Sleep, Norman H. and Kazuya Fujita. 1997. Principles of Geophysics. Blackwell Science, Malden, MA, ISBN 0-86542-076-9, 586 pp., $69.95.
This interesting but difficult book has a preface written by the authors,11 chapters of text (each with annotated references in Selected Readings), and an index (pp.565-586). It was a surprise to me to see how few of the references in the Selected Readings were published in the 1990's. It's understandable that such a book takes time to write and get ready for the market. It is only for the awareness of the reader that I mention this and suggest that one look at journal articles and other recent publications. Because the backgrounds of students and professionals in the origin-of-life sciences vary greatly, you may want to add this book to your reading list. The book is intended for advanced undergraduate and beginning graduate students. Because geophysics is a rapidly changing field, the authors tended to emphasize the basics of the theoretical aspects with some classic examples to illustrate them. The chapter titles are: 1. Solid Earth Geophysics: What, Why, and How?; 2. Gross Properties of the Earth; 3. Geochronology; 4. Gravity; 5. Heat Flow and Geothermics; 6. Magnetism; 7. Paleomagnetism; 8. Travel Time Seismology; 9.Seismic Waves and Other Mechanical Topics; 10. Global Gravity and Geodynamics; 11. Origin of the Planets. I was most interested in chapter 11 so I made the effort to scan the entire book. It wasn't easy!
The authors have made the book understandable--several levels of mathematics and physics are used throughout. The first topics in each chapters are or tend to be qualitative (little geometry and algebra). The level of mathematical difficulty both within chapters and within the book gradually escalates. Thus, at some point, you should be ready to use your experience in differential equations. The alternative is to make the best of it and accept your limitations. Or, you could borrow the math disks from you children or grandchildren and use them to review your algebra, geometry, and calculus.
The book begins with a history of geophysics: the application of physical methods and measurements of the Earth. Since the 1500's, geologists have primarily been interested in determining the distribution of rock types, finding localized deposits of useful minerals, and locating oil. Physicists were concerned with global properties of the Earth, including mass distribution heat flow, magnetic fields, the Earth's core and mantle, and the age of the Earth. Remote sensing, earthquake slippages, and plate tectonics are recent areas of study. Finding lateral homogeneities is only one aspect of this area of study (gross structure and global properties of the Earth). The importance of natural radioactive decay to our area of interest began about 100 years ago (establishing the great antiquity of the Earth). Great improvements in geochronology were the results of research in atomic physics since the 1950s. General relativity and high-energy physics gave us an understanding of the origin and abundance of elements on Earth and in the solar system. Planetology enabled us to improve our understanding of the similarity of the Earth and the other terrestrial planets (and our Moon and Io). The appreciation of the differences between these and gaseous planets also are increasing. The authors do a great job for us in this chapter.
The gross properties of the Earth (spherical shape, rotation, solar orbit, distribution of oceans and continents, etc.) may have been known by a few of the ancient Greeks, but generally this knowledge was not available until the 1500's. Specific knowledge about the ocean basins and plate margins is merely 20 years old. (We need to know more about hydrothermal systems--page 44 and pages 159-160-- but we will have to go elsewhere for that information.)
The geological history of the continents (multiple sedimentary deposition, igneous intrusion, folding, and metamorphism) is much more complex than that of ocean basins. Two-billion-year-old rocks are common on continents while those in the ocean basins are 200 million years old. Figures and photographs are in black and white throughout but are helpful. Figure 2.2 shows the outer and inner core of Earth. "The outer core is believed to be composed of molten metallic iron with nickel and some light elements such as oxygen, hydrogen, carbon, or sulfur. In contrast, the inner core--- is probably composed of solid iron-nickel metal with significantly lesser amounts of light components." The upper and lower mantles overlay the core: the lower mantle having more ferrous iron content. "The upper mantle is divided into additional layers" (classified according to their behavior under stress). "A distinction is made between the shallow, cool, uppermost part of the mantle, which together with the overlying crust forms the lithosphere, and the hot, underlying material called the asthenosphere." This hot zone slowly deforms during the flow associated with the movement of "plates". (Plate tectonics, now widely accepted, is described throughout the book. One of the reviewed topic is sea floor spreading. I was hoping for more information on hydrothermal systems.)
"The concept of plates replaced an earlier concept that continents are active "rafts" that move about while the underlying mantle and oceanic crust remain passive. This "continental drift" hypothesis was formulated by Alfred Wegener in1915. The "plate" concept was established in the 1960s. The research and the data of that period of time greatly modified our thoughts on the subject. The study of the ocean was and remains an interesting side of the origin of life story and the evolution of life that followed.
