|MadSci Network: Earth Sciences|
You have asked a rather good question. Possibly much better than you realize.
The simple answer is that the comparatively thin tectonic plates move and the hotspots, which are from deep in the mantle, donít move with them. A more detailed answer gets messy quickly.
It is relatively easy to determine that two plates are spreading apart along a mid ocean ridge at X cm per year in a given direction. It is considerably harder to decide if the mid ocean ridge is moving relative to the rest of the Earth.
Another factor is where in the mantle do hotspots come from. Some, like the Hawaiian hotspot, appear to come from near the core mantle boundary. Others seem to come from intermediate depths. There appear to be large scale movements of the mantle that can drag a hotspot along.
The best guess at this point is that the hotspots are moving much slower than the plates. There is no reason to expect them to be stationary. There is also no consensus as to how long hotpsots live.
The following abstract from the AGU fall meeting in 2001 may give you some idea of the complexity of your question.
Title: Seismic Structure and Origin of Hotspots and Mantle Plumes Author: Zhao, D University: Ehime University, Geodynamics Research Center, Matsuyama, 790-8577 Japan Abstract: A new model of whole mantle seismic tomography was developed with a novel approach. A grid parameterization was adopted, instead of blocks and spherical harmonic expansions which were used in most of the global tomographic studies. Ray paths and travel times were computed with an efficient 3-D ray tracing scheme [Zhao et al., 1992]. Moreover, the topography of mantle discontinuities at 410 and 660 km depths and the Moho discontinuity [Flanagan and Shearer, 1998; Mooney et al., 1998] were taken into account in the tomographic inversions. The three discontinuities exhibit lateral depth variations of tens of kilometers, which greatly affects the ray path and travel times, hence their depth changes should be taken into account in the inversions. This new approach was applied to a large data set of ISC travel times (P, PP, PcP, pP) which were reprocessed by Engdahl et al. , resulting in a new model of whole mantle P-wave tomography. For the shallow mantle, this new model contains the general features observed in the previous models: a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0-400 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction zone regions, which suggests that most of the slab materials are stagnant for a long time in the transition zone before finally dropping down to the lower mantle. Plume-like slow anomalies are visible under the hotspot regions in most parts of the mantle. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, which suggests that plumes are not fixed in the mantle but can be deflected by the mantle winds. As a consequence, hotspots are not really fixed but can wander on the Earth's surface, as evidenced by the recent geomagnetic and numeric modeling studies. There is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, which suggests that many hotspot plumes may originate from the CMB. However, there may be some small-scaled, weak plumes originating from the transition zone depths. Zhao, D. (2001) Seismic structure and origin of hotspots and mantle plumes, Earth Planet. Sci. Lett., in press.
In the above, CMB is the core mantle boundary. P, PP, PcP, pP describe the paths earthquake waves take. The letters describe the paths taken by the seismic waves. P is a direct arrival. PP bounces once off of the surface. PcP bounces off of the outer core. pP bounces off of the surface near the earthquake. See the Jeffreys-Bullen Travel Time Curves (a pdf file) for a somewhat historical look at seismic waves. A similar study can be done using S (shear) waves. The three mantle discontinuities mentioned occur nearly worldwide. Hot rock has a slower seismic velocity than cold rock of the same composition and depth. The low velocity ring around the Pacific Ocean is sometimes called the Ring of Fire.
For another view see Three distinct types of hotspots in the Earthís mantle. This is also a pdf file.
Keep asking good questions.
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