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Contributors
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- By Mitchell Aboulafia, Frederick Adams, Marilyn McCord Adams, Robert M. Adams, Laird Addis, James W. Allard, David Allison, William P. Alston, Karl Ameriks, C. Anthony Anderson, David Leech Anderson, Lanier Anderson, Roger Ariew, David Armstrong, Denis G. Arnold, E. J. Ashworth, Margaret Atherton, Robin Attfield, Bruce Aune, Edward Wilson Averill, Jody Azzouni, Kent Bach, Andrew Bailey, Lynne Rudder Baker, Thomas R. Baldwin, Jon Barwise, George Bealer, William Bechtel, Lawrence C. Becker, Mark A. Bedau, Ernst Behler, José A. Benardete, Ermanno Bencivenga, Jan Berg, Michael Bergmann, Robert L. Bernasconi, Sven Bernecker, Bernard Berofsky, Rod Bertolet, Charles J. Beyer, Christian Beyer, Joseph Bien, Joseph Bien, Peg Birmingham, Ivan Boh, James Bohman, Daniel Bonevac, Laurence BonJour, William J. Bouwsma, Raymond D. Bradley, Myles Brand, Richard B. Brandt, Michael E. Bratman, Stephen E. Braude, Daniel Breazeale, Angela Breitenbach, Jason Bridges, David O. Brink, Gordon G. Brittan, Justin Broackes, Dan W. Brock, Aaron Bronfman, Jeffrey E. Brower, Bartosz Brozek, Anthony Brueckner, Jeffrey Bub, Lara Buchak, Otavio Bueno, Ann E. Bumpus, Robert W. Burch, John Burgess, Arthur W. Burks, Panayot Butchvarov, Robert E. Butts, Marina Bykova, Patrick Byrne, David Carr, Noël Carroll, Edward S. Casey, Victor Caston, Victor Caston, Albert Casullo, Robert L. Causey, Alan K. L. Chan, Ruth Chang, Deen K. Chatterjee, Andrew Chignell, Roderick M. Chisholm, Kelly J. Clark, E. J. Coffman, Robin Collins, Brian P. Copenhaver, John Corcoran, John Cottingham, Roger Crisp, Frederick J. Crosson, Antonio S. Cua, Phillip D. Cummins, Martin Curd, Adam Cureton, Andrew Cutrofello, Stephen Darwall, Paul Sheldon Davies, Wayne A. Davis, Timothy Joseph Day, Claudio de Almeida, Mario De Caro, Mario De Caro, John Deigh, C. F. Delaney, Daniel C. Dennett, Michael R. DePaul, Michael Detlefsen, Daniel Trent Devereux, Philip E. Devine, John M. Dillon, Martin C. Dillon, Robert DiSalle, Mary Domski, Alan Donagan, Paul Draper, Fred Dretske, Mircea Dumitru, Wilhelm Dupré, Gerald Dworkin, John Earman, Ellery Eells, Catherine Z. Elgin, Berent Enç, Ronald P. Endicott, Edward Erwin, John Etchemendy, C. Stephen Evans, Susan L. Feagin, Solomon Feferman, Richard Feldman, Arthur Fine, Maurice A. Finocchiaro, William FitzPatrick, Richard E. Flathman, Gvozden Flego, Richard Foley, Graeme Forbes, Rainer Forst, Malcolm R. Forster, Daniel Fouke, Patrick Francken, Samuel Freeman, Elizabeth Fricker, Miranda Fricker, Michael Friedman, Michael Fuerstein, Richard A. Fumerton, Alan Gabbey, Pieranna Garavaso, Daniel Garber, Jorge L. A. Garcia, Robert K. Garcia, Don Garrett, Philip Gasper, Gerald Gaus, Berys Gaut, Bernard Gert, Roger F. Gibson, Cody Gilmore, Carl Ginet, Alan H. Goldman, Alvin I. Goldman, Alfonso Gömez-Lobo, Lenn E. Goodman, Robert M. Gordon, Stefan Gosepath, Jorge J. E. Gracia, Daniel W. Graham, George A. Graham, Peter J. Graham, Richard E. Grandy, I. Grattan-Guinness, John Greco, Philip T. Grier, Nicholas Griffin, Nicholas Griffin, David A. Griffiths, Paul J. Griffiths, Stephen R. Grimm, Charles L. Griswold, Charles B. Guignon, Pete A. Y. Gunter, Dimitri