Pure Appl. Geophys. Ó 2013 Springer Basel DOI 10.1007/s00024-013-0653-2
Pure and Applied Geophysics
Book Review ‘‘Core Dynamics’’, Treatise on Geophysics: vol. 8, editor-in-chief G. Schubert, volume editor Peter Olson, Elsevier, 2009; ISBN: 978-0-444-53457-6 MAURIZIO MATTESINI1,2 Core Dynamics is volume eight of the landmark (ten-volumes long) Treatise on Geophysics edited by Prof. Gerald Schubert. It is an excellently written textbook intended to provide an overview of dynamic effects that are taking place at the heart of our planet. Tackled topics include structure and composition of the outer and inner core, energetics of the core, convection, rotation, flow and turbulence in the outer core, solidification of the inner core, dynamo theory and numerical dynamo models, magnetic polarity reversals, laboratory experiments on the core and the geodynamics and the interactions between the core and the other parts of the Earth. The textbook is structured very clearly in 12 chapters, summing up to a total of 358 pages. It starts with a general overview of core dynamics and then explores each topic in detail, thus, making it useful to both experts and graduate-level researchers. It is richly illustrated with clear figures and tables that really aid in understanding key concepts. Extensive reference lists and cross references with related titles in geophysics to facilitate further research are also provided. The book opens with an introductory Chapter 1 written by the volume’s editor, P. Olson, that covers views of the past, present, and future in the field of the Earth’s core. The use of this first chapter to provide the motivation for studying key core dynamics questions is a great addition to the content of the book, and will undoubtedly prove valuable for
1 Departamento de Fı´sica de la Tierra, Astronomı´a y Astrofı´sica I, Universidad Complutense de Madrid, 28040 Madrid, Spain. E-mail:
[email protected] 2 Instituto de Geociencias (UCM-CSIC), Facultad de Ciencias Fı´sicas, Plaza de Ciencias 1, 28040 Madrid, Spain.
students and readers with little prior knowledge in the field. In Chapter 2, F. Nimmo analyses the energetics of the core, which is an important Earth’s issue of longstanding and on-going controversy. The author first derives global balances of energy and entropy giving special attention to the power requirements and the long-term consequences of inner-core growth. A young inner-core scenario is then introduced and discussed in relation to an end-member model in which the core contains appreciable radioactive heat sources. The important elements of the dynamo theory are set out by P. H. Roberts in Chapter 3. The author starts defining what a fluid dynamo is, the meaning of a self-sustaining dynamo, and the difference between kinematic, and MHD dynamos. Dynamo theory starts from the pre-Maxwell equations for electromagnetic induction in a moving, conducting fluid. Then, the bases for kinematic dynamos in both Cartesian and spherical geometries are presented. The chapter also includes a full account of the very influential meanfield electrodynamics, the statistical treatment of the induction effects of turbulent fluid motion. In Chapter 4, R. Holme summarizes what we know about the actual fluid motions in the Earth’s core, both in the present-day and in the past, as far back in time as the geomagnetic record allows inferring them. The author reviews the frozen-flux hypothesis, including its assumptions and limitations, and then goes on to summarize the important aspects of the core flow that the applications of this technique have revealed. Holme also discusses several interesting interactions between the core and the mantle related to core flow, including the decade scale variations in the rate rotations.
M. Mattesini
Pure Appl. Geophys.
