Why are the newest of the Hawaiian Islands the largest? Is tectonic movement slower now than it once was, or is the hotspot more active now?
Careful analyses of the volume of lava erupted by
the plume or hotspot as a function of time show clearly that the youngest
islands are the biggest. Along the entire length of the Hawaii-Emperor
seamount Chain, the record of the plume for the last 60 or 70 million
years, there are also periods of high apparent eruption rate alternating
with periods of low eruptive volumes. The idea that the Pacific plate
velocity could be the reason can be ruled out, because the periods of slow
motion don't correspond with the bulges in volume. Possibly, the output of
the hotspot changes with time, but this is difficult to prove or disprove.
Most theories of plumes and experiments by fluid dynamicists predict a
steady output or blob-like output, but these are simple models. The earth
may be more complicated.
One interesting idea to explain the volume changes with time is an idea by Herb Shaw in 1973 (Bulletin of the Geological Society of America, volume 84, p.1505-1526). Shaw developed the idea that the melting could actually be due directly to the plate motion, with friction between the lithospere and aesthenoshere causing the melting. This mechanism (called shear heating) can explain the volume changes by a feedback mechanism. Basically, shear from plate motion would cause an increase in temperature in the sheared region. In turn, increasing temperature reduces the viscosity of the material, allowing a greater rate of shear, causing a greater temperature rise and more melting. And so on... After escape of the melt, the system would be reset to start the cycle again. Quantitatively, the model seems to work. Nevertheless, the hotspot or plume hypothesis is still the most popular model. In this model, the differences in volume are essentially unexplained. What you have to say is: well, we don't know very much about how plumes work anyway, but apparently the volume of melt they produce can vary with time. I hope this helps.
Dr. Rodey Batiza, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822