Triple junction

In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction).

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.

Triple junctions may be described and their stability assessed without use of the geological details but simply by defining the properties of the ridges, trenches and transform faults involved, making some simplifying assumptions and applying simple velocity calculations.

This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real Earth.

Further conditions must also be met for the triple junction to exist stably – the plates must move in a way that leaves their individual geometries unchanged.

When these are drawn onto the diagram containing the velocity triangle these lines must be able to meet at a single point, for the triple junction to exist stably.

These junctions were classified firstly by the types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc.

An RRR junction is always stable using these definitions and therefore very common on Earth, though in a geological sense ridge spreading is usually discontinued in one direction leaving a failed rift zone.

[7] The Guadeloupe and Farallon microplates were previously being subducted under the North American plate and the northern end of this boundary met the San Andreas Fault.

Material for this subduction was provided by a ridge equivalent to the modern East Pacific Rise slightly displaced to the west of the trench.

The loss of slab pull caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system.

Main tectonic plate boundaries – ridge (red), trench (green), fault (black) – and corresponding triple junctions (yellow dots)
A map of the Afar triangle, showing the East of Africa and the three ridges passing through the Red Sea, the Gulf of Aden and the East African Rift Valley.
A map of the Afar triangle in East Africa, an example of an RRR junction and the only triple junction on Earth that can be seen above sea level.
The Nootka Fault at the triple junction of the North American plate , the Explorer plate , and the Juan de Fuca plate