Plates vs Plumes E-Book

Gillian R. Foulger

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 755

Veröffentlichungsjahr: 2011

Beschreibung

Since the advent of the mantle plume hypothesis in 1971, scientists have been faced with the problem that its predictions are not confirmed by observation. For thirty years, the usual reaction has been to adapt the hypothesis in numerous ways. As a result, the multitude of current plume variants now amounts to an unfalsifiable hypothesis.

In the early 21st century demand became relentless for a theory that can explain melting anomalies in a way that fits the observations naturally and is forward-predictive. From this the Plate hypothesis emerged–the exact inverse of the Plume hypothesis. The Plate hypothesis attributes melting anomalies to shallow effects directly related to plate tectonics. It rejects the hypothesis that surface volcanism is driven by convection in the deep mantle.

Earth Science is currently in the midst of the kind of paradigm-challenging debate that occurs only rarely in any field. This volume comprises its first handbook. It reviews the Plate and Plume hypotheses, including a clear statement of the former. Thereafter it follows an observational approach, drawing widely from many volcanic regions in chapters on vertical motions of Earth's crust, magma volumes, time-progressions of volcanism, seismic imaging, mantle temperature and geochemistry.

This text:

• Deals with a paradigm shift in Earth Science - some say the most important since plate tectonics
• Is analogous to Wegener's The Origin of Continents and Oceans
• Is written to be accessible to scientists and students from all specialities

This book is indispensable to Earth scientists from all specialties who are interested in this new subject. It is suitable as a reference work for those teaching relevant classes, and an ideal text for advanced undergraduates and graduate students studying plate tectonics and related topics.

Visit Gillian's own website at http://www.mantleplumes.org

Leseprobe

Contents

Preface

Geological time scale

1 From plate tectonics to plumes, and back again

1.1 Volcanoes, and exceptional volcanoes

1.2 Early beginnings: Continental drift and its rejection

1.3 Emergence of the Plume hypothesis

1.4 Predictions of the Plume hypothesis

1.5 Lists of plumes

1.6 Testing plume predictions

1.7 A quick tour of Hawaii and Iceland

1.8 Moving on: Holism and alternatives

1.9 The Plate hypothesis

1.10 Predictions of the Plate hypothesis

1.11 Testing the Plate hypothesis

1.12 Revisiting Hawaii and Iceland

1.13 Questions and problems

1.14 Exercises for the student

2 Vertical motions

2.1 Introduction

2.2 Predictions of the Plume hypothesis

2.3 Predictions of the Plate hypothesis

2.4 Comparison of the predictions of the Plume and Plate hypotheses

2.5 Observations

2.6 Plume variants

2.7 Discussion

2.8 Exercises for the student

3 Volcanism

3.1 Introduction

3.2 Predictions of the Plume hypothesis

3.3 Predictions of the Plate hypothesis

3.4 Comparison of the predictions of the Plate and Plume hypotheses

3.5 Observations

3.6 Plume variants

3.7 Discussion

3.8 Exercises for the student

4 Time progressions and relative fixity of melting anomalies

4.1 Introduction

4.2 Methods

4.3 Predictions of the Plume hypothesis

4.4 Predictions of the Plate hypothesis

4.5 Observations

4.6 Hotspot reference frames

4.7 Plume variants

4.8 Discussion

4.9 Exercises for the student

5 Seismology

5.1 Introduction

5.2 Seismological techniques

5.3 Predictions of the Plume hypothesis

5.4 Predictions of the Plate hypothesis

5.5 Observations

5.6 Global observations

5.7 Plume variants

5.8 Discussion

5.9 Exercises for the student

6 Temperature and heat

6.1 Introduction

6.2 Methods

6.3 Predictions of the Plume hypothesis

6.4 Predictions of the Plate hypothesis

6.5 Observations

6.6 Variants of the Plume hypothesis

6.7 Discussion

6.8 Exercises for the student

7 Petrology and geochemistry

7.1 Introduction

7.2 Some basics

7.3 Predictions of the Plume hypothesis

7.4 Predictions of the Plate hypothesis

7.5 Proposed deep-mantle-and core-mantle-boundary tracers

7.6 A few highlights from melting anomalies

7.7 Plume variants

7.8 Discussion

7.9 Exercises for the student

8 Synthesis

8.1 Introduction

8.2 Mantle convection

8.3 An unfalsifiable hypothesis

8.4 Diversity: a smoking gun

8.5 The need for joined-up science

8.6 The future

8.7 Exercises for the student

References

Color plates

Index

This edition first published 2010, © 2010 by Gillian R. Foulger

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Library of Congress Cataloguing-in-Publication Data

Foulger, Gillian R., 1952-

Plates vs plumes: a geological controversy/Gillian R. Foulger.

p. cm.