Gutas, Gary Gutting, Paul Guyer, Kwame Gyekye, Oscar A. Haac, Raul Hakli, Raul Hakli, Michael Hallett, Edward C. Halper, Jean Hampton, R. James Hankinson, K. R. Hanley, Russell Hardin, Robert M. Harnish, William Harper, David Harrah, Kevin Hart, Ali Hasan, William Hasker, John Haugeland, Roger Hausheer, William Heald, Peter Heath, Richard Heck, John F. Heil, Vincent F. Hendricks, Stephen Hetherington, Francis Heylighen, Kathleen Marie Higgins, Risto Hilpinen, Harold T. Hodes, Joshua Hoffman, Alan Holland, Robert L. Holmes, Richard Holton, Brad W. Hooker, Terence E. Horgan, Tamara Horowitz, Paul Horwich, Vittorio Hösle, Paul Hoβfeld, Daniel Howard-Snyder, Frances Howard-Snyder, Anne Hudson, Deal W. Hudson, Carl A. Huffman, David L. Hull, Patricia Huntington, Thomas Hurka, Paul Hurley, Rosalind Hursthouse, Guillermo Hurtado, Ronald E. Hustwit, Sarah Hutton, Jonathan Jenkins Ichikawa, Harry A. Ide, David Ingram, Philip J. Ivanhoe, Alfred L. Ivry, Frank Jackson, Dale Jacquette, Joseph Jedwab, Richard Jeffrey, David Alan Johnson, Edward Johnson, Mark D. Jordan, Richard Joyce, Hwa Yol Jung, Robert Hillary Kane, Tomis Kapitan, Jacquelyn Ann K. Kegley, James A. Keller, Ralph Kennedy, Sergei Khoruzhii, Jaegwon Kim, Yersu Kim, Nathan L. King, Patricia Kitcher, Peter D. Klein, E. D. Klemke, Virginia Klenk, George L. Kline, Christian Klotz, Simo Knuuttila, Joseph J. Kockelmans, Konstantin Kolenda, Sebastian Tomasz Kołodziejczyk, Isaac Kramnick, Richard Kraut, Fred Kroon, Manfred Kuehn, Steven T. Kuhn, Henry E. Kyburg, John Lachs, Jennifer Lackey, Stephen E. Lahey, Andrea Lavazza, Thomas H. Leahey, Joo Heung Lee, Keith Lehrer, Dorothy Leland, Noah M. Lemos, Ernest LePore, Sarah-Jane Leslie, Isaac Levi, Andrew Levine, Alan E. Lewis, Daniel E. Little, Shu-hsien Liu, Shu-hsien Liu, Alan K. L. Chan, Brian Loar, Lawrence B. Lombard, John Longeway, Dominic McIver Lopes, Michael J. Loux, E. J. Lowe, Steven Luper, Eugene C. Luschei, William G. Lycan, David Lyons, David Macarthur, Danielle Macbeth, Scott MacDonald, Jacob L. Mackey, Louis H. Mackey, Penelope Mackie, Edward H. Madden, Penelope Maddy, G. B. Madison, Bernd Magnus, Pekka Mäkelä, Rudolf A. Makkreel, David Manley, William E. Mann (W.E.M.), Vladimir Marchenkov, Peter Markie, Jean-Pierre Marquis, Ausonio Marras, Mike W. Martin, A. P. Martinich, William L. McBride, David McCabe, Storrs McCall, Hugh J. McCann, Robert N. McCauley, John J. McDermott, Sarah McGrath, Ralph McInerny, Daniel J. McKaughan, Thomas McKay, Michael McKinsey, Brian P. McLaughlin, Ernan McMullin, Anthonie Meijers, Jack W. Meiland, William Jason Melanson, Alfred R. Mele, Joseph R. Mendola, Christopher Menzel, Michael J. Meyer, Christian B. Miller, David W. Miller, Peter Millican, Robert N. Minor, Phillip Mitsis, James A. Montmarquet, Michael S. Moore, Tim Moore, Benjamin Morison, Donald R. Morrison, Stephen J. Morse, Paul K. Moser, Alexander P. D. Mourelatos, Ian Mueller, James Bernard Murphy, Mark C. Murphy, Steven Nadler, Jan Narveson, Alan Nelson, Jerome Neu, Samuel Newlands, Kai Nielsen, Ilkka Niiniluoto, Carlos G. Noreña, Calvin G. Normore, David Fate Norton, Nikolaj Nottelmann, Donald Nute, David S. Oderberg, Steve Odin, Michael O’Rourke, Willard G. Oxtoby, Heinz Paetzold, George S. Pappas, Anthony