Title: Plates vs. Plumes: A Geological Controversy
Author: Gillian R. Foulger
- Presents the flaws of a plume theory that has become untestable, hence unscientific
- Proceeds through six lines of evidence, comparing plume theory with alternatives rooted in plate tectonics
- Presents a large body of evidence for competing theories to explain
- Lucid presentation of a very wide range of material
- Intricate argument is highly cross-referenced
- Abundant illustrations from the literature including color plates
- Hundreds of references as recent as 2010
- Ambitious student exercises are more suited for contemplation than completion
- Requires a deep acquaintance with many subfields of geology
Today everyone learns in geology textbooks that Earth's surface is divided into plates, whose motions and interactions account for most of the geology we know. They also learn that some unusual pockets of volcanism exist far from where plate tectonics says they should be, and that geologists call these hotspots. The origin of hotspots, they are told next, is from a deeper realm far below the plates in plumes of extra-hot material that rise from the very base of the mantle.
This two-layer model has been taught and practiced since the 1970s. Hotspot plumes were first proposed shortly after plate tectonics became widely accepted. Plumes and plates are essentially the same age. We know how successful the plate approach has been in its four decades of practice: concepts have been refined, explanations have expanded and the range of predictions, topics and applications has grown ever wider. The founders of plate tectonics would (indeed, do) still feel at home with it today. How is the plume community doing?
In her innovative book Plates vs. Plumes: A Geological Controversy, Gillian Foulger shows that plumist hypotheses have repeatedly run aground. When Jason Morgan first proposed them in 1971, he supposed that they broke up continents, drove plate tectonics, and constituted a network of points that are fixed relative to each other unlike the surface plates. Those predictions failed. Some 40 years after plumes were first proposed, there is still no consensus on (1) how to define plumes, (2) how many there are and (3) where they originate.
Definition: Today plumes are said to have five basic features: their temperatures are high, they rise as big blobs that raise the crust above them, they have narrow "tails" that reach the base of the mantle, they first appear with large flood basalt eruptions, and they produce a trail of progressively younger volcanoes erupting from the plume tail. But not one hotspot shows all five features (and for this reason Foulger discards "hotspot" for the more neutral "melting anomaly"). Other lists use other criteria, such as high ratios of helium-3 to helium-4, seismic images showing soft material 500 kilometers beneath, and so on. But those lists don't always include featureslike the Hawaiian Island chain, Iceland, or Yellowstonethat are universally considered hotspots. "That is," Foulger says, "every one of the five basic features predicted . . . is commonly either considered optional or its absence, even in the face of extensive searching, is considered to be inconclusive." If every hotspot gets a special exemption, what is actually explained?
Number: Morgan began in 1971 with "about twenty deep mantle plumes," but listed only 16. Later totals reached 122, even thousands, but today the various lists tend to cluster around 50. Even so, no list is alike and the criteria are arbitrary, for example by imposing size cutoffs. The smaller plumes resemble ordinary volcanic centers more than Yellowstone or Hawaii. Why not treat volcanic phenomenaall melting, "anomalous" or notas a continuum instead?
Origin: Morgan supposed that plumes arise at the very base of the mantle, where the strange and tantalizing D'' layer sits. But seismic studies show that many hotspots have only shallow roots, and geochemical studies fail to find convincing "core signatures." Odd patterns in trace elements, the one-percent fraction of lavas that are otherwise monotonously alike, are attributed to various "reservoirs" somewhere in the mantle that, other than their positions on various graphs of chemical ratios, have no confirmed physical existence. Some chemical signatures are attributed to subducted crust that has somehow been carried all the way to D'' unchanged. But seismic images of such descending crust are vague and debatable. Other "signals" such as bumps or hollows in the seismic boundaries at 410 and 660 kilometers depth are inconsistent. More recently, plumists have been dividing their plumes into deep, medium and shallow onesall in a mantle that is still poorly constrained by data. Can't shallow melt extraction, the workhorse of ordinary volcanism, do a better job with all this than plumes?
Foulger shows that it can in a systematic walk through the five plume criteria, plus a chapter on geochemistry, in which she puts plumes and plates up against a large body of observational data from around the world. Ordinary volcanism arises where plate tectonics promotes melting by stretching the lithosphere apart thereby releasing pressure on mantle rock, as it does at midocean ridges and continental rifts, or by introducing "fertilizing" material that acts as a flux, as it does at subduction zones. Other well-known mechanisms of extension and fertilization include cracks in plates, changing stresses across plates and bodies of old fertile material in the mantle (placed there by plate tectonics). These are available to offer explanations for both ordinary and anomalous meltingeven in Hawaii, Iceland and Yellowstone. Foulger calls this avenue of research "the Plate hypothesis."
The battle that Foulger has largely led against plumism has taken the form of individual papers and the large compilations Plumes, Plates and Paradigms (2005) and a successor volume, Plates, Plumes, and Planetary Processes (2007). In contrast, Plates vs. Plumes is a 300-page textbook suitable for a graduate-level seminar, the kind of "book club" where seasoned researchers and knowledgeable students can address at leisure the cross-disciplinary questions it raises. To that end, each of the eight chapters has challenging exercises, such as "Develop a plate model for the Emeishan basalts, China" or "Map globally the base of the seismic lithosphere." They are really research projects, but they are in keeping with the rest of the book, which buries plumes on the one hand, yet on the other lays out a body of robust observations and a road map of fruitful research questions for the Plate hypothesis.
An extraordinary aspect of the book is its umbilical link to the webnot to the publisher's site, like many textbooks, but to www.mantleplumes.org, where Foulger and dozens of her peers have been posting position papers and review articles (and I have been visiting) since 2003. Many footnotes are URLs from this site, which provide reviews with more detail and a fixed location for continual updates. And updates will surely follow as this scientific debate continues.