Analyze Triple Junctions

 This example uses the Indian Ocean triple junction.The values on the map are absolute motions in whatever reference frame the plate model uses. You must now convert them to relative motions for each of the three plate-pairs at the triple junction. You can solve for the relative motions graphically or analytically, since they are vector additions. The memo box on the control form shows the magnitudes and directions if you want to work analytically. Note that two of these plates are going in the same direction, but the difference in their velocities indicates the ridge between them.

 India is moving about 24 mm/yr at 66° (the difference between 53 and 29) relative to Africa (shown in blue on the map). This is perpendicular to the trend of the ridge between them. The motion between India and Antarctica is oriented NE-SW (dark green line, again perpendicular to the trend of the ridge, but the motion between Africa and Antarctica (bright green line)  is almost north-south at an angle to the ridge between them.Although drawn on the map, this triangle is actually in velocity space and indicates the relative velocities on points on each of the three plates at the triple junction. In velocity space the plates might not have the same relative orientations they do on the map, although they do here with a ridge-ridge-ridge triple junction.
 Excel spreadsheet to calculate the motions of this triple junction. You must remember to convert the degrees to radians, and that angles in Excel follow math conventions (start in the east and run CCW) rather than compass directions. The final directions were computed by hand (adding or subtracting the ArcTan value from the correct cardinal direction), and are actually not needed.You can achieve the same results more easily with a ruler and protractor, and measure the lengths and orientations of the three relative velocities in the diagram above. The only limitation on the graphical approach is the requirement to insure that the velocity vectors are large enough to measure.
 Velocity diagram of the plates at the triple junction. One plate (AN) is arbitrarily set to 0,0, and the motion of the other two plates relative to the first plate are plotted. Note that Africa moves almost due north from Antarctica, India moves NE, and that the relative motion of India relative to Africa is also correct. This is really just a rescaling of the diagram, with a shift of the coordinate axes to put Antarctica at the origin.

Triple junction motion rules.

 Velocity lines of the three ridges shown in red shade. The lines are parallel to the plate boundaries (ridges). For the AF-IN and AN-IN pairs, this makes them perpendicular to the lines connecting the plate velocities, but that cannot be the case for the AF-AN pair (note from the plate motions and the orientation of the boundary that this spreading must be at an oblique angle, which is reasonable given the orientation of the three ridges where they come together, and is also apparent if you look at the bathymetry in the region.Because the three lines intersect in a point, the triple junction is stable. The triple junction is moving relative to the three plates (they are each spreading away from the triple junction). The analysis could as easily have been done graphically on the map-derived velocity diagram if it is properly scaled.

Additional analysis of this triple junction:

Gulf of California triple junction analysis.