J. Parel, Lydia Patton, R. P. Peerenboom, Francis Jeffry Pelletier, Adriaan T. Peperzak, Derk Pereboom, Jaroslav Peregrin, Glen Pettigrove, Philip Pettit, Edmund L. Pincoffs, Andrew Pinsent, Robert B. Pippin, Alvin Plantinga, Louis P. Pojman, Richard H. Popkin, John F. Post, Carl J. Posy, William J. Prior, Richard Purtill, Michael Quante, Philip L. Quinn, Philip L. Quinn, Elizabeth S. Radcliffe, Diana Raffman, Gerard Raulet, Stephen L. Read, Andrews Reath, Andrew Reisner, Nicholas Rescher, Henry S. Richardson, Robert C. Richardson, Thomas Ricketts, Wayne D. Riggs, Mark Roberts, Robert C. Roberts, Luke Robinson, Alexander Rosenberg, Gary Rosenkranz, Bernice Glatzer Rosenthal, Adina L. Roskies, William L. Rowe, T. M. Rudavsky, Michael Ruse, Bruce Russell, Lilly-Marlene Russow, Dan Ryder, R. M. Sainsbury, Joseph Salerno, Nathan Salmon, Wesley C. Salmon, Constantine Sandis, David H. Sanford, Marco Santambrogio, David Sapire, Ruth A. Saunders, Geoffrey Sayre-McCord, Charles Sayward, James P. Scanlan, Richard Schacht, Tamar Schapiro, Frederick F. Schmitt, Jerome B. Schneewind, Calvin O. Schrag, Alan D. Schrift, George F. Schumm, Jean-Loup Seban, David N. Sedley, Kenneth Seeskin, Krister Segerberg, Charlene Haddock Seigfried, Dennis M. Senchuk, James F. Sennett, William Lad Sessions, Stewart Shapiro, Tommie Shelby, Donald W. Sherburne, Christopher Shields, Roger A. Shiner, Sydney Shoemaker, Robert K. Shope, Kwong-loi Shun, Wilfried Sieg, A. John Simmons, Robert L. Simon, Marcus G. Singer, Georgette Sinkler, Walter Sinnott-Armstrong, Matti T. Sintonen, Lawrence Sklar, Brian Skyrms, Robert C. Sleigh, Michael Anthony Slote, Hans Sluga, Barry Smith, Michael Smith, Robin Smith, Robert Sokolowski, Robert C. Solomon, Marta Soniewicka, Philip Soper, Ernest Sosa, Nicholas Southwood, Paul Vincent Spade, T. L. S. Sprigge, Eric O. Springsted, George J. Stack, Rebecca Stangl, Jason Stanley, Florian Steinberger, Sören Stenlund, Christopher Stephens, James P. Sterba, Josef Stern, Matthias Steup, M. A. Stewart, Leopold Stubenberg, Edith Dudley Sulla, Frederick Suppe, Jere Paul Surber, David George Sussman, Sigrún Svavarsdóttir, Zeno G. Swijtink, Richard Swinburne, Charles C. Taliaferro, Robert B. Talisse, John Tasioulas, Paul Teller, Larry S. Temkin, Mark Textor, H. S. Thayer, Peter Thielke, Alan Thomas, Amie L. Thomasson, Katherine Thomson-Jones, Joshua C. Thurow, Vzalerie Tiberius, Terrence N. Tice, Paul Tidman, Mark C. Timmons, William Tolhurst, James E. Tomberlin, Rosemarie Tong, Lawrence Torcello, Kelly Trogdon, J. D. Trout, Robert E. Tully, Raimo Tuomela, John Turri, Martin M. Tweedale, Thomas Uebel, Jennifer Uleman, James Van Cleve, Harry van der Linden, Peter van Inwagen, Bryan W. Van Norden, René van Woudenberg, Donald Phillip Verene, Samantha Vice, Thomas Vinci, Donald Wayne Viney, Barbara Von Eckardt, Peter B. M. Vranas, Steven J. Wagner, William J. Wainwright, Paul E. Walker, Robert E. Wall, Craig Walton, Douglas Walton, Eric Watkins, Richard A. Watson, Michael V. Wedin, Rudolph H. Weingartner, Paul Weirich, Paul J. Weithman, Carl Wellman, Howard Wettstein, Samuel C. Wheeler, Stephen A. White, Jennifer Whiting, Edward R. Wierenga, Michael Williams, Fred Wilson, W. Kent Wilson, Kenneth P. Winkler, John F. Wippel, Jan Woleński, Allan B. Wolter, Nicholas P. Wolterstorff, Rega Wood, W. Jay Wood, Paul Woodruff, Alison Wylie, Gideon Yaffe, Takashi Yagisawa, Yutaka Yamamoto, Keith E. Yandell, Xiaomei Yang, Dean Zimmerman, Günter Zoller, Catherine Zuckert, Michael Zuckert, Jack A. Zupko (J.A.Z.)
