The relative amount of parent and daughter atoms can be used to determine the age of the rock, within a certain margin of error. The decay rate is measured in terms of half-lifeāthe time it takes for one-half of the parent atoms to decay to form the daughter atoms. Techniques that use radioactive decay are also called radiometric dating. One such method used for determining the age of human ancestors is potassium-argon dating. Potassium 40 (40K) decays to Argon 40 (40Ar) at a half-life of 1.3 billion years. At the time of the rockās formation (time zero), there is only 40K and no 40Ar. After 1.3 billion years, half of the original 40K atoms have been transformed into 40Ar atoms. Similar to this method is Uranium-238, which has a much longer half-life. Another common absolute dating method is Carbon-14, or radiocarbon dating. It is tested on organic materials rather than rock, and depends upon the decay of carbon to nitrogen. It is more limited in its time range, and is only accurate to about 50,000 years ago, although use of newer technology allows older dates to be measured. Dendrochronology (tree ring dating) is another example. Keep in mind that absolute dating does not provide a single precise date but a numerical age range, unlike relative dating that places fossils or geological events in a sequence but without an actual age in years.
FIGURE 12.2 Stratigraphic layers, or strata.
layers, called strata (singular = stratum), also called beds. Each stratum is the result of some previous natural geological event or process, such as a flood or erosion. Over time, the accumulation of these processes causes layers of sediment to pile up and form sedimentary sequences.
According to the well-established principle of superposition, the older strata are deeper in the earth and the younger ones closer to the surface. Keep in mind that geological events may occur to interfere with such neat, clear layers. These include folding events, faulting, erosion, intrusions by other sediment types, and so on.
When fossils have been deposited in sedimentary rocks, the relative ages of the fossils can be determined through comparison of the strata in which they are contained. Thus, relative dating will answer questions about "what's older" or "what's younger" rather than "when did it live?"
Stratigraphy (or biostratigraphy, if life forms are compared among strata) is only one of the many relative dating techniques. Others include paleomagnetism, fluorine dating, and faunal correlation.
Absolute, or Chronometric, Dating
Only after the discovery of radioactivity, around A.D. 1900, were we successful in obtaining absolute dates for geologic events. As a result of techniques that depend upon radioactivity, we know that Earth formed about 4.6 billion years ago, and that dinosaurs became extinct 66 million years ago.
Absolute dating is most commonly used on sediment surrounding fossils rather than the fossils themselves. To use absolute dating on rock, it must contain radioactive isotopes. Radioactive isotopes decay into more stable "daughter" atoms and, in the process, release energy (radiation). Because the decay occurs at a constant rate, it can be used to measure time elapsed.
Plate Tectonics and Climatic Trends in the Cenozoic Era
Earth's plates that underlie the continents are in motion. Now known as plate tectonics, this phenomenon was originally called continental drift. The position of the continents influences the geographical distribution patterns of all living things, including primates. Changes in the paleoclimate are also affected by plate tectonics.
Over the entire Cenozoic era, the primary trend has been one of cooling and drying, but with many fluctuations. These changes in continental positions relative to one another and past climatic conditions influenced past primate distribution, which was quite different than today. In some cases, these distribution patterns may seem quite odd to us, based on the natural distribution of modern primates.
Keep in mind that Earth looked different 66 million years ago as the Paleocene epoch began (Fig. 12.3). North America, Europe, and Asia still formed one huge landmass, called Laurasia. In the Southern Hemisphere, another landmass, Gondwanaland, was in the process of splitting apart into what would become Africa, South America, Antarctica, and Australia. India was an island continent and hadn't yet "docked" at Asia, so the Himalayas did not exist.
In the earliest Cenozoic, much of the planet was warm and humid, with little difference between the northern and