I was in college 14 years, educated mainly in geology. Funny thing: graduate school ruined geology for me as a profession. So I returned to rockhounding as a hobby, and earned my keep by writing software for geologists and other sciences; I had enough classwork in the "hard" sciences to get majors or minors in them all. But reading the writing of Professor Donald R. Prothero made me nostalgic for what might have been. However, I judge that Dr. Prothero is quite a bit more talented than I in big-picture geology, and the long shelf of his books on the subject attests to that. The oldest book of his currently on sale, a major textbook, Interpreting the Stratigraphic Record, published in 1990, still sells for about $128.
Today's book, however, is much more accessible (and less costly!) than the text: The Story of the Earth in 25 Rocks: Tales of Important Geological Puzzles and the People Who Solved Them. (That's not the longest title I've seen, but it is close.) We can take the word "Rocks" in the title rather loosely, since one chapter is on the San Andreas Fault, all 800 miles of it, and another is the first stratigraphic sequence and the first geologic map, produced by William Smith in England in the early 1800's. However, actual rocks aplenty are dealt with, from tiny zircons used to determine the ages of ancient rocks to meteorites, coal and the tin oxide mineral Cassiterite that led to the "tin can" and "tinfoil", before aluminum (aluminium to the English) became cheap enough to replace tin.
Zircons are very valuable geologically because they often contain most of the uranium found in granite and similar igneous rocks, and their robust structure keeps all the daughter elements from uranium's breakdown, so that measurements of the ratio of uranium to lead can tell you the age of the zircon, and thus of the rock it came from. Beware, though: zircon crystals are also found in sedimentary rocks, and do not tell you the age of those rocks, but the age of the rocks in which they first formed. As long as you know this, they are still useful. You just need to know what you are doing.
Just by-the-by, the oldest piece of rock found on Earth is a tiny zircon crystal with an age of 4.4 billion years. The half-lives of U-238 and U-235 are 4.5 and 0.7 billion years, respectively. The final product of U-238 is Pb-206, and that of U-235 is Pb-207. Thus, since the time that ancient crystal was formed, nearly half its U-238 turned to Pb-206, and all but 1.3% of the U-235 turned to Pb-207. A zircon that contains more atoms of lead than of uranium is going to be very old.
The overarching theme of the book is the gradual development of the foundational "sphere" of Earth System science (Geosphere, Hydrosphere, Atmosphere, Biosphere, in decreasing order of mass). When I was a child the development of mountain ranges was explained as the wrinkling of a thin "skin" (Earth's crust) as the planet shrank while it cooled. The analogy was made to an uneaten apple that gets wrinkly as it dries out. When I first took an Earth Science course in high school, there was talk of orogeny (mountain building) as being a side product of geosynclines, based primarily on vertical motions. I do recall someone remarking that the coastlines of Africa and South America seemed to match, and I first heard of "continental drift" at that time, maybe just before 1960. Only after I became a geology major (my third major) in 1970 did I learn of plate tectonics, in which the continents don't just "drift" but are moved along by a "conveyor belt" system, riding in the midst of enormous basaltic plates, driven by a combination of sea-floor spreading at diverging plate boundaries and subduction at converging plate boundaries. Now the analogy is a pot full of thick syrup with a sugary crust on top, heated from the bottom so it convects slowly, moving chunks of the crust about.
At one point or another, every chapter of this book ties back to the plate tectonic system. And why not? It is the whole-Earth process that literally creates geology. To see a planet without tectonic motions, look at Mars. The early loss of that planet's water and 99.4% of its atmosphere pretty much halted major erosion, so that we see a 3-4-billion-year-old landscape with two major kinds of features: one very big canyon (and a few smaller ones), formed as the last of the waters dried up, and a few enormous volcanoes, three times the height of Mount Everest. Oh, and there is a scattering of impact craters gathered over the past few billion years; though there are many, they are much less abundant than craters on the Moon, because Mars did have an eroding atmosphere and hydrosphere for its first billion years or so.
At present, back here on Earth, the Himalayas, the Sierras, the Andes and a few other ranges are growing, the Rockies and Urals are at a standstill, and other ranges such as the Appalachians are eroding away. New features replace old features. This will continue, though at a slightly decreased rate, until the Sun becomes a red giant, and perhaps longer. In another 4.5 billion years, half the present amount of uranium will be gone, and only 8% of the present amount of the radioactive isotope of potassium (K-40) will remain. Crank these figures backward, and we find that radiogenic heating was six times as great as it is now about 4 billion years ago (4Ga in geologist-speak). So plate tectonics rocked along quite a lot faster when Earth was young.
I was rather charmed to notice that most of the photos in the book are credited to Wikimedia Commons. It is now possible to write a book like this one without spending months writing letters to people who might have photos of things you want to illustrate, and permission to use them. However, there are a few other signs of rapid production that caused me a bit of concern. No author should be without a good copy editor, as these three examples (only a few of a dozen or so) attest:
- Page 80, first paragraph: discussing Lord Kelvin's estimate of 100 million years as "the time since the Cambrian", it is stated that this is off by "a factor of almost 50". The Cambrian era began a little more than 500 million years ago, so a factor of 5 would then be the correct one. However, looking up what Kelvin actually wrote, I find that he considered 100 million years to be the age since the Earth cooled from a molten state. Here, the actual factor should indeed be something like 45. So the mention of the Cambrian is the actual error. One's copy editor must know something about the science, not just English usage.
- Page 130, Figure 12.4, a photo of the 4.4 Ga zircon I mentioned above, called a "microphotograph." No, it is a photomicrograph. A microphotograph is what you find in microfilm, where a page of text is rendered to a size of about 1 cm, or even in a "spy dot", in which the page is reduced to 1 mm. A photograph of something through a microscope is a photomicrograph. The error is common, but should not be found in a book by a scientist of this stature.
- Page 221, last paragraph: discussing the news reporter's adage, "If it bleeds, it leads", except in this instance, the word is spelled "ledes". That's one of several dramatic misspellings I found.
OK, I just had to scratch that itch.
Of most importance is the content. This book is a must-read by anyone with the slightest curiosity about the "solid" Earth, about rocks, about how mountains are formed, or about earthquakes. Dr. Prothero is an engaging writer, thoroughly fun to read, who imparts a great lot of information quite painlessly. We need more such teachers among us.
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