rift mountains diminish away from the median valley by additional systems of faults that decrease rather than increase relief. Deeply towed geophysical instruments explored

3024-586: The rift valley floor where most of the dives took place. These efforts observed the zone of crustal accretion aligned along the center of the valley floor. In the FAMOUS area valley floor the accretion zone is marked by several low and elongate volcanic hills about 100–250 m high and 1–2 km long. These are bordered by a fissured terrane where the crust is cracked. Divers observed that these hills are constructed mainly of pillow lavas that are without sediment cover, indicating they are new or young. Sediment covers most of

3087-693: The rift valley of the Mid-Atlantic Ridge. The Woods Hole Oceanographic Institution (WHOI) in Massachusetts provided surface ships and the submersible ALVIN ; the French provided surface ships and the bathyscaph Archimède and submersible CYANA. The British conducted side scan sonar surveys and on-bottom seismic experiments. Lead institutions were WHOI and the French Centre Oceanologique de Bretagne, Brest , France. Project leaders were James Heirtzler, Claude Riffaud, and Xavier Le Pichon . In

3150-405: 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

3213-491: 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 more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in

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3276-725: The south, it is offset westward at Fracture Zone B. A significant obstacle in marine surveys was the use of echo sounders with a wide transmit beam, which smeared-out details of the sea floor features. The crustal accretion or creation process was thought to take place over a few kilometers width of sea floor, which was below the resolution of ship echo sounders. Thus, near-bottom and on-bottom approaches were employed along with new sonar mapping tools. Investigations included airborne magnetics, advanced surface ship sonar, and geophysical measurements, seismology , deep-towed instruments, large format bottom photography, fixed on-bottom instruments, and on-bottom dives with research manned submersibles in

3339-430: The spreading zone while younger rocks will be found nearer to the spreading zone. Spreading rate is the rate at which an ocean basin widens due to seafloor spreading. (The rate at which new oceanic lithosphere is added to each tectonic plate on either side of a mid-ocean ridge is the spreading half-rate and is equal to half of the spreading rate). Spreading rates determine if the ridge is fast, intermediate, or slow. As

3402-408: The square root of its age. 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 T 1 . 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

3465-436: The thermal expansion over z : where α e f f {\displaystyle \alpha _{\mathrm {eff} }} is the effective volumetric thermal expansion coefficient, and h 0 is the mid-ocean ridge height (compared to some reference). The assumption that v is relatively large is equivalent to the assumption that the thermal diffusivity κ {\displaystyle \kappa }

3528-517: The two separating plates. Within the crustal accretion zone is a 1–2 km-wide neovolcanic zone where active volcanism occurs. In the general case, seafloor 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,

3591-445: The use of newly developed narrow-beam and multibeam echo sounders along with deeply-towed instruments and manned submersibles to achieve a new, higher level of resolution of a spreading center. Key to this approach was improved ship navigation with transit satellites to allow detailed mapping, in an era before GPS . This was augmented by use of acoustic on-bottom transponder navigation of ships, instruments, and submersibles. Besides

3654-443: The valley floor as might be expected with the most simple idea of the process. Instead, the computed rate is 7 mm/year to the west and 13.4 mm/year to the east. The higher resolution surveys were able to establish that the median valley is formed by faulting and not volcanism. In the FAMOUS area the median valley displays four provinces: the outer walls of the valley, which are normal faults with vertical movements that border

3717-407: The zone of crustal accretion in the median valley floor. On the large scale, sonar mapping and deep-tow instrument surveys found that the median valley is asymmetric in shape with the rift mountains on the west about 11 km from the deepest part of the valley floor, and those on the east about 20 km from it. This finding indicated that seafloor spreading here is not the same on either side of

3780-461: Was a pulling apart of the plates rather than a pushing apart from the mantle below. These observations; of the architecture of the median valley, of the fracture zone transform faults, and of the crustal accretion zone, mark the first ground-truth data of plate boundaries for a slow rate spreading center. Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary ) located along

3843-554: Was found to extend around Cape Agulhas into the Indian Ocean by the German Meteor expedition . In the 1950s, mapping of the Earth's ocean floors by Marie Tharp , Bruce Heezen , Maurice Ewing , and others revealed that the Mid-Atlantic Ridge had a strange bathymetry of valleys and ridges, with its central valley being seismologically active and the epicenter of many earthquakes . Ewing, Heezen and Tharp discovered that

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3906-417: Was noticed by observing magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with a magnetometer towed 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. By identifying a reversal with a known age and measuring

3969-483: 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 and upper mantle lithosphere begins to form between

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