Chapter 5, written by C. A. Jones, does an outstanding job describing the thermal and compositional convection in the liquid outer-core. After introducing the basic physical principles involved in thermochemical convection in the core, Jones derives the basic equations governing the convection, with emphasis on the effects of compressibility. A systematic tour of the physics of convection is then conducted, starting with a model of high Rayleigh number convection in a non-rotating, electrically insulating fluid, then progressively adding the effects of rotation and magnetic fields, ending up with a model of core convection in which all of the known physical ingredients are present and interacting with each other. In Chapter 6, D. E. Loper gives a connected account of turbulence and the small-scale dynamics of the core. He starts by describing the special conditions for small-scale dynamics in the core, an environment dominated by Coriolis and Lorentz forces due to planetary rotation and the core’s magnetic field, respectively. Interpreting the core as a turbulent MHD system, Loper finds that several dynamically distinct regimes are expected, including complex boundary layers near the inner-core and outer-core boundaries with overlapping magnetic, viscous, and thermal layers. Chapter 7 on rotational dynamics by A. Tilgner is also very nice, as it describes the state of knowledge on rotation-induced core flows. The chapter begins deriving the equations of motion for a fluid rotating about a variable axis, in which the transverse or Poincare´ acceleration appears in addition to the familiar Coriolis and centripetal accelerations. The central problem in this subject is the flow induced by precession. He summarizes the properties of inertial oscillations and the structure and transport properties of viscous Ekman layers. The important problem of the flow induced by precession is also addressed. Tilgner begins by presenting the classical solutions to the inviscid problem, and then introduces the complexities of a real fluid, first the effects of viscosity, then instabilities, tidal excitation and finally nonlinear interaction with convective flows. Chapter 8, written by U. R. Christensen and J. Wicht, then follows with descriptions of numerical dynamo simulations. The authors first review the
governing equations for geodynamo in their basic, Boussinesq form, including the fundamental dimensionless parameters that control the system. They also define the critical dimensionless output parameters that characterize each dynamo model solution and provide the basis for comparison with a geodynamo. The chapter concludes with a description of the main classes of dynamo models found to date, describing their changes in structure as the main controlling parameters. Chapter 9 (G. A. Glatzmaier and R. S. Coe) examines the behaviour of polarity reversals in numerical dynamo models in light of the evidence on the nature of reversals as seen in the paleomagnetic record. The authors analyse some specific numerical examples of reversals types, reaching the general conclusion that chaotic, nearly random reversing dynamos better approximate the observed reversal behaviour as measured by the main geomagnetic polarity timescale. There is also a useful and enlightening discussion about dynamo model sensitivity to mantle heterogeneity, particularly to thermal heterogeneity at the core-mantle boundary. Chapter 10 by I. Sumita and M. I. Bergman covers the inner core dynamics, discussing the various roles the inner core plays in core dynamics and the geodynamo. This chapter first describes the current understanding of the phase diagram of the core, which is critical for properly interpreting the effects of inner-core growth, and for estimating the presentday thermal regime of the core. They then use the information from the phase diagram as the basis for a discussion of the small-scale processes near the innerouter core boundary that affect and are affected by the solidification. The chapter concludes with a description of the various models that have been advanced to explain the recent and most perplexing observations of the inner-core, such as its heterogeneity, anisotropy, and anomalous rotations. Chapter 11 (P. Cardin and P. Olson) includes an excellent description of the important experiments that have shaped our physical intuition about the flows in the core that produce the geodynamo and govern its evolution. The reader is treated to a cursory excursion in the study of fundamental flow structure in rotating fluids (formation of Taylor columns, viscous layers and non-linear effects). It is worth noting
Book Review
the presence of a unique collection of photographic examples of all the types of flow that are believed to be significant in the Earth’s core. Then, the chapter discusses the experiments on precession-induced flows in spheres, spherical shells and spheroids. Importance is also given to the fundamentals of rotating magnetoconvection as it occurs in laboratory fluid metals with such as mercury, gallium, and molten sodium being described, with a focus on the differences between magnetic versus non-magnetic convection in a rotating fluid. Lastly, attention is placed on the recent successes and future prospects for laboratory self-sustaining fluid dynamos. The book closes in Chapter 12 with a final case study: core-mantle interactions, written by B. A. Buffett. The author examines a wide range of possible ways the mantle can affect core dynamics, and the corresponding ways the core can affect the mantle. The electromagnetic core-mantle interactions described in this chapter include the electromotive force exerted by the core field on the mantle through its interaction with electric currents in the mantle. Mechanical interactions were also considered in
terms of core-mantle boundary topography and roughness on core dynamics. Finally, the author summarizes the evidence for and against active chemical interaction between the mantle and the core, including the mechanisms through which mass exchange across the core-mantle boundary might occur. There are several aspects of this book that distinguish it from other similar textbooks, although the most evident one is probably its capacity of identifying, in most of the chapters, the key science questions particular to each area. Its clear topical arrangements make it the perfect desk reference book for researchers concerned with the Earth’s deep interior. The layout is rather good, topics are generally well explained and figures (most of them in colour) are clear and help enhancing concepts explained in the text. To summarize, I have found P. Olson’s Core Dynamics to be a very interesting and valuable book, enjoyable to read, useful not only for researchers working in the field, but also for people in the Earth sciences community, who possess good mathematical and physical skills.