Includes bibliographical references and index.

ISBN 978-1-4443-3679-5 (hardcover: alk. paper) – ISBN 978-1-4051-6148-0 (pbk.: alk. paper) 1. Mantle plumes. 2. Plate tectonics. I. Title.

QE527.7.F68 2011

551.21-dc22

2010012131

ISBN: 978-1-4443-3679-5 (hbk) 978-1-4051-6148-0 (pbk)

A catalogue record for this book is available from the British Library.

Preface

The debate regarding whether or not melting anomalies are fuelled by hot diapirs from the deep mantle – the Plume hypothesis – or whether they arise from shallow processes ultimately related to plate tectonics – the Plate hypothesis – is an extraordinarily rich cross-disciplinary subject with seemingly endless ramifications. Relevant material is scattered far and wide, and because of this, over the last decade, scientists have come together and tried to address this problem using the website www.mantleplumes.org. It grew like Topsy following its founding on 5th March, 2003 and now includes thousands of items contributed by over 500 scientists from many parts of the world. Nevertheless, even when collected together in a single website, making sense of the vast ocean of data relevant to this fascinating subject still presented an extraordinary challenge to the aspiring student. A substantial, coherent work, organized by subject, was still lacking.

I was thus rather easily persuaded, in the summer of 2007, to write the present book. In it I have attempted to summarize where we stand today in some of the subjects most central to the debate. The Plume hypothesis makes five fundamental predictions that can be tested by studying the vertical motions of Earth’s surface, magma volumes, the spatial and temporal pattern of volcanism, seismic imaging of the crust and mantle, and the temperature of the mantle source rocks. The Plate hypothesis makes contrasting predictions in these fields. I have devoted a chapter to each, focusing on how well the observations match the predictions. I include additionally a chapter on geochemistry, which has been extensively applied in an effort to reveal the physical and spatial origins of surface erupted magmas. Deciding what to put in and what to leave out has not been easy, and important subjects such as the effect of pre-existing lithospheric structure on localizing volcanism, the physical origins of the melt source, bolide impacts, and planetary volcanism remain to be treated in more detail.

I am sometimes asked what my own background is, and by what path I arrived at the subject of plumes, “hot spots” and melting anomalies. I have worked my entire career as an earthquake seismologist, with a temporary residency in the parallel universe of GPS surveying applied to crustal deformations and plate motions. I was initiated into scientific research working on geothermal areas in Iceland, where I lived for several years, acquiring the language, an insider’s knowledge of geological lore, and a conviction that cross-disciplinary work is essential for full understanding. I doubt it is really possible to ever truly appreciate the complexities of an area unless one has lived, breathed, and suffered there. What is understood usually finds its way into print, but what is not understood, wherein exciting new discoveries lurk, is only whispered in corridors, perhaps also in an unfamiliar language.

I received early a lesson in listening to the clamoring voices of data during my Ph.D. research. No shear-faulting mechanisms would fit my earthquake data, which seemed intransigent to interpretation. My supervisor told me I must have set the equipment up wrongly. I did not understand. I had calibrated my network in four independent ways and all the calibrations agreed. Six months later I realized that my earthquakes were not caused by shear faulting but by tensile cracks. Everything fell instantly into place. Thermal cracking was happening. An unknown heat source must exist, and a hitherto unknown geothermal reservoir. Geochemical and geological data were simultaneously announced that converged on the same conclusion. The euphoria will always remain with me.

