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“A wonderful primer on geology, and a clear explanation of how the science is done.”
“The story of Earth is told in such geologic forms as rock strata, volcanic ...
“The story of Earth is told in such geologic forms as rock strata, volcanic eruptions, meteor craters, fossils, ocean currents, and ice flows. Macdougall gives these disparate elements voice and puts them into a perspective that emphasizes why Earth science is important in our understanding of both the planet’s history and our role in its tomorrow.”-Jeff Kanipe, author of The Cosmic Connection: How Astronomical Events Impact Life on Earth
Set in Stone
In 1969, when I was a student in California, there was a rash of predictions from astrologers, clairvoyants, and evangelists that there would be a devastating earthquake and the entire state—or at least a large part of it—would fall into the ocean. The seers claimed this would happen during April, although they were not in agreement about the precise date. A few people took the news very seriously, sold their houses, and moved elsewhere. Others, a bit less cautious, simply sought out high ground on April 4, the date of the Big One according to several of the predictors. Cartoonists and newspaper columnists had a field day poking fun at the earthquake scare, and for us geology students the hubbub was amusing but also seemed a bit bizarre. Police and fire stations, along with university geology departments, got thousands of anxious telephone calls from nervous citizens. Ronald Reagan, then the state's governor, had to explain that his out-of-state vacation that month had been planned long in advance and had nothing to do with earthquakes. The mayor of San Francisco planned an anti-earthquake party for April 18, the sixty-third anniversary of the great 1906 San Francisco earthquake. He assured the public that it would be held on dry ground.
California didn't fall into the sea in 1969, of course, nor was there a huge earthquake (although there were earthquakes, as there are every year, most of them quite small). Astrologers can't predict earthquakes (or much else). Even earth scientists, with the best geological information and most up-to-date instrumentation, find precise earthquake prediction elusive, as we will see later in this book. However, the prognosis is much better for many other geological phenomena. And at the core of this geological prediction lies the kind of work geologists have traditionally done: decoding the past.
But how, exactly, do they do that? Where do earth scientists look to find clues to the details of our planet's history, and how do they interpret them? Those questions are at the heart of this book, and the answers are hinted at in the title of this chapter: the clues are found, for the most part, in the stones at the Earth's surface. (There are also many other natural archives of Earth history, such as tree rings and Antarctic ice. Ice cores in particular provide invaluable information about past climates. But these other records tell us only about the relatively recent geological past. Rocks allow us to probe back billions of years.)
To the uninitiated a rock is just a rock, a hard, inanimate object to kick down the road or throw into a pond. Look a little closer and ask the right questions, however, and it becomes more—sometimes much more. Every single rock on the Earth's surface has a story to tell. How did the rock form? When did it form? What is it made of? What is its history? How did it get here, and where did it come from? Why is this kind of rock common in one region and not in another? For a long time in the predominantly Christian countries of the West, answers to questions like these were constrained by religion. The biblical flood was thought to have been especially important in shaping the present-day landscape, and explanations for many geological features had to be built around the presumed reality of this event. However, as the ideas of the Enlightenment took hold during the seventeenth and eighteenth centuries, and as close observation of the natural world became ever more crucial for those seeking to understand the Earth, the sway of religion diminished and more rational explanations began to emerge. For geology especially, a field with its roots in the search for and extraction of mineral resources from the Earth, the pressure of commerce was also important. Those with the best understanding of how gold veins formed, or with the best knowledge of the kinds of geological settings likely to contain such veins, had the best chance of finding the next gold mine.
I will not dwell at length here on the history of geology's development as a science, or on the details of how early geological ideas evolved; these things have been dealt with in many other books. But it is worth pointing out a few key early concepts that revolutionized the way everyone—not just scientists—thought about our planet. Most of these intellectual breakthroughs arose in Europe (especially in Britain) in the eighteenth and early nineteenth centuries, and although there had been independent thinkers in the Middle East and elsewhere who had arrived at similar conclusions much earlier, the European versions would form the bedrock(!) of the emerging field of earth science.
What were these ideas and how did they come about? Without exception they stemmed from examination of rock outcroppings in the field together with observations of ongoing geological processes. One of the new concepts was that different rock types have quite different origins, something that seems obvious enough to us today. But in the eighteenth century a popular concept was that all rocks were formed by precipitation, either from a primordial global ocean or from the waters of the biblical flood. Those who championed this idea were dubbed—for obvious reasons—Neptunists, and they did not give up their theory easily. However, observations like those of Scottish geologist James Hutton, who described outcrops showing clear evidence that some rocks had once been molten, eventually turned the tables. The rock outcrops told Hutton a vivid story: flowing liquid material, now solid rock, had intruded into, and disrupted and heated up, preexisting rock strata. Hutton's descriptions of these once-molten rocks—not to mention the presence of active volcanoes like Vesuvius and Etna in southern Europe—led to the realization that there must be reservoirs of great heat in the planet's interior.
