Editing Triple junction (section)

==Types==
McKenzie and Morgan determined that there were 16 types of triple junction theoretically possible, though several of these are speculative and have not necessarily been seen on Earth. These junctions were classified firstly by the types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and secondly by the relative motion directions of the plates involved. Some configurations such as RRR can only have one set of relative motions whereas TTT junctions may be classified into TTT(a) and TTT(b). These differences in motion direction affect the stability criteria.

McKenzie and Morgan claimed that of these 16 types, 14 were stable with FFF and RRF configurations unstable, however, York<ref>{{cite journal | last=York | first=Derek | title=Evolution of Triple Junctions | journal=Nature | volume=244 | issue=5415 | year=1973 | issn=0028-0836 | doi=10.1038/244341a0 | pages=341–342| bibcode=1973Natur.244..341Y | s2cid=4202607 }}</ref> later showed that the RRF configuration could be stable under certain conditions.

===Ridge–ridge–ridge junctions===
[[Image:Tectonical map of East Africa.png|thumb|alt=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 Depression|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.]]

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 [[aulacogen|failed rift zone]]. There are many examples of these present both now and in the geological past such as the South Atlantic opening with ridges spreading North and South to form the [[Mid-Atlantic Ridge]], and an associated [[aulacogen]], the [[Benue Trough]], in the [[Niger Delta]] region of Africa. RRR junctions are also common as rifting along three fractures at 120° is the best way to relieve stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are believed to be caused by the mantle [[Hotspot (geology)|hotspots]] thought to initiate rifting in continents.

The stability of RRR junctions is demonstrated below – as the perpendicular bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and ca can always be made to meet regardless of relative velocities.

===Ridge–trench–fault junctions===
RTF junctions are less common, an unstable junction of this type (an RTF(a)) is thought to have existed at roughly 12[[Mya (unit)|Ma]] at the mouth of the [[Gulf of California]] where the [[East Pacific Rise]] currently meets the [[San Andreas Fault]] zone.<ref>{{cite web |url=http://www.orfeus-eu.org/Announcements/workshop_utrecht/presentations/zhang.pdf |title=Archived copy |access-date=2009-11-21 |url-status=dead |archive-url=https://web.archive.org/web/20110727153513/http://www.orfeus-eu.org/Announcements/workshop_utrecht/presentations/zhang.pdf |archive-date=2011-07-27 }}</ref> 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. As the ridge itself was subducted an RTF triple junction momentarily existed but subduction of the ridge caused the subducted lithosphere to weaken and 'tear' from the point of the triple junction. The loss of [[slab pull]] caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system. An RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are colinear.

===Trench–trench–trench junctions===
A TTT(a) junction can be found in central Japan where the [[Eurasian plate]] overrides the [[Philippine Plate|Philippine]] and [[Pacific plate]]s, with the Philippine plate also overriding the Pacific. Here the [[Japan Trench]] effectively branches to form the Ryukyu and Bonin [[Volcanic arc|arcs]]. The stability criteria for this type of junction are either ab and ac form a straight line or that the line bc is parallel to CA.