That was the start of my career. Its latest phase has involved work on a much larger scale – the structure of the mantle. In this I was inspired by the pioneering work of H.M. Iyer, with whom I conceived an impossibly ambitious Icelandic project as early as 1989. In the mid1990s I acquired a series of grants from the Natural Environmental Research Council, the National Science Foundation, and the European Community for a project to study “the Iceland plume”. Had I applied for funding to test whether a plume existed, my applications would probably all have been rejected, just as I would have faced rejection in the 1980s had I applied to test whether earthquakes resulted from shear slip on faults.

In 1996 we installed the most ambitious seismic network of its kind yet deployed, covering all Iceland, and we operated it in the face of difficulties that sometimes seemed overwhelming. Nevertheless, a huge data set was recorded, which culminated in my sitting at my desk, late one dark, damp night in the last November of the 20th century, puzzling over a low-wave-speed anomaly that wasn’t the shape we had expected. Everyone got the same answer, not everyone thought it mattered, but I don’t like not understanding things.

It dawned on me that night what the data were shouting at us to hear. Although we could not see the bottom of the anomaly, its odd shape was telling us that it terminated in the transition zone – it did not extend deeper into the lower mantle. What did this mean? We rushed off a paper to Science, only to find that this very week Science published a whole-mantle tomography paper spectacularly confirming our conclusion. I went to the December 1999 American GeophysicalUnion meeting in San Francisco with the message that “the Icelandic plume” was different from what we had hitherto thought. It was an “upper mantle plume”.

Then Bruce Julian introduced me to Don Anderson.

“Maybe there isn’t a plume there,” he suggested. I was dumbstruck with astonishment.

Since that moment I have worked to see for myself whether such an hypothesis can stand that Iceland does not owe its volcanism to a deep mantle plume but to processes rooted at shallow depth. And if such an hypothesis can stand for Iceland, can it also stand for other places traditionally assumed to be underlain by plumes? One does not lightly fly in the face of an almost-universally-assumed, cross disciplinary paradigm of global importance that underpins concepts in almost every branch of Earth science. Nor is it easy to re-examine those concepts with access to little that is not selectively interpreted in terms of the traditional assumptions. I started by turning Iceland inside out. What did it imply for the celebrated “hot spot track”? Could we be certain that the high mantle temperatures assumed to exist there really do? How can we explain the high elevation, apparently vast magmatic volumes and the exotic geochemistry, claimed to come from the lower mantle? Most critically, if a deep mantle plume does not cause Iceland, then what does?

After a long journey, not easily traveled, I am persuaded by the data that melting anomalies arise from processes rooted in the shallow mantle, and not from deep mantle plumes. The data demand it too loudly and clearly to be denied. Extraordinarily few observations fit the Plume hypothesis – fewer, it seems, even than might be expected simply from random chance. The lengths to which the scientific community has had to go in order to cram the distorted plume foot into the glass data slipper, and the unfortunate departures from rigorous scientific practise that this has necessitated, are wholly consistent with this conclusion and have reinforced my conviction that the Plume hypothesis cannot be right.

