Wednesday, March 6, 2013

Plates versus plumes: What do you believe?

For years a little known but heated debate has occurred between geoscientists, pitting geophysics versus geochemistry over the existence of mantle plumes. This theory tries to explain intraplate volcanism, which (unlike most volcanism) is not associated with plate margins and has no obvious source. James Morgan posited in 1971 that these volcanoes resulted from plumes of hot mantle rising from the core-mantle boundary, manifesting on the surface as hotspot volcanoes such as Yellowstone and the Hawaii-Emperor chain. Such chains are thought to age with distance from the current eruptive centre, as if the plate was moving above a fixed source of heat and magma. Plumes are thought to cause many tectonic features, examples being the Ontong Java oceanic lava plateau, the Walvis ridge and Rio Grande rise entering the Atlantic basin from the Parana-Entedekka flood basalts and volcanic rifted margins. Plumes are also considered the main motive force in continental rifting.

The debate has wide implications in understanding many issues, for example whether the mantle is fully convecting, or whether the layers separated by discontinuities at 440 and 660 Km depth convect separately, with only exchange of thermal energy between them. These breaks represent phase changes in minerals as pressure/temperature conditions pass tipping points, but whether matter can pass through them is unknown. Since no one has ever been to the mantle to check, no definite facts are known, and inferences are drawn by both sides, mostly from the relative geochemistry of lavas and seismic maps of the inner Earth.

Plate theorists disagree, postulating that all magma is sourced from the shallow mantle, which is viewed as close to melting, heterogeneous, and having separate convection cells above and below the discontinuities. This means that volcanoes occur wherever there is tectonic extension that allows decompression melting (due to lowered pressure making mineral melting temperatures fall), usually along pre-existing lines of weakness such as sutures between terranes and transform faults. This is a theory of volcanism that posits one basic phenomenon underlying the wide diversity of volcanoes. It accounts for differences between different types of lava as due to upper mantle heterogeneity rather than separate geochemical reservoirs in different parts of the mantle. They also argue that many of the geochemists assumptions about the nature of the mantle are unproved.

They point out correctly that many predictions of plume theory have not been fulfilled despite many years of research, and that the plumes themselves seem to be elusive when sought. Unfulfilled predictions include hot lava geochemistry, domal uplift as the plume hits the bottom of the crust and truly time progressive volcanic trails. This absence of evidence was the stimulus that prompted some geoscientists to think of alternative mechanisms to account for the existing evidence. Some of plate theory predictions have been fulfilled, such as shallow melting (above the discontinuities) and the non-fixity of hotspots.

The debates are very detailed, ranging from geochemical arguments about the sources of different lavas and how different they are from each other to complex arguments about interpretation of seismic data (one side claiming that it reveals plumes, the other that it doesn't). The importance of the debate lies in the quest for a better understanding of the nature of the mantle, its diversity of conditions and the nature of the forces that drive plate tectonics.

They argue that Earth scientists seem to believe unquestioningly in plumes, despite evidence for their existence being arguably limited to non-existent. Those who are working on a new model that fits what is known do not seem so far to have impacted widely so far on the prevalent 'plume faith' despite mustering some convincing evidence. They argue that a nice sounding paradigm is easy to get attached to (I believe that the concept of cognitive dissonance should be the introductory topic in all science courses), but if it doesn't fit the evidence then a scientist must needs abandon it, rather than fit the evidence to the paradigm. Neither side has come up with a definite smoking gun, and both make convincing critiques of the other's assumptions used to underlie their interpretations.

Since we see into the mantle with a glass darkly, this will probably remain the case for the immediate future. Unless advances in resolution or interpretation algorithms for seismic tomography (taking a 3-D picture of the earth using earthquakes as light) occur, or a new geochemical paradigm is built, a clear solution is unlikely to emerge soon.

So far the debate remains unresolved, despite alleged occasional bouts of drunken fisticuffs at conferences. The real joy of it is how much we have left to understand about our wonderful world as our 'geo-eyes' improve.

Illustration courtesy of Don Anderson, Caltech.

A gold mine for all matters plumey:

Morgan' original papers: and

A brief summary of the debate:

The Yellowstone plume debate:

For those who want the gory details:

Don Anderson's New Theory of the Earth and Gil Foulger's Plates Versus Plumes: A Geological Controversy

And the really gory ones...two geodoorstops:

Foulger et al: The Great Plumes Debate, and Plates, Plumes and Paradigms. Geological Society of America Special Publications.

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