Triple Junctions on a Sphere
Triple junction closure on a sphere is similar to triple junction closure on a flat earth except that the sum of the rotation vectors must be zero.
ωBA + ωCB + ωAC = 0
Example: Galapagos Triple Junction
Given the rotation vectors of the Cosos plate relative to the Pacific plate and the Pacific plate relative to the Nazca plate, calculate the spreading rate at 2˚N, 260˚E.
ωCP + ωNC + ωPN = 0 ωNC = -ωCP - ωPN vNC = ω NC × r (θ, φ)
⎮v⎥ - magnitude of spreading rate
Hot Spots and Absolute Plate Motions
So far we have only considered relative plate motions because there was no way to tie the positions of the plates to the mantle. One method of making this connection and thus determining absolute plate motions is to assume that “hot spots” remain fixed with respect to the lower mantle.
A hot spot is an area of concentrated volcanic activity. There is a subset of hot spots that have the following characteristics:
They produce linear volcanic chains in the interiors of the plates.
The youngest volcanoes occur at one end of the volcanic chain and there is a linear increase in age away from that end.
The chemistry of the erupted lavas is significantly different from lava erupted at mid- ocean ridges or island arcs.
Some hotspots are surrounded by a broad topographic swell about 1000 m above the surrounding ocean basin.
These features are consistent with a model where the plates are moving over a relatively fixed mantle plume. After identifying the linear volcanic chains associated with the mantle plumes, it has been shown that the relative motions among hot spots is about 10 times less than the relative plate motions.