I did not travel alone on my journey. Don and I were soon joined by others who also cannot bring themselves to embrace an hypothesis that does not fit the observations. A core group accreted – the PT group – that now numbers 16. Email and the internet, unavailable to most scientists before the 1990s, made possible a cross disciplinary, international, collaboration unique in my experience, that grew to be characterized by the totally free, unrestricted, and unselfish exchange of data, ideas, and mutual support. My journey would have been impossible without the support of this group, and that of a wider community of scientists who want to know the truth. In this context my gratitude is due in particular to David Abt, Ercan Aldanmaz, Andrew Alden, Ken Bailey, Ajoy Baksi, Tiffany Barry, Erin Beutel, Axel Bj örnsson, Scott Bryan, Evgenii Burov, Maria Clara Castro, Fran çoise ChalotPrat, Bob Christiansen, Peter Clift, Valerie Clouard, Piero Comin-Chiaramonti, Gerry Czamanske, Graham Dauncey, Jon Davidson, Don DePaulo, Arwen Deuss, Carlo Doglioni, Tony Doré, Adam Dziewonski, Linda Elkins-Tanton, Wolf Elston, Derek Fairhead, Trevor Falloon, Luca Ferrari, Carol Finn, Godfrey Fitton, Edward Garnero, Laurent Geoffroy, Laurent Gernigon, Phil Gibbard, William Glen, David Green, Jeff Gu, Gudmundur Gudfinnsson, Tristram Hales, Warren Hamilton, Karen Harpp, Robert Harris, Anne Hofmeister, Jack Holden, Bob Holdsworth, Peter Hooper, Stephanie Ingle, Ted Irving, Garrett Ito, Alexei Ivanov, Mark Jancin, Jeremy McCreary, Adrian Jones, Stephen Jones, Brennan Jordan, Fred Jourdan, Bruce Julian, Donna Jurdy, Mehmet Keskin, Scott King, Jun Korenaga, Lotte Melchior Larsen, Thorne Lay, Jean-Paul Lié geois, Erik Lundin, Michele Lustrino, Hidehisa Mashima, Rajat Mazumder, Greg McHone, Anders Meibom, Romain Meyer, Andrew Moore, Jason Morgan, Jim Natland, Ted Nield, Yaoling Niu, Ian Norton, John O’ Connor, Mike O’Hara, Keith Orford, Giuliano Panza, Angelo Peccerillo, Carole Petit, Sé bastien Pilet, Dean Presnall, Chris Reese, Jeroen Ritsema, Sergio Rocchi, Peter Rona, William Sager, Valenti Sallares, David Sandwell, Anders Schersten, James Sears, Hetu Sheth, Tom Sisson, Norman Sleep, Alan Smith, Alexander Sobolev, Carol Stein, Seth Stein, Martyn Stoker, William Stuart, Michael Summerfield, Benoit Tauzin, Marissa Tejada, Ingrid Ukstins Peate, Peter Vogt, Jerry Winterer, and Kerrie Yates.

Geological time scale

542 Ma to present Phanerozoic Eon 66 Ma to 0 Ma    Cenozoic Era 2.6 to 0 Ma       Quaternary Period1 11,400 a to 0 a          Holocene Epoch 2.6 Ma to 11,400 a          Pleistocene Epoch 23 to 2.6 Ma       Neogene Period 5.3 to 2.6 Ma          Pliocene Epoch 23 to 5.3 Ma          Miocene Epoch 66 to 23 Ma       Paleogene Period 34 to 23 Ma          Oligocene Epoch 56 to 34 Ma          Eocene Epoch 66 to 56 Ma          Paleocene Epoch 245 to 66 Ma    Mesozoic Era 146 to 66 Ma       Cretaceous Period 201 to 146 Ma       Jurassic Period 251 to 200 Ma       Triassic Period 544 to 251 Ma    Paleozoic Era 299 to 251 Ma       Permian Period 318 to 299 Ma       Carboniferous Period (Pennsylvanian) 359 to 318 Ma       Carboniferous Period (Mississippian) 416 to 359 Ma       Devonian Period 444 to 416 Ma       Silurian Period 488 to 444 Ma       Ordovician Period 542 to 488 Ma       Cambrian Period 2500 to 542 Ma Proterozoic Eon 1000 to 542 Ma    Neoproterozoic Era 1600 to 1000 Ma    Mesoproterozoic Era 2500 to 1600 Ma    Paleoproterozoic Era 3800 to 2500 Ma Archaean Eon 4570 to 3800 Ma Hadean Eon

1 As a result of a review by the ICS of late Cenozoic subdivisions, and subsequent ratification by the IUGS, the Quaternary was formally made a Period co-terminus with the Pleistocene at 2.6 Ma, in 2009. This settles a longstanding confusion regarding terminology and subdivisions in the late Cenozoic (Head et al., 2008).

1

From plate tectonics to plumes, and back again

Je n’avais pas besoin de cette hypothèse-là.1

-Pierre-Simon Laplace (1749–1827)1

1.1 Volcanoes, and exceptional volcanoes

Volcanoes are among the most extraordinary natural phenomena on Earth. They are powerful shapers of the surface, they affect the make-up of the oceans, the atmosphere and the land on which we stand, and they ultimately incubate life itself. They have inspired fascination and speculation for centuries, and intense scientific study for decades, and it is thus astonishing that the ultimate origin of some of the greatest and most powerful of them is still not fully understood.