The formation of the Earth and smaller bodies in the solar system are described (introduced, on pages 16-18). The authors suggest: formation of the planets occurred from an initially hot nebula of solar composition. Hydrogen and Helium make up 98% of the solar system by mass. Water, methane, carbon dioxide, and hydrogen sulfide along with other simple compounds that can form ices in the cooler regions of the outer solar system, These account for 1.5% of the solar system by mass. The remaining 0.5% (silicates and metallic iron) are in the terrestrial planets. About 0.01% of the solar mass remains as gases and volatile elements that did not accrete with the planets. Four classes of planets exist: 1. the giant planets (Jupiter and Saturn, retain much of their solar gas); 2. the icy giants (Uranus and Neptune, mostly ice -- but with 10 - 15% gas); 3. the icy planets (satellites of Jupiter and other giant planets; and, Pluto with its satellite, Charon); and, 4. the terrestrial planets (Mercury, Venus, Earth, the Moon, Mars, and Io and Europa --- the inner major satellites of Jupiter). The authors describe these in detail. Most meteorites originated on smaller bodies in the solar system, some from the Moon, and others from Mars.
There are two theories for the origin of planets presented (page 534): the planets accreted from a gaseous, solar nebula (origin of the Sun); or, material from the Sun is pulled free (disrupted from the Sun) by a near-miss of a passing object, or is ejected (after a collision) and planets form by accretion. The authors like the first option since gas clouds have been observed to be forming (evolving into) stars resembling the Sun. (Others write of the formation of our solar system from the remnants of a super novae; the explosion of a secondary star containing heavy elements.)
The dust clouds and gases collect on the cold (10 degrees Kelvin) plane of the rotating galaxy (page 534). The icy-rocky components form dust grains. The gravitational collapse of the dust clouds and gases form a rotating system (solar nebula). "The orbital velocity of the gas is less than that of the solid particles because the gaseous pressure resists the fall of the gas into the Sun..." The solids in orbit experience drag. Circular orbits result (for most of the mass). Some "fine dust remains in suspension..." while "slightly larger grains...settle inward..." attracted by the Sun's gravity. Eventually, "...even large orbiting bodies..." spiral inward towards the Sun. The collapse involves almost all the material within a diameter of about one light year. Magnetic fields emerge (around the accretions). This action requires about one million years. Grains of dust in the solar nebula survive in meteorites and are used to determine the age of the solar system's formative events. Comets are believed to form primarily at the distance of Neptune and Uranus by the collapse of icy dust (page 543).
The authors discuss the accretion of the terrestrial planets (beginning with the time required for a swarm of planetesimals to collapse into a planet), the effects of gravity on this process, and the origin of the Moon. If the accretion process is fast, the heat of accretion is retained. The particles accrete because of their size and gravitational attraction. Large units grow in size faster than small units. Many comets are ejected out of the solar system (perturbed by Neptune). "Periodic comets are produced by strong perturbations of the orbit during their close approaches to a giant planet." They have a life-time of about one million years. Asteroids are more stable (in the asteroid belt). Collisions in that region yield meteors, some forming new, unstable orbits the lead to collisions with Earth. The theories on the origin of Earth's Moon (4.4 billion years ago) include: capture from solar orbit; coaccretion of the Earth and Moon, and expulsion of the Moon from the Earth (rotational instability or impact of a very large object). The impact hypothesis is favored (expulsion of a hot Moon). Two ways that this could have happened are: the impact was off-center, the object hitting Earth was large; or, a series of impacts occurred, the impacts ejecting material that later accreted to form the Moon. Thus, you will want to review the history of Gravity (Chapter 4). Humans have known about volcanoes and hot springs (someplace underground there is a great amount of heat). Chapter 5 tells us about that and how we study it. The upward flow of groundwater transports heat to the surface. Plate movements also transfer heat. Heat flows through the oceanic crusts, in the region below the continents, and from the continents (radioactivity in the continental crusts). The histories of magnetism from the 1500's (Chapter 6) and paleomagnetism (Chapter 7) since the 1950's are interesting. The more we learn about it, the more complex the math! Similarly, the more we try to read carefully in the following chapters, the more heavy our eye lids become. Try it. You'll like it.
This illustrated review was prepared by Aristotel Pappelis (professor, Department of Plant Biology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901).