A second important early concept was that slow, inexorable geological processes that can readily be observed (rainwater dissolving rocks, rivers cutting valleys, sedimentary particles settling to the seafloor) follow the laws of physics and chemistry. Once again this seems an obvious conclusion in hindsight, but its implication—this was the revolutionary part for early geologists—was that geological processes in the distant past must have followed these very same laws. This meant that the physical and chemical characteristics of ancient rocks could be interpreted by observing present-day processes. Charles Lyell, the foremost British geologist of his day, promoted this idea as a way of understanding the Earth's history in his best-selling book Principles of Geology, first published in 1830. (The book was so popular it went through numerous editions and is still in print today in the Penguin Classics series.) Lyell himself was not the originator of the concept, but he called it the "principle of uniformitarianism," and the name stuck. Although the phrase itself is no longer in vogue, generations of geology students have learned that it really means "the present is the key to the past." And although the early geologists were primarily interested in working out the Earth's history, Lyell's principle of uniformitarianism can also be turned around: using the same logic, it is true that—to a degree—the past is a key to the future.
Finally, the most revolutionary of the new concepts was that the Earth is extremely old. This flew in the face of both the conventional wisdom of contemporary scholars and the religious dogma of the day. Once again, as with so much early geological thought, the idea of an ancient Earth was formalized by James Hutton, who wrote (in a much-quoted pronouncement about geological time), "we find no vestige of a beginning, no prospect of an end." No single observation led Hutton to the concept of a very old Earth; it was instead a conclusion he drew from a synthesis of all his examinations of geological processes and natural rock outcroppings —observations of things like great thicknesses of rock strata made up of individual sedimentary particles that could only have accumulated slowly, grain by grain, over unimaginably long periods of time.
With a foundation built on these new ideas, which were popularized and widely disseminated through Lyell's book, and with an ever-increasing demand for minerals and resources from the Earth, geology, now mostly free of religious fetters, exploded as a science during the nineteenth century. Countries developed geological surveys to map the terrain and discover resources, and universities founded departments of geological sciences. Decoding the past became a full-time occupation for a legion of geologists.
Today geology is subsumed into the much broader field of earth science, which includes everything from oceanography to mineralogy and environmental science. In a modern university earth science department, it is not uncommon to find researchers in the same building probing subjects as diverse as climate change, biological evolution, the chemical makeup of the Earth's interior, and even the origin of the Moon.
Let's return, however, to those clues to the Earth's past that are inherent in the physical and chemical properties of the planet's rocks—the clues that are set in stone. The challenge for earth scientists is to find ways to extract and interpret them, and in recent years very sophisticated techniques have been developed to do this. Nevertheless, there are also some very simple examples, long used by geologists, that illustrate how the approach works. Take the igneous rocks, those that form from molten material welling up from the Earth's interior. They come in many flavors, from common varieties familiar to most people, like granite or basalt, to more exotic types you may never have heard of, with names like lamprophyre and charnockite. The chemical compositions of these rocks can provide information about how they originated, but chemical analysis requires sophisticated equipment. On the other hand, there is a very simple feature—one that can be assessed quickly by anyone—that provides evidence about where the rocks formed. That characteristic is grain size.
Igneous rocks are made up of millions of tiny, intergrown mineral grains that crystallized as the liquid rock cooled down. How big these grains grow depends crucially on how fast the rock cools; lava flows that erupt on the Earth's surface cool rapidly, and the resulting rocks are very fine-grained. But not all lava makes it to the surface. Some remains in the volcanic conduits, perhaps miles deep in the ground. Well insulated by the overlying rocks, it takes this material a long time to cool, and the slowly growing mineral grains get much bigger than their surface equivalents. For this reason, rocks with exactly the same chemical makeup can have contrasting textures and look very different, depending on how quickly they congeal. This simple characteristic can be used to say something about the depth in the Earth at which the rocks formed.
Less obvious characteristics require more ingenuity to decode, but because the payoff—in terms of what can be learned about the Earth's history—is so great, earth scientists are continually searching for new ways to probe rocks. As we will see in later chapters of this book, geochemistry, especially the fine details of a rock's or an ice core's chemical composition, has become especially important. The behavior of chemical elements such as iron, or sulfur, or molybdenum, for example, depends on the amount of oxygen in their environment. As a result, the minerals formed by these elements are sensitive indicators of oxygen levels when they formed—and in some cases can be used to determine the amount of oxygen in the ancient ocean or atmosphere.
Similarly, analysis of isotopes has become one of the most important ways to extract information about the Earth's past. (Isotopes are slightly different forms of a particular chemical element; almost every element in the periodic table has several isotopes.) Often the conditions that prevailed when a sample formed can be deduced by measuring the abundances of different isotopes of a particular chemical element; we will encounter many examples of this approach in later chapters of this book. In an ice core, for example, oxygen or hydrogen isotope abundances might tell us about the temperature 100,000 years ago; in an ancient rock, isotopes might fingerprint the process that formed the rock, and allow us to investigate how similar or different that process was to those that occur today.