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Contributors
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- By Jane E. Adcock, Yahya Aghakhani, A. Anand, Eva Andermann, Frederick Andermann, Alexis Arzimanoglou, Sandrine Aubert, Nadia Bahi-Buisson, Carman Barba, Agatino Battaglia, Geneviève Bernard, Nadir E. Bharucha, Laurence A. Bindoff, William Bingaman, Francesca Bisulli, Thomas P. Bleck, Stewart G. Boyd, Andreas Brunklaus, Harry Bulstrode, Jorge G. Burneo, Laura Canafoglia, Laura Cantonetti, Roberto H. Caraballo, Fernando Cendes, Kevin E. Chapman, Patrick Chauvel, Richard F. M. Chin, H. T. Chong, Fahmida A. Chowdhury, Catherine J. Chu-Shore, Rolando Cimaz, Andrew J. Cole, Bernard Dan, Geoffrey Dean, Alessio De Ciantis, Fernando De Paolis, Rolando F. Del Maestro, Irissa M. Devine, Carlo Di Bonaventura, Concezio Di Rocco, Henry B. Dinsdale, Maria Alice Donati, François Dubeau, Michael Duchowny, Olivier Dulac, Monika Eisermann, Brent Elliott, Bernt A. Engelsen, Kevin Farrell, Natalio Fejerman, Rosalie E. Ferner, Silvana Franceschetti, Robert Friedlander, Antonio Gambardella, Hector H. Garcia, Serena Gasperini, Lorenzo Genitori, Gioia Gioi, Flavio Giordano, Leif Gjerstad, Daniel G. Glaze, Howard P. Goodkin, Sidney M. Gospe, Andrea Grassi, William P. Gray, Renzo Guerrini, Marie-Christine Guiot, William Harkness, Andrew G. Herzog, Linda Huh, Margaret J. Jackson, Thomas S. Jacques, Anna C. Jansen, Sigmund Jenssen, Michael R. Johnson, Dorothy Jones-Davis, Reetta Kälviäinen, Peter W. Kaplan, John F. Kerrigan, Autumn Marie Klein, Matthias Koepp, Edwin H. Kolodny, Kandan Kulandaivel, Ruben I. Kuzniecky, Ahmed Lary, Yolanda Lau, Anna-Elina Lehesjoki, Maria K. Lehtinen, Holger Lerche, Michael P. T. Lunn, Snezana Maljevic, Mark R. Manford, Carla Marini, Bindu Menon, Giulia Milioli, Eli M. Mizrahi, Manish Modi, Márcia Elisabete Morita, Manuel Murie-Fernandez, Vivek Nambiar, Lina Nashef, Vincent Navarro, Aidan Neligan, Ruth E. Nemire, Charles R. J. C. Newton, John O'Donavan, Hirokazu Oguni, Teiichi Onuma, Andre Palmini, Eleni Panagiotakaki, Pasquale Parisi, Elena Parrini, Liborio Parrino, Ignacio Pascual-Castroviejo, M. Scott Perry, Perrine Plouin, Charles E. Polkey, Suresh S. Pujar, Karthik Rajasekaran, R. Eugene Ramsey, Rahul Rathakrishnan, Roberta H. Raven, Guy M. Rémillard, David Rosenblatt, M. Elizabeth Ross, Abdulrahman Sabbagh, P. Satishchandra, Swati Sathe, Ingrid E. Scheffer, Philip A. Schwartzkroin, Rod C. Scott, Frédéric Sedel, Michelle J. Shapiro, Elliott H. Sherr, Michael Shevell, Simon D. Shorvon, Adrian M. Siegel, Gagandeep Singh, S. Sinha, Barbara Spacca, Waney Squier, Carl E. Stafstrom, Bernhard J. Steinhoff, Andrea Taddio, Gianpiero Tamburrini, C. T. Tan, Raymond Y. L. Tan, Erik Taubøll, Robert W. Teasell, Mario Giovanni Terzano, Federica Teutonico, Suzanne A. Tharin, Elizabeth A. Thiele, Pierre Thomas, Paolo Tinuper, Dorothée Kasteleijn-Nolst Trenité, Sumeet Vadera, Pierangelo Veggiotti, Jean-Pierre Vignal, J. M. Walshe, Elizabeth J. Waterhouse, David Watkins, Ruth E. Williams, Yue-Hua Zhang, Benjamin Zifkin, Sameer M. Zuberi
