Overlapping spreading centers are a feature of spreading centers at

mid-ocean ridges A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverg ...

Appearance

In the normal case adjacent spreading centers not on the same trend, end at a

transform fault A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subduct ...

that appears to offset each center. At an overlapping spreading center the adjacent centers overlap and are not connected by a transform fault. In plan view, overlapping spreading centers look like two people about to shake hands but over-reaching by a distance that is three times the distance between the hands. This 3:1 overlap to offset ratio (observed at a wide range of scales, from hundreds of meters to tens of km) holds the key to their mechanism of formation. In fracture mechanics, when two cracks propagate toward each other, they tend to deflect away from each other, propagate past each other, and then stall out when the ratio is 3:1 because at this point the crack propagation force drops to zero. What results are two spreading centers overlapping, apparently pushing against each other, which would violate one of the rules of

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

: plates are rigid. This paradox is solved by the fact that the overlapping spreading centers are non-steady-state and propagate along the ridge; as one segment lengthens, the neighboring segment shortens. Which segment lengthens, is arbitrary; evidence shows that at times the spreading center tips have alternated migrating directions. The discovery of overlapping spreading centers revealed the time-varying behavior of mid-ocean ridge segments, with implications for magmatism, volcanism and patterns of

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

activity.

Occurrence

Overlapping spreading centers are found mostly on mid-ocean ridge spreading centers in the Pacific Ocean basin where the rates of separation between the plates are higher than in other ocean basins. They occur on all fast-spreading rate centers (>90 mm/yr) including the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located alon ...

, and on most intermediate-rate spreading centers (50–90 mm/yr), such as the Galapagos Spreading Center, the

Juan de Fuca Ridge The Juan de Fuca Ridge is a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the east ...

and the

Pacific-Antarctic Ridge The Pacific-Antarctic Ridge (PAR) is a divergent tectonic plate boundary located on the seafloor of the South Pacific Ocean, separating the Pacific Plate from the Antarctic Plate. It is regarded as the southern section of the East Pacific Rise ...

References

{{Reflist Oceanic ridges Plate tectonics