Page 111 - Global Tectonics

98 CHAPTER 5

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Figure 5.6 The absolute velocities of plates, assuming the hotspot reference frame. The arrows indicate the
displacement of points within the plates if the plates were to maintain their current angular velocities, relative to the
hotspots, for 40 Ma. Filled circles indicate the pole (or antipole) of rotation for the plate if this occurs within the plate.
The medium solid lines are approximate plate boundaries; where barbed, they indicate subduction zones with the barb
on the overriding plate. Note that, because of the Mercator projection, arrows at high latitudes are disproportionately

long compared to those at low latitudes (modified and redrawn from Gripp & Gordon, 2002 with permission from
Blackwell Publishing).

lithosphere relative to the Earth’s deep interior. If the lower mantle, and hence provide a fi xed reference

plate velocities are specified in the no net rotation frame with respect to the lower mantle. This hypothesis
(NNR) reference frame, the integration of the vector is considered further in the next section, and in Chapter
product of the velocity and position vectors for the 12. The hotspot model is attractive to many geologists
whole Earth’s surface will equal zero. By convention, and geophysicists in that the tracks of hotspots across
space geodesists specify absolute plate motions in the face of the Earth offer the possibility of determining
terms of the NNR criterion (Prawirodirdjo & Bock, the absolute motion of plates throughout the past
2004). 200 Ma (Morgan, 1981, 1983).
An alternative model for the determination of abso- The model of Gripp & Gordon (2002) for the current
lute motions utilizes the information provided by vol- absolute motion of plates, based on the trends and rates
canic hotspots on the Earth’s surface. Wilson (1963) of propagation of active hotspot tracks, is illustrated in
suggested that the volcanic ridges and chains of volca- Fig. 5.6. It averages plate motions over the past 5.8 Ma,
noes associated with certain major centers of igneous approximately twice the length of time over which
activity such as Hawaii, Iceland, Tristan da Cunha in the relative velocities are averaged. Two propagation rates
South Atlantic, and Reunion Island in the Indian Ocean, and 11 segment trends from four plates were used in
might be the result of the passage of the Earth’s crust deriving this model.
over a hotspot in the mantle beneath. Morgan (1971) Several other frames of reference for absolute
elaborated on this idea by suggesting that these hot- motions have been suggested, but not pursued. One of
spots are located over plumes of hot material rising from these proposed that the African plate has remained