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6 - The plate mode of convection
- Geoffrey F. Davies, Australian National University, Canberra
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- Mantle Convection for Geologists
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- 03 May 2011
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Summary
Mechanical properties change with temperature, from brittle plate to yielding mantle and back. This strongly affects their dynamical behaviour and their influence on convection. Plates organise the flow. Internal heating versus bottom heating also affects the form of convection. The plate cycle (formation, cooling, subduction, reabsorption) is convection. The plate mode of mantle convection transports a large fraction of Earth's heat budget. Seafloor topography and heat flow can be quantitatively explained with remarkable success.
The convection theory developed in the previous chapter applies to many forms of convection, and it seems to apply reasonably well to mantle convection, but with some important qualifications. Mantle convection takes distinctive forms that in some ways are quite unlike familiar examples of convection such as occur in familiar kinds of fluid. The main reason for the differences is that the mechanical behaviour of mantle rocks changes quite dramatically between the temperature at the Earth's surface and the temperature within the mantle.
The strong lithosphere
The temperature dependence of viscosity shown in Figure 4.4 tells us that reducing the temperature from 1300 °C to 1000 °C will increase the viscosity of mantle rocks by as much as three orders of magnitude – a factor of 1000. However, if the mantle rocks are much cooler than that, they cease to deform like a viscous fluid. Through an intermediate range of temperature they develop ductile shear zones, so that the deformation is concentrated in relatively narrow zones instead of occurring uniformly through the fluid.
Frontmatter
- Geoffrey F. Davies, Australian National University, Canberra
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2 - Context
- Geoffrey F. Davies, Australian National University, Canberra
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Summary
Basic concepts and primary observations. Defining the crust, mantle and core. Distinguishing crust from lithosphere, continents from ocean basins. The distribution of topography and heat flux over the sea floor.
Mantle convection occurs, remarkably enough, in the Earth's mantle. It is affected by the crust, and part of the lithosphere plays a major role. There are peculiarities near the boundary of the mantle with the core that may significantly affect mantle convection, and that certainly tell us some important things about mantle convection. To discuss our subject sensibly, we had better be clear what all these terms refer to: mantle, crust, core, lithosphere and so on. That is one thing this chapter is about. There are also important constraints on mantle convection to be had from the form of the Earth's topography, and from the geographic variation of heat flow from the Earth's interior. These will also be summarised.
