Evidence for moving mantle plumes...  Is nothing stationary? 

The concept of mantle plumes (or hotspots) was first proposed by J. Tuzo Wilson in 1963 (Wilson, 1963), and then described in more detail by Morgan (1972).  Since the early 1970s most geologists have accepted that hotspots are essentially fixed features within the mantle, and that they have left a record of their existence as chains of extinct volcanoes across the plates that have moved over top of them.  The absolute motions of the major plates have been determined on the basis of this assumption, and numerous other inferences about where things were at what time, and in which direction they were moving, have followed.

From the U.S. Geological Survey: A sharp bend in the chain indicates that the motion of the Pacific Plate abruptly changed about 43 million years ago, as it took a more westerly turn from its earlier northerly direction. Why the Pacific Plate changed direction is not known, but the change may be related in some way to the collision of India into the Asian continent, which began about the same time.     

From: http://pubs.usgs.gov/publications/text/Hawaiian.html (October 2003). Another USGS website (http://pubs.usgs.gov/publications/text/hotspots.html ) includes a similar explanation, but has a note at the bottom explaining:  “Since this booklet's publication in 1996, vigorous scientific debate has ensued regarding volcanism at "hotspots." New studies suggest that hotspots are neither deep phenomena nor "fixed" in position over geologic time, as assumed in the popular plume model.”

From a widely used introductory Geology textbook: Notice for example ...that the Hawaiian Island-Emperor Seamount chain bends.  This particular bend in the trace occurred about 40 million years ago when the motion of the Pacific plate changed from nearly due north to its present northwesterly path.

 (Tarbuck and Lutgens, Earth Science, 10th Ed., 2003)

The best-known example of plate motion over a mantle plume is that of the Hawaiian Chain and Emperor Seamounts.  An entire generation of students has been taught that the distribution of the Hawaiian Islands and Emperor Seamounts is the product of the motion of the Pacific Plate over the Hawaii Hotspot, and that the sharp bend between the two chains is the result of an abrupt change in the direction of motion of the Pacific Plate (see box to the right).  

Magnetic data from basalt samples on the Hawaiian Islands have confirmed the view that the hotspot has been essentially stationary for the past 40 m.y., since most samples – even those collected far from the area of active volcanism - have magnetic orientations consistent with the current location of the hotspot at 19º N.  Up to now, however, relatively little good magnetic data have been acquired for the Emperor Seamounts.  Furthermore, some of the data that do exist are not consistent with a stationary plume (Tarduno and Cottrell, 1997).

Ocean Drilling Project Leg 197 (completed in July and August of 2001) was designed to test the theory that the Hawaii Hotspot may not have been stationary prior to 40 m.y. ago.  Bedrock cores were acquired from three main locations within the Emperor chain (namely Detroit, Nintoku and Koko seamounts), and magnetic inclination data were acquired for several hundred samples from each of these holes.  The results are described in a recent report by several members of the research team, including lead author John Tarduno of the University of Rochester (Tarduno et al., 2003).

The new magnetic data show that the rocks of the Detroit, Nintoku and Koko seamounts formed at close to 32, 26 and 21º N respectively.  The interpretation of Tarduno et al. is that the Hawaii Hotspot migrated south at a rate of close 40 mm/y for the period from at least 100 m.y. ago to around 20 m.y. ago, and that it has slowed down over the past 20 m.y. and is now essentially stationary.

These data provide strong support for the idea that the Hawaii hotspot has migrated significantly in the past, although it appears to be stationary now.  The data suggest that the bend in the Hawaii-Emperor chain may be more likely to be related to a change in the sense of motion of the hotspot, than to a change in the direction of the Pacific Plate.

If the Hawaii hotspot can move and then stop moving, then presumably any hotspot can move or stop moving, or start moving.  The result is that we no longer have a reliable stationary frame of reference from which to measure the motions of the plates.


References

Morgan W, 1972, Plate motions and deep mantle convection, Geol. Society of America. Memoir 132, p. 7-22.

Tarduno J and Cottrell R, 1997, Paleomagnetic evidence for motion of the Hawaiian hotspot during formation of the Emperor Seamounts. Earth and Planetary Science Letters, V. 153, p. 171–180.

Tarduno J, Duncan R, Scholl D, Cottrell R, Steinberger B, Thordarson T, Kerr B, Neal C, Frey F, Torii M and Carvallo C, 2003, The Emperor Seamounts: Southward motion of the Hawaiian Hotspot plume in the Earth's mantle, Science, V. 301, p. 1064-1069. (August 2003)

Wilson J, 1963, A possible origin of the Hawaiian Islands, Canadian Journal of Physics, V. 41, p. 863-870.


Steven Earle, Malaspina University College, Nanaimo, Canada, 2003. Return to Earth Science News