Chapter 10: Deep Time: How Old is Old?
Study Plan
This face cliff in Missouri shows two rock units. The sedimentary layers on the left were deposited on the granite to the right. From this relation, we can determine the relative age of the two units-the granite is older.
Credit: Stephen Marshak
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.
As human culture did, the author starts with small units and builds to bigger time divisions. You 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 read about James Hutton and his contributions to establishing the time frame of Earth's history.
Telling 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 age), but they had to apply real numbers (numerical or absolute age) to Earth's materials and events. Relative dating is based on the application of several commonsense principles; numerical dating requires more science. Therefore Earth happenings were put in proper order before they were dated. You'll read about the commonsense principles of relative dating and work with them in this study guide-principles of uniformitarianism, superposition, original horizontality, continuity, cross-cutting relations, inclusions, baked contacts, 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. Several creative lines of logic were applied to the problem; they 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 1950s, observations of the statistical regularity of radioactive decay allowed geologists to assign dates to ancient geologic materials and events that are firmly believed in to this day. The dating 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, half-lives, and parent and daughter isotopes.
Several other nonradioactive procedures have played their parts in dating Earth events. Your author talks briefly about 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-pages missing-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 11 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 zeroes 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.
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