Triple Junctions

Tectonic plates include the crust and the uppermost mantle above the asthenosphere.  The asthenosphere is a very weak layer, but it is not liquid because it transmits earthquake S-waves.  It lies 80-200 km below the surface, much thicker than the thickness of the crust.

At most three plates can come together at a point; this is called a triple junction.  The type of triple junction is named for the nature of the three plate boundaries, like ridge-ridge-ridge.    The boundaries are abbreviated T (trench), ridge (R), and F (transform fault, but T was already taken for trench).   Not all of the 27 possible combinations occur at present, and some of those are geometrically impossible because the motions that are required cannot be sustained, such as an F-F-F boundary.

Not all geometries are stable, and in most cases the triple junction will migrate with respect to at least some of the plates.

Last revision 9/29/2015