Crust, mantle, core
The major division of the Earth's interior is into crust, mantle and core. The boundaries between these regions were detected seismologically, in other words using the internal elastic waves generated by earthquakes, which are detected as they emerge at the Earth's surface. The variation of seismic velocities, and density, with depth in the Earth is shown in Figure 2.1. The boundary between the mantle and the core is at a depth of about 2900 km, where the seismic velocities drop, the shear velocity is zero and the density jumps.
References
- Geoffrey F. Davies, Australian National University, Canberra
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Appendix B - Thermal evolution details
- Geoffrey F. Davies, Australian National University, Canberra
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Mantle Convection for Geologists
- Geoffrey F. Davies
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Mantle convection is the fundamental agent driving many of the geological features observed at the Earth's surface, including plate tectonics and plume volcanism. Yet many Earth scientists have an incomplete understanding of the process. This book describes the physics and fluid dynamics of mantle convection, explaining what it is, how it works, and how to quantify it in simple terms. It assumes no specialist background: mechanisms are explained simply and the required basic physics is fully reviewed and explained with minimal mathematics. The distinctive forms that convection takes in the Earth's mantle are described within the context of tectonic plates and mantle plumes, and implications are explored for geochemistry and tectonic evolution. Common misconceptions and controversies are addressed - providing a straightforward but rigorous explanation of this key process for students and researchers across a variety of geoscience disciplines.
7 - The plume mode of convection
- Geoffrey F. Davies, Australian National University, Canberra
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- Mantle Convection for Geologists
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- 03 February 2011, pp 73-103
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How were plumes conceived? Is there evidence for plumes at the Earth's surface? Hotspot tracks. Inevitable plumes? Hotspot swells, heat and mass transport by the plume mode. Degree of melting. Favoured forms of upwelling and the role of temperature dependence of viscosity. Heads and tails – flood basalts, connecting tracks. Melt production from plume heads, and relation to flood basalts. Irregularities, misfits, puzzles and possibilities – the simple thermal model doesn't explain everything. Thermochemical plumes may account for much of the irregularity.
Scattered across the sea floor are many submarine ridges, seamounts and plateaus (Figure 2.4). These are not obviously related to plate tectonics, as are the mid-ocean ridges and deep ocean trenches. Some of these edifices reach above sea level to form islands. In Chapter 3 I recounted how Tuzo Wilson built on the observations of Darwin and Dana, who had discerned an apparent age progression along island chains in the Pacific, from volcanically active islands through eroded islands to atolls marking where an island had been eroded to sea level. The classic example is the Hawaiian island chain, shown in Figure 7.1. As well as the islands, there is a long chain of seamounts extending to the northwest. There is also a broad swell in the sea floor around the Hawaiian chain. These observations give us important information about processes in the mantle.
1 - Introduction
- Geoffrey F. Davies, Australian National University, Canberra
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Mantle convection is the fundamental agent driving most geology, yet many geologists still have only vague ideas about what mantle convection is, how it works and how it might inform their specialty. Because it is so fundamental, the better every geologist understands mantle convection, the better scientist he or she is likely to be. Of course, not everything is affected by mantle convection, but only by being well informed will a geologist recognise when it is relevant, and what that relevance is.
Misconceptions about mantle convection also seem still to be quite widespread. Some aspects of mantle convection are debated. Much of that debate concerns refinements, so the debate is quite legitimate, but some of the debate is based on misconceptions or incomplete understanding of current theories or observations. The latter debate is not productive. This is not to claim that alternative versions are inconceivable, but just to note that debaters need to be informed about the theories they wish to challenge if they are to make useful contributions.
For these reasons it seems worthwhile to offer an account of our current understanding of mantle convection in terms that are reasonably accessible to most geologists. That means the account should be fairly short, and there should be little mathematics beyond basic algebra and arithmetic. Nor should a strong grasp of physics be assumed, and such physics as is required (notably heat conduction and viscous fluid flow) should be explained in simple and reasonably familiar terms.
