Re: If a hotspot causes islands, why don't they cause one large island?

Andrea,

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, don’t 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. [1998], 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 Earth’s mantle. This is also a pdf file.

Keep asking good questions.

David