Triple Junctions in Plate Tectonics

A triple junction is the point where the boundaries of three tectonic plates meet. There are 25 unique types of triple junctions distributed across the world, defined by the types of boundaries. The motion of one lithospheric plate relative to an adjacent plate result in the deformation of plate boundaries.

Here, the triple junction plays an interesting role in the study of the current plate system. At any boundary between two plates, the plates are either spreading apart (forming mid oceanic ridges), pushing together (forming trenches, fold mountains, and volcanic systems), or sliding past each other (forming transform faults).

Historical Background

The term "Triple Junction" has been traditionally used to discuss only the intersection of three divergent boundaries or spreading ridges, ideally at 120° angles. First-generation researchers of triple junction assume that a point on one plate moves in a circular path relative to an adjacent plate over a finite number of times. In 1969, the first scientific research paper on the triple junction concept was published by W. Jason Morgan, Dan McKenzie, and Tanya Atwater. It stated that the triple junction geometry had been dynamically evolved over time, which invalidated the assumptions of first-generation researchers.

Stable and Unstable Triple Junctions

There is no instance of four or more plates meeting together in the world, which is most unlikely to occur. When three plates meet at the intersection, the plate boundaries bring together their own motions. On the basis of the nature and motion of the plate boundaries, in the triple junction, the interaction between two plate boundaries is of 3 types, namely − Ridge (R), Trench (T), and Fault (F). There are ten possible combinations of R, T, and F in a triple junction, out of which seven match with existing types and the rest three are unstable triple junctions.

As R-R-T and R-R-F could not maintain the same for a long time, they got converted into R-T-T and R-F-F respectively, making the parent combinations unstable triple junctions. Similarly, for a triple junction, an F-F-F combination of plate boundaries cannot co-exist. The following are the seven different combinations of stable triple junction:

Junction R-R-R (Ridge-Ridge-Ridge)

The junction of the Gulf of Aden, Red Sea, and East African Rift is centered at the Afar Triple Junction (the only R-R-R junction above sea level). The Galapagos Triple junction is formed at the conjunction of the Nazca plate, the Cocos plate, and the Pacific plate and thus leads to the formation of the Galapagos Islands. The Rodrigues RRR Triple Junction is formed in the Indian Ocean where the African, Indo-Australian, and Antarctic plates meet.

Junction T-T-T (Trench-Trench-Trench)

The Boso T-T-T Triple Junction is formed off the coast of Japan where the Okhotsk, Pacific, and Philippines Plates meet.

Junction T-T-F (Trench-Trench-Fault)

T-T-F Triple junction is formed off the coast of Chile where the Nazca, South American and Antarctica plates meet.

Junction T-T-R (Trench-Trench-Ridge)

Moresby Island is formed by the T-T-R triple junction of Juan de Fuca, Pacific and North American plates.

Junction R-T-F (Ridge-Trench-Fault)

This type of triple junction is found at the southern end of the Gulf of California.

Junction F-F-R (Fault-Fault-Ridge)

San Andreas fault is a notable example of this kind of triple junction.

Junction F-F-T (Fault-Fault-Trench)

The Mendocino Transform Fault is formed by the F-F-T Triple junction of the Pacific, North American, and the southern tip of Juan de Fuca plates.

Conclusion

Global plate-motion models or GPS/GNSS data help in deriving the real-time velocities of the plate-boundary points. This further helps characterize whether an individual boundary at a triple junction is divergent, convergent, transform or oblique.

Frequently Asked Questions

What is known as Triple Junction?

In addition to the interaction between two plate boundaries, there exists a conjunction of three plate boundaries at a point known as the Triple Junction.

What are Stable and Un-stable triple junctions?

In the triple junction, the interaction between two plate boundaries is of 3 types, namely: Ridge (R), Trench (T), and Fault (F). There are ten possible combinations of R, T, and F in a triple junction, out of which seven match with existing types and the rest three are unstable triple junctions. As R-R-T and R-R-F could not maintain the same for a long time, they got converted into R-T-T and R-F-F respectively, making the parent combinations unstable triple junctions. Similarly, for a triple junction, an F-F-F combination of plate boundaries cannot co-exist.

What are the examples of R-R-R triple junction?

The junction of the Gulf of Aden, Red Sea, and East African Rift is centered at the Afar Triple Junction (the only R-R-R junction above sea level). The Galapagos Triple junction is formed at the conjunction of the Nazca plate, the Cocos plate, and the Pacific plate and thus leads to the formation of the Galapagos Islands. The Rodrigues RRR Triple Junction is formed in the Indian Ocean where the African, Indo-Australian, and Antarctic plates meet.