Contents
- Geoffrey F. Davies, Australian National University, Canberra
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Index
- Geoffrey F. Davies, Australian National University, Canberra
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10 - Mantle chemical evolution
- Geoffrey F. Davies, Australian National University, Canberra
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Trace element heterogeneity of the mantle; apparent ages. Global budgets and a mildly depleted MORB source. Distinct OIB source, no primitive mantle. Major element heterogeneity, sources and survival. Melting reconsidered; physical partitioning, disequilibrium, melt trapping. Recycling oceanic crust and hybrid pyroxenites. Critique of previous abundance estimates. Geochemical modelling using tracers in convection models. Density differences. Residence times. Ages due to remelting, not homogenising. Incorporating noble gases.
The chemistry of the mantle has already entered this presentation implicitly and explicitly. It is explicit in the discussions of compositional differences and radioactive heating, and it is implicit in that the material properties we have called upon depend of course on the composition of the relevant materials. The geochemistry of the mantle is divided for convenience into major elements, trace elements and isotopes. Trace elements and their isotopes give us some key information, but in order to interpret them properly we need to consider the major elements as well.
Trace elements and isotopes give us some important, though indirect, information about the structure of the mantle, and the isotopes give us some time information as well. These are very important kinds of information that are not available from other sources, so we should take advantage of them if we possibly can. It is turning out that the interpretation of these geochemical observations requires a careful consideration of both geochemical and geophysical processes.
3 - Why moving plates?
- Geoffrey F. Davies, Australian National University, Canberra
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How were plates conceived of? How do we know they move? How were mantle plumes conceived?
The main story of the idea of continental drift and its ultimate transformation into the theory of plate tectonics has been well told (e.g. [12, 13]), and will not be recounted in detail here. However, the particular observations that led to the conception of moving plates are perhaps not so well known, as evidenced by the lack of recognition of the person who first conceived of plates. Also, the evidence that persuaded large segments of the geophysical and broader geological community of the reality of moving plates is worth repeating, so we know we are dealing with a theory well based in observations, and not just accepted on the authority of textbooks already a few decades old. (I say ‘already’ because I came into the Earth sciences in 1968, just after the view of the geophysical community had been transformed, and before many geologists had been persuaded, so of course to me it doesn't seem long ago.)
The lead-up
Alfred Wegener conceived of continental drift around 1912 [14] on the basis of the rough match of continental outlines across the Atlantic Ocean, which he was not the first to notice. He supported the concept with geological and palaeontological evidence. He refuted with sound physics the rival theory of land bridges that had risen and sunk, notably that there should be large and observable gravity anomalies associated with such changes.
8 - Perspective
- Geoffrey F. Davies, Australian National University, Canberra
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A fuller picture of mantle convection. Active plates and active plumes. The distinct roles of plates and plumes in heat transport. Plume tectonics cannot replace plate tectonics. How plates and plumes affect each other.
Plumes are not the return mode of plate flow. There is normally no active upwelling under ridges. There is no significant ‘decoupling’ layer. Return flow is not shallow, ‘drag’ is small. There is no seafloor ‘flattening’, though there is some anomalous seafloor elevation. ‘Superswells’ and residual thermal variations. Layered convection?
Rifts and flood basalts. Superplumes? Small-scale modes? Possible, but evidence is marginal. Edge modes. Drips. Mantle wetspots.
Separate but interacting
The picture of mantle convection developed so far is of two thermal boundary layers, each driving a distinctive form of convection. Because the two modes of convection are so different, it has been useful to consider the thermal boundary layers separately. Of course the two modes do interact, but not as strongly as in low-Rayleigh-number ‘textbook’ convection (in which the modes are tightly coupled; Figure 6.2), as we will discuss after a brief assessment of the story so far.
The top thermal boundary layer is very directly implied by all the observations that indicate a steep temperature gradient near the Earth's surface and a shallower gradient further down. The near-surface temperature gradient is directly measured, and the need for a shallower gradient deeper down is implied by the fact that the mantle is not liquid (from seismology) and by temperatures inferred from magmas reaching the surface.
5 - Convection
- Geoffrey F. Davies, Australian National University, Canberra
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Thermal convection is driven by boundary layers. Buoyancy. How to calculate plate velocities – simple mechanical version. Interpretation of an oceanic plate as a thermal boundary layer. Estimation of plate thickness from thermal conduction. Plate velocities – thermo-viscous version. What is a Rayleigh number? Other useful numbers.
