Triple junction

A point where three plate boundaries meet. Kinematic stability means that one instantaneous junction velocity is compatible with all three moving boundaries, so a connected junction can persist. It is not a guarantee of dynamical stability for arbitrary future plate motion.

A junction whose persistence depends on the two overriding-plate trench constraints and the transform velocity line meeting at one point. Specifying only the boundary-type letters does not specify the compatibility geometry or trench polarities.

For pure normal symmetric spreading, the two ridge constraints are perpendicular bisectors of two edges of the plate velocity triangle. The transform constraint lies along the third edge. For a nondegenerate triangle their common intersection exists only when the angle opposite that third edge is a right angle; the junction velocity is then the midpoint of the transform edge.

For a boundary with in-plane normal $n$, an admissible junction velocity $U$ obeys $(U-v_{\mathrm{boundary}})\cdot n=0$. A ridge uses the mean of adjacent plate velocities in the symmetric model, a trench uses the overriding plate velocity, and a transform uses either adjoining plate velocity. Compatibility requires a common intersection of the three velocity-space lines.

The three plate velocities plotted in a common reference frame form a triangle whose directed edges are the pairwise relative velocities. Adding the same reference velocity translates the triangle without changing its edges.