Triple junction

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A triple junction is the point where the boundaries of three tectonic plates 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 volcanic Cameroon line.

Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar Triangle (shaded, center) -- a triple junction where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone (USGS).

The junction of the Red Sea, the Gulf of Aden and the East African Rift centered in the Afar Triangle is an example of a triple junction (the Afar Triple Junction). This is the only Ridge-Ridge-Ridge triple junction above sea level. Another example of a triple junction is the junction between the Arabian Plate , the African Plate , and the Indo-Australian Plate.

Another active example is the Galapagos Triple Junction, an R-R-R triple junction where the Nazca, 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.

The Nootka Fault at the triple junction of the North American Plate, the Explorer Plate, and the Juan de Fuca

Further north on the west coast of North America another unstable triple junction is to be found offshore of Cape Mendocino. There the 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. To the north lies the Cascadia subduction zone, where the section of the Juan de Fuca Plate called the Gorda Plate is being subducted under the margin of the North American Plate at a plate boundary called a trench (T). Another transform fault runs along the boundary between the Pacific Plate and the Gorda Plate called the Mendocino Fault (F). Where the three intersect is a seismically active F-F-T triple junction called the Mendocino Triple Junction.

The North Sea is located at an old triple junction of three former continental plates of the Palaeozoic era: 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.