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Triple Junction

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Definition: A triple junction is 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). A meeting of three plates, then, is also a meeting of three boundaries, each with its own motion.

For convenience, geologists use the notation R (ridge), T (trench) and F (fault) for cataloging triple junctions. Thus a triple junction called RRR could exist at which 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 under 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—that is, RFF, TFF, TTF, RTT. Triple junctions with two ridges (RRT, RRF) cannot exist for more than an instant, splitting into two RTT or RFF triple junctions—that is, they are unstable. That makes ten possible combinations of R, T and F, of which seven match existing types of triple junction and three are unstable.

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