The very first application of isotopes in the earth sciences—aside from the use of radioactive isotopes for dating—still evokes admiration among geochemists and sometimes amazement from those who know nothing about geochemistry. It is a good illustration of how ordinary rocks can be a treasure trove of information about the past when the right questions are asked. In the late 1940s Harold Urey, a Nobel Prize–winning chemist at the University of Chicago, discovered from theoretical considerations that in some compounds the proportions of the different isotopes of oxygen depend on the temperature when the compound formed. In a flash of insight, he realized that this property could be used to deduce the temperature of the ancient ocean—a groundbreaking idea. Urey proposed that measurements of oxygen isotopes in the calcium carbonate shells of fossil marine organisms could be used to calculate the water temperature when these creatures grew. He and his students then verified the theory by making those measurements, and in doing so they pioneered the field of "paleotemperature" analysis. Since that early work, tens of thousands, if not hundreds of thousands, of oxygen isotope measurements have been made to document in fine detail how seawater temperatures have fluctuated in the past. In my humble opinion—perhaps with a slight bias because my own background is in geochemistry—Urey's paleotemperature work ranks among the all-time great advances in the earth sciences.
Different rock types raise different questions about the past, of course, or at least allow different questions to be asked, but well-defined approaches for extracting evidence have been worked out by earth scientists for most rock varieties within the three great categories: igneous, sedimentary, and metamorphic. These familiar subdivisions of the rock kingdom are based on mode of formation: igneous rocks such as granite are formed from molten precursors, as James Hutton was one of the first to realize; sedimentary rocks result from the deposition or precipitation of particles, usually from water; and metamorphic rocks arise when any preexisting rock is changed chemically and/or physically, typically when heated or stressed during a process like deep burial or mountain building. Current theories about how the outer part of the Earth formed and has evolved rest on evidence derived mainly from the chemical properties of igneous and metamorphic rocks, which are the primary components of both the continents and the seafloor. But in many ways sedimentary rocks are the most important for decoding the Earth's history.
Why should that be? There are at least two reasons. First, they form at the Earth's surface, mostly in the sea but sometimes (as in the case of sandstones composed of desert sand) in contact with the atmosphere. This means that, potentially, these rocks incorporate information about the Earth's surface environment in the distant past. And second, many sedimentary rocks contain fossils, the primary record of how life on Earth arose and evolved. Without fossils, our understanding of evolution would be rudimentary.
By putting together thousands upon thousands of stories from studies of individual igneous, sedimentary, and metamorphic rocks and rock outcroppings, earth scientists have gradually woven together a history of the Earth. As for most histories, the details become less sharp the farther back one probes. Some of the most ancient evidence is missing entirely, or made difficult to interpret because geological processes operating over the Earth's long history have altered the rocks' characteristics and muddled the clues they contain. Nevertheless the narrative of our planet's evolution as we know it today is a superb scientific achievement. It is also a story in revision, continually updated as new discoveries are made and improvements in analytical capabilities allow new questions to be asked.
But what about chronology? How have earth scientists determined the timescale of this narrative? Events need to be ordered in time if we are to understand their significance; it isn't very helpful to know the temperature of the seawater in which a fossil animal grew if you have no idea when it lived. Ever since Hutton's "no vestige of a beginning, no prospect of an end"—and even before that—earth scientists have sought ways to determine the ages of rocks and the Earth as a whole. The ultimate goal—the development of techniques that could provide the "absolute" age of rocks in years—came within reach only with the discovery of radioactivity near the end of the nineteenth century. We will come to that shortly. But long before radioisotope dating methods were devised, earth scientists had already developed early versions of the geological timescale, placing important events from the Earth's history in a time sequence (see figure 1 for a modern version; if you are not already familiar with the names of geological eons, periods, etc., you may want to refer to this figure repeatedly as you read this book). How did they do this?
As early as the 1660s Nicolas Steno, a Danish anatomist who had an insatiable curiosity about the natural world, realized that rocks at the bottom of a stack of sedimentary layers must be older than those at the top. Steno was living in Italy at the time, and his observations were made while he was examining sedimentary rocks in the Alps. His insight was that the Alpine sedimentary strata—and the fossils they contain—have time significance. It is only relative time significance, to be sure; Steno could say whether a particular layer was older or younger than neighboring layers, but he couldn't determine its actual age. All this may seem obvious now, but at the time it was a breakthrough. By studying the inert rock layers of the Alps, Steno was able to visualize the nature and timing of their formation. Today he is generally regarded as the founder of the field of stratigraphy, the scientific study of sedimentary rock strata.
Excerpted from Why Geology Matters by Doug Macdougall. Copyright © 2011 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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1. Set in Stone
2. Building Our Planet
3. Close Encounters
4. The First Two Billion Years
5. Wandering Plates
6. Shaky Foundations
7. Mountains, Life, and the Big Chill
8. Cold Times
9. The Great Warming
10. Reading LIPs
11. Restless Giants
12. Swimming, Crawling, and Flying toward the Present
13. Why Geology Matters
Bibliography and Further Reading
Posted April 5, 2013
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