3

Example: Given the above geometry and one spreading velocity V_{AC }what are the other two spreading rates?

= 120 mm/yr,

# VBA

15

85

sum of interior angles = 180˚

VCB

VAC

80

Solving for the lengths of the vectors we find V_{BA }= 118.45 mm/yr and V_{CB = }31.12

mm/yr. One of these flat-earth triple junction closure problems will be on the quiz. The map on the next page shows the other triple junctions. As an exercise, use a bathymetric

map (e.g., Google Earth and this KMZ-file ftp://topex.ucsd.edu/pub/srtm30 plus/SRTM30 PLUS V7.kmz

)

to

determine

the

geometry of another triple junction and use Table 1 (below) to calculate the spreading rate at one of the ridges. The next section develops the mathematics for calculation of plate motions on a spherical earth.