The reasons why spectacular volcanic provinces such as Hawaii, Iceland and Yellowstone exist are currently a major controversy. The fundamental question is the link between volcanism and dynamic processes in the mantle, the processes that make Earth unique in the solar system, and keep us alive. The hunt for the truth is extraordinarily cross-disciplinary and virtually every subject within Earth science bears on the problem. There is something for everyone in this remarkable subject and something that everyone can contribute.

The discovery of plate tectonics, hugely cross-disciplinary in itself, threw light on the causes and effects of many kinds of volcano, but it also threw into sharp focus that many of the largest and most remarkable ones seem to be exceptions to the general rule. It is the controversy over the origin of these volcanoes – the ones that seem to be exceptional – that is the focus of this book.

1.2 Early beginnings: Continental drift and its rejection

Speculations regarding the cause of volcanoes began early in the history of science. Prior to the emergence of the scientific method during the Renaissance, explanations for volcanic eruptions were based largely on religion. Mt Hekla, Iceland, was considered to be the gate of Hell. Eruptions occurred when the gate opened and the Devil dragged condemned souls out of Hell, cooling them on the snowfields of Iceland to prevent them from becoming used to the heat of Hell. Athanasius Kircher (1602–1680) provided an early pictorial representation of then contemporary thought (Fig. 1.1) that has much in common with some theories still popular today (Fig. 1.2). The agent provocateur might be forgiven for wondering how much progress in fundamental understanding we have actually made over the last few centuries.

Figure 1.1 Kircher’s model of the fires of the interior of Earth, from his Mundus Subterraneus, published in 1664 (Kircher, 1664–1678)

The foundations of modern opinion about the origin of volcanoes were really laid by the work of Alfred Wegener (1880–1930). His pivotal book Die Entstehung der Kontinente undOzeane (The Origin of Continents and Oceans; Wegener, 1924), first published in 1915, proposed that continents now widely separated had once been joined together in a single supercontinent. According to Wegener, this super-continent broke up and the pieces separated and drifted apart by thousands of kilometers (Fig. 1.3). The idea was not new, but Wegener’s treatment of it was, and his work ultimately led to one of the greatest paradigm shifts Earth science has ever seen. He assembled a powerful multidisciplinary suite of scientific observations to support continental break-up, and developed ideas for the mechanism of drift and the forces that power it. He detailed correlations of fossils, mountain ranges, palaeoclimates and geological formations between continents and across wide oceans. He called the great mother supercontinent Pangaea (“all land”). He was fired with enthusiasm and energized by an inspired personal conviction of the rightness of his hypothesis.

Figure 1.2 Schematic cross-section of the Earth showing the Plume model (left, modified from Courtillot et al., 2003) and the Plate model (right). The left side illustrates two proposed kinds of plumes – narrow tubes and giant upwellings. The deep mantle or core provides the material and the heat and large, isolated but accessible chemical reservoirs. Slabs penetrate deep. In the Plate model, depths of recycling are variable and volcanism is concentrated in extensional regions. The upper mantle is inhomogeneous and active and the lower mantle is isolated, sluggish, and inaccessible to surface volcanism. The locations of melting anomalies are governed by stress conditions and mantle fertility. The mantle down to ∼ 1000 km contains recycled materials of various ages and on various scales (from Anderson, 2005). See Plate 1

Tragically, during his lifetime, Wegener’s ideas received little support from mainstream geology and physics. On the contrary, they attracted dismissal, ridicule, hostility and even contempt from influential contemporaries. Wegener’s proposed driving mechanism for the continents was criticized. He suggested that the Earth’s centrifugal and tidal forces drove them, an effect that geologists felt was implausibly small. Furthermore, although he emphasized that the sub-crustal region was viscous and could flow, a concept well established and already accepted as a result of knowledge of isostasy, the fate of the oceanic crust was still a difficult problem. A critical missing piece of the jigsaw was that the continental and oceanic crusts were moving as one. Wegener envisaged the continents as somehow to be moving through the oceanic crust, but critics pointed out that evidence for the inevitable crustal deformations was lacking.