David Tyler Replies:

It is an impressive correlation. But is it soundly based? In recent years, it has become apparent that the alleged hot spot under Hawaii is NOT stationary. The data suggests it is moving. This throws a spanner in the works - the satellite measurements give a movement rate higher than would be expected. My source for this is: Geology Today, 14(3), May/June 1998, 101-103.

Misinterpretation in the Pacific?

The Hawaiian-Emperor chain of volcanic islands and seamounts in the Pacific Ocean (Fig. 7) is famous, because it was in that connection that many of the ideas on mantle plumes and hotspots were worked out. The conventional wisdom, reached after much early debate, is that the islands/seamounts are the product of a mantle plume, fixed in (lateral) position relative to the mantle, that gave rise to a series of volcanic hotspots at the surface as the Pacific lithospheric plate moved above it. The chain is also famous for its bend, interpreted as a sudden change in the direction of motion of the Pacific Plate about 43 million years ago.

This story is now so widely accepted that it almost has the status of `fact'. But what if it's not true? What if it's merely wishful thinking? The alternative is perfectly obvious, of course, if not much thought about. Could the bend in the chain be due not to a change in the motion of the Pacific Plate but to a sudden movement of the mantle plume itself? Possible movements of mantle plumes have been much considered in the past and not always ruled out, but envisaged motions have generally been small, barely invalidating the general concept of a `fixed hotspot frame of reference'. Needless to say, any proved deviation from that would be devastating, requiring a major rethink on a number of fronts.

Yet that is what Norton proposed a few years ago (Tectonics, v.14, p.1080, 1995) and is what Tarduno and Cottrell consider in more detail now (Earth & Planetary Science Letters, v.153, p.171, 1997). Specifically, Norton suggested that the hotspot moved southwards up to 43 million years ago, creating the Emperor seamount chain, and then became fixed in the mantle, the Hawaiian part of the chain having been generated by the method outlined in the first paragraph above. As Tarduno and Cottrell now point out, this is testable by palaeomagnetism, for if the hotspot has always remained fixed, the original palaeolatitudes of the now-extinct seamounts of the Emperor chain should be the same as the present latitude of Hawaii, under which the supposed mantle plume now rests. Moreover, thanks to samples collected from Detroit seamount (see Fig. 7) during the Ocean Drilling Program, the palaeomagnetic test can now actually be carried out.

The results are troubling. For one thing, there is an age inconsistency. Hotspot-plate motion models suggest an age of 75-65 million years for Detroit seamont, but new 40Ar/39Ar data indicate an age of about 80 million years. More seriously, the palaeolatitude given by the Detroit rocks is about 36°N, whereas the present latitude of Hawaii is only about 19°N. Taken at face value, that means that the plume could not possibly have remained stationary before 43 million years ago - i.e. the Emperor chain cannot be explained entirely, if at all, by plate motion above a stationary plume. However, the palaeomagnetic data do not have the resolution to allow decisions on whether the Emperor chain is entirely due to plume movement or partly to plume movement and partly to plate motion.

Either way, the palaeomagnetic data, if confirmed (and confirmation is necessary because of the limited sampling of the Emperor chain thus far and the possibility that movement of the Earth's pole may have biased the data to some, unknown, extent), would appear to demolish a cherished hypothesis and thus require `a major change in how we view mantle dynamics and the history of plate tectonics'.

______________________________________________________ Ó 2010 Arthur V. Chadwick, Ph.D.