# Plate Tectonics Defined: Triple Junction

In the field of plate tectonics, a triple junction is a name given to a place where three tectonic plates meet. There are roughly 50 plates on Earth with about 100 triple junctions among them. At any boundary between two plates, they are either spreading apart (making mid-ocean ridges at spreading centers), pushing together (making deep-sea trenches at subduction zones) or sliding sideways (making transform faults). When three plates meet, the boundaries are also bringing together their own motions at the intersection.

For convenience, geologists use the notation R (ridge), T (trench) and F (fault) to define triple junctions. For example, a triple junction known as an RRR could exist when all three plates are moving apart. There are several on Earth today. Likewise, a triple junction called TTT could exist with all three plates pushing together, if they're lined up just right. One of these is located underneath Japan. An all-transform triple junction (FFF), though, is physically impossible. An RTF triple junction is possible if the plates are lined up correctly. But most triple junctions combine two trenches or two faults -- in that case, they are known as RFF, TFF, TTF, and RTT.

### The History of Triple Junctions

In 1969, the first research paper that detailed this concept was published by W. Jason Morgan, Dan McKenzie, and Tanya Atwater. Today, the science of triple junctions is taught in geology classrooms across the globe.

### Stable Triple Junctions and Unstable Triple Junctions

Triple junctions with two ridges (RRT, RRF) cannot exist for more than an instant, splitting into two RTT or RFF triple junctions as they are unstable and do not stay the same over time. An RRR junction is deemed a stable triple junction as it maintains its form as time goes on. That makes ten possible combinations of R, T, and F; and of them, seven matches the existing types of triple junctions and three are unstable.

The seven types of stable triple junctions and some notable locations of them include the following:

• RRR: These are located in the South Atlantic, the Indian Ocean, and west of the Galapagos Islands in the Pacific. The Afar Triple Junction is where the Red Sea, Gulf of Aden and East African Rift meet. It is the only RRR triple junction that is higher than sea level.
• TTT: This type of triple junction is found in central Japan. The Boso Triple Junction off the coast there is where the Okhotsk, Pacific and Phillippine Sea plates meet.
• TTF: There is one of these triple junctions off the coast of Chile.
• TTR: This type of triple junction is located on Moresby Island, western North America.
• FFR, FFT: The triple junction type is found at the San Andreas Fault and the Mendocino Transform Fault in the Western U.S.
• RTF: This type of triple junction is found at the southern end of the Gulf of California.