Convection is not familiar to many geologists, so before we get to mantle convection, which has some unusual features, we need to get to know convection in general. If we start from the right place, then convection becomes understandable in straightforward terms. Instead of it being something vague down there that makes things go around, we can know when to expect convection and what will control it.
The key to convection is that it is driven by boundary layers. I will focus for the moment on thermal convection (there is also compositional convection), so thermal convection is driven by thermal boundary layers. If we can identify a thermal boundary layer, and say something about how it behaves, we are well on the way to understanding the convection that it drives.
In the mantle, the relevant thermal boundary layers form at horizontal boundaries. Later we will look at why that is. For the moment, we can use Figure 5.1 as the simplest realisation of what I have just said. There is a hot thermal boundary layer along the bottom boundary of the fluid layer.
4 - Solid, yielding mantle
- Geoffrey F. Davies, Australian National University, Canberra
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Summary
The mantle is solid and deforming, not molten. How do we know? Viscosity. Estimating the viscosity of the mantle. Temperature dependence of deformation of solids. Inevitable convection.
Prior to the twentieth century, geologists had deduced that the interior of the Earth had to be yielding, in order to accommodate the uplift of mountains. Some geologists assumed the interior was molten, but others acknowledged that a yielding solid would be sufficient. The clearest expression of the latter viewpoint is in the famous 1855 paper by Airy [35] in which he proposed an explanation for mountains being approximately in isostatic balance, namely that mountains have thickened crust beneath them. However, Airy also cited a key observation to support the idea of a deformable, solid interior [35]:
This fluidity may be very imperfect; it may be mere viscidity; it may even be little more than that degree of yielding which (as is well known to miners) shows itself by changes in the floors of subterraneous chambers at a great depth [emphasis added] when their width exceeds 20 or 30 feet [7 or 10m]; and this degree of yielding may be sufficient for my present explanation.
There had been debate about whether the interior is presently fluid or whether it had only been fluid when the Earth was forming. It was Hall [36] in 1859 who established that continuing deformation was required, through his observations that sediments throughout thick sedimentary formations had all been deposited in shallow water.
11 - Assimilating mantle convection into geology
- Geoffrey F. Davies, Australian National University, Canberra
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- 03 February 2011, pp 206-207
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The goal of this book has been to present mantle convection as a topic accessible to all geoscientists so that it can become a routine part of their thinking. This is necessary, because it is so fundamental to so many geological processes. It is also possible, as I hope this presentation has demonstrated.
The understanding of mantle convection that has been developed in the book is fairly well established. There remains some debate about a possible layer in the deep mantle, apart from D″, though the debates usually seem to be conducted in ignorance of the heat flow constraint given in Chapter 8. There is also some debate about the existence of mantle plumes, though this persists mainly in circles that are apparently ignorant of the physics of mantle upwellings given in Chapter 7. In any case readers should be able to follow the essence of such continuing debates, which are a normal part of any scientific topic. Hopefully they will be armed with a better-informed perspective.
Two kinds of implication have also been discussed in this book: for the tectonic evolution of the Earth, and for the chemistry of the mantle. The early tectonic evolution of the Earth is still unclear. Observational constraints are so sparse and possibilities still so diverse that progress will require a continuing conversation among all those involved, and a grasp of the essence of mantle evolution models will be important.
Appendix A - Exponential growth and decay
- Geoffrey F. Davies, Australian National University, Canberra
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- Mantle Convection for Geologists
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At a number of places in this book, a situation has given rise to exponential growth or decay. Although these situations have been analysed without using calculus, as promised, it may be useful to those who know some basic calculus to present the more rigorous solution that calculus allows. Exponential behaviour arises in a standard way, so we can start with a general situation. The resulting solution can then be adapted to particular situations by appropriately identifying the variables. Some of the particular situations will be covered in this appendix, whereas others will be covered in later appendices.
Exponential solution
Suppose something is growing larger, and the rate at which it grows is proportional to its present size. Let's call the something y and its rate of growth v.
Appendix C - Chemical evolution details
- Geoffrey F. Davies, Australian National University, Canberra
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- Mantle Convection for Geologists
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- 03 February 2011, pp 216-217
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