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Chapter 12: Deep Time: How Old Is Old?
Guide to Reading
This chapter deals with geologic time, from mere seconds to the billions of years in an eon, and examines the means by which geologists puzzle out Earth's history. You learn to decipher the clues Earth offers, to date Earth materials and events, and to match this to an appropriate time scale.
The author starts with small units and builds to bigger time divisions. You will read about seconds (which may not be as simple as you think), days, time zones, Greenwich Mean Time, and Coordinated Universal Time. When human society advanced enough to "have time on its hands," it used some of it to speculate about our planet. Many cultures asked, "How old is Earth?" and "What's been happening to Earth throughout all of its existence?" Some persons earned their places in history by trying to answer these questions. You will read about Archbishop James Ussher, Nicolaus Steno, James Hutton, Charles Lyell, William Smith, John Wesley Powell, Lord William Kelvin, Henri Becquerel, and Ernest Rutherford and their contributions to establishing the time frame of Earth's history.
Knowing when something happened is an important part of any historical narrative. Scientists had to figure out not only what came first, last, and in between in Earth's history (relative dating), but they had to apply real numbers (numerical or absolute dating) to Earth's materials and events. Relative dating is based on the application of several common sense principles, whereas numerical dating requires more science, therefore Earth happenings were put in proper order before they were dated. You'll read about these common sense principles of relative dating (principles of uniformitarianism, superposition, original horizontality, continuity, baked zones, cross-cutting relations, inclusions, and fossil succession).
Geologists were quite confident they were getting the events of Earth's history in proper sequence long before they felt much confidence in the numbers they assigned to the events. There were several creative lines of logic applied to the problem, which involved the salinity of oceans, depths of sediments, and temperature of Earth. Unfortunately new data, and newer and better interpretation of old data, always showed fatal flaws in these schemes. Finally, during the early 1900s, observations of the statistical regularity of radioactive decay allowed geologists to assign dates to ancient geologic materials and events that are firmly believed to this day. This method is termed radiometric dating. Your author discusses it thoroughly: the actual procedures used, what the special case of carbon 14 dating is all about, the accuracy of the method and the uncertainty of measurement, and the mechanics of radioactive decay (alpha and beta particles, electron capture, half lives, parent and daughter isotopes, fission, and fission track dating).
Several other nonradioactive procedures have played their parts in dating Earth events. Your author talks briefly about dendrochronology (tree ring dating) and about seasonal influences that result in rhythmic layering of sediments, glacial ice, mineral precipitation, and organic productivity. Rock layers, some with fossils in them, read like pages in a book to reveal Earth's history. Sometimes there are breaks in the rock record—missing pages—called unconformities, which often can be accounted for by finding the missing pages (rock layers) elsewhere in the world (a procedure called correlation).
As time passed and communications got better, correlations worldwide became complete enough to compile a geologic column showing all (or almost all) of Earth's history as written in the rocks. Improved communications also resulted in the development of a dated geologic time scale. Its organization is a bit lacking because it grew by bits and pieces over more than a century, but its terminology is essential to any discussion of Earth's history. This chapter presents the largest, most basic divisions of the scale (Precambrian, Hadean, Archean, Proterozoic, Phanerozoic, Paleozoic, Mesozoic, and Cenozoic). Chapter 13 will go into greater detail.
In a study of geology, you get used to hearing about millions and billions of years. You may be very comfortable with the words and know how many zeros go with each, but as humans we all lead lives that revolve around smaller figures and much less time. Therefore, your author concludes the chapter with an analogy that tries to fit these immense numbers onto a time frame we can feel. He equates all of Earth history to one calendar year. It is a humbling paragraph.