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 Teaching Plates vs. Plumes
Teaching the Plates vs. Plumes Controversy

Gillian R. Foulger

Department of Earth Sciences, Durham University, Durham, UK

 

Introduction

The concept of deep-mantle plumes is typically embedded in Earth science courses at all levels. It is commonly presented as a problem-free fact in most textbooks and popular science books though its basic theory is usually neglected. In contrast, it is common to read statements by educators that the Plate Theory of volcanism (hereafter called "Plate Theory") is too complicated to explain to early career students and must thus be omitted. Whereas there was some justification for this view early on in the development of the theory, in recent years Plate Theory has matured and the argument no longer holds.

The current rivalry between the Plate and Plume theories offers a golden opportunity to introduce science students to the scientific method at the very start of their careers and to present Earth science as more exciting than simply a package of settled facts. It is the objective of this article is to clarify Plate Theory and suggest how the Plates vs. Plumes controversy might be taught in a clear and enriching way.

History

Unlike Plume theory, which was delivered essentially complete to a first order in a single blockbuster paper (Morgan, 1971), Plate Theory for volcanism developed slowly and messily over the first two decades of the current century. Its origins lie with a small number of scientists who felt that the predictions of Plume Theory were too rarely confirmed. In the minds of those scientists, and to quote the late Don Anderson, “Nothing seemed to fit”. Precursory uplift and initial flood basalts were absent or rarely found, evidence for high temperature or a plume tail to the core-mantle boundary were often not found, despite extensive and often expensive searching, and petrological and geochemical observations cited as evidence for a deep-mantle magma source could be equally well be attributed to shallow processes (Lustrino & Anderson, 2015). These problems were typically explained away by ad hoc adaptions of the plume model which were either untestable or, if shown to be wrong, simply replaced with a different ad hoc adaption (Rossetter, 2019). Skeptical scientists abandoned the traditional career crusade to maximize their publication rates and worked to challenge Plume Theory and develop a new hypothesis. Careers slow-tracked as a result.

A period ensued when the feasibility of a shallow source for unusual volcanism was explored. This was initially presented as “top-down tectonics” (Anderson, 2001). A plethora of shallow-sourced processes were suggested that could induce volcanism. These included slab-breakoff, enhanced extension at ridge/transform intersections, removal of the lithospheric mantle by delamination or “dripping” via Rayleigh-Taylor instability, and intraplate rifting. In addition, various shallow-sourced mechanisms for forming melt in the mantle were proposed, e.g., edge-driven convection, as alternatives to melting in hot diapirs from the core-mantle boundary.

This paradigm was justifiably criticized as a complicated melange of many different processes, with a different model for almost every volcanic area. It was argued that while Plume Theory was criticized for proposing a different type of plume for every locality (the number of plume variants now exceeds 100), top-down tectonics did no better – the pot was calling the kettle black.

Plate Theory

Plate Theory rationalized “top-down tectonics” and shallow-sourced volcanic processes into a single, simple, coherent schema. Plate Theory views essentially all volcanism as a consequence of plate tectonics operating in a realistic, complex geological world. The multiple shallow-sourced, plate-tectonic-related processes suggested share in common that they all result in lithospheric extension. The only exception is Impact Theory, which forms a third theory for a small subset of volcanic regions.

Plate Theory for volcanism is far from being a challenging concept and may be simply stated. It is the theory that volcanism on Earth’s surface is a consequence of lithosphere extension that permits melt to escape to the surface. The required extension results from plate tectonics.

Plate Theory offers a fundamental simplification of Earth dynamics. It removes the problem of requiring two independent processes to explain Earth's volcanism – plate tectonics and plumes – and the lack of clarity regarding which volcanic regions are caused by which process. It also renders moot the question of how two dissociated and unrelated convective modes could operate independently in a single convecting body.

A common misunderstanding about Plate Theory is that merely proposing a mechanism by which melt may form in the mantle, e.g., fluxing of the mantle with volatiles, amounts to explaining volcanism. This misunderstanding is predicated on the assumption that the crust and mantle cannot retain melt and so any that forms must immediately drain to the surface. This is incorrect, as demonstrated by the seismic low-velocity layer, which can only be explained by partial melt, and the existence of long-standing crustal magma chambers. Retained melt is widespread in the mantle and can only escape to the surface if permitted to do so by lithospheric extension. The lithosphere is the active element and primary inducer of volcanism – it is not a passive transmitter of whatever lies in the mantle beneath.  

Click here for a fuller description of Plate Theory.

Teaching Plate Theory

Plate Theory can be stated as simply as Plume Theory and introduced to students in the earliest stages of their studies. The fact that this important controversy is playing out now is a golden pedagogic opportunity for students to:

  • experience how science progresses in reality. This is probably different from what they might imagine;
  • understand that science is never settled;
  • learn that significant Earth science research must be multidisciplinary;
  • learn that the data, arguments and assumptions on which theories are based must be understood if they are to be meaningfully judged – assertions cannot be taken at face value;
  • be skeptical of established theories;
  • learn that science is most fertile if done by collaboration between practitioners who respectfully disagree with one another; and
  • accept that it is alright to change one's mind. In the last (1976) version of Sir Harold Jeffreys' famous book The Earth, the author continued to explain mountain belts using contraction theory, rejecting plate tectonics.

The subject clearly is currently at a stage where scientists at every level are divided. Some favor Plate Theory and others Plume Theory with a full spectrum of strengths of opinion ranging from absolute belief in one or the other theory to ambivalence. From the educators in a single department to the world’s most highly honored scientists there is division on the issue. Both Prof. Don Anderson (a strong Plate-theory advocate) and Prof. W. Jason Morgan (a strong Plume-theory advocate) received the US National Medal of Science, from Presidents Bill Clinton and George W. Bush respectively. And yet they cannot both be right – or can they?

If science is divided, this should be taught to students, rather than simply presenting them with one preferred theory. One approach could be to:

  • outline the two theories;
  • have the students read and critique the early, original papers that propose the two theories. Candidate papers are by Morgan (1971) and Foulger & Natland (2003), both of which are short (~ 1,000 words);
  • guide the students to read the introductory parts of relevant scientific articles and to reflect on whether assumptions made are realistic or might be unreliable dogma;
  • define and discuss fundamental concepts such as the geotherm, potential temperature, adiabatic gradient, lithosphere, asthenosphere, upper-lower mantle transition and temperature of the outer core;
  • encourage the students to consider to what extent a theory can be adapted in the light of new findings before it is so different from the original concept that it has become a new theory;
  • introduce, or teach at an appropriate level, the investigative methods and results most relevant to the subject. These methods include seismology, petrology, geochemistry, geochronology, marine geophysics, structural geology, volcanology and the history and philosophy of science;
  • allocate students, singly or in groups, different postulated “hot spots” or volcanic regions to research and, for their particular locality, attempt to falsify both the Plate and the Plume theories.

A suitable list of localities is given in Table 1. These are areas about which a considerable body of research has been published.

Table 1: List of candidate proposed “hot spots” suitable for student-led study.

Teaching resources

Suitable textbooks to support such teaching are:

Click here for Durham University, U.K., teaching materials

Click here for Northwestern University, Illinois, USA, teaching materials

www.mantleplumes.org supports student work. Sets of presentations on this subject by students can be found at http://www.mantleplumes.org/StudentsCorner.html. Here, the best student work addressing the Plates vs. Plumes controversy is showcased.

If students wish their work to be posted in future, they can send a request to the website manager along with the contact details of the educator who taught the class or another qualified academic who may be contacted for a recommendation.

References


last updated 21st May, 2020
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