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A 'triple junction' is the point where the boundaries between three tectonic plates, and three plate margins, meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault and triple junctions can be described according to the types of plate margin that meet at them. Of the many possible types of triple junction only a few are stable through time.
The triple junction concept was developed in 1968 by W. Jason Morgan, Dan McKenzie, and Tanya Atwater. The term has traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles. In plate tectonics theory during the breakup of a continent, one of the divergent plate boundaries would fail (see aulacogen) and the other two would continue spreading to form an ocean. The opening of the south Atlantic Ocean began with a triple junction in the present Gulf of Guinea. The ''failed arm'' of this junction is occupied by the rift system under the Niger Delta area and the Cameroon volcanic line.

The junction of the Red Sea, the Gulf of Aden and the Great Rift Valley of East Africa centered in the Afar Triangle is often given as an example of a triple junction. This is the only Ridge-Ridge-Ridge triple junction above sealevel.
Another active example is the R-R-R triple junction where the Nasca, the Cocos, and the Pacific Plates meet. The East Pacific Rise extends north and south from this junction and the Galapagos Rise goes to the east. This example is made more complex by the Galapagos Microplate which is a small separate plate on the rise just to the southeast of the triple junction.
Further north on the west coast of North America another unstable triple junction is to be found at Cape Mendocino. There the familiar San Andreas fault, a strike-slip fault and transform plate boundary, approaches from the south. The San Andreas fault separates the Pacific plate and the North American plate (Transform Fault = F). To the north lies the Cascadia subduction zone, where the Juan de Fuca plate and the last bit of its lower section, sometimes called the Gorda plate, are being subducted under the margin of the North American plate, defining another plate boundary (trench = T). And west, along the continental shelf, lies the Mendocino fault zone, the transform fault boundary (F) between the main Juan de Fuca plate and the Pacific plate. Where the three come together, a F-F-T triple junction - and another geologically complex zone - results.
The North Sea is located at a triple junction of three continental plates formed during the Palaeozoic: Avalonia, Laurentia and Baltica.
The properties of triple junctions are most easily understood from the purely kinematic point of view where the plates are rigid and moving over the surface of the Earth. No knowledge of the Earth's interior or the geological details of the crust are then needed. Another useful simplification is that the kinematics of triple junctions on a flat Earth are essentially the same as those on the surface of a sphere: despite describing plate motions as involving relative rotations about poles and plate motions on a flat surface being defined by vectors. The relative motions at the triple junction are the same.

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References

★ Oreskes, Naomi, ed., 2003, ''Plate Tectonics: an Insider's History of the Modern Theory of the Earth'', Westview Press, ISBN 0-8133-4132-9

See also

★ Seafloor spreading